MORPHOLOGICAL AND PHYSIOLOGICAL RESPONSES OF VEGETABLE 

TRANSPLANTS, HERBS, AND LEAFY GREENS TO LIGHT QUALITY, QUANTITY, AND 

DURATION 

By 

Charlie Garcia 

A THESIS 

Submitted to 

Michigan State University 

in partial fulfillment of the requirements 

for the degree of 

Horticulture—Master of Science 

2019

 

 

ABSTRACT 

MORPHOLOGICAL AND PHYSIOLOGICAL RESPONSES OF VEGETABLE 

TRANSPLANTS, HERBS, AND LEAFY GREENS TO LIGHT QUALITY, QUANTITY, AND 

DURATION 

By 

Charlie Garcia  

Controlled-environment (CE) production of vegetables, herbs, and leafy greens is quickly 

expanding. However, knowledge gaps exist on how to manage them in CEs. Thus, we evaluated 

developmental parameters of 10 different basil (Ocimum spp.) species and cultivars and 8 herbs 

and leafy greens under a truncated 9-h short day (SD), day-extension lighting, and a 4-h night 

interruption utilizing red+white+far-red (R+W+FR) light-emitting diodes (LEDs). All basil 

cultivars, excluding ‘Red Rubin’ can be classified as day-neutral plants (DNP) under a low 

photosynthetic daily light integral (DLI). Coriander, dill, lavender, and marjoram can be 

classified as facultative long-day plants (LDPs). While watercress, oregano ‘Kirigami’ and 

‘Greek’, and spearmint can be categorized as obligate LDPs. Furthermore, six basil cultivars 

were evaluated under a 9-h SD and 16-h LD utilizing a moderate DLI of ≈13 mol∙m–2∙d–1 and a 

high DLI of ≈23 mol∙m–2∙d–1. Time to visible bud and open flower were hastened under high 

DLIs and node number below the first open flower were reduced indicating that basil exhibited a 

facultative irradiance response (FIR). In a separate experiment, cucumber ‘Elsie’, tomato 

‘Climstar’, and pepper ‘Kathia’ seedlings were grown under LED supplemental lighting (SL) 

providing spectral qualities beyond B and R radiation. Fresh weight and leaf area of all three 

species was greater when G radiation replaced R and B radiation. However, other plant 

parameters evaluated in response to radiation quality were found to be species-specific. Results 

from these studies can provide growers with information on how to manage growth and 

development of vegetable transplants, culinary and ornamental herbs, and leafy greens in CEs.

 

 

ACKNOWLEDGEMENTS 

 

I would like to express my sincere gratitude to my major adviser Dr. Roberto G. Lopez, for 

his academic guidance, patience, and support. I would also like to thank Dr. Erik S. Runkle, and 

Dr. Chieri Kubota for serving on my advisory committee and providing valuable advice 

throughout my experiments and thesis writing. I extremely grateful to Nate DuRussel and Caleb 

Spall for their technical assistance during my experiments. I would also like to thank all of the 

undergraduate greenhouse staff for helping to keep my plants alive, pulling black cloth, and for 

helping with data collection.  

Funding and/or support from the USDA-ARS Floriculture and Nursery Research Initiative, 

and the National Institute of Food and Agriculture, Hatch project MICL02472 are greatly 

appreciated. Supplies provided by industry partners in the form of seeds from Ball Horticultural 

Co. and Syngenta, fertilizer from The Blackmore Co. and JR Peters Inc., substrates from Grodan, 

and LEDs from Heliospectra are also greatly appreciated.  

I would like to thank everyone from the floriculture group for your kind support, and all my 

current graduate student colleagues including Keivan Bahmani, Nathan Kelly, Anthony Soster, 

Prabhjot Kaur, Kellie Walters, and Annika Kohler. A special thank you to Dr. Yujin Park, 

Mengzi Zhang, and Dr. Qingwu Meng for your wonderful friendship and support.  

Finally, I would like thank my family for their never-ending support. For continually pushing 

me to strive onward and never give up. None of this would be possible without your help. Last a 

special thank you to Maggie, for standing by me until the end.     

iii 

TABLE OF CONTENTS 

 

LIST OF TABLES ......................................................................................................................... vi 
 
LIST OF FIGURES ...................................................................................................................... vii 
 
SECTION I .................................................................................................................................... 1 
LITERATURE REVIEW ............................................................................................................ 1 
Literature Review: Horticultural Lighting for Controlled Environments .............................. 2 
Introduction .............................................................................................................................. 2 
Radiation Quality ..................................................................................................................... 3 
Blue radiation .......................................................................................................................... 4 
Green radiation ....................................................................................................................... 5 
Red and far-red radiation ........................................................................................................ 7 
Photoperiodic Lighting ............................................................................................................ 8 
Supplemental Lighting ........................................................................................................... 10 
LITERATURE CITED .............................................................................................................. 13 
 
SECTION II ................................................................................................................................ 21 
FLOWERING DEVELOPMENT PARAMETERS OF CULINARY AND ORNAMENTAL 
HERBS AND LEAFY GREENS IN RESPONSE TO PHOTOPERIOD .............................. 21 
Abstract ................................................................................................................................... 23 
Introduction ............................................................................................................................ 24 
Materials and methods ........................................................................................................... 27 
Plant material ........................................................................................................................ 27 
Greenhouse environment and lighting treatments ................................................................ 29 
Data collection and analysis ................................................................................................. 30 
Results ...................................................................................................................................... 31 
Flowering percentage ............................................................................................................ 31 
Time to first visible bud ......................................................................................................... 32 
Time to first open flower ....................................................................................................... 32 
Node number below the first open flower ............................................................................. 33 
Stem length at first open flower ............................................................................................. 33 
Discussion ................................................................................................................................ 34 
Acknowledgements ................................................................................................................. 38 
APPENDIX .................................................................................................................................. 39 
LITERATURE CITED .............................................................................................................. 47 
 
SECTION III ............................................................................................................................... 51 
PHOTOSYNTHETIC DAILY LIGHT INTEGRAL AND PHOTOPERIOD INFLUENCE 
DEVELOPMENT OF BASIL SPECIES AND CULTIVARS ................................................ 51 
Abstract ................................................................................................................................... 53 
Introduction ............................................................................................................................ 54 
Materials and methods ........................................................................................................... 58 
Plant material (Expt. 1) ......................................................................................................... 58 

iv 

Plant material (Expt. 2) ......................................................................................................... 59 
Greenhouse environment and lighting treatments (Expt 1) .................................................. 59 
Greenhouse environment and lighting treatments (Expt 2) .................................................. 61 
Data collection and analysis ................................................................................................. 61 
Results ...................................................................................................................................... 62 
Expt. 1: Time to first visible bud ........................................................................................... 62 
Expt. 1: Time to first open flower .......................................................................................... 63 
Expt. 1: Node number below the first open flower ................................................................ 63 
Expt. 1: Height at first open flower ....................................................................................... 64 
Expt. 2: Time to first visible bud ........................................................................................... 64 
Expt. 2: Time to first open flower .......................................................................................... 64 
Expt. 2: Node number below the first open flower ................................................................ 65 
Expt. 2: Height at first open flower ....................................................................................... 65 
Discussion ................................................................................................................................ 66 
Acknowledgements ................................................................................................................. 70 
APPENDIX .................................................................................................................................. 71 
LITERATURE CITED .............................................................................................................. 83 
 
SECTION IV ............................................................................................................................... 88 
SUPPLEMENTAL LIGHTING RADIATION QUALITY INFLUENCES CUCUMBER, 
TOMATO, AND PEPPER TRANSPLANT GROWTH AND DEVELOPMENT ............... 88 
Abstract ................................................................................................................................... 90 
Introduction ............................................................................................................................ 91 
Materials and methods ........................................................................................................... 96 
Plant material ........................................................................................................................ 96 
Greenhouse environmental conditions .................................................................................. 97 
Supplemental lighting treatments .......................................................................................... 98 
Plant measurements and experimental design ...................................................................... 99 
Results .................................................................................................................................... 100 
Cucumber ............................................................................................................................ 100 
Tomato ................................................................................................................................. 101 
Pepper ................................................................................................................................. 102 
Discussion .............................................................................................................................. 103 
Acknowledgements ............................................................................................................... 112 
APPENDIX ................................................................................................................................ 113 
LITERATURE CITED ............................................................................................................ 120 
 

v 

LIST OF TABLES 

 

Table II-1. Photoperiod (h), mean (±SD) bench average daily temperature (ADT), and daily light 
integral (DLI) (±SD) during two experimental replications (Rep.) for culinary and ornamental 
herbs and leafy greens. Plants were grown under a truncated 9-h short day (SD) or under a 9-h 
SD extended with red+white+far-red (R+W+FR) light-emitting diode (LED) lamps to achieve 
12-, 13-, 14-, and 16-h photoperiods or a 4-h night interruption (NI) .......................................... 40 
 
Table III-1. Expt. 1. photoperiod (h), mean (±SD) bench average daily temperature (ADT), and 
daily light integral (DLI) (±SD) during two experimental replications (Rep.) for basil. Plants were 
grown under a truncated 9-h short day (SD) and day length was extended with red+white+far-red 
(R+W+FR) light-emitting diode (LED) lamps to achieve 11-, 12-, 13-, 14-, 15-, 16-h or a 4-h 
night interruption (NI)................................................................................................................... 72 
 
Table III-2. Expt. 2. daily light integral (DLI) treatment, photoperiod (h), average DLI (±SD), and 
greenhouse mean (±SD) air average daily temperature (ADT) delivered to six basil cultivars. 
Plants were grown under a truncated 9-h short day (SD) or a 16-h long-day (LD), with moderate 
or high photosynthetic DLIs. Day extension photoperiodic lighting (PL) was delivered by light-
emitting diode (LED) lamps. LED lamps emitted red+white+far-red (R+W+FR) radiation and 
were used to extend the photoperiod by 7 h [16 h (1700 to 2400 HR)]. ........................................ 73 
 
Table IV-1. Supplemental lighting (SL) treatments, replication (Rep.), supplemental radiation, 
average daily light integral (DLI) from SL provided by high-pressure sodium (HPS) lamps or 
light-emitting diodes (LEDs), for 16 h∙d–1 based on an instantaneous threshold [on from 0600 to 
2200 HR when the outside PPFD was below ≈440 µmol·m–2·s–1 (on for a minimum of 25 
minutes and off for a minimum of 20 minutes)], greenhouse canopy air and leaf temperature, and 
nutrient solution pH and electrical conductivity (EC). Cucumber, pepper, and tomato were 
placed under treatments on 26 Sept. 2018, 24 Oct. 2018, and 07 Jan. 2019. ............................. 114 
 

vi 

LIST OF FIGURES 

 

Figure II-1. Spectral distribution, intensity of blue [B (400–500 nm)], green [G (500–600 nm)], 
red [R (600–700 nm)], and far-red [FR (700–800 nm)] radiation, total photon flux density 
(TPFD), light ratio, and estimated phytochrome photoequilibria [PFR/PR+FR (the proportion of 
FR-absorbing phytochromes in the pool of R- and FR-absorbing phytochromes; Sager et al., 
1988)] of R+W+FR light-emitting diode (LED) lamps covered with multiple layers of wire 
mesh. R:FRwide was calculated as 600 to 700 nm:700 to 800 nm; R:FRnarrow was calculated as 655 
to 665 nm:725 to 735 nm .............................................................................................................. 41 
 
Figure II-2. Flowering percentage for eight herbs and leafy greens grown under a truncated 9-h 
short day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-emitting 
diode (LED) lamps to achieve 12-, 13-, 14-, and 16-h photoperiods or a 4-h night interruption 
(NI). Flowering percentage of oregano ‘Greek’ and spearmint ‘Spanish’ are presented by 
replication (Rep.) due to variability between experimental Reps ................................................. 42 
 
Figure II-3. Time to first visible bud (VB) for eight herbs and leafy greens grown under a 
truncated 9-h short day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) 
light-emitting diode (LED) lamps to achieve 12-, 13-, 14-, and 16-h photoperiods or a 4-h night 
interruption (NI). Data were pooled when there was no interaction between replication (Rep.) 
and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across photoperiodic treatments using Tukey-Kramer difference test at P ≤ 0.05. 
Error bars indicate standard error. Mean separations were excluded due to variability between 
experimental Reps. for oregano ‘Kirigami’, oregano ‘Greek’, and spearmint ‘Spanish’... .......... 43 
 
Figure II-4. Time to first open flower (OF) for eight herbs and leafy greens grown under a 
truncated 9-h short day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) 
light-emitting diode (LED) lamps to achieve 12-, 13-, 14-, and 16-h photoperiods or a 4-h night 
interruption (NI). Data were pooled when there was no interaction between replication (Rep.) 
and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. Error bars indicate 
standard error. Mean separations were excluded due to variability between experimental Reps. 
for oregano ‘Greek’, and spearmint ‘Spanish’ .............................................................................. 44 
 
Figure II-5. Node number for eight herbs and leafy greens grown under a truncated 9-h short day 
(SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-emitting diode (LED) 
lamps to achieve 12-, 13-, 14-, and 16-h photoperiods or a 4-h night interruption (NI). Data were 
pooled when there was no interaction between replication (Rep.) and treatment, or if the response 
trends were similar between Reps. Letters indicate mean separations across photoperiodic 
treatments using Tukey-Kramer difference test at P ≤ 0.05. Error bars indicate standard error. 
Mean separations were excluded when there were interactions or due to variability between 
experimental Reps. for oregano ‘Kirigami’, oregano ‘Greek’, and spearmint ‘Spanish’ ............. 45 
 

vii 

Figure II-6. Stem length at first open flower (OF) of eight herbs and leafy greens grown under a 
truncated 9-h short day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) 
light-emitting diode (LED) lamps to achieve 12-, 13-, 14-, and 16-h photoperiods or a 4-h night 
interruption (NI). Data were pooled when there was no interaction between replication (Rep.) 
and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across photoperiodic treatments using Tukey-Kramer difference test at P ≤ 0.05. 
Error bars indicate standard error. Mean separations were excluded due to variability between 
experimental Reps. for oregano ‘Greek’, and spearmint ‘Spanish’ .............................................. 46 
 
Figure III-1. Expt. 1 and 2. spectral distribution, intensity of blue [B (400–500 nm)], green [G 
(500–600 nm)], red [R (600–700 nm)], and far-red [FR (700–800 nm)] radiation, total photon 
flux density (TPFD), light ratio, and estimated phytochrome photoequilibria [PFR/PR+FR (the 
proportion of FR-absorbing phytochromes in the pool of R- and FR-absorbing phytochromes; 
Sager et al., 1988)] of R+W+FR light-emitting diode (LED) lamps covered with multiple layers 
of wire mesh. R:FRwide was calculated as 600 to 700 nm:700 to 800 nm; R:FRnarrow was 
calculated as 655 to 665 nm:725 to 735 nm ................................................................................. 74 
 
Figure III-2. Expt. 1. time to first visible bud (VB) for 10 basil cultivars grown under a truncated 
9-h short-day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-
emitting diode (LED) lamps to achieve 11-, 12-, 13-, 14-, 15- and 16-h photoperiods or a 4-h 
night interruption (NI). Data were pooled when there was no interaction between replication 
(Rep.) and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. Absences of letters 
indicate no significant difference P ≤ 0.05. Error bars indicate standard error ............................ 75 
 
Figure III-3. Expt. 1. time to first open flower (OF) for 10 basil cultivars grown under a truncated 
9-h short-day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-
emitting diode (LED) lamps to achieve 11-, 12-, 13-, 14-, 15- and 16-h photoperiods or a 4-h 
night interruption (NI). Data were pooled when there was no interaction between replication 
(Rep.) and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. Absences of letters 
indicate no significant difference P ≤ 0.05. Error bars indicate standard error ............................ 76 
 
Figure III-4. Expt. 1. node number below the first open flower (OF) for 10 basil cultivars grown 
under a truncated 9-h short-day (SD) or under a 9-h SD extended with red+white+far-red 
(R+W+FR) light-emitting diode (LED) lamps to achieve 11-, 12-, 13-, 14-, 15- and 16-h 
photoperiods or a 4-h night interruption (NI). Data were pooled when there was no interaction 
between replication (Rep.) and treatment, or if the response trends were similar between Reps. 
Letters indicate mean separations across treatments using Tukey-Kramer difference test at P ≤ 
0.05. Absences of letters indicate no significant difference P ≤ 0.05. Error bars indicate standard 
error ............................................................................................................................................... 77 
 
Figure III-5. Expt. 1. height at first open flower for 10 basil cultivars grown under a truncated 9-
h short-day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-emitting 
diode (LED) lamps to achieve 11-, 12-, 13-, 14-, 15- and 16-h photoperiods or a 4-h night 
interruption (NI). Data were pooled when there was no interaction between replication (Rep.) 

viii 

and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. Absences of letters 
indicate no significant difference P ≤ 0.05. Error bars indicate standard error ............................ 78 
 
Figure III-6. Expt. 2. time to first visible bud (VB) for six basil cultivars grown under moderate 
or high photosynthetic daily light integrals (DLIs) and a truncated 9-h short-day (SD) or day-
extension lighting from low-intensity light-emitting diode (LED) lamps emitting red+white+far-
red (R+W+FR) radiation for 7 h (1700 to 2400 HR) to achieve a 16-h photoperiod. Letters 
indicate mean separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. 
Error bars indicate standard error. ................................................................................................ 79 
 
Figure III-7. Expt. 2. time to first open flower (OF) for six basil cultivars grown under moderate 
or high photosynthetic daily light integrals (DLIs) and a truncated 9-h short-day (SD) or day-
extension lighting from low-intensity light-emitting diode (LED) lamps emitting red+white+far-
red (R+W+FR) radiation for 7 h (1700 to 2400 HR) to achieve a 16-h photoperiod. Letters 
indicate mean separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. 
Error bars indicate standard error. ................................................................................................ 80 
 
Figure III-8. Expt. 2. node number below the first open flower (OF) for six basil cultivars grown 
under moderate or high photosynthetic daily light integrals (DLIs) and a truncated 9-h short-day 
(SD) or day-extension lighting from low-intensity light-emitting diode (LED) lamps emitting 
red+white+far-red (R+W+FR) radiation for 7 h (1700 to 2400 HR) to achieve a 16-h photoperiod. 
Letters indicate mean separations across treatments using Tukey-Kramer difference test at P ≤ 
0.05. Error bars indicate standard error. ....................................................................................... 81 
 
Figure III-9. Expt. 2. height at first open flower for six basil cultivars grown under moderate or 
high photosynthetic daily light integrals (DLIs) and a truncated 9-h short-day (SD) or day-
extension lighting from low-intensity light-emitting diode (LED) lamps emitting red+white+far-
red (R+W+FR) radiation for 7 h (1700 to 2400 HR) to achieve a 16-h photoperiod. Letters 
indicate mean separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. 
Error bars indicate standard error. ................................................................................................ 82 
 
Figure IV-1. Spectral quality delivered from high-pressure sodium (HPS) lamps providing 
photoperiodic and supplemental lighting (SL) and light-emitting diode (LED) fixtures delivering 
SL. Blue (B, 400-500 nm) represents the blue photon flux (PF), G (500-600 nm) the green PF, R 
(600-700 nm) the red PF  and FR (700-800 nm) the far-red PF from LEDs. Number subscripts 
after HPS denote the total photon flux density (TPFD) in µmol·m–2·s–1. Number subscripts in the 
LED treatments denote the photon flux density (PFD) in µmol·m–2·s–1 of B, G, R, and FR ..... 115 
 
Figure IV-2. Stem diameter (mm), height (cm), hypocotyl (cm), internode length (cm), and 
average number of nodes per plant of cucumber, tomato, and pepper. Data were collected 28, 35, 
and 35 days after treatment under high-pressure sodium (HPS) or light-emitting diode (LED) 
supplemental lighting (SL) treatments for cucumber, tomato, and pepper, respectively. Data were 
pooled when there was no interaction between replication (Rep.) and treatment, or if the response 
trends were similar between Reps. Letters indicate mean separations across treatments using 

ix 

Tukey-Kramer difference test at P ≤ 0.05. Error bars indicate standard error. Mean separations 
were excluded for un-pooled data ............................................................................................... 116 
 
Figure IV-3. Number of leaves, leaf area (cm2), fresh weight (g), dry weight (g), and chlorophyll 
fluorescence (Fv/Fm) per plant of cucumber, tomato, and pepper. Data were collected 28, 35, and 
35 days after treatment under high-pressure sodium (HPS) or light-emitting diode (LED) 
supplemental lighting (SL) treatments for cucumber, tomato, and pepper, respectively. Data were 
pooled when there was no interaction between replication (Rep.) and treatment, or if the response 
trends were similar between Reps. Letters indicate mean separations across treatments using 
Tukey-Kramer difference test at P ≤ 0.05. Error bars indicate standard error. Mean separations 
were excluded for un-pooled data ............................................................................................... 117 
 
Figure IV-4. Visible bud percentage of 28, 35, and 35 day-old cucumber, tomato, and pepper 
transplants, grown under different high-pressure sodium (HPS) and light-emitting diode (LED) 
supplemental lighting (SL) treatments ........................................................................................ 118 
 
Figure IV-5. The incidence of leaf necrosis resulting from high-pressure sodium (HPS) and light-
emitting diode (LED) supplemental lighting (SL) treatments on tomato (‘Climstar’ L. 
lycopersicum), after 35 days of SL. Percentage of leaves damaged (%) was calculated by 
dividing the number of leaves showing necrotic lesions by the total number of leaves, and 
multiplying by 100 ...................................................................................................................... 119 
 
 

x 

SECTION I  

LITERATURE REVIEW 

1 

Literature Review: Horticultural Lighting for Controlled Environments  

Introduction 

 

Greenhouses and other controlled environments (CEs) are essential for year-round 

production of vegetables, culinary herbs, and leafy greens in northern latitudes. CEs offer many 

advantages over open field production, including increased fruit quality and yield; reduced water 

usage, protection from pest and weed control pressure, and climatic extremes such as drought, 

flooding, and low- and high-temperature stress. Additionally, the ability to control environmental 

parameters such as temperature and light can be used to manage crop timing (McCartney and 

Lefsrud, 2018). As a result, food crop production within greenhouses is quickly expanding in the 

United States (U.S.) (Indoor Crop Production Feeding the Future, 2015). For example, from 

2012 to 2017, the number of operations involved in greenhouse production of fresh cut culinary 

herbs and vegetables increased by 24%, from 8,750 to 10,849, thus resulting in a 15% increase in 

production area (9.1 to 10.5 million m2) (USDA, 2019). Furthermore, during the same period the 

total value of sales increased by 18%, from $634 to $748 million U.S. dollars (USDA, 2019).  

  Due to year-round demand for fresh vegetables, herbs, and leafy greens, production occurs 

during times of the year when solar radiation is limited or excessive in northern regions. 

Therefore, production during the winter months requires supplemental lighting (SL) using high-

intensity discharge (HID) lamps or light-emitting diodes (LEDs) to increase the photosynthetic 

daily light integral (DLI), thus increasing crop growth, yield, and quality. Electromagnetic 

radiation from the sun is a form of energy that is used for plant growth and development. The 

electromagnetic radiation spectrum encompasses a wide range of short and long wavelengths 

including ultraviolet [UV (100-380)], visible (380-770 nm), far-red [FR (700-800 nm)], and 

2 

infra-red [IR (770-2500 nm)] radiation (Bird and Riordan, 1986; Iqbal, 1983). All wavebands 

can influence plant growth and development, however, radiation within the 400 to 700 nm range, 

known as photosynthetically active radiation (PAR) generally has the greatest impact on 

photochemical reactions such as photosynthesis (Lopez et al., 2017; Zhu et al., 2008). 

Photosynthetic pigments such as chlorophyll a and b absorb light energy (photons) within the 

PAR spectrum to convert water and carbon dioxide (CO2) into sugars and oxygen (O2) (van 

Iersel, 2017). These carbohydrates are the building blocks or chemical energy used for the 

production of roots, leaves, storage, and reproductive organs.  

There are three primary properties of the plant light environment including quantity 

(intensity, irradiance), quality (spectrum), and duration (photoperiod). Irradiance refers to the 

amount of photons within a specific waveband falling on a unit area per unit time, and is 

expressed as micromoles per square meter per second (µmol∙m−2∙s−1) (Blanchard et al., 2006). 

Radiation quality refers to the color or wavelength distribution of electromagnetic radiation from 

the sun or electrical lamps. Duration or photoperiod, refers to the number of light hours within a 

24-h period, available for plant growth, and its effect on photoperiodic crops. Generally 

speaking, radiation quantity, quality, and duration are associated with plant biomass 

accumulation, morphology, and flowering, respectively. However, all three properties interact to 

control plant growth and development. 

 

Radiation Quality  

Wavebands within PAR used by plants for photosynthesis include blue [B (400-500 nm)] 

green [G (500-600)], and red [R (600-700)] radiation. They can also utilize UV-A (320-380 nm) 

and FR radiation for other processes. Although PAR accounts for less than half (~43%) of the 

3 

solar spectrum it is the most important waveband utilized for photosynthesis (Lopez et al., 2017). 

Radiation acts as both an energy source and a signal for plant growth and development. For 

instance, photomorphogenesis is a light mediated process by which the radiation spectrum 

(color) affects plant morphology and development. Radiation quality has a greater impact on 

photomorphogenesis, whereas irradiance used as an energy source, has a greater impact on 

photosynthesis (Han et al., 2007; Hernandez and Kubota, 2017).  

 

Specialized photoreceptors such as cryptochromes, phytochromes, phototropins, and more 

recently described UV RESISTANCE LOCUS 8 (UVR8) enable plants to sense and respond to 

external signals (Li et al., 2012; Rizzini et al., 2011). Light signals, perceived by these 

photoreceptor families govern multiple developmental processes including seed germination, 

seedling de-etiolation, phototropism, shade avoidance, circadian rhythms, and flowering time 

(Deng and Quail, 1999; Wang and Deng, 2003; Jiao et al., 2007). Phytochromes primarily absorb 

R and FR wavelengths (and to a lesser extent B radiation), cryptochromes and phototropins 

absorb B and UV-A (320–500 nm), and UVR8 has recently been shown to perceive UV-B (282–

320 nm) (Rizzini et al., 2011). 

Blue radiation 

Cryptochrome mediated plant responses to B radiation include the inhibition of stem 

elongation, promotion of stomatal opening, phototropism, and anthocyanin accumulation 

(Fankhauser and Chory, 1997; Hernandez and Kubota, 2017). Arabidopsis has two cryptochrome 

genes, cry1 and cry2. Both genes are involved in resetting the circadian rhythm, and in de-

etiolation responses (Chen et al., 2004). During stomatal opening, Schwartz and Zeiger (1984) 

discovered that for Asiatic dayflower (Commelina communis) and faba bean ‘Long pod’ (Vicia 

faba) stomatal apertures were higher under B radiation in comparison to white (W) and R 

4 

radiation. Previous studies have reported that with increasing B radiation, stem and hypocotyl 

length were reduced (Brown et al., 1995; Hernandez and Kubota, 2016). For instance, pepper 

‘Hungarian Wax’ (Capsicum annuum) seedlings grown under sole-source lighting (SSL) 

providing a ratio (%) of 10:90 B:R radiation at a photosynthetic photon flux density (PPFD) of 

300 µmol·m–2·s–1, had shorter stem lengths compared to plants grown under 0:100 B:R radiation 

(Brown et al., 1995). Liu et al. (2011) reported that cherry tomato (Lycopersicon esculentum) 

grown under SL providing a ratio (%) of 0:100 B:R radiation with a PPFD of 300 µmol·m–2·s–1 

were 95% taller than those under 50:50 B:R radiation. In a separate study, cucumber ‘Cumlaude’ 

(Cucumis sativus) were grown under SSL providing an increasing B:R radiation ratio, ranging 

from 0:100 to 100:0 B:R at a PPFD of 100 µmol·m–2·s–1. Plant height, hypocotyl, and epicotyl 

length decreased as the proportion of B radiation increased up to 75 (75:25 B:R radiation). 

Interestingly, under 100:0 B:R radiation, hypocotyl length was 69% and 346% greater than under 

0:100 B:R and 75:25 B:R radiation, respectively. The unexpected extension growth observed 

under 100:0 B:R radiation could be due to the lack of the coaction effect, where the absence of R 

radiation, prevents the inhibition of stem elongation that is usually associated with additions of B 

radiation (Hernandez and Kubota, 2016). 

Green radiation 

Green radiation was previously believed to have a minimal effect on plant growth and 

development (McCree, 1972). However, recent research indicates that G radiation may be 

involved in CO2 assimilation and in promoting biomass and thus yield (Smith et al., 2017). 

Furthermore, G radiation can induce shade avoidance responses (Zhang et al., 2011), similar to 

FR light, by inactivating B absorbing cryptochromes (Banerjee et al., 2007; Bouly et al., 2007, 

Sellaro et al., 2010), thus resulting in hypocotyl extension growth (Folta, 2004). However, G 

5 

radiation can also inhibit the accumulation of anthocyanins (Bouly et al., 2007; Zhang and Folta, 

2012) and stomatal opening (Frenchilla et al., 2000). An estimated 10 to 50% of G radiation is 

reflected by chloroplasts (Nishio, 2000; Terashima et al., 2009). The remainder is either 

transmitted to the lower canopy or is absorbed by plant pigments such as chlorophyll and 

accessory pigments including carotenoids. Consequently, inner plant canopies can be rich in G 

radiation, meaning that photosynthesis in lower canopy leaves is mainly driven by G 

wavelengths. Furthermore, G radiation can penetrate deeper into the mesophyll layer of leaves, 

thereby potentially increasing photosynthesis where other wavelengths are limited (Smith et al., 

2017). Chlorophyll exists in two forms chlorophyll a and chlorophyll b (Bollivar, 2006). 

Maximum absorption of dissolved chlorophyll a and b is in the B and R waveband range, and 

more weakly in the G range. Up to 80% of all G radiation is presumed to be transmitted through 

the chloroplast. Thus, G radiation can reach lower into leaf cells due to chlorophyll’s weak 

absorption (Evans and Vogelmann, 2003; Vogelmann and Han, 2000). Vogelmann and Han 

(2000) treated spinach (Spinacia oleracea) with brief 2000 µmol∙m–2∙s–1 flashes of 

monochromatic B, R, and G radiation to determine maximum absorption depths. Adaxial to 

abaxial maximum absorption depths for B, R, and G radiation were 50, 100, and 150 µm, 

respectively.  

Kim et al. (2004) grew lettuce ‘Waldmann’s Green’ (Lactuca sativa) under four lighting 

treatments consisting of B+R, B+R supplemented with G fluorescent lamps, G fluorescent 

lamps, and under cool-white fluorescent (CWF) lamps, which provided 0, 24, 86, and 51% G 

radiation, respectively (18-h photoperiod; 9.7 mol·m–2·d–1). The addition of 24% G radiation to 

the B+R LEDs resulted in an increase for leaf area, shoot fresh weight, and shoot dry weight by 

31, 45 and 47%, respectively. However, leaf area, shoot fresh and dry weight of lettuce was 

6 

significantly reduced when grown under the G fluorescent lamps, compared to both the R+B and 

the R+B+G treatments. A benefit of adding G radiation to B+R radiation is that plants appear G, 

instead of grey or black, and pest, disease and nutritional deficiencies are easier to see.  

Red and far-red radiation  

The phytochrome family consists of five members, phytochromes A, B, C, D, and E, and are 

designated phyA, phyB, phyC, phyD, and phyE (Sharrock and Quail, 1989; Dehesh et al., 1991). 

The phyA gene is the primary photoreceptor involved in the perception and mediation of various 

plant responses to FR radiation, such as stem elongation and the shade avoidance response 

(Dehesh et al., 1993; Parks and Quail, 1993). Phytochrome is a photoreversible pigment with 

peak absorbance in the R and FR range (Rizzini et al., 2011). Phytochrome exists as two 

interconvertible forms, an active and inactive form. The FR absorbing form (PFR) is biologically 

active, and the R absorbing form (PR) is inactive (Hendricks et al., 1962, Sager et al., 1988). 

Therefore, when PFR is illuminated with R radiation or kept in dark conditions, it converts to the 

PR form, and when PR is illuminated with FR radiation, it converts back to the PFR form. 

Additionally, when phytochrome exists at an equilibrium between the active state (PFR) and the 

inactive state (PR), it is referred to as the phytochrome photostationary state (PFR/PTotal, where P = 

PR+ PFR), relative to a specific spectral quality (Sager et al., 1988). PFR formation culminates in 

altered expression of pytochrome responsive genes responsible for driving non-photosynthetic 

radiation responses including a shade avoidance response, seed germination, inhibition of stem 

elongation and flowering (Hendricks et al., 1962; Hendricks and Borthwick, 1967; Mancinelli et 

al., 2007). Reconversion to PR can halt this process. Thus, R and FR emitting LEDs can be used 

to artificially manipulate the photoperiod and either delay or induce flowering responses in 

photoperiodic crops. 

7 

Photoperiodic lighting 

Plants have evolved and developed the ability to perceive changes in day length during the 

growing season. The plant’s developmental response or physiological reaction to changes in the 

length of day and night is known as photoperiodism (Thomas and Vince-Prue, 1997). The 

rotation of the earth and the tilting of the axis results in different amounts of solar radiation 

falling onto the earth, thereby causing different seasons and durations of light hours. Since the 

earth is tilted at an axis of 23.5º, as one moves farther from the equator and closer to the earth’s 

poles the length of day and night can change dramatically. In the northern hemisphere day length 

becomes longer during the summer solstice and shorter during the winter solstice (Jackson, 

2009).  

 

The ability of plants to perceive and respond to these changes in day length proves to be 

beneficial, because it allows them to anticipate adverse environmental conditions. For example, 

in northern latitudes autumnal short days (SD) precede winter. Short days can act as a signal to 

induce bud dormancy and cold hardiness (Thomas and Vince-Prue, 1997). Plant responses 

triggered by seasonal changes of the natural photoperiod include seed germination, leaf 

expansion, stem elongation, tuberization, dormancy, bud set, and most importantly flowering 

(Densmore, 1997; Hay and Heide, 1983; Jackson, 2009, Lopez and Runkle, 2006; Runkle et al., 

2017). 

  Based on photoperiodic flowering responses plants are categorized as long day (LDP), short 

day (SDP), or day-neutral plants (DNP) (Thomas and Vince-Prue, 1997). Plants with a LD 

response will flower when the night length is shorter than the critical duration, while SDPs will 

flower faster when the night length is longer than the critical duration (Runkle et al., 2017). Both 

LDP and SDPs can be further classified as having an obligate (qualitative) or facultative 

8 

(quantitative) response. For example, a facultative SDP will flower faster when the night length 

is long, but will eventually flower under all photoperiods. On the contrary, an obligate LDP will 

only flower when the night length is longer than the critical duration (Blanchard and Runkle, 

2010). DNPs flower regardless of the day or night length (Thomas and Vince-Prue, 1997). 

  Greenhouse growers can use low-intensity (1-2 µmol∙m–2∙s–1) electric lighting when the 

skotoperiod is long or a 4-h night interruption (NI) to induce or prevent flowering of LDPs or 

SDPs, respectively (Blanchard and Runkle, 2010; Mattson and Erwin, 2005). The use of 

blackout systems can also be employed to create truncated SDs to induce flowering of SDPs or 

prevent flowering of LDPs. Low R:FR, promote both stem elongation and flowering of LDPs. 

On the contrary, high R:FR (FR deficient environments) can inhibit flower initiation of LDPs 

such as snapdragon (Antirrhinum majus), campanula (Campanula carpatica), coreopsis 

(Coreopsi grandiflora), and petunia (Petunia ×hybrida) (Downs and Thomas, 1982; Kim et al., 

2002; Runkle and Heins, 2001). FR radiation alone is not usually perceived as a LD, and thus a 

combination of R and FR radiation are commonly used to promote flowering of LDPs. 

Incandescent (INC) lamps emit a mixture of R and FR radiation, that create an intermediate 

phytochrome photoequilibria [PPE (0.60-0.75)], which has been shown to be the most effective 

at promoting flowering of some LDPs (Craig and Runkle, 2012; Thomas and Vince-Prue, 1997). 

Until recently, INC lamps were commonly used for day extension (DE) and NI lighting to 

successfully promote flowering of the LDPs such as campanula, coreopsis, and lavender 

(Lavandula angustifolia) (Damann and Lyons, 1996; Runkle et al., 1998). However, they are 

electrically inefficient and thus have been phased out of production. LED fixtures containing 

R+FR are more efficient, have a longer operating life, and provide a PPE similar to that of INC 

lamps (Yeh and Chung, 2009). Kohyama et al. (2014) compared the flowering response of 

9 

ageratum (Ageratum houstonianum), calibrachoa (Calibrachoa ×hybrida), two cultivars of 

dianthus (Dianthus chinensis) petunia under NI lighting using INC lamps or two LED lamps 

(R+W or R+W+FR). Time to flower, node number and plant height for all crops was similar 

under the INC lamps and R+W+FR LEDs. For example, dianthus ‘Floral Lace Purple’ flowered 

5 days sooner when treated with a NI provided with INC or the R+W+FR lamps compared to a 

SD. Furthermore, time to flower for dianthus, under the R+W lamps was similar to plants under 

SDs. 

 

Supplemental Lighting  

Peak young plant (vegetative cuttings) and seed propagated vegetable and bedding plant 

production in northern latitudes, usually occurs when the ambient photosynthetic DLI can be as 

low as 1 to 5 mol∙m−2∙d−1 (Lopez and Runkle, 2008; Pramuk and Runkle, 2005; Styer, 2003). 

Low DLIs can lead to production issues such as inconsistent and excessive stem elongation, poor 

performance after transplant, and delayed rooting and germination (Lopez and Runkle, 2008). 

Furthermore, the DLI provided to the growing area within the greenhouse can be reduced by 

50% or more, due to glazing materials and other greenhouse structures (Hanan, 1998). Therefore, 

greenhouse operations use SL to increase the DLI, thus increasing plant photosynthesis and 

quality. The most cost effective time to provide SL will depend on the plant density and the type 

of plant species being grown (shade vs. sun), but most importantly, whether or not ambient 

radiation levels are low. This is because at lower radiation levels even small increases in 

irradiance can dramatically increase the net photosynthetic rate (Faust et al., 2017). SL is less 

effective when radiation levels are higher, since the light saturation point will likely be reached 

sooner and thus any further SL will not increase photosynthesis. Thereby indicating that the net 

10 

photosynthetic rate has been maximized and further increases in PAR is not economically 

feasible (Helma et al., 2004).   

  Benefits of SL include increased cutting quality and yields from stock plants (Chong et al., 

2014), shorten production time of plugs, liners, and potted plants (Lopez et al., 2017), and 

compact growth with thicker stems, increased branching and earlier flowering of bedding plants 

(Erwin et al., 2017). To produce high-quality vegetable seedlings for transplant, defined by well-

developed root and shoot systems, compact growth, and thick stems, a DLI ranging from 13 to 

16 mol∙m−2∙d−1 is recommended (Fan et al., 2013; Moe et al., 2006). However mature fruiting 

vegetable plants require even higher DLIs usually ≥30 mol∙m−2∙d−1. Increased DLIs can shorten 

cultivation time, improve productivity and product quality, and allow for year-round production 

(Dorais and Ehret, 2008). Thus, allowing growers to meet year-round consumer demand. 

Similarly, previous research has determined that annual bedding plant seedlings also require a 

DLI of 10 to 12 mol∙m−2∙d−1 for optimal growth (Currey et al., 2012; Lopez and Runkle, 2008; 

Oh et al., 2010). Optimal vegetative growth of culinary herbs such as basil (Ocimum spp.), 

require a DLI of ≥10 mol∙m−2∙d−1 for optimal growth (Beaman et al., 2009; Chang et al., 2008, 

Dou et al., 2018; Walters and Currey, 2018). Young plants provided with SL have increased 

rooting, more branching, thicker stems, reduced extension growth, and potentially more crops 

cycles per season. Additionally, young plants grown under SL generally flower earlier during 

finishing (Lopez et al., 2017). 

Plant height of various bedding plants has been shown to decrease with an increase in DLI. 

For example, height of impatiens ‘Accent Red’ (Impatiens wallerana) and salvia ‘Vista Red’ 

(Salvia splendens) seedlings grown under SSL decreased by 27% and 37%, respectively, as DLI 

increased from 4.1 to 14.2 mol∙m−2∙d−1 (Pramuk and Runkle, 2005). Furthermore, seedling shoot 

11 

dry weight of celosia ‘Gloria Mix’ (Celosia argentea), impatiens, marigold ‘Bonanza Yellow’ 

(Tagetes patula), and pansy ‘Crystal Bowl Yellow’ (Viola) increased by 64, 47, 64, and 68%, 

respectively under the same DLIs (Pramuk and Runkle, 2005). Walters and Currey (2018) 

reported a 144, 205, and 208% increase in fresh weight for greenhouse grown sweet basil 

‘Nufar’ (Ocimum basilicum), lemon basil ‘Lime’ (O. ×citriodorum) and holy basil ‘Holy’ (O. 

tenuiflorum), when the DLI increased from 7 to 15 mol∙m−2∙d−1. Furthermore, dry weight, height, 

node and branch number, and SPAD levels for all three species also increased with the increase 

in DLI. In a separate study, Dou et al. (2018) grew sweet basil ‘Improved Genovese Compact’ 

under varying irradiance. Similar to Walters and Currey (2018), fresh weight of sweet basil 

increased as DLI was increased from 9.3 to 17.8 mol∙m−2∙d−1. For example, fresh weight under 

DLIs of 12.9, 16.5, and 17.8 mol∙m−2∙d−1 was 54, 79 and 78% higher than that of those grown 

under a DLI of 9.3 mol∙m−2∙d−1 (Dou et al., 2018).   

 

12 

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20 

SECTION II  

FLOWERING DEVELOPMENT PARAMETERS OF CULINARY AND ORNAMENTAL 

HERBS AND LEAFY GREENS IN RESPONSE TO PHOTOPERIOD

21 

Flowering development parameters of culinary and ornamental herbs and leafy greens in 

response to photoperiod 

 

Charlie Garcia and Roberto Lopez* 

Department of Horticulture, Michigan State University, 1066 Bogue Street, East Lansing, MI 

48824-1325, USA 

 

*Corresponding author. Tel.: +1 517 353 0342. E-mail address: rglopez@msu.edu (R.G. Lopez) 

22 

Abstract 

Increasingly, retailers and consumers are demanding non-flowering fresh cut culinary herbs 

and leafy greens and flowering potted ornamental herbs. However, few publications have 

documented environmental parameters that promote or inhibit flowering of these crops. 

Therefore, the objectives of this study were 1) to quantify how photoperiod influences flowering 

development parameters of culinary and ornamental herbs and leafy greens and 2) to determine 

the critical photoperiod of those crops that flower in response to photoperiod. Coriander ‘Santo’ 

(Coriandrum sativum), dill ‘Bouquet’ (Anethum graveolens), lavender ‘Bandera Pink’ 

(Lavandula stoechas), marjoram (Origanum majorana), oregano ‘Kirigami’ (O. rotundifolium), 

watercress (Nasturtium officinale), oregano ‘Greek’ (O. vulgare hirtum), and spearmint 

‘Spanish’ (Mentha spicata) were grown in a greenhouse at 20 °C. Photoperiods consisted of a 9-

h short day (SD) or an SD extended to 12, 13, 14, or 16 h with red+white+far-red (R+W+FR) 

light-emitting diode (LED) lamps (R:FR = 0.8) providing a total photon flux density of ≈2 

µmol·m–2·s–1. Additionally, an SD with a 4-h night interruption (NI) from the same R+W+FR 

lamps was included. Plants were assessed daily for time to first visible flower bud (VB) and open 

flowers (OF). Node number below the first OF and plant height were recorded at first OF. Node 

number at flowering was greater under 9-h SDs for coriander, dill, lavender, and marjoram, and 

generally decreased as day length increased to 16 h or under an NI. Coriander and dill grown 

under a 9-h SD were more compact at flowering in comparison to plants grown under 

photoperiods ≥13 h. Time to first VB occurred more rapidly under LDs for coriander, dill, 

lavender, and marjoram. Watercress and oregano ‘Kirigami’ only produced VBs and OFs under 

≥13-h day lengths. Oregano ‘Greek’ and spearmint only flowered under 16 h or NI treatments. 

Therefore, coriander, dill, lavender, and marjoram can be classified as facultative long day plants 

23 

(LDPs). While watercress, oregano ‘Kirigami’, oregano ‘Greek’, and spearmint can be 

categorized as obligate LDPs, requiring ≥14-h, ≥13-h, and 16, or NI, respectively, for VB 

formation and flowering to occur. 

 

Keywords: critical photoperiod, day-extension lighting, far-red radiation, light-emitting diodes, 

long-day plants, night-interruption lighting. 

 

Abbreviations: ADT, average daily temperature; B, blue; CWF, cool-white fluorescent; DE, day-

extension lighting; DLI, daily light integral; DN, day-neutral; FR, far red; G, green; HPS, high-

pressure sodium; INC, incandescent; LED, light-emitting diode; LD, long-day; LDP, long-day 

plant; MH, metal halide; NI, night-interruption; OF, open flower; PPE, phytochrome 

photoequilibria; PPFD, photosynthetic photon flux density; PSS, phytochrome photostationary 

state; R, red; Rep., replication(s); SD, short-day; SDP, short-day plant; TPFD, total photon flux 

density; VB, visible bud; W, white. 

 

Introduction 

An increased health conscious attitude and curiosity in culinary cuisine from different 

cultures have increased the popularity and consumption of culinary herbs and leafy greens (Behe 

et al., 2013; Simon, 1990; Yue et al., 2012). To meet this growing demand, the number of 

operations involved in greenhouse vegetable, fresh cut culinary herb and leafy green production 

in the U.S. more than doubled to 8,750 from 2007 to 2012 (USDA, 2014). This has resulted in a 

59% increase in production area under glass or other protection (USDA, 2014). In the same time 

period, sales increased by more than $81 million (+16%) (USDA, 2014).   

24 

 

Fresh culinary herbs and leafy greens are valued for their overall visual quality, flavor, 

texture, and aroma, and used as garnish, while ornamental herbs are valued for their flowers or 

essential oils. Additionally, herbs such as lavender (Lavendula spp.), mint (Mentha spp.), basil 

(Ocimum spp.), coriander (Coriandrum sativum), marjoram (Origanum majorana), and oregano 

(Origanum spp.) have cosmetic and/or medicinal properties (Aburjai and Natsheh, 2003; Baratta 

et al., 1998; Cavanagh and Wilkinson, 2002). For example, lavender is used for its essential oils 

in soaps, lotions, and perfumes (Aburjai and Natsheh, 2003). Furthermore, lavender, mint, basil, 

coriander, and oregano are known to have antimicrobial and/or anti-inflammatory properties 

(Baratta et al., 1998; Cavanagh and Wilkinson, 2002). Depending on the market, flowers can be 

advantageous or undesirable. Flower initiation and development is often undesirable during fresh 

cut production of crops such as mint, coriander, watercress (Nasturtium officinale), and oregano. 

For instance, premature flowering is such a major issue in coriander that seed companies are now 

offering slow-to-bolt cultivars. Although these cultivars are less susceptible to premature 

flowering, none are completely unresponsive to temperature and long days (LD) (Simon, 1990). 

Additionally, consumers may not buy, and retailers may reject fresh cut herbs and leafy greens 

with flower buds or flowers. Studies have shown that the composition of essential oils changes 

with the onset of flowering, leading to potential changes in flavor, and thereby affecting their use 

as a culinary herb or leafy green (Skrubis and Markakis, 1976; Vazquez and Dunford, 2005). On 

the contrary, if herbs are grown for ornamental purposes or for essential oil production, then 

flower initiation and development may be desired, since an increase in the number of flowers and 

oil yield can increase profitability (Burbot and Loomis, 1957; Hassiotis et al., 2014).  

Greenhouse growers of ornamental crops often delay or promote flowering by manipulating 

the natural photoperiod or temperature. Photoperiod refers to the number of light hours within a 

25 

24-h period, and acts as an environmental signal for bud dormancy and break, formation of 

storage organs, and flowering of some crops (Thomas and Vince-Prue, 1984). However, the 

length of the dark period (or skotoperiod) is the determining factor of the flowering response 

(Thomas and Vince-Prue, 1997). Plants can be categorized as having an LD, short-day (SD), or 

day-neutral (DN) flowering response to day length (Thomas and Vince-Prue, 1997). Long-day 

and SD plants (LDPs and SDPs) can be further classified as qualitative or obligate if a certain 

photoperiod is absolutely required for flowering, or quantitative or facultative if the photoperiod 

accelerates but is not required for flowering (Blanchard and Runkle, 2010).  

The use of red (R) and far-red (FR) radiation at low intensities (1-2 µmol∙m–2∙s–1) can be 

applied at the end of the day [day-extension lighting (DE)], during the middle of the night [night-

interruption light (NI)], or before dawn to promote flowering responses in LDPs or inhibit 

flowering of SDPs (Runkle et al., 2012; Whitman et al., 1998). Before the introduction of light-

emitting diode (LED) lamps, incandescent (INC) lamps were the conventional choice for DE and 

NI lighting because they were inexpensive and emitted an effective mixture of R and FR 

radiation that promoted flowering of LDPs (Vince-Prue and Canham, 1983). Unfortunately, they 

were also highly energy-inefficient, had a short life span, and were subsequently phased out of 

production (Waide, 2010). Although more expensive, LED flowering lamps have a longer-life 

span, are more energy efficient and provide a similar R to FR ratio as INC lamps are an 

alternative for greenhouse growers (Mitchell et al., 2015; Owen et al., 2018).  

The majority of photoperiodic studies on specialty crops have been conducted on ornamental 

greenhouse crops. However, Downs et al. (1958) conducted photoperiodic studies with dill 

(Anethum graveolens) and found that when R and FR radiation were used as DE lighting, 

flowering was accelerated. Subsequently, dill and peppermint have been classified as LDPs 

26 

(Burbott and Loomis, 1967; Thomas and Vince-Prue, 1997). Bleasdale (1964) conducted a study 

on watercress, where plants were placed under 9-, 13-, 16-, and 20-h day lengths. Flower bud 

initiation was hastened under a 20-h photoperiod compared to a 13-h photoperiod, and no plants 

formed visible buds (VBs) under a 9-h SD. Spanish lavender ‘Chica Purple’, ‘Chica Rose’, 

‘Coco Purple’, and ‘Coco Blue and White’ (Lavandula stoechas) was classified as a facultative 

LDP that did not require vernalization (Whitman and Padhye, 2009). However, another study 

conducted on Lavandula angustifolia determined that an increase in the duration of vernalization 

increased flowering percentage. Furthermore, flowering percentage also increased with a 4-h NI, 

in comparison to a 9-h SD (Whitman et al., 1996). Although photoperiodic studies have been 

conducted on some common culinary and ornamental herbs and leafy greens, the critical 

photoperiod of new cultivars and many other genera have not yet been determined. Therefore, 

the objectives of this study were 1) to determine how photoperiod influences flowering 

development parameters of economically important culinary and ornamental herbs and leafy 

greens and 2) to determine the critical photoperiod of those crops that flower in response to 

photoperiod. 

 

Materials and methods 

Plant material 

Cuttings of spearmint ‘Spanish’ (M. spicata) were placed in 72-cell trays filled with a 

propagation substrate composed of (v/v) 50% perlite (Horticultural Perlite; Perlite Vermiculite 

Packaging, North Bloomfield, OH) and 50% soilless medium containing 70% peat moss, 21% 

perlite, and 9% vermiculite (Suremix; Michigan Grower Products, Inc., Galesburg, MI) on 25 

Jan. 2018 and 17 Dec. 2018. The trays were then placed in a glass-glazed greenhouse under a 16-

27 

h photoperiod with air and substrate temperature set points of 21 °C and 27 °C, respectively. 

After 14 d, rooted cuttings were transplanted into round 15-cm diameter (1,300-mL) containers 

filled with a soilless medium (Suremix; Michigan Grower Products Inc., Galesburg, MI) and 

then placed under their respective photoperiodic treatments. Seven days later, shoot tips were 

pinched, leaving 3 nodes per plant.  

On 08 Feb. 2018 and 07 Jan. 2019, seeds of coriander ‘Santo’, oregano ‘Greek’ (O. vulgare 

hirtum), dill ‘Bouquet’, watercress, and marjoram were sown into 128-cell (12.0-mL) plug trays. 

Cells were filled with a seed sowing mix composed of 50% vermiculite (Vermiculite Premium 

Grade; Sungro Horticulture, Agawam, MA) and 50% of the soilless medium (Suremix; Michigan 

Grower Products Inc., Galesburg, MI) previously mentioned. Each tray was placed onto capillary 

mats in their respective treatments. After germination, seedlings were thinned to one plant per 

cell.  

On 08 Mar. 2018 and 04 Feb. 2019 dill and watercress were transplanted into 11-cm round 

(600-mL) containers and subsequently transplanted into 15-cm round (1,300-mL) containers on 

20 Apr. 2018 and 19 Mar. 2019. Coriander, oregano ‘Greek’, and marjoram were transplanted 

into 11-cm round containers on 09 Mar. 2018 and 05 Feb. 2019. Because of low germination 

rates in Rep. 1, cuttings of oregano ‘Greek’ were taken on 02 Feb. 2019 from pre-established 

stock plants of the same cultivar. The same environmental parameters and methods utilized for 

spearmint were utilized to root oregano ‘Greek’ cuttings.  

Plugs of oregano ‘Kirigami’ (O. rotundifolium) and lavender ‘Bandera Pink’ (L. stoechas) 

obtained from a young plant propagator (Raker-Roberta's Young Plants, Litchfield, MI) were 

transplanted on 09 Mar. 2018 and on 05 Jan. 2019 into 11-cm containers. Lavender plants had 

the apical meristem excised 7 d after transplant, to 4 nodes per plant.  

28 

 

Plants were top irrigated with reverse osmosis water supplemented with water-soluble 

fertilizer (mg∙L‒1) 60 nitrogen (N), 23 phosphorus (P), 60 potassium (K), 28 calcium (Ca), 5 

magnesium (Mg), 1 iron (Fe), 0.6 manganese (Mn), 0.6 zinc (Zn), 0.6 copper (Cu), 0.4 boron 

(B), and 0.1 molybdenum (Mo) (MSU Plug Special; Blackmore Company, Kankakee, IL) during 

the seedling stage and after transplant with reverse osmosis water supplemented with 14N-3P-

14K water-soluble fertilizer (mg·L–1) 125 N, 12 P, 100 K, 65 Ca, 12 Mg, 1.0 Fe and Cu, 0.5 Mn 

and Zn, 0.3 B, and 0.1 Mo (MSU Orchid RO Water Special; Blackmore Company). 

Greenhouse environment and lighting treatments 

Plants were grown in a glass-glazed greenhouse at Michigan State University (East Lansing, 

MI; lat. 42º N) with exhaust fans, evaporative-pad cooling, radiant hot-water heating, and 

supplemental lighting controlled by an environmental control system (Priva Office version 725-

3030; Priva North America, Vineland Station, ON, Canada). The greenhouse air temperature set 

point was a constant 20 °C. An aspirated thermocouple [36-gauge (0.127-mm diameter) type E] 

located in the middle of each bench measured the air temperature at plant height every 10 s, and 

a data logger (CR1000; Campbell Scientific, Logan, UT) recorded hourly means. The data logger 

controlled a 1500-W electric heater underneath each bench to provide supplement heat when the 

nighttime temperature was <18.9 °C. The air average daily temperature (ADT) (±SD) for the 

greenhouse during the duration of the study was 20.8 ± 3.5 °C for replication (Rep.) 1 and 19.6 ± 

2.9 °C for Rep. 2 (data not provided). The actual bench ADT at plant height of each treatment 

during the two Reps. of the experiment is provided in Table 1. 

Supplemental lighting provided by R+white (W) LEDs (Philips GP-TOPlight DRW-MB; 

Koninklijke Philips N.V., Eindhoven, the Netherlands) delivered a supplemental photosynthetic 

photon flux density (PPFD) of 90 ± 14 µmol·m–2·s–1 between 0800 and 1700 HR when the 

29 

outdoor light intensity was below ≈440 µmol·m–2·s–1. On each bench, a line quantum sensor (LI-

191R, LI-COR, Lincoln, NE) or a quantum sensor (LI-190R, LI-COR, Lincoln, NE) positioned 

horizontally at plant height measured PPFD every 10 s and a datalogger recorded hourly 

averages. The daily light integral (DLI) of each treatment during the two Reps. of the experiment 

were calculated and are provided in Table 1. 

 Each day, opaque black cloths were pulled over each bench at 1700 HR and opened at 0800 

HR to create a truncated 9-h SD. Treatments consisted of the 9-h SD, or a 9-h day extended by 

four R+W+FR LED lamps (GreenPower LED flowering DR/W/FR 14 W, E26; Philips, 

Eindhoven, The Netherlands) beginning at 1700 HR to create 12-, 13-, 14-, or 16-h photoperiods 

or a 4-hour NI (from 2200 to 0200 HR). Each LED fixture was covered with multiple layers of 

aluminum wire mesh to achieve a total photon flux density (TPFD) of ≈2 µmol∙m–2∙s–1. The 

spectral distribution of the LED lamps was measured in five random locations throughout each 

bench with a spectroradiometer (PS-200; Stellar-Net, Tampa, FL), and the phytochrome 

photoequilibrium was estimated according to Sager et al.1988 (Fig. 1). 

Data collection and analysis 

All plants were monitored daily and the date of first VB and first open flower (OF) were 

recorded for all plants under each treatment. Plants that did not flower after 20 weeks were 

considered non-flowering. The number of nodes below the first OF, and plant height (from the 

media surface to the plant apical meristem) were recorded at first OF.  

The experiment was a randomized complete block design. The two Reps. were performed 

over time and considered the blocking factor in the experiment. The six photoperiodic treatments 

were the single factor in the experiment. Eight or 10 plants (sub-units) of spearmint and all other 

plant species, respectively, were randomly assigned to each treatment. Data were separately 

30 

analyzed for each species and the interactions between species were not evaluated. Data were 

analyzed with the SAS version 9.4 (SAS Institute, Inc., Cary, NC) mixed-model (PROC 

GLIMMIX) procedure and pairwise comparisons between treatments were performed with 

adjusted Tukey-Kramer difference test (P ≤ 0.05). Data were pooled when there was no 

interaction between Rep. and treatment, or if the response trends were similar between 

replications. Statistics were not included for spearmint and oregano ‘Greek’ because of 

variability between experimental replications.  

 

Results 

Flowering percentage  

The percentage of coriander and dill that flowered ranged from 95 to 100% for all treatments 

(Fig. 2A and B). Flowering percentage for all other species generally increased with increasing 

day lengths (Fig. 2C-H). For example, flowering percentage of lavender was 0, 40, 40, 90, 100, 

and 100 under a 9-, 12-, 13-, 14-, and 16-h photoperiod or a 4-h NI, respectively (Fig. 2C). The 

flowering percentage of watercress was 25, 80, and 90% under 14 and 16-h photoperiods and NI 

lighting, respectively (Fig. 2F). For marjoram, apart from the 9-h SD, there was an increase in 

flowering percentage from 12 to 16 h and the NI treatment (Fig. 2D). For oregano ‘Kirigami’ the 

percentage of plants that flowered was 53, 100, 100, and 100 under 13-, 14-, and 16-h 

photoperiods and a 4-h NI, respectively (Fig. 2E). During Rep. 1 flowering percentage of 

oregano ‘Greek’ was 40% and only flowered under the 16-h photoperiod, during Rep. 2 

flowering percentage was 50% and only flowered under NI lighting (Fig. 2G and 4G). Spearmint 

flowering percentage was 100% under the 16 h and NI treatments during Rep. 1, but did not 

flower under any treatment during Rep. 2 (Fig. 2H and 4H). 

31 

Time to first visible bud 

Generally, time to first VB occurred most rapidly under photoperiods ≥13 h compared to 9 h 

for coriander, dill, lavender, marjoram, and watercress (Fig. 3A-D, and F). As day length 

increased, time to first VB for coriander, dill, and lavender decreased, but not for marjoram (Fig. 

3D). For marjoram, time to first VB increased from 95 to 107 d as the day length increased from 

9 to 12 h, but then progressively decreased to 80 d as the photoperiod increased to 16 h (Fig. 

3D). For coriander, dill, and lavender, time to first VB was hastened by 7, 6, and 36 d, 

respectively, under a 16-h LD compared to a 9-h SD (Fig. 3A, B, and C). For watercress, no VBs 

formed under day lengths <14 h. Additionally, VBs appeared 10 and 11 d earlier when 

watercress was grown under a 16-h LD or 4-h NI, respectively, compared to a 14-h photoperiod 

(Fig. 3F). Only data for Rep. 2 is presented for oregano ‘Kirigami’ as missing data prevented us 

from including Rep. 1. Photoperiod influenced time to first VB for oregano ‘Kirigami (Fig. 3E). 

For oregano ‘Greek’, time to first VB was 3 d faster under the NI treatment for Rep. 2 when 

compared to the 16-h treatment in Rep. 1 (Fig. 3G). For spearmint, time to VB was hastened by 4 

d under the 16-h LD in comparison to the NI treatment for Rep. 1 (Fig. 3H). 

Time to first open flower 

Time to first OF of coriander decreased as day length increased, with a 6-d difference 

between plants under 16-h and 9-h photoperiods (Fig. 4A). Under a 16-h photoperiod, dill and 

marjoram flowered 7 and 13 d faster than under a 9-h SD, respectively (Fig. 4B and D). 

Lavender followed the same general trend, but plants did not flower when grown under a 9-h SD 

(Fig. 4C). As photoperiod increased from 13 to 16 h, time to first OF of oregano ‘Kirigami’ 

decreased from 62 to 45 d (Fig. 4E) Time to first OF was not significantly different for 

coriander, dill, lavender, marjoram, oregano ‘Kirigami’, and watercress grown under a 16-h 

32 

photoperiod or NI lighting (Fig. 4A-F). Watercress did not flower under 9-, 12-, or 13-h 

photoperiods and no significant difference for time to first OF were observed for plants grown 

under the 14, 16 h or NI treatments (Fig. 4F). Although all species flowered faster under LDs, 

watercress, oregano ‘Greek’, and spearmint only flowered under photoperiods ≥14-h long (Fig. 

4F, G, and H). Nevertheless, as previously mentioned, both oregano ‘Greek’ and spearmint had 

inconsistent flowering during Reps. 1 and 2.  

Node number below the first open flower 

At flowering, coriander, dill, and marjoram had the greatest number of nodes below the first 

OF when grown under a 9-h SD and node number generally decreased with increasing day 

length (Fig. 5A, B, D). For instance, dill grown under a 9-h photoperiod had two more nodes 

than plants grown under 13-, 14-, 16-h and NI treatments (Fig. 5B). Marjoram grown under a 9-, 

12-, 13-, 14-, or 16-h photoperiod or under NI lighting averaged 21, 20, 19, 17, 15, and 14 nodes, 

respectively (Fig. 5D). For lavender, no plants flowered under a 9-h SD; from 12 to 16 h or 

under a NI, node number decreased by 1 to 5 nodes (Fig. 5C). Watercress did not flower under 

9-, 12-, or 13-h photoperiods and there was no significant difference in node number for plants 

under 14- or 16-h photoperiods, or under NI lighting (Fig. 5F). 

Stem length at first open flower 

Stem length at flowering of 9-h grown coriander and dill was 24.4 (25%) to 32.2 cm (31%) 

and 23.1 (17%) to 32.9 cm (23%) shorter compared to all the other treatments, respectively. (Fig. 

6A and B). Stem length of 16 h and NI grown oregano ‘Kirigami’ was significantly shorter in 

comparison to 13-h grown plants (Fig. 6E). Lavender and watercress stem length was not 

affected by photoperiod (Fig. 6C and F). 

 

33 

Discussion 

Phytochrome are photoreversible pigments found in all higher plants used to detect radiation, 

with peak absorbance in the R (600 to 700 nm) and FR (700 to 800 nm) radiation wavebands, 

and to a lesser extent in the blue [B (400 to 500 nm) waveband (Sager et al., 1988; Smith, 1983). 

They exist as two interconvertible forms, an active biological form (PFR) and an inactive form 

(PR) (Hendricks et al., 1962, Sager et al., 1988). When the R:FR is low it exists as the PR form, 

and shade avoidance responses, such as stem elongation and leaf expansion occur. When the 

R:FR is high, PR is converted to PFR culminating in altered expression of phytochrome-

responsive genes resulting in a flowering response in LDPs (Hendricks et al., 1962; Hendricks 

and Borthwick, 1967; Mancinelli et al., 2007). 

Depending on the radiation quality provided, PFR and PR reach an equilibrium, which is 

referred to as the phytochrome photostationary state (PSS) or phytochrome photoequilibrium 

[PPE (PFR/PTotal, where PTotal = PR+ PFR)] (Sager et al., 1988). A PPE of 0.10-0.50 creates a low 

R:FR (high FR spectrum), while a high PPE of 0.75-0.89 creates a high R:FR (high R spectrum). 

A PPE between 0.60-0.75 is considered an intermediate PPE (Sager and Mc Farlane, 1997). 

Incandescent lamps emit a low R:FR, which creates an intermediate PPE. The LED lamps 

employed in our study had a PPE of 0.66, thereby providing an intermediate PPE that promoted 

flowering of the LD culinary and ornamental herbs and leafy greens evaluated. Craig and Runkle 

(2016) reported similar findings; as snapdragon ‘Liberty Classic Cherry’ (Antirrhimun majus) 

and three cultivars of petunia ‘Shock Wave Ivory’, ‘Easy Wave White’, and ‘Wave Purple 

Improved’ (Petunia × hybrida) flowered fastest under NI lighting provided by INC or LED 

lamps with a PPE of 0.64 or 0.63, respectively (intermediate PPE).  

34 

Excessive stem elongation and suppressed lateral branching can be a major drawback of 

utilizing FR radiation, since stem breakage and undesirable phenotypic traits can occur 

(Bleasdale, 1964; Moe and Heins, 1990; Whitman et al., 1998). For example, Torres and Lopez 

(2011), reported that stem elongation of the LDP Tecoma stans ‘Mayan Gold’ increased by 38% 

when photoperiod was extended to 12 or 16 h under INC lamps providing a TPFD of ≈2 

µmol·m–2·s–1. Whitman et al. (1998) found that Campanula carpatica ‘Blue Clips’ and 

Coreopsis grandiflora ‘Early Sunrise’ were both taller when grown under INC lamps in 

comparison to cool-white fluorescent (CWF), metal halide (MH), or high-pressure sodium (HPS) 

lamps providing 7-h DE lighting. Additionally, Kohyama et al. (2014) compared growth and 

development of dianthus ‘Super Parfait Strawberry’ (Dianthus chinensis) under a 9-h SD and NI 

lighting utilizing INC lamps or LED lamps emitting R+W radiation with or without FR radiation. 

They reported that plants grown under INC or R+W+FR radiation were ≈24 to 33% taller than 

those grown under SD or R+W LEDs (without FR radiation). Photoperiods >9 h increased stem 

length for coriander and dill. For instance, stem length of coriander and dill was 32.2 and 27.4 

cm greater, respectively, for plants grown under a 16-h LD in comparison to a 9-h SD (Fig. 6A, 

and B). The increase in stem length is directly associated to the increased duration of exposure to 

FR radiation under the 16-h LD. Conversely, stem elongation may not always be associated with 

low R:FR for certain species. For example, Whitman et al. (1998) reported that stem of 

Coreopsis verticillata ‘Moonbeam’ grown under INC or HPS lamps were significantly shorter 

than those grown under CWF and MH lamps.  

Time to VB was generally comparable to time to first OF for all species evaluated (Fig. 3A-

H, and 4A-H). Visible bud formation and subsequent flowering of coriander, dill, and marjoram 

was generally hastened under ≥13 h photoperiods (Fig. 3A, B, and D, 4A, B, and D). Watercress, 

35 

oregano ‘Greek’, and spearmint only produced VBs and flowers under LDs ≥14 h or a NI (Fig. 

3F-H and 4F-H). Although oregano ‘Kirigami’ flowered under 13 h, flowering percentage was 

incomplete compared to ≥14 h or a NI, under which flowering was 100% (Fig. 2E). Thus, 

oregano ‘Kirigami’ requires ≥14 h or an NI for uniform flowering. Our results for watercress are 

similar to those previously mentioned by Bleasdale (1964), who reported VB formation was 

hastened under 20-h LDs versus shorter photoperiods. Similarly, VBs were not observed under a 

9-h SD in either study. However, they reported that watercress produced VBs when grown under 

a 13-h photoperiod achieved with INC lamps, whereas in the current study, VBs and OFs did not 

initiate under 9 to 13-h photoperiods created with LEDs (Fig. 3F and 4F). Our lavender 

flowering data is in agreement to an industry report by Whitman and Padhye (2009), who 

classified Spanish lavender as a facultative LDP. Although plants produced VBs under a 9-h SD 

in our study, no OFs were observed and furthermore time to VB and OF decreased with 

increasing day length or a 4-h NI (Fig. 3C and 4C).   

Although there were inconsistencies between Reps. related to flowering for oregano ‘Greek’ 

and spearmint, VBs and OFs only developed under either a 16-h or NI treatment (Fig. 3G-H and 

4G-H). Langston and Leopold (1954) suggested that peppermint (M. piperita var. vulgaris) is an 

LDP, with a critical day length of 16 or 18 h required for floral differentiation and development. 

This contrasts with Burbott and Loomis (1957) who reported that peppermint (M. piperita var. 

Black Mitcham) flowered under a 14-h photoperiod created from a combination of fluorescent 

and INC lamps. Furthermore, Allard (1941) conducted DE photoperiodic studies utilizing natural 

sunlight or INC lamps for 18 h to evaluate multiple Mentha species. Spearmint (M. spicata) 

remained vegetative under 10 h but did flower when exposed to a 12-h photoperiod, whereas 

peppermint (M. piperita) produced few flowers when grown under a 14-h day length.  

36 

After 120 d, only 40 and 50% of oregano ‘Greek’ flowered under a 16-h photoperiod (Rep. 

1) or a 4-h NI (Rep. 2), respectively (Fig. 2G). These inconsistent flowering responses between 

Reps. may have been influenced by the methods of propagation employed between Rep. 1 and 2. 

In Rep. 1, germination was low and seedling growth was slow. Therefore, we harvested cuttings 

in Rep. 2 from our stock plants that were maintained under a 9-h SD. Nevertheless, further 

studies are required to evaluate the length of the juvenile phase, temperature, and initial 

establishment (seed vs. cuttings) and their effect on flower development of LDPs.        

Similar to other studies, we have shown that a 4-h NI can be as effective at promoting 

flowering of LDPs to DE lighting providing a 16-h photoperiod. Under a 4-h NI, flowering 

percentage was 100 and statistically occurred at the same time as plants under the 16-h 

photoperiod for coriander, dill, lavender, marjoram, oregano ‘Kirigami’, and spearmint (Fig. 2A-

E, H, 4A-E, H). Runkle and Heins (2003) found similar results for pansy ‘Crystal Bowl Yellow’ 

(Viola ×wittrockiana), where flowering percentage was similar under DE (16-h photoperiod) and 

NI lighting created by INC lamps. 

In conclusion, coriander ‘Santo’, dill ‘Bouquet’, lavender ‘Bandera Pink’, and marjoram can 

be considered facultative LDPs because time to VB and OF were generally hastened under LDs 

(≥13 h) or an NI, in comparison to a 9-h SD (Fig. 3A-D, 4A-D). Furthermore, node number 

below the first OF for coriander, dill, lavender, and marjoram decreased as photoperiod increased 

from 9 to 16 h or under an NI (Fig. 5A-D). Also, flowering percentage of lavender, marjoram, 

oregano ‘Kirigami’, and watercress increased from 9 to 16 h or under an NI (Fig. 2C-F). From 

our study oregano ‘Kirigami’, watercress, oregano ‘Greek’, and spearmint ‘Spanish’ can be 

classified as obligate LDPs, requiring ≥14-h for flowering to occur (Fig. 4E-H). Additionally, the 

critical photoperiod of watercress, oregano ‘Kirigami’, oregano ‘Greek’ and spearmint ‘Spanish’ 

37 

is 14, 13, 16, and 16 h, respectively. Although time to VB and OF of oregano ‘Greek’ and 

spearmint was inconsistent between Reps., both species exhibited obligate LDP responses, 

requiring either 16 h or an NI to form buds and subsequently flower (Fig. 3G, H, 4G, and H). 

Results from this study can prove beneficial to growers who seek to prevent or delay flowering 

of herbs, while also providing valuable information for the promotion of flowering of ornamental 

crops such as lavender or oregano ‘Kirigami’. 

 

Acknowledgements 

We gratefully acknowledge support by The USDA-ARS Floriculture and Nursery Research 

Initiative, Ball Horticultural Co. for seed, the Blackmore Company for fertilizer and Nathan 

DuRussel for technical assistance. The use of trade names in this publication does not imply 

endorsement by Michigan State University of products named nor criticism of similar ones not 

mentioned. This work was supported by the USDA National Institute of Food and Agriculture, 

Hatch project MICL02472. 

 

38 

APPENDIX

39 

 

Table II-1. Photoperiod (h), mean (±SD) bench average daily temperature (ADT), and daily light integral (DLI) (±SD) during two 
experimental replications (Rep.) for culinary and ornamental herbs and leafy greens. Plants were grown under a truncated 9-h short 
day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-emitting diode (LED) lamps to achieve 12-, 13-, 14-, 
and 16-h photoperiods or a 4-h night interruption (NI). 
Photoperiod (h)   Rep. 1 bench ADT           

Rep. 2 DLI 
(mol·m‒2·d‒1) 

[mean ± SD (ºC)]  

Rep. 2 bench ADT           
[mean ± SD (ºC)]  

Rep. 1 DLI 
(mol·m‒2·d‒1) 

9 
12 
13 
14 
16 
NI 

20.6 ± 2.3 
20.7 ± 2.3 
20.4 ± 2.1 
20.8 ± 2.5 
20.5 ± 2.3 
20.9 ± 2.5 

20.9 ± 2.0 
20.5 ± 2.4 
20.7 ± 1.9 
20.7 ± 1.9 
21.0 ± 2.0 
20.3 ± 2.5 

-z 

10.7 ± 4.6 
10.8 ± 4.5 
10.2 ± 4.8 
  9.7 ± 5.6 
  9.1 ± 4.3 

-z 

9.1 ± 4.3 
10.5 ± 4.9 
10.0 ± 4.5 
  9.4 ± 4.4 
  9.9 ± 4.6 

z No data collected 

40 

 

 

 

R+W+FR 

0.14 
0.26 
0.82 
1.08 
2.31 

 

 

0.17 
0.13 
0.75 
0.83 
0.66 

 
Light ratio 
   B:R 
   B:FR 
   R:FRwide 
   R:FRnarrow 
   PFR:PR+FR 

Parameter 
Light intensity (% of the TPFD) 
   B (400 to 500 nm) 
   G (500 to 600 nm) 
   R (600 to 700 nm) 
   FR (700 to 800 nm) 
TPFD (400 to 800 nm) 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure II-1. Spectral distribution, intensity of blue [B (400–500 nm)], green [G (500–600 nm)], 
red [R (600–700 nm)], and far-red [FR (700–800 nm)] radiation, total photon flux density 
(TPFD), light ratio, and estimated phytochrome photoequilibria [PFR/PR+FR (the proportion of 
FR-absorbing phytochromes in the pool of R- and FR-absorbing phytochromes; Sager et al., 
1988)] of R+W+FR light-emitting diode (LED) lamps covered with multiple layers of wire 
mesh. R:FRwide was calculated as 600 to 700 nm:700 to 800 nm; R:FRnarrow was calculated as 655 
to 665 nm:725 to 735 nm. 

41 

 

Figure II-2. Flowering percentage for eight herbs and leafy greens grown under a truncated 9-h 
short day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-emitting 
diode (LED) lamps to achieve 12-, 13-, 14-, and 16-h photoperiods or a 4-h night interruption 
(NI). Flowering percentage of oregano ‘Greek’ and spearmint ‘Spanish’ are presented by 
replication (Rep.) due to variability between experimental Reps.  

 

42 

 

Figure II-3. Time to first visible bud (VB) for eight herbs and leafy greens grown under a 
truncated 9-h short day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) 
light-emitting diode (LED) lamps to achieve 12-, 13-, 14-, and 16-h photoperiods or a 4-h night 
interruption (NI). Data were pooled when there was no interaction between replication (Rep.) 
and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across photoperiodic treatments using Tukey-Kramer difference test at P ≤ 0.05. 
Error bars indicate standard error. Mean separations were excluded due to variability between 
experimental Reps. for oregano ‘Kirigami’, oregano ‘Greek’ and spearmint ‘Spanish’. 

 

43 

 

Figure II-4. Time to first open flower (OF) for eight herbs and leafy greens grown under a 
truncated 9-h short day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) 
light-emitting diode (LED) lamps to achieve 12-, 13-, 14-, and 16-h photoperiods or a 4-h night 
interruption (NI). Data were pooled when there was no interaction between replication (Rep.) 
and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. Error bars indicate 
standard error. Mean separations were excluded due to variability between experimental Reps. 
for oregano ‘Greek’ and spearmint ‘Spanish’.

 

44 

 

Figure II-5. Node number for eight herbs and leafy greens grown under a truncated 9-h short day 
(SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-emitting diode (LED) 
lamps to achieve 12-, 13-, 14-, and 16-h photoperiods or a 4-h night interruption (NI). Data were 
pooled when there was no interaction between replication (Rep.) and treatment, or if the response 
trends were similar between Reps. Letters indicate mean separations across photoperiodic 
treatments using Tukey-Kramer difference test at P ≤ 0.05. Error bars indicate standard error. 
Mean separations were excluded when there were interactions or due to variability between 
experimental Reps. for oregano ‘Kirigami’, oregano ‘Greek’ and spearmint ‘Spanish’. 

 

45 

 

Figure II-6. Stem length at first open flower (OF) of eight herbs and leafy greens grown under a 
truncated 9-h short day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) 
light-emitting diode (LED) lamps to achieve 12-, 13-, 14-, and 16-h photoperiods or a 4-h night 
interruption (NI). Data were pooled when there was no interaction between replication (Rep.) 
and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across photoperiodic treatments using Tukey-Kramer difference test at P ≤ 0.05. 
Error bars indicate standard error. Mean separations were excluded due to variability between 
experimental Reps. for oregano ‘Greek’ and spearmint ‘Spanish’.  

 

46 

 

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47 

 

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50 

 

SECTION III  

PHOTOSYNTHETIC DAILY LIGHT INTEGRAL AND PHOTOPERIOD INFLUENCE 

DEVELOPMENT OF BASIL SPECIES AND CULTIVARS 

51 

 

Photosynthetic daily light integral and photoperiod influence development of basil species and 

cultivars 

 

Charlie Garcia and Roberto Lopez* 

Department of Horticulture, Michigan State University, 1066 Bogue Street, East Lansing, MI 

48824-1325, USA 

 

*Corresponding author. Tel.: +1 517 353 0342; E-mail address: rglopez@msu.edu (R.G. Lopez) 

 

52 

 

Abstract 

Limited controlled environment studies have been conducted on common culinary herbs such 

as basil (Ocimum spp.) to determine how changes in day length and daily light integral (DLI) 

influence developmental parameters. Therefore, the objectives of this study were to quantify how 

photoperiod and DLI influence flowering development parameters of 10 basil species and 

cultivars. For experiment (Expt.) 1, photoperiodic response of O. basilicum ‘Genovese’, ‘Sweet 

Thai’, ‘Cinnamon’, ‘Red Ruben’, ‘Sweet Dani Lemon’, and ‘Nufar’, O. ×citriodorum ‘Lime 

Basil’, O. tenuiflorum ‘Holy Basil’, O. basilicum var. citriodora ‘Mrs. Burns’ Lemon’, and O. 

minimum ‘Pluto’ were quantified. In Expt. 2 responses to photoperiod and DLI were evaluated 

for ‘Genovese’, ‘Sweet Thai’, ‘Red Ruben’, ‘Nufar’, ‘Lime Basil’ and ‘Holy Basil’. In both 

Expts. seeds were germinated in plug trays and grown at 25 °C. Photoperiods for Expt. 1 

consisted of a 9-h short-day (SD) or a 9-h SD extended to 11-, 12-, 13-, 14-, 15-, 16-h or a 4-h 

night interruption (NI) utilizing red+white+far-red (R+W+FR) light-emitting diode (LED) lamps 

(R:FR = 0.8) providing a total photon flux density (TPFD) of ≈2 µmol·m–2·s–1. In Expt. 1, the 

DLI was low ≈8 mol∙m–2∙d–1. For Expt. 2, a 9-h SD and a 16-h long-day (LD) were created under 

a moderate DLI of ≈13 mol∙m–2∙d–1 and high DLI of ≈23 mol∙m–2∙d–1. All cultivars, with the 

exception of ‘Red Rubin’ can be classified as day-neutral plants (DNPs) under a low DLI, 

because there were no discernible flowering trends among treatments. During Expt. 1 ‘Red 

Rubin’ exhibited a facultative SD response and a day neutral response under a moderate and high 

DLI. Under moderate and high DLIs, ‘Genovese’, ‘Nufar’, and ‘Sweet Thai’ exhibited a 

facultative LD response as time to first open flower was hastened under a 16-h photoperiod. In 

Expt. 2, time to first open flower and node number below the first open flower were reduced 

when the DLI was increased from a moderate to a high DLI for all cultivars excluding ‘Lime 

53 

 

Basil’, thus indicating a facultative irradiance response.  

 

Keywords: Culinary herbs, day-neutral plants, DLI, facultative, long-day plants, Ocimum 

basilicum, short-day plants 

 

Abbreviations: ADT, average daily temperature; B, blue; CE, controlled environment; CWL, 

cool white fluorescent; DE, day-extension; DLI, daily light integral; DNP, day-neutral plant; 

Expt., experiment(s); FIR, facultative irradiance response; FR, far red; G, green; HPS, high-

pressure sodium; LD, long-day; LDP; long-day plant; LED, light-emitting diode; NI, night 

interruption; OF, open flower; PFD, photon flux densities; PL, photoperiodic lighting; PPFD, 

photosynthetic photon flux density; R, red; SD; short-day; SDP, short-day plant; SL, 

supplemental lighting; TPFD, total photon flux density; VB, visible bud; W, white. 

 

Introduction 

The genus Ocimum, composed of 30 (Simon et al., 1999) to 150 (Evans, 2009) species, is 

collectively known as basil, and is a member of the mint family, Lamiaceae. Basil is believed to 

be native to India, Asia, and Africa (Darrah, 1980; Putievsky and Galambogi, 1999; Simon, 

1985), but its exact origin is uncertain. Ocimum spp. have several uses, including as a medicinal 

herb, for religious purposes, for the production of essential oils, and as a landscape ornamental or 

cut flower (Dole and Wilkins, 2005; Grayer et al., 1996; Kalita and Khan, 2013; Simon et al., 

1990; Simon et al., 1999). However, sweet basil (O. basilicum) is the most common species and 

is used as a culinary herb, either as a fresh-cut, container grown, or dried product.  

54 

 

The market for fresh culinary herbs in the United States (U.S.) has increased by 10 to 12% 

annually from 2004 to 2014 (USAID, 2014). Total production of field and controlled 

environment (CE) grown fresh cut herbs is valued at $109 million (certified organic) and $200 

million (not certified organic) (USDA, 2015). Of this production, CE production expanded by 

190% between 1998 and 2014, and the number of commercial operations increased by 173% to 

524 operations (USDA, 2015). Basil is the most popular culinary herb grown in CE greenhouses 

and indoor vertical farms due to its high market value (DeKalb et al., 2014). This includes both 

hydroponic and container production (Adam, 2005). However, statistics individual commodities 

such as basil not collected.  

 

Prevention of flower initiation and development in culinary herbs such as basil is important 

(Davis, 1995), as retailers often will not accept fresh-cut herbs if buds or flowers are present. 

Retailers and consumers perceive fresh-cut basil with buds and flowers as old, off flavored, and 

bitter (Williamson, 2019). In contrast, field producers of other Ocimum spp. used for their 

essential oils will want their crops to flower quickly to maximize aromatic oil concentrations 

(Simon, 1995), since the greatest amount of essential oils are primarily accumulated in the leaves 

and flowers (Nurzynska-wierdak et al., 2013).  

 

Flowering of many horticulture crops can be influenced by changes in day length and 

photosynthetic daily light integral (DLI) that occur with seasonal changes (Erwin and Warner, 

2002; Mattson and Erwin, 2005, Runkle and Heins, 2003). Photoperiod refers to the number of 

light hours within a 24-hour period. However, the skotoperiod or critical night length regulates 

flowering responses (Thomas and Vince-Prue, 1997). Seasonal responses, as a result of 

photoperiodism include the onset of dormancy, the formation of storage organs, asexual 

55 

 

reproduction, leaf development, stem elongation, germination, and the initiation or prevention of 

flowering (Taiz and Zeiger, 2010; Thomas and Vince-Prue, 1984).  

Horticultural crops are classified into three major photoperiodic classes including short-day 

(SDP), long-day (LDP) and day-neutral plants (DNP) (Runkle et al., 2017; Thomas and Vince-

Prue, 1997). Both LDP and SDP can be further classified depending on whether the photoperiod 

is required for flowering (qualitative or obligate) or if the photoperiod accelerates but is not 

required for flowering (quantitative or facultative) (Blanchard and Runkle, 2010). In CEs, 

growers can manipulate the day length based on the flowering response desired. For example, 

day-extension (DE) or night interruption (NI) lighting can be employed to induce flowering of 

LD species such as coreopsis ‘Moonbeam’ (Coreopsis verticillata), bellflower ‘Deep Blue Clips’ 

(Campanula carpatica), and coreopsis ‘Early Sunrise’ (Coreopsis grandiflora) when natural day 

lengths are short (Padhye and Runkle, 2011; Whitman et al., 1998). While the use of blackout 

systems can be used to truncate natural LDs to provide inductive conditions for SD species, such 

as creeping zinnia (Sanvitalia procumbens) and Mexican sunflower ‘Sundance’ Tithonia 

rotundifolia (Mattson and Erwin, 2005). DNPs such as New Guinea impatiens (Impatiens 

hawkeri), summer snapdragon (Angelonia angustifolia), and tomato (Lycopersicon esculentum) 

are insensitive to day length and flower regardless of the photoperiod provided (Currey et al., 

2011).  

A limited number of studies have examined how photoperiod influences growth and 

development of culinary herbs. Burbott and Loomis (1967) reported that peppermint (Mentha 

piperita) grown under a 12-h day length remained vegetative. Langston and Leopold (1954), 

reported that flower induction of peppermint occurred when day length was extended to 14 or 18 

h. When sweet basil was grown at 30/12 ºC, 24/12 ºC, and 18/12 ºC (day and night temperatures) 

56 

 

and under 10- or 16-h photoperiods, plants flowered faster under LDs and higher temperatures 

(Putievsky, 1983). In a separate study, sweet basil grown under an 18-h photoperiod flowered 8, 

5, and 3 d faster compared to plants grown under a 9-, 12-, and 15-h photoperiod, respectively 

(Skrubis and Markakis, 1976).  

Herb production within greenhouses and other semi CEs can occur year-round, however 

during the winter months in northern latitudes ambient DLIs can be low. Thus, the only feasible 

way to appreciably increase the DLI is with the use of supplemental lighting (SL) (Currey et al., 

2017). Beaman et al. (2009) reported a yield increase for sweet basil, cilantro (Coriandrum 

sativum), dill (Anethum graveolens), and parsley (Petroselenium crispum) by 144, 154, 241, and 

120%, respectively when the DLI was increased from 6.3 to 7.5 mol∙m–2∙d–1 to 14.5 to 19.2 

mol∙m–2∙d–1. Walters and Currey (2018) reported fresh and dry weight, height, and node number 

of O. basilicum ‘Nufar’ increased by 144, 178, 20, and 18%, respectively, as DLI increased from 

7 to 15 mol∙m–2∙d–1. In a separate study, sweet basil ‘Improved Genovese Compact’ grown under 

sole-source lighting providing 5 different DLIs resulted in larger and thicker leaves when the 

DLI was increased (Dou et al., 2018).  

Previous studies have established that an increase in DLI can accelerate time to first open 

flower (OF) of certain bedding crops (Erwin et al., 2017; Mattson and Erwin, 2005; Oh et al., 

2009). For example, time to flower of cyclamen (Cyclamen persicum) ‘Metis Scarlet Red’ 

decreased from 133 to 75 d as DLI was increased from 1.4 to 17.3 mol∙m–2∙d–1 (Oh et al., 2009). 

For sweet pea ‘Royal White’ (Lathyrus odoratus), leaf number below the first flower decreased 

from 16 to 11, and time to flower decreased from 78 to 57 d when grown under an 18-h 

photoperiod with a DLI of 9.7 mol∙m–2∙d–1 provided by high-pressure sodium (HPS) lamps in 

57 

 

comparison to a NI treatment (Mattson and Erwin, 2005). Furthermore, flowering did not occur 

under 9-h SDs, regardless of supplemental lighting.    

Studies investigating how photoperiod and DLI interact to affect developmental parameters 

of popular basil species grown in CEs are needed; considering there are more than 60 cultivars 

(Simon, 1995) of O. basilicum, not including other Ocimum spp., (Evans, 2009) and that 

previous studies have focused on growth (fresh and dry weight, height, leaf area) and not 

flowering parameters. Therefore, the objectives of this study were to: 1) quantify how 

photoperiod influences flowering of 10 basil species and cultivars and 2) determine if 

photoperiod and DLI interact to influence flowering responses. 

 

Materials and methods 

Plant material (Expt. 1) 

Seeds of various genotypes of basil including O. basilicum ‘Genovese’, ‘Sweet Thai’, 

‘Cinnamon’, ‘Red Ruben’, ‘Sweet Dani Lemon’, and ‘Nufar’, O. tenuiflorum ‘Holy Basil’, O. 

basilicum var. citriodora ‘Mrs. Burns’ Lemon’, O. ×citriodorum ‘Lime Basil’, and O. minimum 

‘Pluto’ were sown on 13 Oct. 2017 and 20 Aug. 2018 into 128-cell plug trays (2.7 × 2.7 cm; 

12.0-mL cell volume). Each cell was filled with a soilless substrate composed of 50% 

vermiculite (Vermiculite Premium Grade; Sungro Horticulture, Agawam MA) and 50% soilless 

medium containing 70% peat moss, 21% perlite, and 9% vermiculite (Suremix; Michigan 

Grower Products, Inc., Galesburg, MI).  

 

Seeded trays of each cultivar were placed under each lighting treatment and on capillary 

mats. Seeded trays were overhead irrigated as needed with reverse osmosis water supplemented 

with water-soluble fertilizer (mg∙L‒1) 60 nitrogen (N), 23 phosphorus (P), 60 potassium (K), 28 

58 

 

calcium (Ca), 5 magnesium (Mg), 1 iron (Fe), 0.6 manganese (Mn), 0.6 zinc (Zn), 0.6 copper 

(Cu), 0.4 boron (B), and 0.1 molybdenum (Mo) (MSU Plug Special; Blackmore Company, 

Kankakee, IL). After cotyledon emergence, seedlings were thinned, so that only one plant per 

cell remained. 

Once plugs were pullable, 10 randomly selected seedlings were transplanted into 11-cm 

round (600-mL) containers filled with a soilless medium (Suremix; Michigan Grower Products 

Inc.). Twenty-one days after sowing ‘Genovese’, ‘Cinnamon’, ‘Sweet Dani Lemon’, ‘Nufar’, 

‘Mrs. Burns’ Lemon’, ‘Lime Basil’, and ‘Pluto’ were transplanted and ‘Red Rubin’, ‘Sweet 

Thai’, and ‘Holy Basil’ were transplanted 28 d after sowing. After transplant, plants were 

irrigated as needed with reverse osmosis water supplemented with water-soluble fertilizer 

(mg·L–1) 125 N, 12 P, 100 K, 65 Ca, 12 Mg, 1.0 Fe and Cu, 0.5 Mn and Zn, 0.3 B, and 0.1 Mo 

(MSU Orchid RO Water Special; Blackmore Company). 

Plant material (Expt. 2) 

Seeds of ‘Genovese’, ‘Holy Basil’, ‘Lime Basil’, ‘Nufar’, ‘Sweet Thai’, and ‘Red Ruben’ 

were sown on 10 Sept. 2018 and 12 Mar. 2019. Sowing, germination, thinning, and irrigation of 

seedlings and transplants followed the same protocol as in Expt. 1. However, once plugs were 

pullable after 21 d, 9 seedlings were randomly selected and transplanted into round 15-cm 

diameter (1,300-mL) containers filled with the same soilless medium (Suremix; Michigan 

Grower Products Inc.). 

Greenhouse environment and lighting treatments (Expt. 1) 

Plants were grown in a glass-glazed greenhouse with exhaust fans, evaporative-pad cooling, 

radiant hot-water heating, and SL controlled by an environmental control system (Priva Office 

version 725-3030; Priva North America, Vineland Station, ON, Canada). The greenhouse air 

59 

 

average daily temperature (ADT) set point was a constant 25 ºC. Space heaters provided 

supplemental heat for each bench under black cloth when the air temperature fell under 24.8 °C. 

The photoperiod was 9 h (0800 to 1700 HR) consisting of natural photoperiods (lat. 42º N) and 

supplemental lighting from 200-W light-emitting diodes (LEDs) (Philips GP-TOPlight DRW-

MB; Koninklijke Philips N.V., Eindhoven, the Netherlands) that provided a photosynthetic 

photon flux density (PPFD) (±SD) of 90 ± 14 µmol·m–2·s–1 when the outdoor light intensity was 

below ≈440 µmol·m–2·s–1 to achieve a DLI of <8 mol·m–2·d–1, hence forward referred to as a low 

DLI (Table 1). The 100-nm waveband ratios (%) of the LED fixtures, defined by their blue [B 

(400-500 nm)], green [G (500-600 nm)], and red [R (600-700 nm)] photon flux densities (PFD), 

were 10:5:85. 

Opaque black cloths were pulled over each bench daily at 1700 HR and opened at 0800 HR to 

create a truncated 9-h short day (SD) for all treatments. Treatments consistent of the 9-h SD or a 

9 h extended by four R+W+Far-red (R+W+FR) LED lamps (GreenPower LED flowering 

DR/W/FR 14 W, E26; Philips) on each bench to create 11-, 12-, 13-, 14-, 15-, or 16-h 

photoperiods or a 4-h NI from 2200 to 0200 HR. Each LED lamp was covered with multiple 

layers of aluminum wire mesh to achieve an average total photon flux density (TPFD) of ≈2 

µmol∙m–2∙s–1 between 400 and 800 nm. The 100-nm waveband ratios (%) of the R+W+FR LED 

lamps, defined by their B, G, R, and FR radiation, were 7:12:35:46. The spectral distribution of 

the LED lamps was measured in five random locations throughout each bench by a 

spectroradiometer (PS-200; Stellar-Net, Tampa, FL), and the phytochrome photoequilibrium was 

estimated according to Sager et al. (1988) (Fig. 1). 

 

 

60 

 

Greenhouse environment and lighting treatments (Expt 2).  

Plants were grown in two separate glass-glazed greenhouse compartments. Environmental 

parameters such as heating, cooling, and SL were controlled by the same environmental control 

system previously mentioned from Expt. 1. The greenhouse air ADT was set at a constant 25 ºC. 

Continuous SL was provided to one of the greenhouse compartments for 9 h∙d–1 (0800 to 1700 

HR) utilizing six 200-W LED arrays (Philips GP-TOPlight DRW-LB and DRW-HB; Koninklijke 

Philips N.V., Eindhoven, the Netherlands) that provided a supplemental PPFD (±SD) of ≈250 ± 

1.7 µmol·m–2·s–1, to provide a DLI of ≈23 mol·m–2·d–1, hence forward referred to as a high DLI 

(Table 2). The 100-nm waveband ratios (%) of the LED arrays, defined by the combined B (peak 

wavelength 449 nm) and R (peak wavelength 665 nm) PFDs, was 15:85. All LED arrays 

providing SL were placed ≈147 cm above the benches. The second greenhouse compartment did 

not receive any SL, and the average DLI was ≈13 mol·m–2·d–1, hence forward referred to as a 

moderate DLI (Table 2).  

There were two benches per greenhouse compartment, over which two photoperiodic lighting 

(PL) treatments were delivered. Treatments consisted of a truncated 9-h SD created by pulling an 

opaque black cloth as previously described. DE lighting provided by R+W+FR LED lamps (as 

previously described) were used to create a 16-h (0800 to 2400 HR) photoperiod.  

Data collection and analysis 

In both experiments, plants were assessed daily and the date of first visible bud (VB) and 

date of first OF were recorded. At first OF, the number of nodes below the first OF and plant 

height were recorded (from the media surface to the plant apex). Plants that did not flower 105 d 

after sowing were considered non-flowering. A shielded and aspirated 0.13-mm type E 

thermocouple (Omega Engineering, Stamford, CT, USA) at canopy height recorded the air 

61 

 

temperatures on each bench and a line quantum sensor (LI-191R, LI-COR, Lincoln, NE) 

(Apogee Instruments, Inc., Logan, UT) or a quantum sensor (LI-190R, LI-COR, Lincoln, NE) 

placed at canopy height recorded the light intensity. A CR-1000 datalogger (Campbell Scientific, 

Logan, UT) collected the environmental data every 15 s and hourly means were recorded. Actual 

mean air temperatures and DLI during Expt. 1 and 2 are reported in Table 1 and 2, respectively.  

Expt. 1 was a randomized complete block design (RBCD) with 10 plants of each cultivar 

assigned to each treatment. Each Rep. was regarded as a blocking factor. Benches with different 

photoperiodic treatments were considered the experimental units and pots randomly placed 

throughout the benches were considered sub samples. Expt. 2 was also a RCBD with 9 plants per 

cultivar. Each plant was considered an experimental unit in the data analysis. Experiment one 

had one factor (photoperiodic treatment) and eight levels (9, 11, 12, 13, 14, 15, 16 h and NI) and 

Expt. 2 had two factors 1) photoperiod, and 2) light intensity, each with two levels (9 and 16-h 

photoperiods, and presence or absence of SL). For both Expt., data were separately analyzed for 

each cultivar, therefore species and cultivars were not considered another factor. Data for Expt. 1 

was pooled if no interaction between Rep. and treatments was present, or if similar response 

trends were observed between Reps. Data for both Expts. was analyzed with the SAS version 9.4 

(SAS Institute, Inc., Cary, NC) using the PROC GLIMMIX procedure and pairwise comparisons 

between treatments were performed with adjust Tukey-Kramer difference test (P ≤ 0.05). 

 

Results 

Expt. 1: Time to first visible bud 

  No significant differences for time to first VB were observed for ‘Cinnamon’, ‘Genovese’, 

‘Lime Basil’, ‘Pluto’, and ‘Sweet Thai’ under a low DLI (Fig. 2A, B, D, G, and J). Although 

62 

 

there were significant differences among photoperiodic treatments, for ‘Nufar’ and ‘Sweet Dani 

Lemon’ there were no discernable trends (Fig. 2F and I). For ‘Holy Basil’ time to first VB 

occurred 9 d faster under a 9-h SD in comparison to a 16-h LD (Fig. 2C). Similarly, time to first 

VB was hastened by 7 and 6 d for ‘Red Ruben’ grown under a 9- or 12-h SD in comparison to a 

16-h LD (Fig. 2H). 

Expt. 1: Time to first open flower 

Under a low DLI (<8 mol·m–2·d–1) no significant difference for time to first OF were 

observed for ‘Cinnamon’, ‘Genovese’, “Holy Basil’ and ‘Pluto’ (Fig. 3A-C, and G). Although 

there were significant differences, no discernable trend was evident for time to first OF for the 

remaining cultivars (Fig. 3D-F, I, and J). However, time to first OF occurred most rapidly for 

‘Red Rubin’ grown under 9 and 12-h SDs (Fig. 3H). For instance, time to first OF for ‘Red 

Rubin’ was hastened by 7 d for plants grown under 9 and 12-h photoperiods, when compared to 

plants grown under the 16-h LD.     

Expt. 1: Node number below the first open flower 

There was no significant difference in the number of nodes below the first OF for 

‘Genovese’, ‘Lime Basil’, ‘Pluto’, and Sweet Dani Lemon’ under a low DLI (Fig. 4B, D, G, and 

I). For the remaining cultivars, there were significant differences in the node number below the 

first open flower (Fig. 4A, C, E, F, H, J). However, differences were minor, ranging from 1 to 3 

nodes. For ‘Holy Basil’, with the exception of the 13-h photoperiod, plants grown under the 9-h 

SD had the least number of nodes compared to all other photoperiodic treatments (Fig. 4C). ‘Red 

Rubin’ under the 9-h SD had one less node below the first OF compared to the 16-h LD (Fig. 

4H).  

 

63 

 

Expt. 1: Height at first open flower 

Under a low DLI, excluding ‘Genovese’, significant differences in height were observed for 

all species and cultivars (Fig. 5A-J). Height at first open flower was significantly shorter under 

the 9-h SD compared to all other photoperiodic treatments for ‘Holy Basil’, ‘Mrs. Burns’ 

Lemon’ and ‘Sweet Dani Lemon’ (Fig. 5C, E, and I). For example, under a 9-h SD ‘Holy Basil’ 

and ‘Mrs. Burns’ Lemon’ plants were 18 to 26% and 18 to 23% shorter, respectively in 

comparison to all other photoperiodic treatments (Fig. 5C and E). Furthermore, excluding the 4-h 

NI treatment, ‘Nufar’, ‘Red Rubin’, and ‘Sweet Thai’ were shortest under a 9-h SD compared to 

the remaining DE lighting treatments (Fig. 5F, H, and J).   

Expt. 2: Time to first visible bud  

Time to first VB was hastened under a 16-h LD with a high DLI compared to a 9-h SD with a 

moderate DLI for ‘Genovese’, ‘Holy Basil’, ‘Nufar’, ‘Red Rubin’ and ‘Sweet Thai’ (Fig. 6A, B, 

D-F). Under a moderate DLI photoperiod significantly affected time to first VB of ‘Holy Basil’, 

‘Nufar’, and ‘Sweet Thai’ (Fig. 6B, D, and F). Where time to first VB occurred more rapidly for 

‘Holy Basil’ under a 9-h SD compared to a 16-h LD (Fig. 6B). Whereas, time to first VB was 

hastened under a 16-h LD and delayed under a 9-h SD for ‘Nufar’ and ‘Sweet Thai’ (Fig. 6D and 

F). Under a high DLI there was no significant difference among photoperiodic treatments for all 

basil cultivars (Fig. 6A-F).  

Expt. 2: Time to first open flower  

For all cultivars, except ‘Lime Basil’ time to first OF occurred most rapidly when plants were 

grown under a 16-h LD and a high DLI (≈23 mol·m–2·d–1) in comparison to the 9-h SD and a 

moderate DLI (≈13 mol·m–2·d–1) (Fig. 7A-F). Time to first OF for ‘Nufar’, under a moderate and 

high DLI was hastened by 10 and 6 d under a 16-h LD compared to a 9-h SD, respectively (Fig. 

64 

 

7D). ‘Genovese’ and ‘Sweet Thai’ followed similar trends to that of ‘Nufar’ (Fig. 7A, D, F). 

Under a moderate DLI, time to first OF for ‘Holy Basil’ occurred most rapidly (8 d) under a 9-h 

SD, compared to a 16-h LD. However, under a high DLI there was no significant difference 

between LD and SDs (Fig. 7B). For ‘Red Rubin’ under moderate and high DLIs, photoperiod did 

not significantly affect time to first OF. However, an increase in DLI hastened flowering of both 

SD and LD grown plants compared to the 9-h SD under a moderate DLI (Fig. 7E).  

Expt. 2: Node number below the first open flower 

Node number below the first OF was reduced under a high DLI regardless of photoperiod, 

compared to a moderate DLI for ‘Lime Basil’ and Sweet Thai’ (Fig. 8C and F). Under a 

moderate and high DLI photoperiod did not significantly affect node number below the first OF, 

for all cultivars, excluding ‘Holy Basil’ and ‘Nufar’ under a moderate DLI (Fig. 8A-F). For 

instance, under a moderate DLI and 16-h LD, node number below the first OF increased by two 

nodes for ‘Holy Basil’compared to a 9-SD with a moderate DLI (Fig. 8B). Whereas, under a 

moderate DLI and 16-h LD, node number below the first OF decreased by two nodes for ‘Nufar’ 

compared to a 9-h SD with a moderate DLI (Fig. 8D).   

Expt. 2: Height at first open flower 

Regardless of DLI, photoperiod did not significantly affect height of ‘Holy Basil’, ‘Nufar’, 

‘Red Rubin’, and ‘Sweet Thai’ at first OF (Fig. 9B, D-F). However, with an increase in DLI, 

height of ‘Holy Basil’ and ‘Nufar’ was reduced (Fig. 9B and D). For instance, under a high DLI, 

the combined average height of 9- and 16-h grown ‘Holy Basil’ decreased by 19.8 (22%), 

compared to the combined average height of 9- and 16-h grown plants under a moderate DLI 

(Fig. 9B). Under a high DLI, height of ‘Genovese’, decreased by 15.2 (18%) cm when grown 

under a 16-h LD compared to a 9-h SD (Fig. 9A).  

65 

 

Discussion 

Flower inhibition or induction of basil is desirable depending on its intended use; be it for 

culinary, ornamental, or the production of essential oils. An understanding of the environmental 

parameters that influence developmental parameters could be beneficial for growers who seek to 

induce flowering when basil is to be used as an ornamental or for essential oil production. 

However, inhibition of flowering could be desired by CE growers. Numerous studies conducted 

on bedding plants have demonstrated how DE and NI lighting can effectively induce or inhibit 

flowering of LDPs and SDPs, respectively, while truncated SDs using blackout systems can 

promote or inhibit flowering (Mattson and Erwin, 2005; Torres and Lopez, 2011). Based on the 

results for time to first OF from Expt. 1, under a low DLI (<8 mol·m–2·d–1) all cultivars 

excluding ‘Red Rubin’ exhibited DNP responses (Fig 3A-J). Generally, there were no significant 

differences in time to first OF, and when there were significant differences, no discernable trends 

were observed. ‘Red Rubin’ exhibited facultative SD responses under a low DLI as flowering 

occurred under all photoperiodic treatments and the NI, however time to first OF occurred more 

rapidly under a 9 and 12-h SD (Fig 3H).  

Results from Expt. 1 for ‘Genovese’ and ‘Lime Basil’ are in agreement with Erwin et al. 

(2017), who describe these two cultivars as DNP; however, information regarding specific 

photoperiodic treatments were not provided. In contrast to our results, Skrubis and Markakis 

(1976) reported that O. basilicum var. citriodora displayed LD responses, since flower 

development and subsequent anthesis occurred more rapidly under an 18-h photoperiod 

compared to 9-, 12-, and 15-h photoperiods, utilizing cool white fluorescent (CWF) lamps. 

However, in their study plants were grown under ambient sunlight for 9 h (0800 to 1700 HR), and 

then moved into a growth chamber where the CWF lamps provided DE lighting. The ambient 

66 

 

DLI was not provided and DE lighting did not increase the final DLI by more than 0.6 mol∙m–

2∙d–1 for those treatments. 

 

In addition to photoperiod, DLI can also influence flowering of plants (Mattson and Erwin, 

2005; Owen et al., 2018, Zhang et al., 1996). For instance, Zhang et al. (1996) reported a 

decrease in time to flower of the facultative LDP yarrow ‘Summer Pastels’ (Achillea millefolum) 

from 57 to 37 d as DLI increased from 5.8 to 17.3 mol∙m–2∙d–1 under a 16-h photoperiod. 

Similarly, an increase in DLI from 5 to 20 mol∙m–2∙d–1 hastened flowering of the LDP petunia 

(Petunia ×hybrida Vilm.-Andr.) (Faust et al., 2005). In Expt. 2, time to first OF of ‘Genovese’ 

under a moderate DLI occurred 7 d faster under a 16-h LD compared to a 9-h SD. Under a high 

DLI time to first OF occurred 9 d faster under a 16-h LD compared to a 9-h SD. Therefore, under 

moderate and high DLIs ‘Genovese’ exhibited facultative LDP responses (Fig. 7A). ‘Nufar’ and 

‘Sweet Thai’ followed similar trends and can also be classified as facultative LDPs, under 

moderate and high DLIs (Fig. 7D and F).  

  Although ‘Red Rubin’ displayed facultative SD responses under a low DLI in Expt. 1, the 

response was diminished under moderate and high DLIs (Expt. 2) as there were no significant 

differences in time to first OF among the photoperiodic treatments (Fig. 3H and 7E). ‘Lime 

Basil’ responded as a DNP under the low DLI treatment in Expt. 1 and under the moderate and 

high DLIs in Expt. 2 (Fig. 3D and 7C).  

An increase in DLI has been associated with the acceleration of growth and development of 

numerous crops due to an increase in photosynthesis (Nemali and van Iersel, 2004). The 

hastening of time to VB and OF in our study can be attributed to an increase in photosynthesis 

resulting from the additional irradiance that would increase metabolic activity, thus accelerating 

plant growth and development and possibly reducing the juvenile phase (Adams et al., 1999). A 

67 

 

facultative irradiance response (FIR) could further explain and validate this hypothesis. An FIR 

can occur within the three primary photoperiodic response groups, SDP, LDP, and DNP (Erwin 

et al., 2017). An FIR referrers to a decrease in time to first OF and node number below the first 

OF, resulting from an increase in DLI (Mattson and Erwin, 2005). Plants exhibiting an FIR 

flower faster because the juvenile phase is shortened under high radiation levels (Adams et al., 

1999). Warner (2010) described faster flowering and fewer nodes in four Petunia spp. (Petunia 

axillaris, P. exserta, P. ×hybrida ‘Mitchell’, and P. integrifolia) grown under 16-h LDs with a 

DLI of 16.5 mol∙m–2∙d–1 compared to 9-h SDs at 11.4 mol∙m–2∙d–1. For instance, node number 

below the first flower for P. axillaris and P. exserta decreased from 38 and 26 to 20 and 13, 

while time to flower decreased from 70 and 64 d to 45 and 53 d, respectively, as irradiance and 

day length increased. Therefore, ‘Genovese’, ‘Holy Basil’, ‘Nufar’, ‘Red Rubin’, and ‘Sweet 

Thai’ exhibited an FIR, as time to first OF was hastened, and node number below the first OF 

was reduced, when the DLI was increased from a moderate to a high DLI (Fig. 7A, B, D-F, 8A, 

B, D-F).  

From Expt. 1, the increase in plant height at flowering generally observed under DE lighting 

can be associated to an increased exposure to FR radiation from the R+W+FR LED lamps. The 

use of FR radiation promotes stem elongation and suppresses lateral branching (Moe and Heins, 

1990) of crops such as watercress (Bleasdale, 1964), pansy ‘Matrix Yellow’ (Viola 

×wittrockiana), and petunia ‘Dreams Midnight’ (P. ×hybrida) (Owen et al., 2018) when grown 

under incandescent (INC) or R+W+FR LEDs. Similarly, in our study, all cultivars excluding 

‘Genovese’ were significantly taller when grown under a 16-h LD compared to a 9-h SD (Fig. 

5A-J). Therefore, the increase in height was likely evoked by the shade avoidance response, 

mediated by the R and FR absorbing photoreceptor phytochrome. Plants in a greenhouse are 

68 

 

generally exposed to nearly equal amounts of B, G, R, and FR radiation from the sun. However, 

the R+W+FR LED lamps employed during our study had a low R:FR (R:FR ratio = 0.8) which 

created a FR rich environment, resulting in taller plants. 

Generally, photoperiod did not significantly affect plant height at first OF during Expt. 2 

(Fig. 9A-F). However, ‘Holy Basil’ and ‘Nufar’, plant height decreased with an increase in DLI, 

regardless of photoperiodic treatment (Fig. 9B and D). Similar to our findings for these to 

cultivars, Hurt et al. (2019) reported a decrease in stem length under SL in comparison to PL and 

ambient light for impatiens ‘Accent Premium Salmon’ (I. walleriana) and petunia ‘Ramblin 

Peach Glo’ (P. ×hybrida) plants. A decrease in stem height resulting from an increase in DLI can 

be of great benefit to growers who struggle with canopy management, by helping to reduce stem 

breakage, which would otherwise be common for herbs grown under FR rich environments.  

In conclusion, under a low DLI we have classified O. basilicum ‘Genovese’, ‘Sweet Thai’, 

‘Cinnamon’, ‘Sweet Dani Lemon’, and ‘Nufar’, O. ×citriodorum ‘Lime Basil’, O. tenuiflorum 

‘Holy Basil’, O. basilicum var. citriodora ‘Mrs. Burns’ Lemon’, and O. minimum ‘Pluto’  as 

DNPs since photoperiod either did not significantly affect time to first OF or no discernable 

trend was evident under DE or NI lighting. However, under a low DLI ‘Red Rubin’ can be 

classified as a facultative SDP because time to first OF occurred most rapidly under SDs in 

comparison to a 16-h LD (Fig. 3H). These results were diminished in Expt. 2, under moderate 

and high DLIs as ‘Red Rubin’ exhibited DN responses (Fig. 7E). Under moderate and high DLIs 

‘Genovese’, ‘Nufar’, and ‘Sweet Thai’ exhibited facultative LD responses (Fig. 7A, D, and F). 

Furthermore, in Expt. 2 under a 9-h SD all cultivars excluding ‘Lime Basil’ exhibited an FIR, 

where higher DLIs induced faster flowering while reducing the number of nodes below the first 

OF (Fig. 7A-F, 8A-F). Our results indicate that photoperiod alone is not a feasible option for 

69 

 

reproductive management of the basil cultivars evaluated. However, DLI generally had a greater 

impact on time to first OF, VB, node number, and height. Thus, the management of DLI could be 

a more effective strategy for managing plant development. Additionally, we have demonstrated 

that basil should not be grown under DLIs <13 mol∙m–2∙d–1 in order to avoid excessive stem 

elongation, especially if growing under a FR radiation rich environment. Under high DLIs, basil 

should be harvested earlier to prevent undesirable flowering.  

 

Acknowledgements 

We gratefully acknowledge support by The USDA-ARS Floriculture and Nursery Research 

Initiative, Ball Horticultural Co. for seed, the Blackmore Company for fertilizer and Nathan 

DuRussel for technical assistance. The use of trade names in this publication does not imply 

endorsement by Michigan State University of products named nor criticism of similar ones not 

mentioned. This work was supported by the USDA National Institute of Food and Agriculture, 

Hatch project MICL02472. 

70 

 

APPENDIX

71 

 

Table III-1. Expt. 1. photoperiod (h), mean (±SD) bench average daily temperature (ADT), and 
daily light integral (DLI) (±SD) during two experimental replications (Rep.) for basil. Plants were 
grown under a truncated 9-h short day (SD) and day length was extended with red+white+far-red 
(R+W+FR) light-emitting diode (LED) lamps to achieve 11-, 12-, 13-, 14-, 15-, 16-h or a 4-h 
night interruption (NI).   

Photoperiod (h) 

[mean ± SD (ºC)]  

[mean ± SD (ºC)]  

(mol·m‒2·d‒1) 

(mol·m‒2·d‒1) 

Rep 1 ADT           

Rep 2 ADT           

Rep 1 DLI 

Rep 2 DLI 

9 
11 
12 
13 
14 
15 
16 
NI 

z No data collected. 
 

23.6 ± 1.3 
24.0 ± 1.2 
24.5 ± 1.2 
24.1 ± 1.6 
23.9 ± 1.5 
24.2 ± 1.4 
24.0 ± 1.3 
23.1 ± 1.8 

24.4 ± 2.1 
24.7 ± 2.0 
24.2 ± 1.9 
24.9 ± 2.4 
24.3 ± 2.0 
24.3 ± 2.3 
24.6 ± 2.2 
24.3 ± 1.8 

- z 
- 
-  

7.2 ± 4.4 

-  

6.6 ± 4.5 

-  

7.5 ± 4.1 

-  

7.1 ± 3.6 
6.8 ± 3.1 

-  

6.2 ± 3.0 

- 

6.7 ± 3.5 

-  

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

72 

 

Table III-2. Expt. 2. daily light integral (DLI) treatment, photoperiod (h), average DLI (±SD), and 
greenhouse mean (±SD) air average daily temperature (ADT) delivered to six basil cultivars. 
Plants were grown under a truncated 9-h short day (SD) or a 16-h long-day (LD), with moderate 
or high photosynthetic DLIs. Day extension photoperiodic lighting (PL) was delivered by light-
emitting diode (LED) lamps. LED lamps emitted red+white+far-red (R+W+FR) radiation and 
were used to extend the photoperiod by 7 h [16 h (1700 to 2400 HR)].  

DLI 

treatment 
Moderate 

 

High 

 

Photoperiod (h) 

(mol·m‒2·d‒1)  

[mean ± SD (ºC)] 

Avg. DLI           

Greenhouse air ADT  

9 
16 
9 
16 

12.8 ± 8.0  
13.7 ± 8.0  
22.2 ± 7.9 
23.2 ± 8.0 

25.7 ± 1.2 
25.7 ± 1.2 
25.2 ± 2.4 
25.2 ± 2.4 

73 

 

 

R+W+FR 

0.15 
0.27 
0.80 
1.03 
2.25 

 

 

0.19 
0.15 
0.78 
0.88 
0.66 

 
Light ratio 
   B:R 
   B:FR 
   R:FRwide 
   R:FRnarrow 
   PFR:PR+FR 

Parameter 
Light intensity (% of the TPFD) 
   B (400 to 500 nm) 
   G (500 to 600 nm) 
   R (600 to 700 nm) 
   FR (700 to 800 nm) 
TPFD (400 to 800 nm) 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure III-1. Expt. 1 and 2. spectral distribution, intensity of blue [B (400–500 nm)], green [G 
(500–600 nm)], red [R (600–700 nm)], and far-red [FR (700–800 nm)] radiation, total photon 
flux density (TPFD), light ratio, and estimated phytochrome photoequilibria [PFR/PR+FR (the 
proportion of FR-absorbing phytochromes in the pool of R- and FR-absorbing phytochromes; 
Sager et al., 1988)] of R+W+FR light-emitting diode (LED) lamps covered with multiple layers 
of wire mesh. R:FRwide was calculated as 600 to 700 nm:700 to 800 nm; R:FRnarrow was 
calculated as 655 to 665 nm:725 to 735 nm. 
 
 

74 

 

Figure III-2. Expt. 1. time to first visible bud (VB) for 10 basil cultivars grown under a truncated 
9-h short-day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-
emitting diode (LED) lamps to achieve 11-, 12-, 13-, 14-, 15- and 16-h photoperiods or a 4-h 
night interruption (NI). Data were pooled when there was no interaction between replication 
(Rep.) and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. Absences of letters 
indicate no significant difference P ≤ 0.05. Error bars indicate standard error. 

 

75 

 

 
Figure III-3. Expt. 1. time to first open flower (OF) for 10 basil cultivars grown under a truncated 
9-h short-day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-
emitting diode (LED) lamps to achieve 11-, 12-, 13-, 14-, 15- and 16-h photoperiods or a 4-h 
night interruption (NI). Data were pooled when there was no interaction between replication 
(Rep.) and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. Absences of letters 
indicate no significant difference P ≤ 0.05. Error bars indicate standard error. 

76 

 

Figure III-4. Expt. 1. node number below the first open flower (OF) for 10 basil cultivars grown 
under a truncated 9-h short-day (SD) or under a 9-h SD extended with red+white+far-red 
(R+W+FR) light-emitting diode (LED) lamps to achieve 11-, 12-, 13-, 14-, 15- and 16-h 
photoperiods or a 4-h night interruption (NI). Data were pooled when there was no interaction 
between replication (Rep.) and treatment, or if the response trends were similar between Reps. 
Letters indicate mean separations across treatments using Tukey-Kramer difference test at P ≤ 
0.05. Absences of letters indicate no significant difference P ≤ 0.05. Error bars indicate standard 
error. 

 

77 

 

Figure III-5. Expt. 1. height at first open flower for 10 basil cultivars grown under a truncated 9-
h short-day (SD) or under a 9-h SD extended with red+white+far-red (R+W+FR) light-emitting 
diode (LED) lamps to achieve 11-, 12-, 13-, 14-, 15- and 16-h photoperiods or a 4-h night 
interruption (NI). Data were pooled when there was no interaction between replication (Rep.) 
and treatment, or if the response trends were similar between Reps. Letters indicate mean 
separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. Absences of letters 
indicate no significant difference P ≤ 0.05. Error bars indicate standard error. 
 

 

 

78 

 

Figure III-6. Expt. 2. time to first visible bud (VB) for six basil cultivars grown under moderate 
or high photosynthetic daily light integrals (DLIs) and a truncated 9-h short-day (SD) or day-
extension lighting from low-intensity light-emitting diode (LED) lamps emitting red+white+far-
red (R+W+FR) radiation for 7 h (1700 to 2400 HR) to achieve a 16-h photoperiod. Letters 
indicate mean separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. 
Error bars indicate standard error. 

 

79 

 

Figure III-7. Expt. 2. time to first open flower (OF) for six basil cultivars grown under moderate 
or high photosynthetic daily light integrals (DLIs) and a truncated 9-h short-day (SD) or day-
extension lighting from low-intensity light-emitting diode (LED) lamps emitting red+white+far-
red (R+W+FR) radiation for 7 h (1700 to 2400 HR) to achieve a 16-h photoperiod. Letters 
indicate mean separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. 
Error bars indicate standard error. 

 

80 

 

Figure III-8. Expt. 2. node number below the first open flower (OF) for six basil cultivars grown 
under moderate or high photosynthetic daily light integrals (DLIs) and a truncated 9-h short-day 
(SD) or day-extension lighting from low-intensity light-emitting diode (LED) lamps emitting 
red+white+far-red (R+W+FR) radiation for 7 h (1700 to 2400 HR) to achieve a 16-h photoperiod. 
Letters indicate mean separations across treatments using Tukey-Kramer difference test at P ≤ 
0.05. Error bars indicate standard error. 

 

 

81 

 

Figure III-9. Expt. 2. height at first open flower for six basil cultivars grown under moderate or 
high photosynthetic daily light integrals (DLIs) and a truncated 9-h short-day (SD) or day-
extension lighting from low-intensity light-emitting diode (LED) lamps emitting red+white+far-
red (R+W+FR) radiation for 7 h (1700 to 2400 HR) to achieve a 16-h photoperiod. Letters 
indicate mean separations across treatments using Tukey-Kramer difference test at P ≤ 0.05. 
Error bars indicate standard error. 
 

 

 
 

82 

 

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83 

 

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SECTION IV  

SUPPLEMENTAL LIGHTING RADIATION QUALITY INFLUENCES CUCUMBER, 

TOMATO, AND PEPPER TRANSPLANT GROWTH AND DEVELOPMENT 

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Supplemental lighting radiation quality influences cucumber, tomato, and pepper transplant 
growth and development 

 

Charlie Garcia and Roberto Lopez* 

Department of Horticulture, Michigan State University, East Lansing, 1066 Bogue Street, MI 

48824-1325, USA 

 

*Corresponding author. Tel.: +1 517 353 0342. E-mail address: rglopez@msu.edu (R.G. Lopez) 

89 

 

Abstract 

Supplemental lighting (SL) is required for the production of high-quality vegetable 

transplants in greenhouses when photosynthetic daily light integral (DLI) is low. Light-emitting 

diodes (LEDs) are a promising alternative to high-pressure sodium (HPS) lamps. However, there 

are a limited number of studies that have evaluated how LED SL spectral quality beyond blue 

(B) and red (R) radiation influence plant growth and development. Seeds of hybrid greenhouse 

seedless cucumber ‘Elsie’ (Cucumis sativus), tomato ‘Climstar’ (Lycopersicon esculentum Mill.), 

and pepper ‘Kathia’ (Capsicum annuum) were sown and placed into a dark growth chamber until 

radicle emergence. Seedlings were grown in a greenhouse at a 25 °C constant temperature set 

point and under five lighting treatments. The SL treatments delivered a total photosynthetic 

photon flux density (TPFD) of 120 μmol·m−2·s−1 for 16 h·d–1 based on an instantaneous 

threshold from HPS lamps or LEDs [three treatments composed of B (400–500 nm), R (600–700 

nm), white and/or far-red (700–800 nm) LEDs] and a control that delivered 25 μmol·m−2·s−1 

from HPS lamps with matching photoperiod. The LED treatments defined by their wavebands 

(photon flux density in μmol·m–2·s–1) of B, green (G, 500–600 nm), R and far-red (FR) radiation 

were B20G10R75FR15, B25R95, and B30G30R60, whereas the HPS treatments emitted B7G57R47FR9 

(HPS120) and B1G13R9FR2 (HPS25).  Generally, cucumber, tomato, and pepper transplants under 

B30G30R60 and HPS120 SL had the greatest stem diameter. Fresh weight and leaf area of all three 

species was significantly greater when G radiation replaced R or B radiation. For example, leaf 

area and fresh weight of cucumber, tomato, and pepper increased by 33, 49, and 22% and 35, 56, 

and 14%, respectively, for plants under B30G30R60 SL compared to plants under B25R95 SL. 

Generally, both cucumber and pepper transplants were most compact under B25R95 SL and 

tomatoes under the HPS25 (low intensity control) and B25R95 SL. The inclusion of FR radiation 

90 

 

reduced the incidence of leaf necrosis. From this study, one can conclude that plant responses to 

SL quality are generally species-specific, and therefore high-wire transplants should be grown 

separately to optimize production, however more studies are required. 

 

Keywords: Capsicum annuum, Cucumis sativus, greenhouse, hydroponic, Lycopersicon 

esculentum, seedlings. 

 

Abbreviations: ADT, average daily temperature; B, blue; CE, controlled-environment; DLI, daily 

light integral; FR, far red; G, green; HPS, high-pressure sodium; INC, incandescent; IR, infra-

red; LED, light-emitting diode; PL, photoperiodic lighting; PPFD, photosynthetic photon flux 

density; PSII, photosystem, R, red; SL, supplemental lighting; SSL, sole-source lighting TPFD, 

total photon flux density; UV, ultraviolet; W, white. 

 

Introduction 

Controlled-environment (CE) greenhouse and protected production of fruiting vegetable 

crops offers many advantages over open field production, including: increased fruit quality and 

yield; predictable crop timing; year-round production; reduced water usage, pest and weed 

control pressure, and climatic extremes such as drought, flooding, and low- and high-temperature 

stress (McCartney and Lefsrud, 2018). Therefore, CE production of food crops, within 

greenhouses and other protected structures, is gaining interest in the United States (U.S.) (Indoor 

Crop Production Feeding the Future, 2015). From 2009 to 2014, CE and protected food crop 

production area increased by 31% from 6.6 million to over 8.6 million m2 (USDA, 2010, 2015). 

Additionally, during the same period, the total value of sales of food crops grown under 

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protection increased by 44%, from $553 to $797 million (USDA, 2010, 2015). Of the more than 

265,000 metric tons of produce grown under protection in 2014, high-wire fruiting vine crops 

such as tomato (Solanum lycopersicum), cucumber (Cucumis sativus), and pepper (Capsicum 

spp.) accounted for 37, 14, and 1%, respectively (USDA, 2015). From 2010 to 2014, cucumber 

and pepper production increased by 174% and 323%, respectively, (USDA, 2010, 2015); and 

although tomato production decreased by 40%, greenhouse-grown tomatoes accounted for as 

much as 70% of tomato sales (Greenhouse Management, 2013). The combined wholesale and 

retail value of these three commodities totaled over $484 million, or 61% of all CE food crop 

sales in 2014 (USDA, 2015). 

 

Successful CE and field vegetable production is dependent on high-quality young plants, 

commercially referred to as transplants (Mitchell et al., 2015). For example, earlier and multiple 

harvests per growing season, better stand development, and increased profitability are a result of 

using high-quality transplants (Schrader, 2000). Consequently, the demand for vegetable 

transplants is increasing (Kubota et al., 2004; Kwack et al., 2016). In 2014, the number of 

operations commercially producing vegetable and strawberry transplants increased to 693; 543 

of which utilized greenhouses or other protected structures (USDA, 2015). In 1988, sales from 

transplants were valued at $50.7 million; by 2014, sales were nearly $372 million. Total sales of 

pepper and tomato transplants (grown under protection and from open field) accounted for 22% 

of all transplant sales in 2014 (USDA, 2015).  

  Nonetheless, the need for a reliable domestic source of high-quality grafted transplants for 

CE production has out-paced current availability (Kubota et al., 2004). As a result, greenhouse 

vegetable growers are purchasing grafted transplants from specialized producers, mainly sourced 

from Canadian propagators (Kubota and Kroggel, 2004). Imported transplants can potentially to 

92 

 

be denied entry into the U.S. based on phytosanitary grounds. Furthermore, prolonged shipping 

by truck increases the risk of transplant deterioration, resulting in successive delayed growth, 

flower abortion and delayed fruit development from low light, ethylene exposure, chilling, and/ 

or freezing injury (Kubota et al., 2004; Kwack et al., 2016).  

Transplants used for CE production are grown in soilless media (i.e. rock wool or coco coir) 

(Boyhan and Granberry, 2017), irrigated using subirrigation systems (i.e. ebb and flow or flood 

floors), and grown under high-intensity supplemental lighting (SL) (Demers and Gosselin, 2002; 

Hernandez, and Kubota, 2012; Mitchell et al., 2015; McCall, 1992). Supplemental lighting is 

used to increase the daily light integral (DLI) in greenhouses when solar radiation intensities and 

day lengths are limited, especially during winter months (Hernandez, and Kubota, 2012; Mitchell 

et al., 2015; McCall, 1992). To produce high-quality vegetable transplants, a DLI of 13 mol·m–

2·d–1 or greater is recommended (Dorais et al., 2017; Fan et al., 2013). However, in greenhouses 

located in northern latitudes, the DLI can average <5 mol·m–2·d–1 during winter months (Fausey 

et al., 2005; Lopez and Runkle, 2008). Under low light conditions, stem diameter is reduced, 

extension growth is promoted, flowers are aborted, and subsequently fruit abortion can occur 

leading to economic losses (Dorais et al., 2017). However, compact transplants with short 

internodes and thick stems (Mitchell et al., 2015), and shortened production times, can be 

produced under SL (Fisher et al., 2017). 

  High-pressure sodium (HPS) lamps have been the industry standard for greenhouse SL for 

many decades. However, the recent availability of high-intensity and energy-efficient, light-

emitting diode (LED) fixtures, have made them a promising alternative as their prices continue 

to decrease and energy efficacies increase. LEDs are solid-state, semiconducting devices 

(Bourget, 2008), with a narrow- or broad-band radiation spectrum (Currey and Lopez, 2013; 

93 

 

Mitchell et al., 2015). Their long lifetime, small size, and cool-emitting temperatures can make 

them well suited for CE horticultural production (Mitchell et al., 2015).   

  Blue [B (400-500 nm)] and red [R (600-700)] LEDs have dominated horticultural lighting 

because B and R radiation are considered the most photosynthetically efficient wavebands 

(McCree, 1972). However, plants perceive and use a broader range of radiation for growth and 

development, including green [G (500 to 600 nm)] and far-red [FR (700-800 nm)] radiation. 

Furthermore, when B and R radiation are used together, plants appear purple or gray to the 

human eye, making pest and nutritional deficiencies difficult to identify (Massa et al., 2008). 

One solution could be the addition of G or white (W) radiation. When added to R and B 

radiation, it increases the color-rendering index, thereby creating a more pleasant working 

environment without compromising plant growth (Terashima et al., 2009; Snowden et al., 2016).  

Limited SL studies have been published to evaluate vegetable seedling responses to different 

supplemental radiation qualities (Mitchell et al., 2015). One such study sought to evaluate high-

wire cucumber ‘Cumlaude’ transplant growth under a low solar DLI (6.3 mol∙m–2∙d–1) with HPS 

or monochromatic B or R LED SL that increased the DLI by 3.7 mol∙m–2∙d–1 (Hernandez and 

Kubota, 2015). No significant differences in shoot fresh and dry mass or number of leaves were 

observed between plants grown under SL providing B or R radiation. Conversely, plants grown 

under HPS lamps had 28 and 32% greater shoot fresh mass than plants grown under LEDs that 

provided B or R radiation, respectively. Presumably the increased fresh mass was due to higher 

leaf temperatures, resulting from infra-red (IR) radiation emitted from HPS SL. More 

importantly, when SL contributes <40% of the DLI (Hurt et al., 2019), morphological responses 

may be less pronounced than under sole-source lighting (SSL) (Hernandez and Kubota, 2015) 

and maybe species dependent (Hernandez and Kubota, 2014b).   

94 

 

Green radiation was long considered the least efficient waveband within the PAR spectrum 

for photosynthesis (McCree, 1972). However, G radiation’s low absorption rate allows for better 

penetration into the plant canopy and can potentially further increase photosynthesis and plant 

growth (Klein, 1992). Green radiation can reduce hypocotyl elongation and increase leaf area, 

fresh and dry mass of cucumber hybrid ‘Mandy’ transplants (Novičkovas et al., 2012). However, 

plant responses to SL containing G radiation have been contradictory. For example, Kim et al. 

(2004) found that a high percentage of green radiation [>50% of total photosynthetic photon flux 

density (PPFD)] reduced growth of lettuce (Lactuca sativa), while a lower percentage (24%) of 

G radiation in combination with B and R LEDs increased leaf area and shoot fresh and dry 

weight. In another study with tomato and French marigold (Tagetes patula), fresh and dry 

weight, height of plants and lengths of peduncle were increased when G radiation was filtered 

from W radiation provided by cool-white fluorescent (CWF) lamps. Furthermore, when 

supplementary G radiation was added to W light the fresh weight of marigold was reduced, but 

height was unaffected (Klein et al., 1965).  

The objective of this study was to build upon previous SL studies by quantifying 

physiological and morphological responses to different B, G, R, and FR radiation intensities for 

the production of economically significant cultivars of cucumber, tomato, and pepper for 

transplant production. We postulate that supplemental lighting including green or far-red 

radiation will increase vegetable transplant quality by increasing leaf area and fresh mass, but 

will also increase stem length and decrease stem diameter.  

 

 

 

95 

 

Materials and methods 

Plant material 

Hybrid greenhouse cucumber ‘Elsie’, tomato ‘Climstar’, and pepper ‘Kathia’ (Capsicum 

annuum) seeds (Syngenta Seeds, Inc. Minneapolis, MN) were sown into 120-cell rockwool plug 

trays (2 × 2.7 cm; 8.5-mL individual cell volume) (GroPlug; Grodan, Roermond, The 

Netherlands) and covered with a layer of vermiculite on 24 Sept. 2018, 22 Oct. 2018, and 05 Jan. 

2019. Seeded plug trays were placed into a dark growth chamber that had an air average daily 

temperature (ADT) and relative humidity set point of 28 °C and 60%, respectively until radicle 

emergence. The trays were overhead irrigated as needed with a nutrient solution consisting of 

reverse osmosis water supplemented with a combination of magnesium sulfate [25 mg∙L–1 sulfur 

(S)] and 12N-4P-16K water-soluble fertilizer, supplying (mg∙L‒1): 100 nitrogen (N), 33 

phosphorus (P), 133 potassium (K), 58 calcium (Ca), 36 magnesium (Mg), 27 S, 0.1 boron (B), 

0.4 copper (Cu), 1 iron (Fe), 0.4 manganese (Mn), 0.1 molybdenum (Mo), and 0.4 zinc (Zn) (RO 

Hyro FeED; JR Peters Inc., Allentown, PA). The pH and EC were adjusted to 6.0 and 0.88 dS∙m–

1, respectively.   

Upon radicle emergence, trays of each species were randomly assigned to one of four SL 

treatments or a low intensity control, each within one of five separate glass-glazed greenhouse 

compartments in the Plant Science Research Greenhouse ranges at Michigan State University 

(East Lansing, MI; lat. 42 °N). Plants were rotated daily to mitigate any positional effects within 

the greenhouses. After ten days under SL for cucumber and fourteen days for tomato and pepper, 

10 seedlings per species were transplanted into rockwool cubes (10 × 10 × 6.5 cm; 650-mL 

individual volume) (Delta Blocks; Grodan). After transplant, plants were irrigated daily using an 

ebb and flow system with reverse osmosis water supplemented with the same nutrient solution 

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previously mentioned. Each greenhouse compartment had a 208-L tote reservoir (HDX, The 

Home Depot, Atlanta, GA), and a submersible water pump (Kedsum-3500 65 Watt pump; 

Xiolan, China) that delivered 49-L∙m‒1 to their respective flood bench. The pH and EC of the 

nutrient solution within reservoirs were adjusted daily using a hand-held meter (HI 991301 

pH/EC/TDS Meter: Hanna Instruments). The pH was adjusted to 6.0 using sulfuric acid (pH 

down) and potassium bicarbonate (pH up). The EC was adjusted using reverse osmosis water to 

0.88 dS∙m‒1. The mean pH and EC (±SD) during the experiment were 6.1 ± 0.1 and 0.9 ± 0.01 

dS∙m‒1, respectively. Actual values pH and EC values, per greenhouse compartment are reported 

in Table 1. 

Greenhouse environmental conditions 

Whitewash (ReduSol; Baarle-Nassau, Netherlands) was applied to the glass exterior of the 

five east-to-west orientated greenhouse sections, to decrease radiation intensity and improve 

uniformity. To avoid radiation contamination from adjacent SL treatments, whitewash was also 

applied to the glass between greenhouse compartments. Radiation intensity in each section was 

measured by a quantum sensor, (LI-190/R; LI-COR, Lincoln, NE) placed at plant canopy height. 

A shielded and aspirated 0.13-mm type E thermocouple (Omega Engineering, Stamford, CT, 

USA) measured air temperature at canopy height, and an IR sensor (Type T, OS36-01; Omega 

Engineering) measured leaf temperature. A CR-1000 datalogger (Campbell Scientific, Logan, 

UT) collected environmental data in each compartment every 15 s, and hourly means were 

recorded. Exhaust fans, roof vents, evaporative pad cooling, and radiant hot-water heating were 

controlled by an environmental control system (Integro version 725-3030; Priva North America, 

Vineland Station, ON, Canada) to maintain an air ADT set point of 25 °C. The actual air ADT 

and leaf temperatures are reported in Table 1. 

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Supplemental lighting treatments 

Four SL treatments and a low intensity control, providing a photoperiodic lighting (PL) 

treatment were delivered for 16 h∙d–1 based on an instantaneous threshold [on from 0600 to 2200 

HR when the outside PPFD was below ≈440 µmol·m–2·s–1 (on for a minimum of 25 minutes and 

off for a minimum of 20 minutes)]. Four 400-W HPS lamps (LR48877; P.L. Light Systems, 

Beamsville, Ontario, Canada) or three 600-W LEDs fixtures (LX601G TL1002 R2A or LX601C 

HLB607-l2-B-L1-RC; Heliospectra, Göteborg, Sweden) (42.5 cm L × 21.9 cm W × 19.9 cm H; 

286 diodes) per treatment provided a total photon flux density (TPFD) of 120 µmol·m–2·s–1 in an 

experimental area of 1.9 m2. The low intensity control was delivered by one 150 W-HPS light 

fixture (HPS25) (LU150; Acuity Lithonia Lighting, Conyers, GA) and provided a TPFD of 25 

µmol·m–2·s–1 in an experimental area of 1.9 m2. The 100-nm waveband ratios of the three LED 

SL treatments, delivered by cool W and R LEDs and defined by their wavebands (photon flux 

density in μmol·m–2·s–1) of B, G, R, and FR radiation, were B20G10R75FR15, B25R95, and 

B30G30R60. Additionally, the B30G30R60 SL treatment also provided 2 μmol·m–2·s–1 of FR 

radiation. The HPS lamps emitted intensities of B1G13R9FR2 (HPS25) and B7G57R47FR9 (HPS120). 

For cucumber, tomato, and pepper, the total number of lamp hours of operation was 325, 407, 

and 370 (Rep 1), 426, 521, and 478 (Rep 2), and 431, 522, and 468 (Rep 3). For all sections, 

spectral quality and TPFD at plant height were measured in twelve separate locations throughout 

the growing area with a portable spectroradiometer (PS-200; Stellar-Net, Tampa, FL) (Figure 1). 

Spectral scans and radiation intensity measurements of the SL were taken at night, at the 

beginning or end of each replication to ensure consistency from one replication to another. The 

total DLIs are reported in Table 1. The phytochrome photoequilibrium was estimated for all five 

lighting treatments following Sager et al. (1988). 

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Plant measurements and experimental design 

Excluding germination, cucumber plants were harvested after 28 days under SL, while 

tomato and pepper plants were harvested after 35 days. Plant height (measured from the medium 

surface to apical meristem) and hypocotyl length were measured using a ruler. Stem diameter, ≈1 

cm below the cotyledons, was measured using a digital caliper (41101 DigiMax; Wiha 

Switzerland). The number of nodes and fully expanded leaves (greater than 1 cm in length) per 

plant were also recorded at harvest. Internode length was calculated by dividing the number of 

nodes by plant height. Total leaf area per plant was measured using a leaf area meter (LI-300; LI-

COR, Lincoln, NE). The reproductive status of each plant was also recorded. Plants were 

deemed either reproductive or non-reproductive, depending on the presence or absence of visible 

flower buds. The number of tomato leaves per plant with necrotic lesions was recorded for the 

second and third replications, and the number of leaves with necrotic lesions were divided by the 

total number of leaves per plant to calculate incidence of leaf necrosis (%). Plants were excised 

just above the medium, and total shoot (stems and leaves) fresh weight was measured using a 

digital scale. Stems and leaves were put into paper envelopes and placed inside a drying oven set 

at ≥70 °C for ≥ 6 d. Dry weights for each plant were recorded after drying. Prior to the 

destructive plant measurements, chlorophyll fluorescence of five plants, per species, per 

treatment were measured using a portable chlorophyll fluorescence meter (Handy Plant 

Efficiency Analyzer (PEA); Hansatech Instruments Ltd., King’s Lynn, Norfolk, U.K.). Fully 

expanded leaves were dark-acclimated for a minimum of 15 min, using the manufacturer’s 

plastic and foam clips before measurements were recorded. Fluorescence was measured by 

opening the shutter of the dark-acclimating clip and exposing the leaf to R radiation (peak 

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wavelength of 650 nm at 1,200 µmol∙m−2∙s−1) for 5 s to saturate photosystem II (PSII). 

Chlorophyll fluorescence was expressed as Fv/Fm.  

The experiment was a randomized complete block design (RCBD), with three replications 

over time. Plants were blocked by SL treatments with 10 experimental units (individual plants) 

of each species per treatment and replication. The data for each plant species were analyzed 

separately using the PROC GLIMMIX procedure in SAS (version 9.4; SAS Institute, Cary, NC). 

In the analysis, the SL treatment was considered the fixed factor, whereas replication was 

regarded as a random factor in the analysis. Mean separations were analyzed using adjusted 

Tukey-Kramer HSD (P = 0.05). Data were pooled when there was no interaction between Rep. 

and treatment, or if the response trends were similar between Reps.  

 

Results 

Cucumber 

All data parameters, with the exception of dry weight, were analyzed and presented as pooled 

results. Data for dry weight was pooled for Rep. 1 and 2, while Rep. 3 was analyzed separately 

due to inconsistencies between Reps. Transplants grown under HPS120 and B30G30R60 SL 

exhibited the greatest stem diameter (Fig. 2A). The stem diameter of plants under HPS25 

(control) SL was significantly lower than all the other treatments. Height, hypocotyl, and 

internode length were greatest under the control, averaging 35.3, 8.1, and 5.0 cm in length, 

respectively (Fig. 2D, G, and J). The greatest number of nodes and leaves were under HPS25 and 

B30G30R60 SL, and the least were recorded under B25R95 SL (Fig. 2M and 3A). Leaf area was 

similar among transplants under HPS25, HPS120, and B30G30R60 SL (Fig. 3D). SL providing 

B20G10R75FR15 and B25R95 significantly reduced leaf area. For example, leaf area of transplants 

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grown under B25R95 was 225 (25%), 227 (25%), and 231 (25%) cm2 less than transplants grown 

under the control, B30G30R60, and HPS120, respectively (Fig. 3D).  

Transplants grown under HPS120 and B30G30R60 SL exhibited the greatest fresh weight (Fig. 

3G). In the pooled results for Reps. 1 and 2, the greatest seedling shoot dry weight was under 

HPS120 and B30G30R60 and the lowest was under the low intensity control. In Rep. 3, the greatest 

shoot dry weight was under B20G10R75FR15 and B30G30R60 SL (Fig. 3J). The efficiency of PSII, 

denoted by Fv/Fm, was greatest for transplants grown under the control (0.84), and not 

significantly different between HPS120 (0.82), B20G10R75FR15 (0.82), and B30G30R60 (0.83) (Fig. 

3M). After 28 d of SL, the percentage of cucumber plants having visible flower buds averaged 

53, 63, 80, and 87% for B25R95, B20G10R75FR15, B30G30R60, and HPS120, respectively, and 73% 

for the control (Fig. 4A). 

Tomato 

All data parameters, with the exception of stem diameter, were analyzed and presented as 

pooled results. From the pooled results (Reps. 1 and 2), tomato seedlings under B30G30R60 and 

HPS120 SL had the greatest stem diameter (6.3 and 6.3 mm) (Fig. 2B). However, for Rep. 3, 

plants grown under the control had the greatest stem diameter (6.2 mm) (Fig. 2B). No significant 

difference in height was observed among transplants grown under HPS120, B20G10R75FR15, and 

B30G30R60 SL (Fig. 2E). Transplants grown under the control and B25R95 were significantly 

shorter than all the other treatments. For example, transplants grown under the B25R95 were 9.7 

(22%), 10.7 (24%), and 11.6 (25%) cm shorter than those grown under HPS120, B20G10R75FR15, 

and B30G30R60 SL, respectively (Fig. 2E). Hypocotyl length was greatest under the control (5.1 

cm) and shortest under B30G30R60 (4.2 cm) (Fig. 2H). Internode length was greater under HPS120, 

B20G10R75FR15, and B30G30R60, and shorter under the control and B25R95 (Fig. 2K). Additionally, 

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internode length of B25R95 grown transplants was significantly less in comparison to all other 

treatments. Transplants grown under HPS120, B20G10R75FR15, and B30G30R60 SL did not differ 

significantly in the number of nodes, and averaged between 9 to 10 nodes (Fig. 2N).  

Transplants under B30G30R60 SL had the greatest leaf area (986cm2), and fresh (52.4 g) and 

dry weight (5.0 g) (Fig. 3E, H, and K). There was no significant difference in leaf area, fresh and 

dry weight between transplants grown under HPS120 and B20G10R75FR15 SL (Fig. 3E, H, and K). 

Under the control, transplants had the fewest number of leaves, and lowest fresh and dry weight 

(Fig. 3B, H and K). For example, fresh weight of plants grown under the control was 5.8 

(17.1%), 17.2 (38.2%), 17.4 (38.4%), and 24.5 g (46.7%) less than those under B25R95, 

B20G10R75FR15, HPS120 and B30G30R60 SL, respectively (Fig. 3H). Fv/Fm was slightly higher for 

plants grown under HPS25, but was not significantly different from B20G10R75FR15, and 

B30G30R60 grown plants (Fig. 3N). After 35 days, all plants under SL had 100% visible flower 

buds, while plants under the low intensity control only had 6.7% (Fig. 4B). For tomato plants 

grown under HPS120 and B30G30R60 SL, the incidence of leaf necrosis was 20.7% and 16.9%, 

respectively (Fig. 5). 

Pepper 

Data for stem diameter, internode length, leaves, leaf area, fresh weight, Fv/Fm and visible 

flower bud formation were pooled (Fig. 2C, L, 3C, F, I, O, and 4C). Data from Rep. 2 and 3 were 

pooled together for height, hypocotyl, and dry weight (Fig. 2F, I, and 3L). For node number, 

Rep. 1 and 2 were pooled and Rep. 3 was analyzed separately (Fig. 2O). Stem diameter of 

pepper transplants grown under the control was 12 to 20% smaller than transplants grown under 

B20G10R75FR15, B25R95, B30G30R60, and HPS120 (Fig. 2C). Height of pepper transplants from Rep. 

1 and 2 was significantly reduced under B25R95 SL, but there was no significant difference in 

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height among treatments in Rep. 3 (Fig. 2F). Hypocotyl length, from the pooled results (Reps. 2 

and 3), was greatest for HPS25 grown plants (Fig. 2I). From the un-pooled results hypocotyl 

length was still the greatest under the control (4.1 cm) (Fig. 2I). Internode length was greatest 

under the control, and was significantly greater compared to HPS120, B25R95, and B30G30R60, but 

not B20G10R75FR15 grown transplants (Fig. 2L). There was no significant difference in the 

number of nodes for the pooled data (Reps. 1 and 2) among all SL treatments (Fig. 2O). 

However, in Rep. 3 HPS120 had the greatest number of nodes (9), while B25R95 had the least (6) 

(Fig. 2O).  

Pepper transplants grown under the control had the least number of leaves, and there was no 

significant difference in leaf number among the transplants under HPS120, B25R95 and B30G30R60 

SL (Fig. 3C). Leaf area was greatest among transplants grown under HPS120 and B30G30R60, 

while those under B20G10R75FR15 and B25R95 SL had the lowest leaf area (Fig. 3F). In Reps. 2 and 

3, HPS120 grown transplants had greater dry weight than all other SL treatments and the control 

(Fig. 3L). From the un-pooled results, the greatest dry weight was observed in plants grown 

under B25R95, but was not significantly different from plants grown under HPS120 and B30G30R60 

SL (Fig. 3L). Fv/Fm of transplants grown under the control averaged 0.83 and was significantly 

greater than all other treatments (Fig. 3O). The visible flower bud percentages for pepper were 

25, 85, 95, 100, and 100% for transplants grown under HPS25, B20G10R75FR15, B30G30R60, B25R95 

and HPS120 SL (Fig. 4C). 

 

Discussion 

High-quality ornamental transplants are defined as having large stem diameters, are compact 

in size, fully rooted, and have high root and shoot dry mass (Oh et al., 2010; Randall and Lopez, 

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2014). Similar morphological characteristics define a high-quality greenhouse grown vegetable 

transplant, including well-developed leaves, straight stems, and deep-green leaves (Gomez and 

Mitchell, 2015). However, certain features can differ depending on the intended use of the 

transplant (Chia and Kubota, 2010). For example, seedlings can be used as rootstocks, scions, or 

as non-grafted transplants (Lee, 1994). Grafted seedlings benefit from extended hypocotyl 

length, since it helps to increase grafting success and hence survival rate, and reduce rooting 

from the scion after transplant (Chai and Kubota, 2010). However, elongated hypocotyls are not 

desired for non-grafted seedlings, because it may lead to weak transplants (Gomez and Mitchell, 

2015; Jones, 2008) and logistical challenges for shipping.  

Numerous studies have documented the positive effects of SL during both ornamental and 

vegetable young plant production in greenhouse environments (Currey and Lopez, 2013; Gomez 

and Mitchell, 2015; Poel and Runkle, 2017; Hernandez and Kubota, 2012). Gomez and Mitchell 

(2015) evaluated multiple tomato cultivars under varying DLIs throughout the year. Ambient 

DLIs were supplemented with HPS lamps or LEDs providing (%) 20:80, 5:95, or 0:100 B:R 

radiation at a PPFD of 61 ± 2 μmol·m−2·s−1 for 23 h·d−1 to achieve a SL DLI of 5.1 mol·m−2·d−1. 

For tomato cultivars grown under SL, hypocotyl diameter, epicotyl length, leaf number, leaf 

area, and shoot dry weight increased, while hypocotyl elongation was decreased (Gomez and 

Mitchell, 2015). In another study conducted by Poel and Runkle (2017), petunia (Petunia 

×hybrida) ‘Wave Misty Lilac’, snapdragon (Antirrhinum majus) ‘Montego Yellow’, and tomato 

‘Supersweet’ seedlings were grown under PL providing 10 µmol·m–2·s–1 or SL providing a 

PPFD of 90 µmol·m–2·s–1 for 16 h·d−1. A 16 to 40% increase in DLI from SL generally 

increased the number of leaves, dry root and shoot weight. For tomato seedlings, shoot and root 

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dry weight was 27 and 38% lower, respectively, when grown under PL in comparison to HPS SL 

(Poel and Runkle, 2017).  

Our study confirms that an increase in DLI from SL has a positive impact on many of the 

morphological traits measured. For instance, under higher DLIs (≈11.8 mol·m–2·d–1) stem 

diameter of cucumber and pepper increased, compared to the low intensity control (≈6.1 mol·m–

2·d–1) by 17 to 27% and 14 to 25%, respectively (Fig. 2A, C). Fan et al (2013) observed similar 

results when cherry tomato seedlings were grown under LED SSL providing a 1:1 of B:R 

radiation at a PPFD of 50 to 550 µmol·m–2·s–1. Stem diameter increased incrementally, by 14 to 

23%, as the DLI increased from 2.2 to 23.8 mol·m–2·d–1. Randall and Lopez (2015) compared 

growth of vinca ‘Titan Red Dark’ (Catharanthus roseus), impatiens ‘Super Elfin XP Blue Pearl’ 

(Impatiens walleriana), and geranium ‘Bullseye Red’ (Pelargonium ×hortorum) under ambient 

solar radiation (control) to SL from HPS lamps or LEDs providing a ratio (%) 13:87 B:R 

radiation at a PPFD of 70 µmol·m–2·s–1. Additionally, the same species were grown under SSL 

consisting of (%) 13:87 B:R or 30:70 B:R radiation at 185 µmol·m–2·s–1 to evaluate if there were 

differences in plant quality between plants grown under SSL and greenhouse SL. Under SL and 

SSL, stem diameter of vinca, impatiens, and geranium was 12% to 17%, 26% to 45%, and 8% to 

15% greater, respectively, compared with those seedlings under the control. 

From our study, as DLI was increased from ≈6.1 to ≈11.8 mol·m–2·d–1 shoot dry weight of 

tomato transplants increased by 107 to 183% compared to the low intensity control (Fig. 3K) The 

same trend was generally observed for the pooled results of cucumber and pepper, however 

minor variabilities existed (Fig. 3J and L). The increase in shoot dry weight can be attributed to 

increased biomass accumulation from SL that increased the DLI (Hernandez and Kubota, 2014; 

Pramuk and Runkle, 2005). For example, Pramuk and Runkle (2005) found that average shoot 

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dry weight increased as DLI increased from 4.1 to 14.2 mol·m–2·d–1 for celosia, impatiens, 

marigold, and viola by 64, 47, 64, and 68%, respectively.  

Both SSL and SL studies have shown how the use of specific radiation wavebands can be 

used to manipulate hypocotyl length, plant height, stem diameter and length, and leaf area, 

among many other morphological properties of ornamental and vegetable plants (Chia and 

Kubota, 2010; Currey, and Lopez, 2013; Fan et al., 2013; Klein et al., 1965; Liu et al., 2011; 

Lopez and Runkle, 2008; Massa et al., 2008; McCall, 1992). However, studies have not always 

reported morphological changes in response to SL radiation quality. For instance, Poel and 

Runkle (2017), reported few (if any) differences when SL contributed 20 to 40% of the total 

DLI. Conversely, Randall and Lopez (2014; 2015), found that morphological responses to SL for 

bedding plant seedlings were observed when 40 to 60% of the DLI was provided by SL. Given 

this, Hurt et al. (2019) hypothesized that greater than 40% of the total DLI needs to come from 

SL to elicit morphological responses. For instance, when LED SL provided 40 to 55% of the 

total DLI, compact growth of gerbera (Gerbera jamesonii) ‘Jaguar Deep Orange’, impatiens 

‘Accent Premium Salmon’, and petunia ‘Ramblin Peach Glo’ were observed. In the current 

study, SL provided ≈43% of the total DLI for cucumber, tomato, and pepper transplants. 

Hernandez and Kubota (2014a) also reported that under B and R LED SL spectral quality 

treatments only elicited morphological responses when the solar DLI was low. 

Leaf area and fresh weight of cucumber, tomato, and pepper increased by 33, 49, and 22% 

and 35, 56, and 14%, respectively, under the B30G30R60 SL treatment compared to the B25R95 

(Fig. 3D-I). A possible explanation could be the replacement of R or B radiation for G. The 

increased leaf area and fresh weight can be in part attributed to G radiation’s ability to be 

transmitted more deeply into the plant canopy compared to R and B radiation (Klein, 1992; 

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Smith, 1993). For example, chlorophyll weakly absorbs G radiation, meaning that up to 80% G 

radiation is transmitted through the chloroplast (Terashima et al., 2009), thereby allowing more 

G photons to pass deeper into the mesophyll (Smith et al., 2017; Sun et al., 1998) and thus 

helping to further increase photosynthetic efficiency, biomass accumulation, and yield (Smith et 

al., 2017). Kim et al. (2004) observed similar results, where growth of lettuce (Lactuca sativa 

‘Waldmann’s Green’) was compared under LED SSL providing 150 µmol·m–2·s–1 of B and R 

radiation or B and R radiation supplemented with G fluorescent lamps. An increase of 32, 45, 

and 47% in leaf area, shoot fresh weight, and shoot dry weight, respectively, were reported as a 

result of replacing R or B radiation with G radiation.   

Spectral quality manipulation can be an effective alternative to using plant growth regulators 

(PGRs) (Currey and Lopez, 2011; Lopez and Runkle, 2007) or day/night differentials (DIF) 

(Ecke et al., 2004) for the control of extension growth or plant height (Randall and Lopez, 2014; 

Wollaeger and Runkle, 2015). For instance, under monochromatic red LEDs hypocotyl 

elongation of cucumber transplants was promoted (Hernandez and Kubota, 2016). Taller 

transplants can make handling and transportation more difficult because they run a greater risk of 

stem breakage (Kubota et al., 2004; Kwack et al., 2016; Pramuk and Runkle 2005). Therefore, 

the addition of B to R radiation can prevent excessive stem elongation (Hernandez and Kubota, 

2016; Randall and Lopez, 2014; Wollaeger and Runkle, 2015).  

Previous studies have reported that with increasing B radiation, stem and hypocotyl length 

are reduced (Brown et al., 1995; Hernandez and Kubota, 2016; Liu et al., 2011). For instance, 

Brown et al. (1995) reported shorter pepper seedlings under SSL providing 10:90 B:R radiation 

at a PPFD of 300 µmol·m–2·s–1 in comparison to monochromatic R radiation. Similarly, cherry 

tomato plants grown under SL providing 0:100 B:R radiation at a PPFD of 300 µmol·m–2·s–1 

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were 95% taller than plants grown under 50:50 B:R radiation (Liu et al., 2011). When cucumber 

plants were grown under SSL providing an increasing B:R radiation ratio, ranging from 0:100 to 

100:0 B:R at a PPFD of 100 µmol·m–2·s–1, plant height, hypocotyl, and epicotyl length 

decreased as the proportion of B radiation increased up to 75:25 B:R radiation. However, under 

monochromatic B LEDs, height increased by 69% compared to monochromatic R LEDs, and 

increased by 346% compared to the 75:25 B:R SSL treatment (Hernandez and Kubota, 2016). 

Similarly, we observed reductions in height of vegetable transplants grown under B25R95, which 

were not as apparent in other LED treatments that contained G and FR radiation (Fig. 2D-F).  

SL and SSL radiation intensity can have varying results on extension growth and height 

depending on the plant species (Randall and Lopez, 2014; 2015; Poel and Runkle, 2017). We 

found that by increasing the DLI from ≈6.1 to 11.8 mol·m–2·d–1, height of cucumber and pepper 

transplants was generally more compact under SL compared to the low intensity control (Fig. 2D 

and F). For instance, under SL, cucumber transplants were 38 to 52% shorter than those under 

the low intensity control (Fig. 2D). On the contrary, increasing the DLI resulted in a 23 to 29% 

increase in plant height of tomato transplants under SL (excluding B25R95), compared to the low 

intensity control (Fig. 2E). Similarly, Pramuk and Runkle (2005) found species-dependent 

responses for height in ornamental seedlings when the DLI was increased from 4.1 to 14.2 

mol·m–2·d–1 under SSL. For instance, height of impatiens ‘Accent Red’ and Salvia splendens 

‘Vista Red’ decreased by 27% and 37%, respectively, while the height of Tagetes patula 

‘Bonanza Yellow’ and Celosia argentea var. plumosa ‘Gloria Mix’ increased by 10% and 12%, 

respectively. Contrary to our results for tomato, but consistent with our results for cucumber, Fan 

et al. (2013) reported a reduction in height as DLI increased. For example, tomato was 28 to 47% 

shorter when grown under DLIs ranging from 6.5 to 23.8 mol·m–2·d–1, in comparison to 2.2 

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mol·m–2·d–1. The underlying difference between the two studies is that Fan et al. (2013) grew 

plants under SSL while the current study was conducted under SL in a greenhouse with 

background solar radiation that provided different spectral radiation qualities. 

Height of plants is influenced by internode length. For instance, internode length of 

cucumber transplants was greatest under the control, averaging ≈5.0 cm, resulting in the tallest 

plants (Fig. 2D and J). When DLI was increased from ≈6.1 to 11.8 mol·m–2·d–1 internode length 

was reduced by 31 to 40%, thus resulting in a significant decrease in height (Fig. 2D and J). A 

similar reduction of internode length was observed for pepper transplants, however there was no 

significant difference between the low intensity control and B20G10R75FR15 (Fig. 2L). For tomato 

transplants, excluding B25R95, increasing the DLI resulted in a 9 to 13% increase in internode 

length, and thus taller plants were observed (Fig. 2E and K). 

Considering all three species are categorized as DNPs, and because the duration of SL and 

PL were equal (16 h∙d‒1), differences in visible bud percentage observed for tomato and pepper 

can be associated with an increase in DLI. For example, visible flower bud percentage for tomato 

under the low intensity control was 7%, compared to 100% for plants under SL, regardless of the 

spectral quality (Fig. 4). For cucumber and pepper there was a small reduction in visible flower 

bud percentage under B25R95 and B20G10R75FR15 SL, respectively. Further studies are required to 

evaluate the spectral quality influence on flower initiation and development of vegetable 

transplants.  

Considering tomato plants can develop physiological disorders such as chlorosis and necrosis 

under photoperiods >16 h or continuous lighting, it has been suggested that long photoperiods 

with low radiation intensities can be an alternative method to prevent symptoms on young 

transplants (Gomez and Mitchell, 2015). In the current study, the incidence of small and irregular 

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necrotic lesions were observed on tomato leaves under all SL treatments, with the highest 

percentage of symptoms observed under HPS120 and B30G30R60 SL. The incidence of leaf 

necrosis was 2% for tomato transplants grown under the low intensity control providing 25 

µmol·m–2·s–1. Therefore, the reduction in leaf necrosis can be associated to the lower DLI, in 

comparison to the SL treatments, which averaged ≈11.8 mol·m–2·d–1 across treatments (Fig. 5). 

Additionally, low fertility can be ruled out as the reason for necrotic symptoms since, low N 

during transplant production is considered between 50 to 75 mg∙L‒1, and we provided 100 mg∙L‒

1. The optimal range for tomato transplant production is 100 to 150 mg∙L‒1 (Whipker et al., 

2018).  

No biotic factors were identified by a diagnostic lab that might have caused these symptoms; 

therefore, we suspect that these symptoms were caused by a physiological disorder such as 

intumescence. This non-pathogenic disorder has been observed previously in susceptible 

cultivars of tomato grown under ultraviolet (UV)- and/or B-deficit light environments, such as 

under SSL LEDs (Lang et al. 1983; Eguchi et al. 2016). Under UV-deficit environments it has 

also been suggested that phytochrome is involved in the regulation of intumescence development 

in tomato (Morrow, 1987; Morrow and Tibbitts, 1988). In a study by Morrow (1987), 

intumescence developed under R radiation, but was inhibited under combinations of R and FR. It 

was also reported that tomato ‘Early Girl’ plants did not develop intumescence injury when 

grown under SSL LEDs providing only B radiation (Wollaeger and Runkle, 2014).  

HPS lamps have been the industry standard for providing SL to vegetable transplants 

(Gomez and Mitchell, 2015; Mitchell et al., 2015). However, HPS lamps primarily emit radiation 

within the 565 to 700 nm range (Randall and Lopez, 2014). LEDs have emerged as a viable 

alternative SL source, due to their electrical efficacy (Nelson and Bugbee, 2014; Wallace and 

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Both, 2016) and ability to provide specific light spectra for the optimization of transplant 

production (Gomez and Mitchell, 2015). From our study, LED SL provided by the B30G30R60 

provided similar results for many of the parameters evaluated for all three species (Fig. 2A-O 

and 3A-O). For instance, stem diameter, internode length, and number of nodes for all three 

species were similar when grown under the HPS120 and B30G30R60 (Fig. 2A-C, J-O). 

Additionally, for cucumber and pepper, no significant differences were observed for leaf area 

and fresh weight when grown under HPS120 or B30G30R60 (Fig. 3D, F, G, and I). Furthermore, 

leaf area, fresh and dry weight of tomato was greater when grown under B30G30R60 compared to 

HPS120 (Fig. 3E, H, and K). Based on results from our study, transplant quality was equivalent or 

greater when grown under B30G30R60 compared to HPS120 SL. Thus, the use of LEDs for SL are 

a viable and economically feasible alternative to HPS lamps for the production of vegetable 

transplants in CEs.   

In conclusion, the results from our study help to quantify how SL quality influences the 

morphological and physiological properties of vegetable transplants. Spectral quality 

significantly influenced the parameters evaluated for cucumber, tomato, and pepper. Increasing 

the DLI resulted in an increase of stem diameter of cucumber and pepper transplants. For 

example, stem diameter of cucumber was greatest under the HPS120 and B30G30R60. Stem 

elongation of cucumber was promoted under the low intensity control, while it was reduced for 

tomato transplants under the low intensity control. Furthermore, the replacement of B or R 

radiation with G and/or FR radiation increased plant height of cucumber and tomato. Generally, 

B25R95 was the most effective at reducing internode length of all three species. Fresh weight of 

cucumber and pepper was greatest under HPS120 and B30G30R60 SL. Leaf area and fresh weight of 

tomato was greatest under the B30G30R60 SL. The results from our study, suggest that the B25R95 

111 

 

SL produces the most compact cucumber and tomato transplants, which is a desired trait for 

preventing breakage during transport. However, parameters such as leaf area and fresh weight 

were negatively impacted under the B25R95 SL for both species. Finally, the B30G30R60 LED 

treatment was equally effective as the HPS120 for the promotion of desirable traits for vegetable 

transplants. Thus, indicating that LED SL is both a viable and economically feasible alternative 

to the current industry standard.  

 

Acknowledgements 

We gratefully acknowledge Syngenta for seeds, Grodan for substrates, Heliospectra for 

LEDs, JR Peters Inc. for fertilizer and Nathan DuRussel for technical assistance. The use of trade 

names in this publication does not imply endorsement by Michigan State University of products 

named nor criticism of similar ones not mentioned. This work was supported by the USDA 

National Institute of Food and Agriculture, Hatch project MICL02472. 

 

112 

 

APPENDIX

113 

 

Table IV-1. Supplemental lighting (SL) treatments, replication (Rep.), supplemental radiation, average daily light integral (DLI) from 
SL provided by high-pressure sodium (HPS) lamps or light-emitting diodes (LEDs), for 16 h∙d–1 based on an instantaneous threshold 
[on from 0600 to 2200 HR when the outside PPFD was below ≈440 µmol·m–2·s–1 (on for a minimum of 25 minutes and off for a 
minimum of 20 minutes)], greenhouse canopy air and leaf temperature, and nutrient solution pH and electrical conductivity (EC). 
Cucumber, pepper, and tomato were placed under treatments on 26 Sept. 2018, 24 Oct. 2018, and 07 Jan. 2019. 

Lighting 
treatment 

HPS25 
 
 
HPS120 
 
 
B25R95 
 
 
B30G30R60 
 
 
B20G10R75FR15 
 
  

Supplemental 

Rep. 

radiation                

(µmol·m–2·s–1) 

Total DLI   
(mol·m–2·d–1) 

Temperature (°C) 

Air 

Leaf 

Nutrient solution 

Nutrient solution 

pH 

EC (dS∙m–1) 

1 
2 
3 
1 
2 
3 
1 
2 
3 
1 
2 
3 
1 
2 
3 

- 

   25 ± 2.7 
   25 ± 3.1 
120  ± 3.3 
123  ± 2.5 
124 ± 5.7 
119 ± 6.6 
121 ± 6.8 
120 ± 5.3 
119 ± 5.3 
120 ± 4.5 
122 ± 6.1 
119 ± 3.0 
120 ± 4.0 
120 ± 3.0 

  6.9 
  5.3 
  6.2 
12.0 
11.5 
11.9 
12.2 
11.9 
12.0 
12.6 
11.6 
11.6 
13.2 
10.7 
10.5 

24.1 ± 1.4 
25.2 ± 0.9 
25.0 ± 1.4 
24.1 ± 1.3 
24.6 ± 1.4 
24.8 ± 3.3 
24.8 ± 1.5 
24.6 ± 1.4 
23.4 ± 3.4 
25.5 ± 1.1 
25.1 ± 1.1 
24.5 ± 2.6 
23.5 ± 1.2 
24.1 ± 1.9 
23.7 ± 3.3 

25.9 ± 1.5 
27.2 ± 1.0 
26.4 ± 2.2 
25.9 ± 1.3 
26.0 ± 1.5 
25.9 ± 3.3 
25.5 ± 1.6 
24.7 ± 1.5 
24.5 ± 3.5 
26.8 ± 1.5 
25.9 ± 1.4 
25.8 ± 2.8 
25.1 ± 1.5 
25.4 ± 2.0 
24.8 ± 3.5 

6.13 ± 0.07 
6.06 ± 0.08 
6.08 ± 0.07 
6.09 ± 0.07 
6.06 ± 0.10 
6.06 ± 0.09 
6.05 ± 0.11 
6.05 ± 0.12 
6.04 ± 0.08 
6.05 ± 0.11 
6.05 ± 0.11 
6.03 ± 0.10 
6.09 ± 0.10 
6.06 ± 0.09 
6.06 ± 0.09 

0.89 ± 0.03 
0.89 ± 0.02 
0.89 ± 0.02 
0.90 ± 0.02 
0.89 ± 0.03 
0.89 ± 0.03 
0.89 ± 0.02 
0.88 ± 0.03 
0.89 ± 0.03 
0.89 ± 0.03 
0.89 ± 0.03 
0.89 ± 0.02 
0.89 ± 0.02 
0.89 ± 0.02 
0.89 ± 0.03 

114 

 

Figure IV-1. Spectral quality delivered from high-pressure sodium (HPS) lamps providing 
photoperiodic and supplemental lighting (SL) and light-emitting diode (LED) fixtures delivering 
SL. Blue (B, 400-500 nm) represents the blue photon flux (PF), G (500-600 nm) the green PF, R 
(600-700 nm) the red PF  and FR (700-800 nm) the far-red PF from LEDs. Number subscripts 
after HPS denote the total photon flux density (TPFD) in µmol·m–2·s–1. Number subscripts in the 
LED treatments denote the photon flux density (PFD) in µmol·m–2·s–1 of B, G, R, and FR.

 

115 

 

Figure IV-2. Stem diameter (mm), height (cm), hypocotyl (cm), internode length (cm), and 
average number of nodes per plant of cucumber, tomato, and pepper. Data were collected 28, 35, 
and 35 days after treatment under high-pressure sodium (HPS) or light-emitting diode (LED) 
supplemental lighting (SL) treatments for cucumber, tomato, and pepper, respectively. Data were 
pooled when there was no interaction between replication (Rep.) and treatment, or if the response 
trends were similar between Reps. Letters indicate mean separations across treatments using 
Tukey-Kramer difference test at P ≤ 0.05. Error bars indicate standard error. Mean separations 
were excluded for un-pooled data.  

 

116 

 

Figure IV-3. Number of leaves, leaf area (cm2), fresh weight (g), dry weight (g), and chlorophyll 
fluorescence (Fv/Fm) per plant of cucumber, tomato, and pepper. Data were collected 28, 35, and 
35 days after treatment under high-pressure sodium (HPS) or light-emitting diode (LED) 
supplemental lighting (SL) treatments for cucumber, tomato, and pepper, respectively. Data were 
pooled when there was no interaction between replication (Rep.) and treatment, or if the response 
trends were similar between Reps. Letters indicate mean separations across treatments using 
Tukey-Kramer difference test at P ≤ 0.05. Error bars indicate standard error. Mean separations 
were excluded for un-pooled data.

 

117 

 

Figure IV-4. Visible bud percentage of 28, 35, and 35 day-old cucumber, tomato, and pepper 
transplants, grown under different high-pressure sodium (HPS) and light-emitting diode (LED) 
supplemental lighting (SL) treatments.

 

118 

 

Figure IV-5. The incidence of leaf necrosis resulting from high-pressure sodium (HPS) and light-
emitting diode (LED) supplemental lighting (SL) treatments on tomato (‘Climstar’ L. 
lycopersicum), after 35 days of SL. Percentage of leaves damaged (%) was calculated by 
dividing the number of leaves showing necrotic lesions by the total number of leaves, and 
multiplying by 100.

 

119 

 

 
 

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