MANIPULATING LIGHT QUALITY TO IMPROVE GROWTH ATTRIBUTES OF 

HIGH-VALUE SPECIALTY CROPS IN CONTROLLED ENVIRONMENTS 

By 

Mengzi Zhang 

A THESIS 

Submitted to 

Michigan State University 

in partial fulfillment of the requirements 

for the degree of 

Horticulture—Master of Science 

2018

 

 

ABSTRACT 

MANIPULATING LIGHT QUALITY TO IMPROVE GROWTH ATTRIBUTES OF  

HIGH-VALUE SPECIALTY CROPS IN CONTROLLED ENVIRONMENTS 

By 

Mengzi Zhang 

Manipulating light quality with light-emitting diodes (LEDs) can regulate crop traits and 

responses including plant architecture, quality characteristics, and flowering time. We 

investigated how different radiation wavebands, especially far-red (FR; 700-800 nm), could be 

delivered in controlled environments to produce specialty crops with desired attributes. We 

utilized end-of-day (EOD) photoperiodic lighting in greenhouses to determine whether FR 

radiation would promote extension growth without influencing the flowering of poinsettia. EOD 

FR lighting promoted poinsettia extension growth, and at a low-intensity it was not perceived as 

a long day, but it was at a higher intensity. In another study, we determined the impacts of 

photoperiodic lighting (to increase leaf area) and supplemental lighting (SL) (to increase 

photosynthesis) on growth, pigmentation, and sensory attributes of greenhouse-grown lettuce. 

EOD lighting had few effects on lettuce growth, but greenhouse SL, especially from blue (B; 

400-500 nm) and red (R; 600-700 nm) LEDs, increased lettuce growth and coloration but 

sensory attributes were less preferred by consumers than plants grown without SL. In a third 

study, we quantified how a variety of floriculture crop seedlings respond to the addition of FR 

radiation to B+R sole-source LED lighting. The addition of 40 µmol·m–2·s–1 of FR radiation 

promoted stem elongation of some species and accelerated the flowering of snapdragon. 

Collectively, this research shows the potential applications of including FR in a radiation 

spectrum to elicit specific plant growth, quality, and flowering attributes of horticultural crops.

 
 

ACKNOWLEDGEMENTS 

 

 

I would like to express my sincere gratitude for my major advisor, Dr. Erik Runkle, for 

all his encouragement, patience, and the continuous great support and guidance on research and 

life. I also wish to thank Dr. Roberto Lopez and Dr. Frank Telewski for serving on my advisory 

committee and provide their valuable inputs on my M.S. project and thesis writing. Thanks to 

Nate DuRussel and Catherine Whitman for their technical assistance with my experiments in the 

greenhouse.   

 

I appreciate all the encouragement and support from our floriculture team and I value the 

great friendship with our graduate students: Yujin Park, Qingwu Meng, Kellie Walters, Charlie 

Garcia, Allison Hurt, and Qiuxia Chen. I wish to thank our greenhouse undergraduate student 

employees for their help with maintenance in the research greenhouse and with experiment data 

collection. I would also like to thank HOGS and the HortReport team to provide diverse 

experience to enrich my graduate student life and provide a chance to meet with other good 

friends. I want to express my thanks to Dr. Ron Perry for providing an opportunity for me to 

develop my hobby in wine and being a teaching assistant for the wine class. 

 

Finally, I would like to express my special gratitude to my beloved family for their 

endless love. Special thanks to Han Wang for the companion, support, and encouragement and 

for all the happy memories we share.  

 

 

iii 

TABLE OF CONTENTS 

 

LIST OF TABLES ......................................................................................................................... vi 
 
LIST OF FIGURES ...................................................................................................................... vii 
 
SECTION I .................................................................................................................................... 1 
LITERATURE REVIEW ............................................................................................................ 1 
Literature Review: Horticultural Lighting ................................................................................ 2 
Introduction ............................................................................................................................... 2 
Horticultural lighting ................................................................................................................ 4 
Supplemental Lighting .............................................................................................................. 9 
Photoperiodic Lighting ........................................................................................................... 11 
Light quality ............................................................................................................................ 15 
Red and far-red light ............................................................................................................. 16 
Blue light................................................................................................................................ 18 
Green light ............................................................................................................................. 21 
Considerations for Lighting Fixtures .................................................................................... 22 
LITERATURE CITED .............................................................................................................. 24 
 
SECTION II ................................................................................................................................ 32 
REGULATING FLOWERING AND EXTENSION GROWTH OF POINSETTIA USING 
RED AND FAR-RED LEDS FOR END-OF-DAY LIGHTING ............................................ 32 
Abstract .................................................................................................................................... 34 
Introduction ............................................................................................................................. 35 
Materials and Methods ........................................................................................................... 38 
Plant materials ...................................................................................................................... 38 
Lighting treatments ................................................................................................................ 39 
Environmental conditions ...................................................................................................... 40 
Plant measurements ............................................................................................................... 41 
Experimental design and data analysis ................................................................................. 41 
Results ...................................................................................................................................... 41 
Discussion ................................................................................................................................. 43 
APPENDIX .................................................................................................................................. 48 
LITERATURE CITED .............................................................................................................. 54 
 
SECTION III ............................................................................................................................... 58 
MANIPULATING GROWTH, COLOR, AND TASTE ATTRIBUTES OF FRESH CUT 
LETTUCE BY GREENHOUSE SUPPLEMENTAL LIGHTING ........................................ 58 
Abstract .................................................................................................................................... 60 
Introduction ............................................................................................................................. 61 
Materials and Methods ........................................................................................................... 64 
Expt.1: Supplemental greenhouse lighting ............................................................................ 64 
Plant materials ................................................................................................................... 64 
Environmental conditions .................................................................................................. 64

 

iv 

Lighting treatments ............................................................................................................ 65 
Expt. 2: Sensory evaluation ................................................................................................... 66 
Plant materials and growth conditions .............................................................................. 66 
Sample preparation and consumer sensory evaluation ..................................................... 67 
Expt. 3: Supplemental greenhouse lighting at different growing stages ............................... 67 
Plant material .................................................................................................................... 67 
Environmental conditions .................................................................................................. 68 
Lighting treatments ............................................................................................................ 68 
Plant measurements and experimental design ................................................................... 68 
Results ...................................................................................................................................... 69 
Expt. 1: Supplemental greenhouse lighting ........................................................................... 69 
Plant growth....................................................................................................................... 69 
Leaf coloration ................................................................................................................... 69 
Expt. 2: Sensory evaluation ................................................................................................... 70 
Expt. 3: Supplemental greenhouse lighting at different growing stages ............................... 70 
Plant growth....................................................................................................................... 70 
Leaf coloration ................................................................................................................... 71 
Discussion ................................................................................................................................. 72 
APPENDIX .................................................................................................................................. 76 
LITERATURE CITED .............................................................................................................. 82 
 
SECTION IV ............................................................................................................................... 86 
INVESTIGATING THE INTERACTION OF BLUE AND FAR-RED RADIATION ON 
GROWTH AND SUBSEQUENT FLOWERING OF FLORICULTURE CROPS ............. 86 
Abstract .................................................................................................................................... 88 
Introduction ............................................................................................................................. 89 
Materials and Methods ........................................................................................................... 92 
Plant material ........................................................................................................................ 92 
Radiation treatments and environmental conditions during the seedling stage ................... 93 
Environmental conditions during the finish stage ................................................................. 95 
Data collection and analysis ................................................................................................. 96 
Results ...................................................................................................................................... 97 
Seedling stage ........................................................................................................................ 97 
Finish stage............................................................................................................................ 98 
Discussion ................................................................................................................................. 99 
APPENDIX ................................................................................................................................ 104 
LITERATURE CITED ............................................................................................................ 112 
 

 

v 

 

LIST OF TABLES 

 

Table II-1. Means (±SD) of greenhouse air temperature during the two experimental replications 
(Rep.) as measured by aspirated thermocouples. Plants were grown under a 9-h short day (15-h 
dark period; black bar) without or with end-of-day lighting from red+white+far-red (R+FR; 
white bar) or far-red (FR; grey bar) light-emitting diodes at 2 to 3 μmol·m–2·s–1 for 2 or 4 h. In 
Rep. 2, an additional treatment of a high-intensity far-red (H-FR) at 13 μmol·m–2·s–1 was 
included. ........................................................................................................................................ 49 
 
Table II-2. Effect of end-of-day (EOD) lighting treatments on the percentage of plants with bract 
color and reaching anthesis of two cultivars of poinsettia (‘Marble Star’ and ‘White Glitter’). See 
Table 1 for descriptions of lighting treatments. ............................................................................ 50 
 
Table III-1. Means (± SD) of greenhouse air temperature and daily light integral (DLI) during the 
two experimental replications (rep) in Expt. 1 and 3 as measured by aspirated thermocouples and 
quantum sensors. The supplemental lighting treatments were none, high-pressure sodium (HPS) 
and red+blue (R+B) light-emitting diodes (LEDs), and blue (B) LEDs. The 4-h end-of-day 
lighting provided by red+white+far-red flowering lamps at 2 to 5 μmol·m–2·s–1 in Expt. 2. ....... 77 
 
Table III-2. Sensory attributes and mean scores of lettuce grown under three different greenhouse 
light conditions in Expt. 2. Samples with different letters within each cultivar and the attribute 
column are significantly different at the 95% confidence level.  For ‘Green Incised’, n=45 (12 
male and 33 female).  For ‘Green Sweet Crisp’, n=39 (12 male and 27 female). ........................ 78 
 
Table IV-1. Spectral characteristics and average photosynthetic daily light integral (DLI) of nine 
sole-source radiation treatments in two replications (rep) delivered indoors by blue (B, 400-500 
nm), red (R, 600-700 nm), far-red (FR, 700-800 nm), and warm white (WW) light-emitting 
diodes (LEDs) or one greenhouse treatment with sunlight supplemented by high-pressure sodium 
(HPS) fixtures. The subscript values that follow each LED type indicate their photon flux density 
in µmol·m–2·s–1. .......................................................................................................................... 105 
 
Table IV-2. Average leaf area, dry shoot weight, and dry root weight of nine species at the end of 
the seedling stage. Lighting treatments had no statistical differences on all nine species and data 
for each species was pooled from all lighting treatments. Data for each species represent the 
mean (±SE) of two replications with 10 subsamples (plants) per replication.............................. 106 
 
Table IV-3. Days to flower after transplant, visible bud or inflorescence number, plant height, 
and leaf number of eight species at the finishing stage. Lighting treatments had no statistical 
differences on eight of the species and data for each species was pooled from all lighting 
treatments. Data for each species represent the mean (±SE) of two replications with 10 
subsamples (plants) per replication. ............................................................................................ 107 
 

 

 

 

vi 

 

LIST OF FIGURES 

 

Figure II-1. Spectral distribution of two types of end-of-day (EOD) lighting treatments delivering 
red (R, 600 to 700 nm), and/or far-red (FR, 700 to 800 nm) radiation. The photon flux density of 
the two EOD lighting treatments was 2 to 3 µmol·m–2·s–1. .......................................................... 51 
 
Figure II-2. Days to first bract color and anthesis of two cultivars of poinsettia grown under a 9-h 
short-day (control) and seven red and/or far-red end-of-day lighting treatments (see Table 1 for 
treatment descriptions). All treatments were delivered at a photon flux density (400 to 800 nm) 
of 2 to 3 µmol·m–2·s–1, except 4-h H-FR, which was delivered at 13 µmol·m–2·s–1. All data were 
pooled from two replications, except 4-h H-FR, which was only conducted in the second 
replication. Pairwise comparison was made within each cultivar. Means sharing a letter are not 
statistically different by Tukey’s honest significant difference test at P ≤ 0.05. Error bars indicate 
standard error. ............................................................................................................................... 52 
 
Figure II-3. Increase in plant height, node number, and average internode length of two cultivars 
of poinsettia grown under a 9-h short-day (control) and seven end-of-day red and/or far-red 
lighting treatments (see Table 1 for treatment descriptions). All treatments were delivered at a 
photon flux density (400 to 800 nm) of 2 to 3 µmol·m–2·s–1, except 4-h H-FR, which was 
delivered at 13 µmol·m–2·s–1. Pairwise comparisons were made within each cultivar. Means 
sharing a letter are not statistically different by Tukey’s honest significant difference test at P ≤ 
0.05. Error bars indicate standard error. ....................................................................................... 53 
 
Figure III-1. The spectral distribution of supplemental lighting treatments (top) delivered by 
high-pressure sodium (HPS) fixtures, red+blue (R+B) light-emitting diodes (LEDs) (in Expt. 1 
and 2), and B LEDs (in Expt. 2), and end-of-day lighting (bottom) provided by red+white+far-
red LEDs. The peak wavelengths (in nm) are provided for each fixture type. ............................. 79 
 
Figure III-2. Growth characteristics of five cultivars of lettuce grown without supplemental 
lighting (none) or with supplemental lighting from high-pressure sodium (HPS) or light-emitting 
diodes (LED) in Expt. 1. Leaf number of ‘Salanova Green Butter’ included leaves ≥ 7 or 3.5 cm 
in length for replication 1 (Rep 1) or replication 2 (Rep 2), respectively, while leaf number for 
the other four cultivars included leaves ≥ 3 cm in both replications. Within the same replication, 
means with the same letter are not statistically different by Tukey’s honest significant difference 
test at P ≤ 0.05. Error bars indicate standard error. ...................................................................... 80 
 
Figure III-3. Leaf coloration characteristics of two cultivars of lettuce grown without 
supplemental lighting (none) or with supplemental lighting from high-pressure sodium (HPS) or 
light-emitting diodes (LED) in Expt. 1. In replication (Rep) 1, a single measurement was taken 
on one outer leaf per plant while in Rep 2, measurements were taken on three outer leaves per 
plant. Within the same replication, means with the same letter are not statistically different by 
Tukey’s honest significant difference test at P ≤ 0.05. Error bars indicate standard error. .......... 81 
 

 

vii 

 

Figure IV-1. The spectral distribution of sole-source lighting treatments delivered from mint 
white (MW), red (R), blue (B), and green (G) light-emitting diodes (LEDs) at total 
photosynthetic photon flux density (PPFD) = 160 µmol∙m−2∙s−1. The subscript values after each 
LED type indicate the percentages of the total PPFD delivered from each LED type. ............. 108 
 
Figure IV-2. Stem length of seedlings grown under nine sole-source lighting treatments delivered 
by blue (B, 400-500 nm), red (R, 600-700 nm), far-red (FR, 700-800 nm), and warm white (WW) 
light-emitting diodes (LEDs) indoors and one greenhouse treatment (GH) with supplemental 
lighting from high-pressure sodium fixtures. The subscript values that follow each LED type 
indicate their intensity in µmol·m–2·s–1. The top, middle, bottom row represents long-day plants, 
short-day plants, and day-neutral plants, respectively. Data for each species represent the mean 
of two replications with 10 subsamples (plants) per replication. Means sharing a letter are not 
statistically different by Tukey’s honest significant difference test at P ≤ 0.05 and NS indicates 
nonsignificance. Error bars indicate the standard error and was calculated by 20 subsamples from 
two replications. .......................................................................................................................... 109 
 
Figure IV-3. SPAD index value of petunia and zinnia seedlings grown under nine sole-source 
lighting treatments delivered by blue (B, 400-500 nm), red (R, 600-700 nm), far-red (FR, 700-
800 nm), and warm white (WW) light-emitting diodes (LEDs) indoors and one greenhouse 
treatment (GH) with supplemental lighting from high-pressure sodium fixtures. The subscript 
values that follow each LED type indicate their intensity in µmol·m–2·s–1. Data for each species 
represent the mean of two replications with 10 subsamples (plants) per replication. Means 
sharing a letter are not statistically different by Tukey’s honest significant difference test at P ≤ 
0.05 and NS indicates nonsignificance. Error bars indicate the standard error and was calculated 
by 20 subsamples from two replications. .................................................................................... 110 
 
Figure IV-4. Flowering characteristics of snapdragon at finishing stage when seedlings were 
grown under nine sole-source lighting treatments delivered by blue (B, 400-500 nm), red (R, 
600-700 nm), far-red (FR, 700-800 nm), and warm white (WW) light-emitting diodes (LEDs) 
indoors and one greenhouse treatment (GH) with supplemental lighting from high-pressure 
sodium fixtures. The subscript values that follow each LED type indicate their intensity in 
µmol·m–2·s–1. Data represent the mean of two replications with 10 subsamples (plants) per 
replication. Means sharing a letter are not statistically different by Tukey’s honest significant 
difference test at P ≤ 0.05 and NS indicates nonsignificance. Error bars indicate the standard 
error and was calculated by 20 subsamples from two replications. ............................................ 111 
 

 

viii 

 

 

SECTION I  

LITERATURE REVIEW

1 

 

 

Introduction 

Literature Review: Horticultural Lighting 

Greenhouses are commonly used in the commercial production of flowering plants (e.g., 

bedding plants, herbaceous perennials, and potted plants) and vegetables. The primary purpose 

of growing plants in greenhouses is to better control environmental factors that regulate plant 

growth and development so that growers can develop and implement production schedules to 

produce crops reliably and for specific market dates. Temperature in the greenhouse is relatively 

easily controlled while light, as one of the most important environmental factors for plant growth, 

is difficult to manage (Albright et al., 2000).  

The natural photoperiod and photosynthetic daily light integral (DLI) depend on a 

number of factors, especially the time of year and the location. Photoperiod, which is the number 

of hours of light during a 24-h period, is involved in several physiological responses of plants 

including flowering. Plants can perceive light at a very low photosynthetic photon flux density 

(PPFD), for example at 0.1 µmol·m–2·s–1 (Whitman et al., 1998), so the biological photoperiod 

can be defined as the period from sunrise to sunset, plus 30 to 40 minutes of twilight (Faust and 

Heins, 1995). The photoperiod changes little at the equator but the seasonal variation increases as 

latitude increases. For example, the biological photoperiod at 30 degrees latitude varies from 

about 11 h during the winter to 14.5 h during the summer, while at 50 degrees, it varies from 9 to 

17 h (Runkle et al., 2017).  

Sunlight is the major light source inside most greenhouses. Radiation emitted by the sun 

passes through the earth’s atmosphere, which modifies the spectrum once it reaches the earth’s 

surface depending on the time of day and year, latitude, and cloud cover (Ehret et al., 1989). The 

DLI describes the total amount of photosynthetically active radiation (PAR; 400 to 700 nm) 

 

2 

 

delivered during a 24−h period in a square meter, and has the unit of mol·m–2·d–1. In the northern 

part of the U.S. and Canada, the maximum DLI is about 60 mol·m–2·d–1 outdoors on a cloudless 

summer day, and is less than 5 mol·m–2·d–1 in the winter on a dark, cloudy day. In greenhouses, 

within the U.S., variation in DLI is greatest between the north and south during the fall and 

winter, while during spring and summer, the DLI variation is greatest between the eastern and 

western regions (Korczynski et al., 2002). PAR accounts for approximately 43% of the total 

solar photosynthetic radiation energy (for around 1960 µmol·m–2·s–1) (Thimijan and Heins, 1983) 

and is further reduced by 10-40% because of the attenuation of light from the glazing material, 

structural frame, hanging baskets, and other obstructions (Monk and Molnar, 1987). Thus, the 

average DLI that enters the greenhouse varies from 5 to 30 mol·m–2·d–1 (Runkle, 2006). 

Generally, glass has ≥10% greater light transmission than plastic films (Nijskens et al., 1985). 

Single layer glass provides a direct light transmittance of 90% compared to 80% for polyethylene 

(Both and Faust, 2017). Thus, during the winter and early spring, the DLI is often a limiting 

factor for plant growth in greenhouses >40 ºN latitude (Aldrich et al., 1969). 

Plant growth typically increases with an increase in DLI. For example, the total dry 

weight of a wide range of crops, including ageratum (Ageratum houstonianum), begonia 

(Begonia ×semperflorens-cultorum), marigold (Tagetes erecta), petunia (Petunia ×hybrida), 

salvia (Salvia coccinea), vinca (Catharanthus roseus), and zinnia (Zinnia elegans) increased as 

the DLI increased from 5 to 43 mol·m–2·d–1 at an average temperature of 23 ºC (Faust et al., 

2005). Flower number of petunia, salvia, vinca, and zinnia also increased, by 419% to 558%, as 

the DLI increased from 5 to 43 mol·m–2·d–1. In addition, shoot dry mass of snapdragon 

(Antirrhinum majus), ruddles (Calendula officinalis), impatiens (Impatiens wallerana), mimulus 

(Mimulus ×hybridu), and torenia (Torenia fournieri) increased (102% to 240%) and flowering 

 

3 

 

time decreased (by 9% to 38%) as the DLI increased from 10.5 to 21.8 mol·m–2·d–1 at 20 ºC 

(Warner and Erwin, 2005). 

Increasing DLI in greenhouses is usually achieved by delivery of supplemental, high-

intensity lighting. For example, supplemental lighting increased photosynthetic rate more than 2-

fold compared to non-lighted strawberries (Fragaria ×ananassa), resulting in an increased 

average fruit weight in ‘Benihoppe’ and fruit number in ‘Benihoppe’, ‘Sagahonoka’, and 

‘Akihime’ (Hidaka et al., 2015). In lettuce (Lactuca sativa), day-extension lighting from 16 to 24 

h at a PPFD of 50-100 µmol·m–2·s–1 increased biomass by 20% and reduced the growth cycle by 

7 d during the winter (Gaudreau et al., 1994). The phytochemicals of baby leaf lettuce including 

anthocyanins, carotenoids, and phenolics also increased, depending on the quality of 

supplemental lighting (Li and Kubota, 2009). Although supplemental lighting is effective for 

plant production, its utility depends, in part, on the location of the greenhouses, crop(s) grown, 

and lamp type. For example, supplemental lighting can significantly increase the DLI for a 

northern U.S. greenhouse, but not for a greenhouse in the Southwest (Korczynski et al., 2002).  

 

Horticultural Lighting 

Electrical lighting is usually used by growers to increase plant growth, regulate flowering, 

or both. Supplemental, photoperiodic, and sole-source lighting are three types of the most 

common lighting applications in horticulture. Supplemental lighting refers to delivery of 

moderate to high-intensity light in the greenhouse, usually from above. This system increases the 

DLI and promotes photosynthesis, resulting in increased plant growth of shoots, roots, flowers, 

and fruits, especially during low-light conditions. Photoperiodic lighting is typically delivered at 

a low intensity to regulate flowering; it accelerates flowering of long-day plants (LDPs) and 

inhibits flowering of short-day plants (SDPs) and thus, helps to regulate crop production 

 

4 

 

schedules and reduce production durations. Sole-source lighting is the delivery of a moderate to 

high intensity of light that is the only light source for plants. Sole-source lighting has been used 

successfully in vertical farming (plant factories) in Japan for over two decades, and for even 

longer inside growth chambers for controlled-environment research and for germination of 

seedlings by commercial growers.  

Incandescent (INC) and fluorescent (FL) lamps have been traditionally used in 

greenhouse crop production, primarily as photoperiodic lighting. INC works on the principle of 

heating a metal to a high temperature to emit light. Commonly used as photoperiodic lighting to 

regulate flowering of SDPs and LDPs, it has a relatively low red (R, 600-700 nm) to far-red (FR, 

700-800 nm) ratio (R:FR) of around 0.6, and thus, promotes stem elongation of most crops. 

However, INC has a very low electrical efficacy due to the loss of more than 90% of electric 

input into heat (Runkle and Both, 2017). Although INC is inexpensive to purchase and easy to 

install, it has been gradually phased out of production due to its inefficiency in converting 

electricity into radiation useful to plant production. Compact fluorescent lamps (CFL), which 

generate light by sending an electrical discharge through an ionized gas, were designed as a 

replacement for INC. It emits more blue (B; 400 to 500 nm) light and less FR light than INC, has 

an R:FR ratio around 6.0, and is more electrically efficient than INC lamps (Runkle and Both, 

2017). However, petunia flowered 2-3 weeks later under CFL than INC, which indicates that for 

at least some LDPs, CFL lamps are less effective at promoting flowering than INC (Oh and 

Runkle, 2016; Runkle et al., 2012). In addition, the lifespan of CFLs depends on the number of 

on/off cycles and therefore, is not an appropriate product for cyclic lighting (Runkle and Both, 

2017). In addition, FL lamps are sometimes used indoors for germination shelves to provide light 

without excessive heat (Mattson, 2015).   

 

5 

 

High-intensity discharge lamps are the most widely used lamps in modern greenhouse 

crop production. These lamps, which include high-pressure sodium (HPS) and metal halide 

(MH), work on the principle of passing an electric current through a gas. HPS is used for 

supplemental lighting to increase photosynthesis and thus promote the growth and yield. For 

example, the total yield of greenhouse tomato (Lycopersicon esculentum) fruit increased by 16% 

to 37% under 140 µmol·m–2·s–1 HPS compared to non-supplemented (ambient) light, and as the 

PPFD under HPS increased from 70 to 140 µmol·m–2·s–1, the weight of tomato fruit increased 

by 20-30% (McAvoy and Janes, 1984). HPS can also be effective as photoperiodic lighting to 

regulate flowering. For example, bellflower (Campanula carpatica), coreopsis (Coreopsis 

grandiflora), petunia, and black-eyed susan (Rudbeckia hirta) were studied under a 4-h NI with 

cyclic HPS lamps during a 15-h night (Blanchard and Runkle, 2010). All species promoted 

flowering under HPS lamps, while none of the long-day plants flowered under a 9-h short-day. 

However, HPS has an R:FR ratio of around 6.3 and thus when delivered at a low intensity, may 

not be as effective as lamps with a lower R:FR ratio at promoting the flowering of LDPs (Runkle 

and Both, 2017).  

MH lamps are sometimes used in retail garden centers because they emit more of a white 

light than HPS lamps, which provides more true plant colors to consumers (Fisher et al., 2017). It 

has also been used as supplemental lighting to increase plant growth. However commercially, 

HPS is more suitable for large-scale supplemental lighting than FL and MH lamps because of its 

higher efficacy of transforming electricity into photosynthetic photons (Moe et al., 2005), as well 

as its longer bulb life. 

Light-emitting diodes (LEDs) are a relatively new lighting technology that is increasingly 

being used in horticulture. LEDs work on the principle of light emission produced by passing a 

 

6 

 

current through different semiconductor materials such as indium gallium nitride (InGaN), 

aluminum gallium indium phosphide (AlGaInP), and aluminum gallium arsenide (AlGaAs). 

Early LEDs could only emit low-intensity infrared wavelengths while modern LEDs can emit 

wavebands of photosynthetic radiation (from 400 to 700 nm), as well as ultraviolet and far-red 

radiation, in somewhat narrow or broad wave bands. With the development, LEDs have a 

significantly longer lifetime than conventional lamps, ranging from 20,000 to 55,000 h compared 

with lifetimes for INC of 1,000 h, CFL of 8,000 to 10,000 h (Tähkämö et al., 2012), and HPS of 

10,000 h (Nelson and Bugbee, 2014). In addition, LEDs are also developed to be safer than other 

bulb types because there is no fragile glass envelope to break, is no high touch temperature, and 

are fewer hazardous materials (Olle and Viršilė, 2013).  

There are several advantages of LEDs compared to other lamp types, including their 

relatively high electrical efficacy that continues to increase. LEDs emit relatively little radiant 

heat, so they can be placed closer to the crop compared to conventional fixtures such as HPS 

lamps (Fisher et al., 2017). In addition, because light emitted is more directional than 

conventional lamps, there’s the potential to focus the light and reduce illuminated areas not 

occupied by plants. The efficacy of LEDs continues to increase as the technology develops. In 

2014, the efficacy of LED fixtures ranged from 0.9 to 1.7 µmol·J–1 (Nelson and Bugbee, 2014), 

and more recently, fixtures with efficacies of >2.0 µmol·J–1 have been developed (Fisher et al., 

2017). The efficacy of LED intracanopy lighting for converting electricity into high-wire tomato 

biomass was 75% higher than overhead lighting from HPS lamps (Gómez et al., 2013). In 

addition, the electrical cost for the average fruit under HPS overhead lighting was 403% higher 

than from LED intracanopy lighting, without a compromise in fruit yield.   

 

7 

 

However, LED arrays used for horticultural applications are a relatively new technology 

and there are still barriers to commercial implementation. The relatively high cost of investment 

for LEDs is the greatest obstacle to the expansion of LED technology in horticulture. Thus, 

LEDs will become more popular as the technology develops, the efficacy continues to increase, 

and the cost is further reduced. A hybrid lighting system that combines some of the features of 

LEDs with conventional lamps could be an economically efficient option, depending on the crop 

and lamp characteristics desired for production (Olle and Viršilė, 2013). Additional 

considerations for LEDs to be used in greenhouses include adequate dissipation of heat (an 

excessively high temperature can overheat the LED package, reducing light output and fixture 

longevity) and tolerance to high humidity. 

Different types of LEDs can be combined on arrays to emit a specific spectrum for 

supplemental or sole-source lighting to promote growth and yield. For example, lamb's lettuce 

(Valerianella locusta) was grown under 90% of red LEDs (peak = 660 nm) plus 10% blue LEDs 

(90R/10B), warm white LEDs, or HPS at a supplemental PPFD of 200 μmol·m−2·s−1 in the 

greenhouse (Wojciechowska et al., 2015). The yield was the greatest under 90R/10B and was 29-

33% higher than under HPS or warm-white LEDs. In a study with hydroponically grown Boston 

lettuce, plants were grown in growth chambers under red+blue+white LEDs (RBW) and 

red+blue LEDs (RB) at a PPFD of 210 μmol·m−2·s−1. Lettuce grown under RBW had 20% and 

14% greater shoot fresh and dry weight as well as 48% and 44% greater root fresh and dry 

weight compared to that of RB (Lin et al., 2013). 

LEDs can also be used to deliver photoperiodic lighting to regulate flowering responses. 

Manipulating the spectrum of LEDs can influence flowering attributes as well as increase the 

efficacy of delivering the long-day condition. For example, several LDPs were grown under 9-h 

 

8 

 

short-days (SDs) with or without a 2 μmol·m−2·s−1 4-h NI from LEDs with or without FR. NI 

including FR (R+W+FR) significantly promoted flowering of coreopsis, petunia, and snapdragon 

by ~12, 6, and 13 d, respectively, compared to those grown under NI without the FR (such as R, 

R+B, cool-white, and warm-white) (Meng and Runkle, 2015). In a separate study, under a 

photoperiod of 16 h, marigold and salvia had 43% and 100% more flower buds under 1:1 R+B 

LEDs than FL when delivered at a PPFD of 90 μmol·m−2·s−1 (Heo et al., 2006). In addition, the 

number of open flowers of ageratum was 100% greater under R+B than L.  

 

Supplemental Lighting 

Supplemental lighting in greenhouses provides additional light when the solar DLI is not 

sufficient to maintain high yield and quality for crop production. Supplemental lighting increases 

photosynthesis, crop growth, and yield most efficiently when the DLI is low, especially from 

mid-November to mid-February in the Northern hemisphere. Typically, 50 to 75 μmol·m−2·s−1 is 

delivered to floriculture crops, and a higher intensity of 100 to 200 μmol·m−2·s−1 is provided to 

high-wire vegetables and cut flowers (Fisher et al., 2017). Specifically for vegetable crops, a 

photoperiod of 12 to 16 h of supplemental lighting at 50-75 μmol·m−2·s−1 for lettuce, 75-100 

μmol·m−2·s−1 for sweet pepper (Capsicum annum), and 75-175 μmol·m−2·s−1 for cucumber 

(Cucumis sativus) has been recommended for vegetable seedlings (Dorais and Gosselin, 2002). 

For tomatoes, 83 μmol·m−2·s−1 was generally favorable for seedling production and a DLI of ≥ 

30 mol·m−2·d−1 was recommended for fruiting crops (Dorais, 2003).  

Plant growth and biomass increase under supplemental lighting compared to natural light 

mainly because of greater photosynthesis that increases growth and yield. For example, Dorais 

and Gosselin (2002) reported that increasing the light intensity from 100 to 150 μmol·m−2·s−1 

 

9 

 

increased yield by 4 to 30%, depending on the plant species. In a separate study, shoot dry 

weight of celery (Apium graveolens), lettuce, broccoli (Brassica oleracea), and tomato 

transplants increased by 22%, 40%, 19%, and 24%, and root dry weight by 97%, 42%, 38%, and 

21%, respectively, under 400-W HPS that delivered a PPFD of 100 μmol·m−2·s−1 compared to 

natural light (Masson et al., 1991). Similarly, the biomass of Boston-type lettuce increased up to 

270%, along with a greater head firmness and a ~30% reduction of production cycle length, 

under 100 μmol·m−2·s−1 HPS lamps with a 24-h photoperiod than under natural light (Gaudreau 

et al., 1994). In cucumber, seedlings were grown under a 16-h photoperiod from 400-W HPS 

lamps with a PPFD of 65 μmol·m−2·s−1 at planting, and increased to 350 μmol·m−2·s−1 when the 

plants reached the overhead wire (Hao and Papadopoulos, 1999). Supplemental lighting 

increased leaf chlorophyll content (by 14-23%), plant biomass accumulation (by 9-18%), and 

yield including 25-56% more fruit, 34-60% greater fruit weight, 11-23% increased fruit size, 10-

12% greater fruit dry mass content, and 16-43% higher fruit skin chlorophyll content compared 

to ambient light conditions.  

Although commercial producers sometimes use photoperiodic lighting to manipulate 

flowering of daylength-sensitive crops, supplemental lighting can also influence the flowering 

process. For example, supplemental lighting at 90 μmol·m−2·s−1 from HPS lamps during either 

the last two-thirds or the entire plug stage accelerated subsequent flowering of petunia and pansy 

(Viola ×wittrockiana) by 5-6 d compared to the control, which was photoperiodic lighting at 3 

μmol·m−2·s−1 provided by HPS (Oh et al., 2010). Similarly, days to the first flower of cyclamen 

decreased by 44% as DLI provided by sole-source cool-white FL lamps increased from 1.4 to 

17.3 mol·m−2·d−1 (Oh et al., 2009). However, supplemental lighting from conventional lamps can 

also increase plant temperature by non-photosynthetic radiation (700(cid:173)50,000 nm) (Faust and 

 

10 

 

Heins, 1997). For example, the shoot-tip temperature of vinca that received 100 μmol·m−2·s−1 of 

supplemental HPS lighting was 1.7 °C higher than that without supplemental lighting (Faust and 

Heins, 1997). Thus, a reduction in time to flowering under supplemental lighting can at least 

partly be attributed to a higher plant temperature. Therefore, supplemental lighting can improve 

plant production and/or accelerate flowering by increasing photosynthesis as well as slightly 

increasing plant temperature when conventional lamps are used.  

The spectral distribution of supplemental lighting can also influence branching, as well as 

other quality attributes of horticultural crops. Light with a high R:FR typically promotes 

branching, while a lower R:FR suppresses it. For example, poinsettia (Euphorbia pulcherrima) 

branching decreased by an average of 4.2 branches per plant under an R filter (creating a low 

R:FR), and increased it by 1.5 branches under an FR filter (creating a higher R:FR), compared to 

plants under a non-selective screen (Clifford et al., 2004). Cut rose (Rosa ×hybrida) produced 49% 

and 64% more flowering shoots under 70-75 μmol·m−2·s−1 HPS and MH, respectively, compared 

to HPS filtered to provide a relatively low R:FR ratio (Roberts et al., 1993). Similarly, the 

axillary shoot number of rhododendron grown under a filter creating an R:FR ratio of 0.4 was 33% 

less than under an R:FR ratio of 2.0, both with a PPFD of around 25 μmol·m−2·s−1 (Marks and 

Simpson, 1999).  

 

Photoperiodic Lighting 

Flowering is one of the most important developmental stages in a plant’s life cycle. 

Photoperiodism, which refers to a physiological reaction of the plant to day length, is one of 

several factors that can mediate flowering responses. Day length is perceived primarily by the 

leaves, and for photoperiodic crops that have obtained the capacity to flower (i.e., are not 

 

11 

 

juvenile), an inductive photoperiod leads to transmission of the flowering signal from the leaves 

to the shoot meristem. Upon perceiving this signal, plants switch from vegetative to reproductive 

growth and initiate flower buds (Taiz and Zeiger, 2010). Circadian rhythms allow plants to 

program molecular events at specific times of the day. Photoperiod perception and circadian 

rhythms work collaboratively to drive the response to photoperiod and regulate the flowering 

process.  

Different species, and even different cultivars of the same species, can have different 

inductive photoperiods, but photoperiodic flowering responses can generally be divided into five 

groups (Thomas and Vince-Prue, 1997). Flowering in SDPs is determined primarily by the 

duration of darkness (Taiz and Zeiger, 2010). They only flower, or flower earlier, when exposed 

to a long, uninterrupted dark period of a critical length, which is at least 12 h for most 

ornamental SDPs (Runkle and Fisher, 2004). LDPs are dark dominant and only flower, or flower 

earlier, when the length of the dark period is less than 8-11 h. Within each category (SDPs, 

LDPs), photoperiodically responsive plants have an obligate (qualitative) or a facultative 

(quantitative) response. Obligate plants only flower under a particular photoperiod, while 

facultative plants can flower under any photoperiod but flower earlier under some particular 

photoperiod. Day-neutral plants (DNPs) flower regardless of the day/night length (Thomas and 

Vince-Prue, 1997). In addition, intermediate day plants only flower when the day length is 

neither too long nor too short, while ambiphotoperiodic-day plants require a dual day length (e.g., 

short days and long days in a sequence) to initiate flowering.  

In the commercial production of many photoperiodic floriculture crops, flowering is 

regulated by photoperiodic (low-intensity) lighting. For day-extension lighting, the threshold 

PPFD to regulate flowering of several herbaceous perennials ranged from 0.05 to 0.4 

 

12 

 

μmol·m−2·s−1, while the saturation PPFD ranged from 0.2 to 1 μmol·m−2·s−1, depending on the 

species (Whitman et al., 1998). Generally, a PPFD of 1-3 μmol·m−2·s−1 during the night is 

recommended to accelerate flowering of LDPs and inhibit flowering of SDPs (Runkle, 2015). 

Using photoperiodic lighting to control flowering is crucial to coordinate marketability of crops 

with the time of high demand. For example, the production of poinsettia and carnation (Dianthus 

caryophyllus), which are targeted for Christmas and Mother’s Day, respectively, utilizes 

photoperiod manipulation (Vince-Prue and Canham, 1983). Photoperiodic lighting can reduce 

production time, which can reduce overhead costs such as heating and cooling, as well as other 

inputs such as water and labor.  

The day length is manipulated in commercial greenhouses in various ways. SD can be 

created using light-excluding materials to cover plants, such as blackout cloth or black plastic. 

However, heat absorbed from the sun can accumulate under the black cloth, which can delay 

flowering of at least some SDP such as poinsettia and chrysanthemum (Chrysanthemum 

morifolium). In contrast, there are two general strategies to create long-days (LDs). To deliver 

long days, short days can be extended to 14-16 h, an approach known as end-of-day (EOD) or 

day-extension (DE) lighting, or lighting can be provided during the middle of the dark period, 

which is known as night interruption (NI) lighting (Armitage, 1994; Vince-Prue and Canham, 

1983). Although DE can effectively regulate flowering, it can require more electricity than NI 

when the natural days are short because operation time can be as long as 6 or 7 h.  

NI lighting is typically delivered during the middle of a long (12-16 h) night (Lane et al., 

1965; Thomas and Vince-Prue, 1997). NI lighting for a short duration (e.g., 30 min) and low 

irradiance (1 μmol·m−2·s−1) is sufficient to keep some (but not all) SDPs vegetative (Lane et al., 

1963; Vince-Prue and Canham, 1983). In contrast, LDPs usually require a longer exposure (>2 h) 

 

13 

 

and/or higher irradiance to trigger flowering. Therefore, a 4-h NI at a minimum intensity of 2 

μmol·m−2·s−1 is recommended to ensure saturation of the flowering response for most SDPs and 

LDPs (Runkle et al., 1998). Typically, an NI from 22:00-02:00 HR is widely used in commercial 

greenhouse crop production (Runkle and Fisher, 2004). However, the specific time period for 

this 4 h NI can be different. For example, dianthus (Dianthus chinensis), zinnia, and geranium 

(Pelargonium zonale) were investigated under 3-5 μmol·m−2·s−1 from white FL with NI during 

three time periods (18:00-22:00 HR, 22:00-02:00 HR, or 02:00-06:00 HR) (Park et al., 2013). In 

the LDP dianthus, NI from 02:00-06:00 HR was the most effective in promoting flowering (by 5 

d) and in delaying flowering of the SDP zinnia (by 14 d). The flowering of geranium was not 

affected by the NI application time, which is not surprising because it is day neutral.  

Cyclic lighting, which is the delivery of light on an intermittent basis, provides an 

opportunity to create LD while reducing electrical cost. There are three ways to create cyclic 

lighting (Runkle, 2007): INC lamps that turn on for 6 min and off for 24 min for 4 to 6 h during 

the night; HPS/MH/LEDs mounted on an irrigation boom that moves over the plants at least 

once every 15 min for at least 4 h; and a stationary HPS lamp with a rotating reflector. Cyclic 

INC with a timer is effective on most SDPs, but may not promote flowering of some LDPs as 

much as continuous NI (Runkle et al., 1998). For example, Blanchard and Runkle (2010) grew 

petunia and black-eyed susan under either a cyclic INC (6 min on every 30 min) or a continuous 

INC, each for 4 h, and at a PPFD of 3 μmol·m−2·s−1. The flowering of petunia and black-eyed 

susan was delayed by 9 to 16 d when grown under cyclic INC compared to continuous INC.  

Different radiation wavebands can have distinctly different roles in the photoperiodic 

regulation of flowering. Generally, R is most effective at inhibiting flowering of SDPs, while the 

flowering of a wide range of LDPs is promoted most when R is combined with FR. For example, 

 

14 

 

LEDs with an R:FR ≥0.66 or INC (with an R:FR= 0.59), both at a photon flux density from 600-

800 nm of 1.3-1.6 μmol·m−2·s−1, inhibited flowering of SDPs when used as a 4-h NI (Craig and 

Runkle, 2013). It delayed flowering of chrysanthemum, dahlia (Dahlia hortensis), and marigold 

by 42 d, 11 d, and 10-20 d respectively. In contrast, an 8-h DE from INC accelerated flowering 

of the LDP henbane (Hyoscyamus niger) by 9 to 18 d compared to cool-white FL with a high 

R:FR (Downs and Thomas, 1982). Similarly, a 2- or 4-h NI or 6-h DE from INC lamps, at a 

PPFD of 2.3 μmol·m−2·s−1, promoted flowering of pansy by up to 5-6 d compared to CFL lamps 

(Oh and Runkle, 2016). The efficacy of B light as a DE or NI varies among species. In a study 

with cyclamen, plants were grown under a 9-h short day with a 4-h NI provided by R, B, or R+B 

at a PPFD of 4 μmol·m−2·s−1 in the greenhouse (Shin et al., 2010). NI with R+B, R, or B 

promoted flowering by 19, 12, and 11 d, respectively, compared to those without NI. However, 

in other SDPs [e.g., cosmos (Cosmos sulfureus)] and LDPs (e.g., dianthus), low intensity (0.6˗1.6 

μmol·m−2·s−1) B light, alone or when added to R light as a 4˗h NI, did not influence flowering, 

and plants were similar to those under a 9-h SD (Meng and Runkle, 2015).  

 

Light Quality 

Plants perceive light quality with four main classes of photoreceptors: phytochromes, 

cryptochromes, phototropins, and UVR8. Phytochromes primarily absorb R and FR light, and to 

a much smaller extent, B light. Phytochromes interact with cryptochromes to regulate multiple 

physiological responses including photomorphogenesis, shade avoidance, circadian rhythms, 

flowering, and phototropism. Cryptochromes and phototropins are considered B light 

photoreceptors because they primarily absorb B, although cryptochromes also absorb ultraviolet 

A (UV-A, 320 to 400 nm). Phototropins are reported to regulate phototropism, chloroplast 

 

15 

 

movement, and stomatal opening. Peak absorption of UVR8 is by ultraviolet B (UV-B, 280 to 

320 nm), and it mediates responses including UV-B acclimation and tolerance and hypocotyl 

growth inhibition (Favory et al., 2009).  

Red and far-red light  

The relative quantum efficiency curve for photosynthesis generated by McCree (1972) 

peaks in the orange-red region. Therefore, R is photosynthetically efficient at promoting plant 

growth. For instance, hydroponic lettuce under monochromatic R LEDs at either a low or high 

PPFD (85 or 170 μmol·m−2·s−1) developed an average of four more leaves compared to plants 

under monochromatic B LEDs at the same PPFD (Yanagi et al., 1996). In addition, plants under 

R LEDs were 77% and 52% taller than those under B LEDs at the high or low PPFD, 

respectively. While the proportion of R light from conventional lamps ranges from 24-40%, the 

percentage is commonly 75-85% in LED arrays developed for horticulture applications (Runkle, 

2016).  

Red and FR light are perceived by phytochromes, which are present in two forms: the R 

light-absorbing form (referred to as Pr) and the FR light-absorbing form (referred to as Pfr) (Taiz 

and Zeiger, 2010). The interconversion between these two forms of each phytochrome in plants 

is known as photoreversibility, and can regulate seed germination, extension growth, and 

flowering. The Pr form is a blue-colored, biologically inactive form, which can convert to the Pfr 

form, a blue-green biologically active form, and vice versa. One form cannot be fully converted 

into the other because their absorption spectra overlap. Depending on the light quality, these two 

forms of phytochromes reach an equilibrium called the photostationary state or phytochrome 

photoequilibrium. The ratio between Pfr and the total phytochrome pool (Pfr/Ptotal) can be an 

indicator of phytochrome states, which can be estimated by the light spectrum (Sager, 1988). 

 

16 

 

By definition, FR is outside of the PAR waveband, and it promotes stem elongation and 

leaf expansion independently from photosynthesis. Unlike R, which is mostly absorbed by leaves, 

most FR is reflected or transmitted, creating an FR-enriched (low R:FR) environment under a 

plant canopy, especially under high plant density. Once a plant perceives a shade signal, stems 

elongate and leaves expand to compete with nearby plants in an attempt to capture more light. 

This phenomenon is called the shade-avoidance response and is mediated by phytochrome. Thus, 

the inclusion of FR light in electrical lights can indirectly promote plant growth. For example, 

the stem dry mass of pepper was 70% greater under R+FR (peaks at 660+735 nm) compared to 

R alone, both at a PPFD of 300 μmol·m−2·s−1 (Brown et al., 1995). In addition, pepper under 

R+FR was 55% taller than under R. In a separate study, geranium, petunia, snapdragon, and 

impatiens were grown under different sole-source lighting treatments from LEDs in a growth 

chamber (Park and Runkle, 2017). LED lighting provided 32 μmol·m−2·s−1 of B light (peak at 

447 nm) either with a total photon flux of 128 μmol·m−2·s−1 from R (peak at 660 nm) and FR 

(peak at 731 nm), or 128 μmol·m−2·s−1 of R with additional intensities of FR light. The stem 

length of geranium, petunia, snapdragon, and impatiens decreased by 41%, 95%, 41%, and 24%, 

respectively, as the estimated phytochrome photoequilibria increased from 0.69 to 0.88. The 

addition of FR to the same PPFD increased total leaf area of geranium and snapdragon by 7% 

and shoot dry weight by 28-50%. The promotion of extension growth from FR is often 

undesirable in ornamental production, but it can be beneficial in cases where stem extension is 

often desired, such as in cut flower production. 

Although plant growth regulators are widely used in commercial floriculture production, 

manipulating light quality, especially R and FR, can be an alternative way to regulate extension 

growth. Generally, the more FR light the plant receives (lower R:FR), the taller the plant. For 

 

17 

 

example, five LDP species [bellflower, coreopsis, pea (Pisum sativum), lobelia (Lobelia 

×speciosa), and pansy] were studied under natural, R-light deficient (Rd) or FR-light deficient 

(FRd) environments with a similar DLI (Runkle and Heins, 2001). Rd had a low R:FR while FRd 

had a high R:FR. The Rd environment increased plant height from the onset of the experiment to 

visible bud (VB) by 65% in bellflower and 23% in pea compared to plants in the natural light 

environment. The FRd environment suppressed stem extension in pea and pansy by 20% and 

14%, respectively, compared to natural light. In addition, stem extension in bellflower, coreopsis, 

pea, and pansy showed a negative linear relationship with the R:FR.  

In addition to the influence on stem elongation, a lack of FR can delay flower initiation 

and development in some LDPs. In the same experiment that Runkle and Heins (2001) 

conducted, FRd filters delayed time to VB by 2 d in bellflower and 14 d in coreopsis, as well as 

delayed flower development of pansy by 21 d compared to natural light. Similarly, petunia 

grown under FR-absorbing films (R:FR=1.51) delayed flowering by 11 d compared to plants 

under clear films (R:FR=1.05) (Ilias and Rajapakse, 2005). In addition, there were 42% fewer 

petunia flowers under FR-absorbing films compared to the clear films, respectively. 

Blue light 

The effects of B light on stem elongation, which are mediated by cryptochrome, are not 

fully understood. Contradictory results have been reported under B light with or without the 

presence of R light. Generally, with the presence of R light, the addition of blue inhibits 

extension growth as its intensity (or proportion) increases. For example, in a study with lettuce, 

spinach (Spinacia oleracea) and mustard (Sinapis alba), plants were grown under MH, HPS, 

MH+tungsten halogen lamps, and HPS+MH at the same PPFD of either 320 or 700 µmol·m–2·s–

1 (Tibbitts et al., 1983). There was a strong negative relationship between B photon flux density 

 

18 

 

and hypocotyl length under both PPFDs. Similarly, stem length (from the cotyledon to the center 

of the apical bud) of lettuce seedlings grown under a PPFD of 80 µmol·m–2·s–1 significantly 

decreased (from 14 cm to 6 cm) as the B light proportion increased from 10% to 40% (Okamoto 

et al., 1996). In another study with five LDP, a blue deficient (Bd) environment increased stem 

length from the onset of the experiment to VB by 100%, 72%, 17%, and 16% in bellflower, 

coreopsis, lobelia, and pea respectively, compared to natural light (Runkle and Heins, 2001). In 

addition, a Bd environment increased stem elongation in coreopsis, lobelia, and pansy by 31%, 

11%, and 10% respectively, from VB to flowering.  

However, cryptochrome˗mediated responses cannot occur without the presence of active 

phytochrome, which is activated by R light (Ahmad and Cashmore, 1997). Therefore, B light 

cannot trigger photomorphogenic responses alone. For example, marigold and salvia grown 

under 90 µmol·m–2·s–1 of monochromatic B LEDs with a 16-h photoperiod were both taller than 

plants grown under FL providing the same PPFD (Heo et al., 2002). In a study with tomato 

seedlings, the hypocotyl length linearly decreased as the B light proportion increased from 10% 

to 75% (Hernández et al., 2016). However, under 100% B light, hypocotyl length was 34-56% 

greater than seedlings grown under 20-75% B treatments.  

Plant growth responses under monochromatic R compared to R+B are species specific. 

When added to R light, B light typically promotes plant growth and increases biomass. For 

example, when lettuce, radish (Raphanus sativus), and spinach were grown under 300 µmol·m–

2·s–1 from R LEDs with B FL lamps (peaks at 660+450 nm, 10% B light), additional light 30 

µmol·m–2·s–1 from B LEDs increased dry mass by 70%, 200%, and 150%, respectively, 

compared to additional light from R LEDs at the same PPFD (Yorio et al., 2001). In ornamental 

plants, the leaf area of both dumbcane (Dieffenbachia amoena) and rubber fig (Ficus elastic) 

 

19 

 

under 30 µmol·m–2·s–1 monochromatic R or B for 50 days was similar compared to natural light, 

but increased by 23% and 26% respectively, under R+B with the same PPFD (Heo et al., 2010). 

However, in a separate study, impatiens, salvia, and petunia seedlings were grown under only R 

LEDs or R+B LEDs at a PPFD of 160 µmol·m–2·s–1 and an 18-h photoperiod. Fresh shoot 

weight of impatiens was 53%-78% greater under R compared to R+B (≥ 50% B) and salvia was 

65-98% greater under R compared to R+B (≥ 25% B). In addition, salvia had a 70-133% greater 

dry weight under R compared to R+B (≥ 25% B) and petunia had 33-91% greater dry weight 

under R compared to R+B (≥ 13% B). These findings indicate that sole-source lighting with a 

mixture of R and B is generally effective at promoting growth for some, but not all crops, and 

responses could depend on the total PPFD delivered, the photon flux densities of B and R, or the 

B:R.  

Anthocyanins are one of the most abundant pigments in higher plants and its synthesis is 

highly dependent on light quality. These compounds can add color to petals, leaves, and fruits, 

and have benefits for human health when consumed. Activation of the anthocyanin pathway 

along with the accumulation of pigmentation involves the regulation of light signals, including B 

light. The anthocyanin concentration of lettuce increased by 31% when grown under white LEDs 

(W) supplemented with B compared to W alone, both at a PPFD of 300 µmol·m–2·s–1 in the 

growth chamber (Li and Kubota, 2009). In a separate study, red-leaf lettuce was grown under 

HPS lamps with or without two different B peak wavebands (455 nm and 470 nm) at 200 

µmol·m–2·s–1 with a 16-h photoperiod in the greenhouse (Samuolienė et al., 2012). The total 

anthocyanin concentration increased by 35-106% under the two HPS+B treatments compared to 

plants grown under HPS alone. These responses could be attributed to the activation of the chs 

 

20 

 

gene, which is a flavonoid biosynthetic gene, under UV-A/blue light and thus promotes 

anthocyanin accumulation in plant tissues (Christie and Jenkins, 1996).  

Green light  

For a long time, green light (G) has been believed to be less effective at driving 

photosynthesis than R or B radiation, for two reasons: it has a relatively higher leaf reflectance 

and canopy penetration rate, and in vitro chlorophyll absorbs much more B and R than G 

radiation (Klein, 1992). However, G has different impacts on plant growth, depending in part on 

the photon flux density delivered. In some plants, G light can promote leaf expansion and shoot 

biomass at a lower intensity but inhibit it at a higher intensity. For example, lettuce grown under 

R+B LEDs with or without 7 µmol·m-2·s-1 supplemental G LEDs at a similar PPFD had no 

significant differences in leaf area and shoot fresh and dry weight (Kim et al., 2004). In a 

separate study, lettuce was grown under an 18-h photoperiod in a growth chamber with R+B, 

R+B with green FL lamps, green FL lamps, and cool-white FL lamps (Kim et al., 2004). The 

fraction of G in those four lighting treatments was 0%, 24%, 51%, and 86% respectively, with a 

total PPFD of 150 µmol·m-2·s-1. As the photon flux density of G increased from 0 to 36 or 77 

µmol·m-2·s-1 (the G fraction increased from 0% to 24% or 51%), lettuce developed 31% or 13% 

larger leaves and had 47% or 14% greater shoot dry weight, respectively. However, as the G 

light intensity continued to increase to 129 µmol·m-2·s-1 (the G fraction increased to 86%), there 

was a 30% and 46% decrease in leaf area and shoot dry weight, respectively, compared to plants 

under no G.  

In addition to affecting plant growth, including G in the spectrum has several potential 

benefits. The addition of G to R+B light allows plants to appear their typical green color rather 

than a purplish color, which makes it easier to visually detect nutritional or physiological 

 

21 

 

disorders, diseases, and/or pest issues (Wollaeger and Runkle, 2014). G light also penetrates 

deeper into a plant canopy, which could lead to less loss of lower leaves under the canopy.  

 

Considerations for Lighting Fixtures 

Fixture efficacy (i.e., electrical efficiency) is among the most important factors to 

consider when selecting lamp types for indoor or greenhouse production of plants. LEDs can 

convert up to 45% of electrical energy into photosynthetic light, depending on the color, while 

traditional single-ended (module) HPS and INC convert about 30% and 6%, respectively, into 

light (Michigan State University, 2011). Among LED types, the most electrically efficient ones 

emit B and R, as well as B LEDs coated with phosphors to create a cool-white spectrum (Nelson 

and Bugbee, 2014). Photosynthetic photon efficacy (PPE) is the appropriate metric to describe 

the PAR photon output divided by the energy input, with the unit of µmol·J-1 (Runkle and 

Bugbee, 2017). In 2014, the PPE of the most efficient LED arrays, double-ended HPS, ceramic 

MH, and FL were 1.70, 1.70, 1.46, and 0.95 µmol·J-1, respectively (Nelson and Bugbee, 2014), 

but the PPE of LEDs continues to increase because of technology improvements. 

In addition to PPE, canopy photon capture efficiency can also influence fixture efficacy. 

As the plant growth area under the lighting fixture decreases, the amount of radiation that strikes 

areas without plants increases (Nelson and Bugbee, 2014), leading to a decreased canopy photon 

capture efficiency. Therefore, greenhouse characteristics need to be considered when selecting 

appropriate lamp types and their distribution of light. Precision luminaires or LEDs with 

adjustable angles can be used to deliver more directional lighting to plants, especially in small-

scale greenhouses with widely spaced benches (Nelson and Bugbee, 2014).  

 

22 

 

In addition to lamp efficacy and the distribution of light, several other factors merit 

consideration when selecting lamps including cost, spectrum, longevity and durability, light 

uniformity, tolerance to damp conditions, supply and cost of electricity, and shading from the 

lighting fixtures (Runkle, 2017). Non-uniform lighting can potentially cause uneven plant growth 

and flowering, creating non-uniform plants with varying watering demands (Fisher et al., 2001). 

FL lamps generally have a large shadow considering its low photon output (Nelson and Bugbee, 

2014) and thus, are not commonly used in greenhouses. In contrast, LEDs (and to a lesser extent, 

HPS and MH lamps) can be mounted on the greenhouse structure and provide light with less 

shading.  

 Maintenance cost for a lighting system is highly dependent on the life expectancy of the 

fixture. The estimated life of conventional lamps is around 10,000 h [for example, 10,000 h for 

double-ended HPS lamps (Nelson and Bugbee, 2014) and 8,000-10,000 for CFL (Tähkämö et al., 

2012)], but LEDs have a life expectancy of as much as 55,000 h if used in an appropriate 

environment (Nelson and Bugbee, 2014). However, the purchase and replacement cost of LEDs 

is usually substantially more expensive than conventional lamps.  

 

23 

 

 

LITERATURE CITED

24 

 

LITERATURE CITED 

 

Ahmad, M., and A.R. Cashmore. 1997. The blue-light receptor cryptochrome 1 shows functional 

dependence on phytochrome A of phytochrome B in Arabidopsis thaliana. Plant J. 
11:421–427. 

 
Albright, L.D., A.-J. Both, and A.J. Chiu. 2002. Controlling greenhouse light to a consistent 

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SECTION II  

REGULATING FLOWERING AND EXTENSION GROWTH OF POINSETTIA USING 

RED AND FAR-RED LEDS FOR END-OF-DAY LIGHTING 

32 

 

Regulating Flowering and Extension Growth of Poinsettia Using Red and Far-red LEDs for End-

of-day Lighting 

 

Mengzi Zhang1 and Erik S. Runkle2 

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

48824 

 

We gratefully acknowledge the USDA-ARS Floriculture and Nursery Research Initiative and 

Philips Lighting for supporting this project, C. Raker and Sons and Syngenta Flowers for the 

donation of plant material, and Nate DuRussel for technical assistance. This work was supported 

by the USDA National Institute of Food and Agriculture, Hatch project 192266. 

 

1Graduate student. 

2Corresponding author. Email address: runkleer@msu.edu.

 

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Abstract 

 

Manipulating light quality is a potential alternative method of regulating plant height in 

the commercial production of ornamental crops. In particular, end-of-day (EOD) lighting with a 

high red (R, 600-700 nm) to far-red (FR, 700–800 nm) ratio (R:FR) can suppress extension 

growth while a low R:FR can promote it. We investigated the impacts of the R:FR and duration 

of EOD lighting in regulating extension growth and flowering of two poinsettia cultivars, ‘White 

Glitter’ and ‘Marble Star’. Plants were grown at 20 °C under 9-h short days with or without EOD 

lighting provided by two types of light-emitting diode (LED) bulbs: R+white+FR (subsequently 

referred to as R+FR) and FR only. The R:FR ratios were 0.73 and 0.04 respectively, and the 

photon flux density between 400 to 800 nm was adjusted to 2-3 μmol·m–2·s–1 at plant canopy. 

The six end-of-day lighting treatments were R+FR or FR for 2 or 4 h, 2h R+FR followed by 2 h 

FR, and 4 h R+FR followed by 2 h FR. We also investigated the impact of a 4-h high-intensity 

(13 μmol·m–2·s–1) EOD FR treatment in the second replication. EOD lighting generally increased 

poinsettia extension growth, with the greatest promotion under the longest lighting periods. 

There were no differences in days to first bract color and days to anthesis when the 9-h day was 

extended by 2 h, but they were delayed under 4- or 6-h EOD treatments except 2-h R+FR + 2-h 

FR and 4-h FR. Four hours of high-intensity EOD FR greatly promoted extension growth and 

delayed or prevented bract coloration in both cultivars. We conclude that EOD lighting promotes 

extension growth of poinsettia and specifically, EOD FR at a low intensity is not perceived as 

long-day signal whereas a higher intensity of FR is perceived and delays flowering.  

 

Keywords: Euphorbia pulcherrima, light quality, morphogenesis, photoperiod, photoreversibility, 

phytochrome 

 

34 

 

Introduction 

Poinsettia (Euphorbia pulcherrima) is a popular potted-flowering crop, especially for the 

Christmas holiday, because of its colorful and showy bracts. Approximately one-third of the 

global poinsettia market is in the U.S., while the remaining market is mostly in Europe (Taylor et 

al., 2011). Poinsettia was the second most valuable potted flowering plant after orchids in 2015, 

accounting for $140 million in wholesale sales and representing 17% of the total value of potted 

flowering plants sold in the U.S. (USDA, 2016). Coordinated control of flowering of poinsettia is 

crucial so that commercial crops are marketable for pre-determined dates for the Christmas 

holiday. It is a short-day (SD) plant with a critical photoperiod of around 12 hours and 20 

minutes (night length of 11 hours and 40 minutes) and thus, is naturally induced to flower in the 

northern hemisphere beginning in late September (Ecke et al., 2004). 

Height control is one of the major challenges in commercial greenhouse production of 

poinsettia. High-quality poinsettia requires the plants to be marketed at specific target heights, 

which is typically 36 to 41 cm for plants grown in 15-cm containers (Taylor et al., 2011). 

Excessively tall poinsettia can develop branches that are more susceptible to breakage and 

increase transportation cost (Clifford et al., 2004). Therefore, plant growth retardants (PGRs) are 

commonly used to suppress stem extension to produce more compact plants. Chlormequat 

chloride, daminozide, uniconazole, paclobutrazol, and flurprimidol, used either as foliar spray or 

drench, are effective at inhibiting stem elongation (Currey and Lopez, 2011; Latimer and 

Whipker, 2013; Lopez and Runkle, 2007). 

For ornamentals, although the promotion of stem growth is usually not desired in 

commercial production, it can be beneficial in some situations, such as in cut flower production 

or when plants are shorter than desired. Gibberellic acid is commonly applied by commercial 

 

35 

 

growers to increase plant height and promote leaf expansion of poinsettia when an excessive 

concentration of a PGR was previously applied (Lopez and Runkle, 2007). However, the use of 

PGRs is restricted to various degrees by different governments (Rajapakse et al., 1999). Different 

strategies can be used as an alternative way to manipulate extension growth, such as cooler night 

than day temperatures, less crop spacing, and avoiding phosphorus deficiency (Ecke et al., 2004; 

Runkle, 2005). 

Light quality manipulation is an alternative strategy to manage plant height, especially 

the ratio of red (R, 600-700 nm) and far-red (FR, 700-800 nm) radiation. R and FR radiation are 

perceived by phytochrome photoreceptors, which have two forms: a R light-absorbing (Pr) form 

and a FR light-absorbing (Pfr) form (Butler et al., 1959). The interconversion between these two 

forms of each phytochrome in plants is known as photoreversibility, and can at least partially 

regulate seed germination, extension growth, and flowering (Borthwick, 1952; Schopfer et al., 

1982; Borthwick and Downs, 1964). Spectral filters have been used experimentally to determine 

the effects of different light qualities on plant height. For example, poinsettia were grown under 

cladding materials of neutral density (control) (R:FR = 1.07) or FR, R, and blue (B, 400-500 nm) 

plastic filters creating an FR deficient, R deficient, and B deficient environment with an R:FR of 

1.74, 0.04, and 1.05, respectively, under sunlight (Clifford et al., 2004). The internode lengths 

were increased by 71% and 9% under R and B deficient filters, respectively, and decreased by 20% 

under the FR deficient filter compared to the control.  

End-of-day (EOD) lighting can also be used to regulate extension growth by 

manipulating the R:FR ratio. Generally, plants elongate under a low R:FR ratio. For example, 

chrysanthemum (Chrysanthemum×morifolium) was grown under a 9-h short day (SD) with EOD 

lighting for 30 min at a photosynthetic photon flux density (PPFD) of 1-3 μmol·m−2·s−1 with 

 

36 

 

either a high (2.4) or low (0.4) R:FR (Lund et al., 2007). Plant height after 3 weeks increased by 

50-75% with EOD lighting at the low R:FR compared to the high ratio. In a separate study, 

impatiens (Impatiens walleriana), geranium (Pelargonium ×hortorum), and petunia (Petunia 

×hybrida) were grown under an 8.5-h day with 30 min of EOD lighting at 20 μmol·m−2·s−1 with 

an R:FR of 0.9 or 8.4 (Randall, 2014). Stem length of impatiens, geranium, and petunia was 

promoted by 29%, 20%, and 44%, respectively, after 21 d under the lower R:FR compared to the 

higher ratio.  

End-of-day lighting can also be used to extend the natural photoperiod and create long 

days to regulate flowering of photoperiodic crops. The threshold irradiance to regulate flowering 

of several herbaceous perennials ranged from 0.05 to 0.4 μmol·m−2·s−1, while the saturation 

irradiance ranged from 0.2-1.0 μmol·m−2·s−1, depending on the species (Whitman et al., 1998). 

Generally, an intensity of 1-3 μmol·m−2·s−1 is recommended to accelerate flowering of long-day 

plants (LDPs) and inhibit flowering of short-day plants (SDPs) (Runkle, 2015).  

Red light is typically effective at inhibiting flowering of SDPs. For example, the 

flowering of chrysanthemum, dahlia (Dahlia hortensis), and marigold (Tagetes erecta) was 

delayed by 42, 11, and 10-20 d when grown under 4-h night interruption (NI) provided by LEDs 

with an R:FR ≥0.66 at a photon flux density of 1.3-1.6 μmol·m−2·s−1 compared to SD control 

(Craig and Runkle, 2013). The inclusion of FR with R radiation, delivered as EOD or NI lighting, 

had little to no effect on regulating flowering of SDPs, but did promote flowering of some LDPs. 

However, a low-intensity of FR alone as an NI is generally not perceived as a long-day (LD) 

signal by LDPs and SDPs. For example, the SDP marigold flowered 10-19 d earlier under SDs or 

NI with only FR compared to NI with a combination of R and FR (Craig and Runkle, 2013).  

 

37 

 

R/FR photoreversibility is one of the characteristic features for phytochrome responses 

and was first discovered in a lettuce (Lactuca sativa) seed germination experiment (Borthwick et 

al., 1952). R light promoted lettuce seed germination, whereas subsequent FR inhibited it. This 

kind of photoreaction response was also established on flower initiation of some model plants. 

For example, a short period of R light as an NI prevented the flower initiation of the SDP 

soybean (Glycine max) and pigweed (Amaranthus caudatus) and induced the flowering of the 

LDP henbane (Hyoscyamus niger), while subsequent FR irradiation reversed the effects of R 

light (Downs, 1956). Although there are three action modes for phytochrome responses, 

photoreversibility is a characteristic of low-fluence response (Li et al., 2011). This suggests that 

phytochrome-mediated flowering responses could be affected by the last NI lighting treatment 

and potentially show photoreversibility in at least some light-sensitive plants.  

The objective of this study was to investigate whether EOD FR would promote extension 

growth without influencing flowering of poinsettia. We also wondered whether flowering could 

be inhibited by delivering FR after R given the phytochrome photoreversibility response. We 

postulated that a low intensity of FR as EOD lighting would promote extension growth but not 

regulate flowering and thus, could be used to increase plant height without influencing crop 

scheduling. We also anticipated that ending EOD lighting with FR would at least partly reverse 

the effects of R light at inhibiting flowering. 

 

Materials and Methods 

Plant materials 

Unrooted cuttings of two cultivars of poinsettia, ‘White Glitter’ and ‘Marble Star’, were 

obtained from C. Raker & Sons (Litchfield, MI) for replication 1 and Syngenta Flowers (Gilroy, 

 

38 

 

CA) for replication 2. They were rooted at 23 °C under a 16-h long day in a propagation 

greenhouse at Michigan State University (MSU, East Lansing, MI). Rooted cuttings were 

transplanted into 15-cm pots containing 70% peatmoss, 21% perlite, and 9% vermiculite potting 

media (SUREMIX; Michigan Grower Products, Inc., Galesburg, MI) and grown in a glass-

glazed greenhouse. All plants were grown under a 16-h photoperiod consisting of natural 

daylengths with supplemental lighting at a PPFD of 70 μmol·m–2·s–1 provided from 400-W 

high-pressure sodium fixtures (LR48877; P.L. Light System, Beamsville, ON, Canada) for 

replication 1 or red+white LEDs (210 W, peak wavelength = 660 nm; GreenPower LED top 

lighting DR/W/MB; Philips, Eindhoven, The Netherlands) for replication 2, from transplanting 

until the start of treatments. Supplemental lighting was automatically switched on when the 

outside ambient solar PPFD was <437 μmol·m–2·s–1, as controlled by a greenhouse 

environmental control system (Integro 725; Priva North America, Vineland, Ontario, Canada). 

Plants were pinched back to six nodes from the soil surface about three weeks after transplanting. 

About four weeks after pinching, 70 (replication 1) or 80 (replication 2) pots of the most uniform 

poinsettias of each cultivar were selected and randomly assigned to seven (replication 1) or eight 

(replication 2) benches with different lighting treatments in the same greenhouse. Plants were 

irrigated as necessary with reverse osmosis water supplemented with a water-soluble fertilizer 

containing (mg·L–1) 250 N, 25 P, 239 K, 154 Ca, 38 Mg, 3.4 Fe, 0.8 Cu, 0.8 Zn, 1.7 Mn, 0.4 B, 

and 0.4 Mo (EC = 2.0). 

Lighting treatments 

 

Ten plants of each cultivar were grown under a 9-h truncated natural day created by 

pulling the black cloth over each bench daily at 1700 HR and opening it at 0800 HR. They 

received six (replication 1) or seven (replication 2) EOD lighting treatments or no EOD 

 

39 

 

treatment (control). The EOD lighting treatments were delivered by R+white+FR LEDs 

(GreenPower DR/W/FR flowering lamps; Philips) (subsequently referred to as R+FR) with an 

R:FR of 0.73, and FR LEDs (GreenPower FR flowering lamps; Philips) with an R:FR of 0.04 

(Fig. 1). The EOD lighting treatments were: 2 h of R+FR or FR (1700 HR to 1900 HR), 2 h of 

R+FR followed by 2 h of FR (1700 HR to 1900 HR + 1900 HR to 2100 HR), 4 h of R+FR or FR 

(1700 HR to 2100 HR), and 4 h of R+FR followed by 2 h of FR (1700 HR to 2100 HR + 2100 HR to 

2300 HR) (Table 1). Layers of wire mesh were placed over the lamps so that the average photon 

flux density (from 400 to 800 nm) at plant canopy was between 2 and 3 μmol·m–2·s–1. In addition, 

4 h of FR (1700 HR to 2100 HR) (subsequently referred to as H-FR) at 10-15 μmol·m–2·s–1 was 

provided as an additional treatment in replication 2. Photon flux measurements were made with a 

spectroradiometer (PS-200; StellerNet, Inc. Tampa, FL) at four representative positions 18 cm 

above the bench. The average photon flux density of the R+FR, FR, and H-FR lighting 

treatments were 2.6, 2.5, and 13.4 μmol·m–2·s–1, respectively.  

Environmental conditions 

Plants were grown at a constant temperature setpoint of 20 °C. Whitewash (Kool Ray 

Classic; Continental Products Co., Euclid, OH) was applied to the greenhouse glazing to deliver 

a more similar daily light integral (DLI) between the two replications. The PPFD at bench height 

was recorded by a quantum sensor (LI-190SA; LI-COR, Lincoln, NE). Air temperature was 

recorded on benches by aspirated thermocouples (Type E; Omega Engineering, Stamford, CT) 

near canopy height. Air temperature and the PPFD were collected by a data logger (CR-10; 

Campbell Scientific, Logan, UT) every ten seconds and average values were recorded hourly. 

Daily temperatures were checked routinely and averages are reported in Table 1. The average 

DLI during the study was 7.4 and 10.5 mol·m–2·d–1 for replication 1 and 2, respectively.  

 

40 

 

Plant measurements 

Plant height (from the substrate surface to the tallest meristem) was measured at the 

initiation of the lighting treatments. Height (from the substrate surface to the bottom of the 

flower bud) was measured at anthesis for flowering plants, and for non-flowering plants, height 

was recorded after 15 weeks of treatments. Date of first bract color (when the first leaf on each 

plant showed partial color), date of first anthesis (when the first flower showed pollen tubes), and 

node number were also recorded. Average internode length was calculated by final plant height 

divided by node number only in replication 2. 

Experimental design and data analysis 

The experiment was performed twice, with treatments initiated in October 2016 and April 

2017. The experiment was a randomized complete block design with subsamples. Two 

replications were considered two blocks to account for seasonal changes in environmental 

conditions. Each bench was regarded as the experimental unit for the EOD lighting treatment. 

Within the experimental unit, ten individual plants per cultivar were the subsamples. Data were 

pooled from two replications and were analyzed with the SAS (SAS Institute, Inc., Cary, NC) 

mixed-model (PROC MIXED) and glimmix-model (PROC GLIMMIX) procedures. Pairwise 

comparisons between treatments were performed using Tukey’s honest significant difference test 

at P ≤ 0.05. 

 

Results 

All plants of both cultivars showed bract coloration and reached anthesis except for some 

plants under the 4-h R+FR, 4-h R+FR + 2-h FR, and 4-h H-FR treatments (Table 2). The general 

trends for bract coloration and anthesis were similar in both cultivars (Fig. 2). When the day was 

 

41 

 

extended for ≤4 h of lighting, days to first bract coloration and anthesis were similar to plants 

grown under the 9-h SD (control), except for plants under the 4-h R+FR treatment. The 4-h 

R+FR delayed bract coloration and anthesis by 34 and 29 d in ‘Marble Star’ and 28 and 23 d in 

‘White Glitter’, respectively, compared to the SD control, whereas the 4-h FR EOD did not delay 

flowering. In addition, the 4-h R+FR + 2-h FR and 4-h H-FR treatments delayed first bract 

coloration of ‘Marble Star’ by 45 and 55 d and anthesis by 34 and 38 d, respectively, compared 

to the SD control. Similarly, in ‘White Glitter’, bract coloration and anthesis were delayed by 17 

and 34 d, respectively, under 4-h R+FR + 2-h FR EOD lighting. The days to bract coloration and 

anthesis are statistically similar under 4-h R+FR and 4-h R+FR + 2-h FR, therefore, no 

phytochrome photoreversibility was observed. Under the 4-h R+FR + 2-h FR and 4-h H-FR 

lighting treatments, ‘Marble Star’ reached anthesis shortly after first bract coloration, and the 

interval was much shorter than for plants under the other lighting treatments.  

Among the plants under treatments that fully flowered, stem length generally increased as 

the duration of light increased in both cultivars (Fig. 3). Plants of both cultivars were tallest 

under the longest EOD lighting treatment with a low-light intensity (4-h R+FR + 2-h FR). The 

increase in stem length at the end of the experiment for ‘Marble Star’ and ‘White Glitter’ was 

161% and 207% greater, respectively, under the longest EOD lighting compared to plants grown 

under the SD control. In addition, extension growth at anthesis of ‘Marble Star’ under the 4-h 

R+FR was 125% greater than that of the control. Stem elongation of 4-h H-FR for 15 weeks was 

statistically similar with those under 4-h R+FR followed by 2-h FR. Therefore, 4-h of high-

intensity FR was as effective as the 6-h EOD lighting treatment in increasing plant height, and 

thus plants were also the tallest among all the lighting treatments. 

 

42 

 

‘Marble Star’ had an average of 14 or 15 nodes under all treatments 15 weeks after the 

initiation of the treatments except for the 4-h R+FR, 4-h R+FR + 2-h FR, and 4-h H-FR 

treatments, which developed 20, 20, and 27 nodes, respectively, before anthesis (Fig. 3). 

Similarly, ‘White Glitter’ had an average of 15 to 19 nodes overall but developed 6 and 9 more 

nodes under the 4-h R+FR + 2-h FR and 4-h H-FR treatments, respectively, compared to the SD 

control. Average internode length of most EOD lighting treatments was significantly greater than 

plants grown under the 9-h day. ‘Marble Star’ and ‘White Glitter’ grown under 2-h R+FR + 2-h 

FR developed a 22% and 19% longer internode, respectively, compared to those under 2-h R+FR. 

Similarly, ‘White Glitter’ developed a 21% greater internode length, respectively, when grown 

under 4-h R+FR + 2-h FR compared to 4-h R+FR. In addition, the average internode length for 

both cultivars was significantly greater for plants under 4-h H-FR compared to 4-h FR, with a 

promotion of 13% in ‘Marble Star’ and 30% in ‘White Glitter’. 

 

Discussion 

We tested the hypothesis that flowering of the SDP poinsettia would not be influenced by 

delivering FR light during an otherwise long night. Several LDPs including black-eyed susan 

(Rudbeckia hirta) and fuchsia (Fuchsia hybrida) were reportedly insensitive to 4-h NI with FR 

alone at 1.3-1.6 μmol·m–2·s–1 for accelerating flowering compared to NI with R+FR at the same 

photon flux density (Craig and Runkle, 2016). In SDPs, the flowering time of chrysanthemum, 

dahlia, and marigold exposed to a 4-h NI with 1.3 to 1.6 μmol·m–2·s–1 of FR alone were similar 

to that of the SD control treatment (Craig and Runkle, 2013). Consistent with these responses, 

this study showed that both cultivars of poinsettia grown under a 9-h day with 2- or 4-h EOD 

lighting of low-intensity FR alone showed bract color and reached anthesis at a similar time as 

 

43 

 

the SD control. In addition, the previous study showed that a moderate to high R:FR (≥ 0.66) was 

the most effective NI for the SDPs for delaying the flowering (Craig and Runkle, 2013). In this 

study, 4-h EOD lighting with a R:FR of 0.73 delayed the poinsettia bract color and anthesis 

development. Therefore, a low-intensity of FR alone as EOD lighting was not perceived as an 

LD signal in poinsettia whereas a mixture of R and FR inhibited flowering. 

Interestingly, the flowering percentage decreased in both cultivars when grown under 4-h 

EOD lighting of 13 μmol·m–2·s–1 FR compared to the SD control. Days to first bract color and 

anthesis of ‘Marble Star’ were significantly delayed and ‘White Glitter’ still remained vegetative 

when the experiment ended. In contrast, Higuchi et al. (2012) grew the SDP chrysanthemum 

under cool white fluorescent tubes at 150 μmol·m–2·s–1 for 12 hours per day with or without a 4-

h NI provided by either R or FR monochromatic LEDs. A 62 μmol·m–2·s–1 of FR NI did not 

inhibit the flowering of chrysanthemum. These results suggest that the reproductive process of 

poinsettia is relatively sensitive to FR radiation and even a moderate dose of FR is sufficient to 

inhibit the flowering of poinsettia but not in other SDPs such as chrysanthemum.  

The critical photoperiod of poinsettia is around 12 hours and 20 minutes (Ecke et al., 

2004). As expected, plants grown with 2 h of R+FR EOD lighting, creating an 11-h photoperiod, 

flowered similar to those under the SD control. We also expected that the 2-h R+FR + 2-h FR 

EOD lighting treatment would create an SD because the low-intensity FR would not be 

perceived as an LD signal, and indeed, that occurred. Surprisingly, plants under the 4-h R+FR 

and 4-h R+FR + 2-h FR treatments (creating a 13-h or 15-h photoperiod, respectively) flowered, 

although some plants did not flower in Rep. 2 and flowering was delayed. However, unlike 

plants under SD conditions, we noticed that the primary stems of some plants grown under ≥ 4-h 

EOD lighting remained vegetative, while bract color and anthesis occurred on lower branches 

 

44 

 

that were at least partly shaded by the primary stems (data not collected). We postulate that the 

leaves on the primary stem prevented R light from reaching lower branches and thus, those 

branches were induced to flower.  

The flowering of plants grown under 4-h R+FR EOD lighting was delayed, and we did 

not observe a reversing effect of subsequent 2-h FR EOD lighting that one might expect with 

phytochrome photoreversibility. Phytochrome photoreversibility responses are reportedly low-

fluence responses with saturating photon fluxes of around 1 to 1,000 μmol·m–2 (Li et al., 2011). 

In this study, the total photon fluence of R light during one night was 11,500 to 17,500 μmol·m–2, 

which exceeded the response saturation range. In addition, once receiving the R light, 

phytochrome would change from the biologically inactive Pr form to biologically active Pfr form. 

The Pfr transition action can vary from slow to fast in responses such as stem elongation, seed 

germination, and flower induction (Kendrick and Kronenberg, 2012). For example, the transition 

action can vary from 9 h of darkness between R and FR required for a 50% loss of 

photoreversibility in germination of lettuce seeds, to 30-60 min before Pfr drops and induces 

flowering of the SDP cocklebur (Xanthium stunarium) (Borthwick et al., 1954; Kendrick and 

Kronenberg, 2012; Salisbury, 1981). Therefore, 2 h of FR at a low fluence immediately 

following R may not be a sufficient dose to reverse the flowering inhibition by 4-h of R light.  

Generally, plant height increased as the lighting duration increased, with the greatest 

promotion under the longest photoperiod or under a higher intensity of EOD FR lighting. Longer 

stems can result from longer internodes, more nodes because of the delay in the development and 

flower initiation, or both. Craig and Runkle (2013) reported that SDPs grown under NI with a 

moderate R:FR were taller than those grown under NI with FR alone because plants under FR-

only NI entered the reproduction stage earlier in development. Similarly, chrysanthemum and 

 

45 

 

marigold under 4-h NI that delayed the flowering resulted in significantly taller plants compared 

to the SD treatment (Meng and Runkle, 2015). In this study, both cultivars of poinsettia grown 

under 4-h FR initiated the flowering process similar to that of plants under the SD control, 

whereas flowering under 4-h R+FR, 4-h R+FR + 2-h FR, and 4-h H-FR was delayed and plants 

developed more nodes before flowering. Therefore, the flower initiation was later in the 

development and the overall extension of plants under those treatments was greater than the 

others.  

Delivering low-intensity EOD FR successfully promoted poinsettia extension growth 

without delaying flowering. Promotion of stem length can be achieved by providing EOD 

lighting, and a low R:FR is typically more effective than a high R:FR. For example, the 

hypocotyl length of tomato seedlings under 12-min EOD lighting treatment, regardless of light 

quality, was 20-44% greater than that without an EOD treatment (Chia and Kubota, 2010). In 

addition, the hypocotyl length under EOD lighting with a R:FR of 0.05 was 20% greater than 

under EOD lighting with a R:FR of 0.47. In another poinsettia study, two cultivars (‘Christmas 

Spirit’ and ‘Christmas Eve’) were exposed to R or FR EOD lighting for 30 min at 10 μmol·m–

2·s–1 following a 10-h SD (Islam et al., 2014). The internode length after 11 weeks was 55-107% 

greater under EOD-FR compared to EOD-R. In our study, plant height in ‘Marble Star’ under the 

2-h EOD lighting treatments (regardless of light quality) was statistically similar to the SD 

control, while in ‘White Glitter’, plants were significantly taller. In addition, no differences in 

height and internode length were observed for both cultivars grown under 2- and 4-h EOD-FR 

compared to R+FR. Therefore, the sensitivity of response to R/FR in stem elongation might be 

cultivar specific. This is supported by Rajapakse et al. (1993), who reported that a 15 min EOD-

 

46 

 

FR treatment did not influence plant height of one cultivar of chrysanthemum but increased 

height by ~50% in another cultivar.  

 

47 

 

 

APPENDIX

48 

 

Table II-1. Means (±SD) of greenhouse air temperature during the two experimental replications 
(Rep.) as measured by aspirated thermocouples. Plants were grown under a 9-h short day (15-h 
dark period; black bar) without or with end-of-day lighting from red+white+far-red (R+FR; 
white bar) or far-red (FR; grey bar) light-emitting diodes at 2 to 3 μmol·m–2·s–1 for 2 or 4 h. In 
Rep. 2, an additional treatment of a high-intensity far-red (H-FR) at 13 μmol·m–2·s–1 was 
included. 

Average air temperature (°C) 

Lighting treatment 
9-h control 
2-h R+FR 
2-h FR 
2-h R+FR + 2-h FR 
4-h R+FR 
4-h FR 
4-h R+FR + 2-h FR 
4-h H-FR 

 

Rep. 1 

20.0 ± 1.5 
20.0 ± 1.9 
19.8 ± 2.2 
19.4 ± 1.3 
19.9 ± 1.4 
19.8 ± 1.3 
19.4 ± 1.4 

-- 

 

 

 

 

 

 

 

 

Rep. 2 

23.7 ± 4.2 
21.5 ± 3.1 
21.9 ± 3.2 
21.7 ± 3.2 
22.0 ± 3.1 
22.0 ± 3.1 
21.9 ± 3.3 
23.6 ± 4.1 

49 

 
 

 

 

 

Table II-2. Effect of end-of-day (EOD) lighting treatments on the percentage of plants with bract color and reaching anthesis of two 
cultivars of poinsettia (‘Marble Star’ and ‘White Glitter’). See Table 1 for descriptions of lighting treatments. 

Bract color 

Anthesis  

Rep 1 

Rep 2 

Rep 1 

Rep 2 

‘Marble 

Star’ 
100 
100 
100 
100 
100 
100 
80 
-- 

‘White 
Glitter’  

100 
100 
100 
100 
100 
100 
100 
-- 

‘Marble 

Star’ 
100 
100 
100 
100 
40 
100 
30 
40 

‘White 
Glitter’  

100 
100 
100 
100 
90 
100 
40 
20 

‘Marble 

Star’ 
100 
100 
100 
100 
100 
90 
50 
-- 

‘White 
Glitter’  

100 
100 
100 
100 
100 
100 
80 
-- 

‘Marble 

Star’ 
100 
100 
100 
100 
30 
100 
20 
30 

‘White 
Glitter’  

100 
100 
100 
100 
30 
100 
20 
0 

Lighting treatment 

 

 

 

 

 

 

 

* 

*Only performed in Rep 2.   
 
 

 

50 

 

Figure II-1. Spectral distribution of two types of end-of-day (EOD) lighting treatments delivering 
red (R, 600 to 700 nm), and/or far-red (FR, 700 to 800 nm) radiation. The photon flux density of 
the two EOD lighting treatments was 2 to 3 µmol·m–2·s–1.  

 

 

 

 

 

51 

 

 
Figure II-2. Days to first bract color and anthesis of two cultivars of poinsettia grown under a 9-h 
short-day (control) and seven red and/or far-red end-of-day lighting treatments (see Table 1 for 
treatment descriptions). All treatments were delivered at a photon flux density (400 to 800 nm) 
of 2 to 3 µmol·m–2·s–1, except 4-h H-FR, which was delivered at 13 µmol·m–2·s–1. All data were 
pooled from two replications, except 4-h H-FR, which was only conducted in the second 
replication. Pairwise comparison was made within each cultivar. Means sharing a letter are not 
statistically different by Tukey’s honest significant difference test at P ≤ 0.05. Error bars indicate 
standard error.

 

52 

 

Figure II-3. Increase in plant height, node number, and average internode length of two cultivars 
of poinsettia grown under a 9-h short-day (control) and seven end-of-day red and/or far-red 
lighting treatments (see Table 1 for treatment descriptions). All treatments were delivered at a 
photon flux density (400 to 800 nm) of 2 to 3 µmol·m–2·s–1, except 4-h H-FR, which was 
delivered at 13 µmol·m–2·s–1. Pairwise comparisons were made within each cultivar. Means 
sharing a letter are not statistically different by Tukey’s honest significant difference test at P ≤ 
0.05. Error bars indicate standard error. 

 

 

53 

 

 

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54 

 

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57 

 

 

SECTION III  

MANIPULATING GROWTH, COLOR, AND TASTE ATTRIBUTES OF FRESH CUT 

LETTUCE BY GREENHOUSE SUPPLEMENTAL LIGHTING

58 

 

Manipulating Growth, Color, and Taste Attributes of Fresh Cut Lettuce by Greenhouse 

Supplemental Lighting 

 

Mengzi Zhang1, Catherine M. Whitman, and Erik S. Runkle2 

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

48824 

 

We gratefully acknowledge RecoveryPark and the Michigan Economic Development 

Corporation for financially supporting this project and Nate DuRussel for technical assistance. 

This work was supported by the USDA National Institute of Food and Agriculture, Hatch project 

192266. 

 

1Graduate student. 

2Corresponding author. E-mail address: runkleer@msu.edu.

 

59 

 

Abstract 

Supplemental lighting (SL) is often used in greenhouse lettuce production to increase 

yield when the solar daily light integral (DLI) is low. High-pressure sodium (HPS) fixtures are 

traditionally used to deliver SL, but light-emitting diodes (LEDs) have become a viable, more 

energy-efficient replacement. Red (R, 600-700 nm) and blue (B, 400-500 nm) light are especially 

effective in promoting photosynthesis and increasing crop growth, while far-red (FR, 700-800 

nm) light has the potential to increase yield by promoting leaf expansion. We performed three 

greenhouse experiments with HPS or LED fixtures to determine the impacts of SL on lettuce 

growth, coloration, and sensory attributes. We grew lettuce at 20 °C without or with 

supplemental lighting from HPS or R+B LEDs with a 16-h photoperiod (Expts. 1 and 2). In Expt. 

3, we grew plants without or with HPS SL without or with R+FR photoperiodic lighting at the 

end of the day during the growing (first) stage, and/or SL from R+B or B LEDs during the 

finishing (second) stage. SL enhanced lettuce growth and yield by 32-104% regardless of light 

quality. Four hours of 2 to 5 μmol·m−2·s−1 end-of-day R+FR lighting had few effects on lettuce 

growth or quality attributes. Compared to no SL, lettuce provided with R+B LED SL for 7 d 

before harvest was 27% darker, 76-82% redder, and 39-55% less yellowish. Consumers tended 

to prefer lettuce grown under natural daylight more than plants grown with SL, and had a less 

bitter, sweeter, and a more likable aftertaste. We conclude that providing different types of 

greenhouse SL at different growing stages can improve lettuce growth and coloration but could 

be less preferred by consumers.  

 

Keywords: Blue light, end-of-day lighting, far-red light, Lactuca sativa, light quality, red light 

 

 

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Introduction 

Lettuce (Lactuca sativa) is the second most consumed vegetable (only to potato) in the 

American diet (Produce for Better Health Foundation, 2015), of which 45% is leaf and romaine 

lettuce (USDA and Economic Research Service, 2018). Most lettuce is commercially produced 

outdoors in desert regions of CA and AZ, but an increasing amount is grown in controlled 

environments. Lettuce growth can be slow when the daily light integral (DLI) is low, such as in 

greenhouses during the winter in northern latitudes. Therefore, supplemental lighting (SL) is 

often used to increase the DLI for greenhouse lettuce production. Typically, 12 to 16 h of SL at 

50-75 μmol·m−2·s−1 for lettuce seedlings is recommended (Dorais and Gosselin, 2002). The 

recommended DLI for finished plant production in greenhouses is somewhat cultivar specific, 

with 17 mol·m−2·d−1 for Boston bibb lettuce, but a maximum of 15 mol·m−2·d−1 for other 

cultivars to prevent a calcium deficiency that results in tipburn (Brechner and Both, 1996).  

High-pressure sodium (HPS) fixtures are widely used for SL in greenhouses while arrays 

of light-emitting diodes (LEDs) are increasingly being used because of their technological 

advantages. LEDs have a significantly longer lifetime than conventional fixtures, ranging from 

20,000 to 55,000 h compared with the lifetime for HPS bulbs of 10,000 h (Nelson and Bugbee, 

2014). The photosynthetic photon efficacy of the most efficient LED and double-ended HPS 

fixture in 2014 was similar (1.7 µmol·J–1), but the photosynthetic photon efficacy of LEDs 

continues to increase because of technology improvements (Fisher et al., 2017). For example, in 

2018, the two LEDs tested with the greatest efficacy were approximately 50% greater than a 

1,000-W double-ended HPS fixture (Radetsky, 2018). In addition, LEDs can provide a crop-

specific spectrum to elicit desirable photomorphogenic and quality attributes. However, the 

relatively high investment cost for LEDs is the greatest obstacle to expanding implementation by 

 

61 

 

the commercial greenhouse industry. A hybrid lighting system that combines some of the 

features of LEDs with conventional HPS fixtures could be an economically efficient option (Olle 

and Viršilė, 2013).   

Far-red (FR, 700 to 800 nm) light regulates leaf morphology and promotes leaf expansion. 

Lettuce grown under white fluorescent lamps with supplemental FR developed a 44% longer and 

15% wider leaf, resulting in a 65% larger leaf compared to without FR, both at a photosynthetic 

photon flux density (PPFD) of 300 µmol·m−2·s−1 (Li and Kubota, 2009). Providing a low dose 

and a short period of FR as end-of-day (EOD) lighting can also increase leaf area in some 

species. For example, a 5 min EOD-FR pulse for 18 d elongated tobacco (Nicotiana tabacum) 

leaves by 11% (Kasperbauer and Peaslee, 1973). In Easter lily (Lilium longiflorum), leaf area of 

‘Nellie White’ increased by 17% when exposed to 1-h EOD-FR at a photon flux density of 0.2 to 

6.5 µmol·m−2·s−1 compared to those without EOD-FR (Blom et al., 1995). Therefore, we 

postulated that low-fluence EOD-FR to increase leaf expansion early in development, followed 

by high-intensity SL to increase biomass and coloration, could be an economic strategy to 

improve crop quality in greenhouse lettuce production.  

The biomass or harvestable yield of many horticultural crops is related to the average 

DLI. For example, the biomass of Boston-type lettuce increased up to 270%, along with a greater 

head firmness and a ~30% reduction of production cycle length, under continuous greenhouse 

SL at a PPFD of 100 μmol·m−2·s−1 from HPS fixtures compared to without SL (Gaudreau et al., 

1994). In another study, the fresh mass of kale (Brassica oleracea) grown indoors increased from 

8 to 93 grams per plant as the DLI increased from 10.8 to 43.2 mol·m−2·d−1 (Lefsrud et al., 2006). 

Similarly in romaine lettuce, shoot fresh weight was enhanced by 14%, 24%, and 27% as the 

 

62 

 

DLI increased from 5 to 10, 20, and 30 mol·m−2·d−1, respectively, when grown indoors (Fu et al., 

2012).  

Anthocyanins are one of the most abundant pigments in higher plants, and they contribute 

to leaf coloration as well as antioxidant concentration. Their biological synthesis is highly 

dependent on light quality, especially blue (B; 400-500 nm) and ultraviolet (UV; 280- 400 nm) 

radiation through cryptochrome responses (Mancinelli, 1985). For example, the total anthocyanin 

concentration in red-leaf lettuce grown under HPS fixtures increased by 35-106% with 

supplemental B LEDs at 200 µmol·m–2·s–1, depending on the peak wavelengths (455, 470, 505, 

535 nm), compared to without supplemental B (Samuolienė et al., 2012). Similarly in grapes 

(Vitis labrusca × V. vinifera), the total anthocyanin concentration of the skin increased by 46% 

when measured 77 d after full bloom and when treated with 6-h day extension lighting at 13-17 

μmol·m−2·s−1 from B LEDs (peak = 450 nm) compared with red (R; 600-700 nm) LEDs (peak = 

660 nm) (Kondo et al., 2014). However, other studies indicate a combination of R and B are 

more effective than monochromatic B in increasing anthocyanin concentration. For example, 

red-leaf lettuce ‘Ruby Sky’ was grown under monochromatic R, B, or R+B LEDs at a 

supplemental PPFD of 100 μmol·m−2·s−1 under a 16-h photoperiod in a greenhouse (Owen and 

Lopez, 2015). Maximum leaf coloration during the finishing stage occurred 7 d earlier under 

R+B than R or B alone. In addition, the relative redness (a* value) of ‘Ruby Sky’ under R+B 

was 12-151% higher than R or B LEDs alone at the end of the experiment.  

The objectives of this study were to 1) assess the increase in growth of lettuce from SL 

provided by HPS or LED fixtures, 2) determine if SL influences consumer sensory properties of 

lettuce during greenhouse production, and 3) quantify and compare the impacts of different types 

of greenhouse SL during the different growth phases on the quality and quantity of cut lettuce 

 

63 

 

production.   

 

Materials and Methods 

Expt.1: Supplemental greenhouse lighting 

Plant materials 

 

Seeds of five lettuces (‘Salanova Red Incised’, ‘Salanova Red Sweet Crisp’, ‘Salanova 

Green Incised’, ‘Salanova Green Sweet Crisp’, and ‘Salanova Green Butter’) were obtained from 

Johnny’s Selected Seeds (Winslow, ME). They were sown in 128-cell plug trays and grown in a 

research greenhouse at 20 °C under a 16-h photoperiod (from 6:00 to 22:00 HR) until ready for 

transplant. The most uniform 30 seedlings of each cultivar were selected and then transplanted 

into 11-cm pots containing 70% peatmoss, 21% perlite, and 9% vermiculite potting media 

(SUREMIX; Michigan Grower Products, Inc., Galesburg, MI) and placed on greenhouse 

benches and under lighting treatments (Table 1). Plants were irrigated as necessary with reverse 

osmosis water supplemented with a water-soluble fertilizer containing (mg·L–1) 125 N, 12 P, 100 

K, 65 Ca, 12 Mg, 1 Fe and Cu, 0.5 Mn and Zn, 0.3 B, and 0.1 Mo (MSU RO Water Special; 

GreenCare Fertilizers, Inc., Kankakee, IL). 

Environmental conditions  

The experiment was performed at 20 °C in three nearly identical compartments of the 

Michigan State University (MSU) Plant Science Research Greenhouses and light fixtures were 

installed in two of the compartments. Roof vents and fans were controlled by a greenhouse 

environmental control system (Integro 725; Priva North America, Vineland, Ontario, Canada). 

One heavy whitewash (Kool Ray Classic; Continental Products Co., Euclid, OH) was applied in 

late March to the exterior of the greenhouse to reduce the ambient light intensity. The PPFD at 

 

64 

 

plant canopy height was measured in each treatment and recorded by a quantum sensor (LI-

190SA; LI-COR, Lincoln, NE). Air temperatures were recorded on benches in each treatment by 

aspirated thermocouples (Type E; Omega Engineering, Stamford, CT) near canopy height. 

Temperature and the PPFD were collected by a data logger (CR-10; Campbell Scientific, Logan, 

UT) every ten seconds and average values were recorded hourly. Average daily temperature and 

DLI are reported in Table 1. 

Lighting treatments 

After transplanting, plants were randomly placed under ambient daylight (no SL), 

supplemental HPS, or supplemental R+B LEDs until harvest (for 34 and 38 d in the two 

experimental replications). Four HPS fixtures (400W; LR48877; P.L. Light System, Beamsville, 

ON, Canada) and six R+B LED research fixtures (Light Speed USA; Grand Rapids, MI) were 

placed ~ 1.4 m and ~ 1.2 m, respectively, above the benches in each of two compartments. 

Fixture heights were adjusted to provide a similar PPFD in the two SL treatments. SL was 

turned on automatically when the outside ambient solar light level fell below 223 W·m–2 (a 

PPFD of 437 μmol·m–2·s–1). The average PPFD (± SD) from the SL treatments was 84 ± 8 

µmol∙m–2∙s–1 under the HPS fixtures and 78 ± 6 µmol∙m–2∙s–1 under the LEDs. Photon flux 

measurements of the lighting fixtures were made on the greenhouse benches with a 

spectroradiometer (PS-200; Apogee Instruments Inc., Logan, UT; Fig. 1). 

Plant measurements and experimental design. The experiment was performed twice, with the 

treatments initiated in Mar. and Apr. 2016. The experiment was a randomized complete block 

design with subsamples. Two replications were considered two blocks to account for seasonal 

changes in environmental conditions. After harvest, leaf number (leaves ≥ 7 or 3.5 cm for 

‘Salanova Green Butter’ in replication 1 and 2, respectively, and leaves ≥ 3 cm for other cultivars 

 

65 

 

in both replications) and shoot fresh weight (using an A&D Weighing GR-1000 balance, San 

Jose, CA) were recorded. In addition, the leaf area of the ten largest leaves was measured by a 

leaf area meter (LI-3000; LI-COR) and leaf color was measured using a colorimeter (CR-

400/410; Konica Minolta Sensing Americas Inc., Ramsey, NJ) after calibration to a standard 

white plate (L* = 97.9, a* = –0.56, b* = 2.88). L* indicates the lightness and darkness (white: L* 

= 100; black: L* = 0). Chromametric a* and b* are the ratio of greenness to redness (green: a* = 

–60; red: a* = +60) and blueness to yellowness (blue: b* = –60; yellow: b* = +60), respectively. 

The average leaf size was that measured for the ten largest leaves and the average leaf fresh 

weight was calculated by shoot fresh weight divided by leaf number. Data were analyzed for two 

replications separately with the SAS (SAS Institute, Inc., Cary, NC) mixed-model (PROC 

MIXED) procedure. Pairwise comparisons between treatments were performed using Tukey’s 

honest significant difference test at P ≤ 0.05 within each replication. 

 

Expt. 2: Sensory evaluation 

Plant materials and growth conditions 

Seeds of ‘Salanova Green Incised’ and ‘Salanova Green Sweet Crisp’ were obtained from 

Johnny’s Selected Seeds and sown into 128-cell trays on 25 Aug. 2016. Growth conditions, 

lighting treatments, and environmental monitoring were as described in Expt. 1 unless otherwise 

noted. To improve crop uniformity, plants were cut back on 20 Oct. and again on 10 Nov. From 

11 Nov. to the harvest day, the average DLIs were 3.8 mol∙m–2∙d–1 in the non-supplemented 

daylight treatment and 9.1 mol∙m–2∙d–1 in the SL treatments. On average, SL contributed ~58% 

of the DLI in the HPS and R+B LED treatments. Average daily air temperature was 19.6 °C and 

varied by < 0.3°C among the greenhouse compartments. 

 

66 

 

Sample preparation and consumer sensory evaluation 

Immediately after harvest on 01 Dec. 2016, lettuce was rinsed in reverse-osmosis water, 

dried in a salad spinner, cut into bite-size pieces, stored at 4 °C in plastic bins, and remained 

refrigerated overnight. Consumer acceptance testing was conducted the following day at the 

Sensory Evaluation Laboratory in the Department of Food Science and Human Nutrition at MSU. 

Eighty-four panelists recruited by E-mail and flyers were all product consumers. Portions of 

lettuce were equivalent to approximately 4 to 6 cm2 and were served to panelists in random order 

with 3-digit blinding codes following the protocol described by Szczygiel et al. (2017). A 9-point 

hedonic scale (1= dislike extremely; 9= like extremely) was used to measure overall 

acceptability, appearance, flavor, and aftertaste, while a 5-point hedonic scale (1= not at all 

bitter/sweet; 5= extremely bitter/sweet) was used to measure bitterness and sweetness. The taste 

test was performed once and consumer sensory data were collected using SIMS (vs. 6.0) sensory 

software (SIMS, Berkeley Heights, NJ, USA) as Szczygiel et al. (2017). Data were analyzed as 

in Expt. 1.  

 

Expt. 3: Supplemental greenhouse lighting at different growing stages 

Plant material 

Seeds of three lettuces (‘Salanova Red Incised’, ‘Salanova Red Sweet Crisp’ and 

‘Salanova Green Incised’) were obtained from Johnny’s Selected Seeds and sown in 98-cell plug 

trays and grown as previously described for 21 d after seed sow. The most uniform 70 seedlings 

of each cultivar were transplanted and assigned to different lighting treatments (Table 1), which 

consisted of a growing stage for three weeks and a finishing stage for ten days before harvest, 

with ten plants for each treatment.  

 

67 

 

Environmental conditions 

The experiment was performed in five nearly identical compartments of the MSU Plant 

Science Research Greenhouses and light fixtures were installed in four of the compartments. 

Environmental conditions and monitoring were as described in Expt. 1 unless otherwise noted. 

Two greenhouse compartments contained HPS fixtures, one contained R+B LEDs, and another 

contained only B LEDs (peak wavelength = 450 nm; GreenPower LED research module blue; 

Philips, Eindhoven, The Netherlands). The average daily temperature and DLI in each treatment 

and replication are reported in Table 1. 

Lighting treatments 

During the growing stage, plants were grown under ambient light (control) or ambient 

light with SL from HPS fixtures at a PPFD of 98 μmol·m–2·s–1 with or without a 4-h EOD 

lighting treatment (from 22:00 to 02:00 HR). The EOD treatment was delivered by 

red+white+far-red LEDs (GreenPower DR/W/FR flowering lamps; Philips) with an R:FR of 

0.73 (Fig. 1) and a PPFD of 2-5 μmol·m–2·s–1. After three weeks, ten plants of each cultivar 

grown under SL were then provided with one of two finishing stage lighting treatments: the R+B 

LED arrays at a PPFD of 89 μmol·m–2·s–1 or B LED arrays at a PPFD of 95 μmol·m–2·s–1.  

Plant measurements and experimental design 

The experiment was performed twice, with the treatments initiated in Jan. and Apr. 2017, 

and data collection and analysis were as in Expt. 1 unless otherwise noted. At harvest, leaf 

number (leaves ≥ 4 cm), shoot fresh weight, leaf area of the three largest leaves, leaf color, and 

shoot dry weight (after plants were dried in an oven at 80 °C for 5 d) of each cultivar were 

measured and recorded. The average leaf size was that for the three largest leaves.  

 

 

68 

 

Results 

Expt. 1: Supplemental greenhouse lighting 

 Plant growth 

Generally, plants developed more leaves under SL treatments than without, but the type 

of SL had little to no impact (Fig. 2). SL increased leaf number by 8 to 12 in ‘Salanova Red 

Incised’, 9 to 14 in ‘Salanova Red Sweet Crisp’, ~8 in ‘Salanova Green Incised’, ~7 in ‘Salanova 

Green Sweet Crisp’, and 24 to 36 in ‘Salanova Green Butter’ in the two replications. In 

replication 2, the average leaf size of ‘Salanova Green Incised’ and ‘Salanova Green Sweet Crisp’ 

increased by 23-30% and 10-21%, respectively, under SL compared to no SL.  

In most instances, shoot fresh weight of all five cultivars was significantly greater under SL (Fig. 

2). In ‘Salanova Green Incised’ and ‘Salanova Green Butter’, plants grown under SL treatments 

were 43-84% and 30-46% heavier, respectively, compared to those grown without SL. ‘Salanova 

Green Sweet Crisp’ had the greatest fresh weight among the lettuce varieties tested. In ‘Salanova 

Red Sweet Crisp’, plants grown under LEDs were 33% heavier than no SL in replication 1, while 

HPS or LED SL increased the fresh weight by 54% in replication 2. In addition, the average leaf 

fresh weight of ‘Salanova Green Incised’ and ‘Salanova Green Sweet Crisp’ increased by 38% 

and 32-50%, respectively, under SL treatments compared to no SL in replication 2. 

Leaf coloration 

Both red-leaf cultivars developed a darker leaf color under SL, with the lightness (L*) 

value reduced by 9-14% and 13-25%, in ‘Salanova Red Sweet Crisp’ and ‘Salanova Red Incised’ 

respectively, compared to those grown under ambient light (Fig. 3). The R+B LED SL increased 

the degree of redness (a* value) in both red-leaf cultivars, by 59% in ‘Salanova Red Sweet Crisp’ 

and 90% in ‘Salanova Red Incised’, compared to the control in replication 2. In addition, SL 

 

69 

 

treatments decreased the degree of yellowness (b* value) in both cultivars, with the greatest 

reduction under the R+B LED treatment.  

 

Expt. 2: Sensory evaluation 

 In ‘Salanova Green Incised’, the overall consumer acceptability was slightly higher for 

lettuce from the no SL treatment than from the R+B LED treatment, and lettuce under HPS 

lamps was rated in between (Table 2). There were no statistical differences in appearance, but 

the overall flavor of lettuce grown without SL was preferred to plants grown under the SL 

treatments. Lettuce grown without SL was less bitter, sweeter, and had a more likable aftertaste 

compared to plants grown under the SL treatments. There were no flavor differences between 

lettuce grown under the HPS and LED treatments. In ‘Salanova Green Sweet Crisp’, bitterness 

was the only attribute for which panelists detected differences between the lighting treatments.  

Plants grown with R+B LED lighting were perceived as more bitter than plants grown without 

SL, and plants grown with HPS lighting were rated between the other treatments. 

 

Expt. 3: Supplemental greenhouse lighting at different growing stages 

Plant growth 

In the three cultivars tested, SL increased leaf number (by 5 to 10) only in ‘Salanova 

Green Incised’ (Fig. 4). ‘Salanova Red Incised’ and ‘Salanova Green Incised’ had 20-35% and 

41-62% greater average leaf size under SL, respectively, while ‘Salanova Red Sweet Crisp’ 

maintained a similar leaf size under all treatments.  

Generally, fresh shoot weight of ‘Salanova Red Sweet Crisp’ was statistically similar 

under the treatments.  However, plants grown under HPS lamps during both the growing and 

 

70 

 

finishing stages, and HPS+EOD lighting followed by R+B LED lighting, developed a 55% and 

59% greater fresh shoot weight, respectively, than no SL (Fig. 4). In ‘Salanova Green Incised’, 

most SL treatments significantly increased the shoot and the average leaf fresh weights, but there 

were no differences among SL treatments. In addition, SL promoted shoot dry weight by 66-

102%, 68-100%, and 125-180% of ‘Salanova Red Incised’, ‘Salanova Red Sweet Crisp’, and 

‘Salanova Green Incised’, respectively, but again, there were no differences among SL 

treatments (data not shown).  

Leaf coloration 

All SL treatments significantly decreased the lightness (L* value) for ‘Salanova Red 

Sweet Crisp’ compared to no SL (L* = 41.3), with the greatest reduction for plants grown under 

HPS lamps during the growing stage followed by R+B for the finishing stage (Fig. 5). Similarly, 

‘Salanova Red Incised’ developed significantly darker leaves when grown under R+B during the 

finishing stage. All SL treatments increased the degree of redness (a* value) by 29-89% and 40-

83% for ‘Salanova Red Sweet Crisp’ and ‘Salanova Red Incised’, respectively. The greatest 

promotion for leaf redness was under R+B for the finishing stage in both red leaf cultivars. The 

average degree of yellowness (b* value) for ‘Salanova Red Sweet Crisp’ and ‘Salanova Red 

Incised’ under ambient light was 20.4 and 27.4 CIELAB units, respectively, and b* values were 

significantly decreased by 19-40% and 29-60%, respectively, under the SL treatments. In 

‘Salanova Red Sweet Crisp’, plants were 50-60% less yellow when grown under R+B LEDs 

during the finishing stage. SL generally did not have effects on the lightness (L* value), degree 

of redness (a* value) and degree of yellowness (b* value) for ‘Salanova Green Incised’ 

regardless of light quality (data not shown).  

 

 

71 

 

Discussion 

 

In Expt. 1, we determined the impacts of SL on greenhouse lettuce growth and postulated 

that SL will increase crop yield compared with no SL. Previous research has shown that Boston 

lettuce hydroponically grown under SL HPS or LEDs for four weeks developed a 15-39% and 

22-29% greater fresh and dry mass, respectively, compared to plants without SL (Martineau et 

al., 2012). In a separate study, lamb’s lettuce (Valerianella locusta) were grown under ambient 

solar light with or without white or R+B LED SL at a PPFD of 100 μmol·m–2·s–1 in the 

greenhouse (Wojciechowska et al., 2013). Both SL treatments greatly increased fresh weight, by 

25-120% under white LEDs and 46-144% under R+B LEDs, respectively, compared to those 

without SL. Gaudreau et al. (1994) also indicated that day-extension lighting from 16 to 24 h at a 

PPFD of 50-100 µmol·m–2·s–1 increased lettuce biomass by 20 %. Consistent with these 

responses, this study showed that SL generally increased lettuce leaf number (by up to 60%), 

average leaf size (by up to 51%), and fresh weight (by up to 104%).  

We anticipated that SL would increase crop yield similarly if the PPFD is similar among 

treatments, regardless of the photon flux distribution within the photosynthetically active 

radiation waveband. Consistent with our hypothesis, we did not observe any growth differences 

under HPS and R+B LEDs among all cultivars in Expts. 1 and 3 except ‘Salanova Red Incised’ 

in replication 1 of Expt. 1. This was supported by Brazaitytė et al. (2006) that the fresh mass and 

photosynthetic productivity of lettuce ‘Grand Rapids’ was similar under HPS and R+B LEDs 

when grown indoors. In contrast, previous studies have also shown that crop growth and biomass 

can be influenced by the quality of SL. For example, lettuce ‘Frillice’ grown under R+B LED SL 

in the greenhouse had a 53% greater fresh weight compared to that under HPS, both at a PPFD 

of 120-140 μmol·m–2·s–1 (Pinho et al., 2007). Similarly, the yield of lamb’s lettuce under 

 

72 

 

monochromatic R or R+B SL LEDs increased by 18-26% and 18-41%, respectively, in two 

consecutive winters compared to that under HPS, both at a PPFD of 200 μmol·m–2·s–1 

(Wojciechowska et al., 2015). Therefore, lettuce sensitivity to the quality of SL may be cultivar-

specific, and more studies are merited. In addition, SL from HPS and LEDs contributed 23-26% 

and 16-23% of DLI, respectively, in Expt. 1, and 40-61% and 18-49%, respectively, in Expt. 3. 

Therefore, the majority of the light which came from the sunlight may diminish the impacts on 

growth differences from SL with different light qualities.  

Among the five cultivars we tested, ‘Salanova Green Butter’ had the greatest leaf number 

and developed 52 to 61 more leaves than the other four cultivars. However, its average leaf size 

and average leaf fresh weight were significantly lower than the others and therefore, did not 

result in the greatest yield. ‘Salanova Green Sweet Crisp’ had the greatest harvestable yield, and 

the fresh shoot weight was 33-79% and 34-71% greater than the other four cultivars under 

natural daylight and SL, respectively. Within the red-leaf cultivars, ‘Salanova Red Sweet Crisp’ 

had a 25-35% greater yield than ‘Salanova Red Incised’ in both experiments.  

In Expt. 3, we tested the effects of EOD lighting (R:FR = 0.73) on lettuce growth, with 

the expectation that R+FR will promote leaf expansion and therefore increase fresh and dry 

weight of lettuce. However, we did not observe any differences in leaf size and yield between 

plants provided with or without 4-h of EOD lighting. Similar results were reported by Kubota et 

al. (2011) that fresh and dry weight of baby leaf lettuce ‘Red Salad’ was similar without or with 

EOD-FR with a dose of 1.2 to 9.2 mol·m–2·d–1 in the greenhouse. In a separate study, lettuce 

‘Cherokee’ and ‘Waldmann’s Green’ were grown under natural daylight with or without 50 

μmol·m–2·s–1 EOD lighting provided by B, R, and white (Chinchilla et al., 2018). None of the 

EOD lighting treatments influenced leaf area, leaf number, fresh weight, or dry weight, 

 

73 

 

regardless of light quality, except EOD-B increased fresh weight by 18% compared to the natural 

daylight control. Together, these studies indicate that lettuce growth is relatively insensitive to 

EOD low-intensity lighting. 

Anthocyanins, which are antioxidant compounds that influence red pigmentation, are 

higher in concentration in red-leaf cultivars than in green (Samuolienė et al., 2012). Providing 

SL is one of the ways to increase lettuce quality including its appearance and nutritional value, 

especially when the DLI is low. As in previous studies (Giliberto et al., 2005; Owen and Lopez, 

2015), this study demonstrates that SL improves lettuce leaf coloration. This was also supported 

by Mancinelli (1985) that the production of small concentration of anthocyanins can be induced 

by single short irradiance, while the accumulation of larger concentrations of anthocyanins 

requires a prolonged period at a high PPFD, such as SL.  

Anthocyanin concentration can also be increased by delivering B SL (Giliberto et al., 

2005; Samuolienė et al., 2012) and therefore, we postulated that lettuce leaf pigmentation would 

be greatest under B SL during the finishing stage. However, this study indicates that a mixture of 

R and B is more effective at increasing leaf coloration than monochromatic B or HPS fixtures. 

This is in agreement with Owen and Lopez (2015), who reported that the relative redness (a* 

value) was 45-370% greater in four cultivars of lettuce with 100 µmol·m–2·s–1 R+B SL (R:B = 

1:1) compared to plants under monochromatic R or B. Similarly, the anthocyanin concentration 

of red-leaf lettuce ‘Banchu Red Fire’ seedlings under 100 µmol·m–2·s–1 R+B was 350% and 80% 

higher than monochromatic R and B, respectively, at a similar PPFD in a growth chamber 

(Johkan et al., 2010). In a separate study, lettuce grown indoors under 300 µmol·m–2·s–1 of R+B 

had a 265% higher anthocyanin concentration than when grown under R alone at a similar PPFD 

(Stutte et al., 2009). Mancinelli (1985) suggested that the spectral sensitivity of anthocyanin 

 

74 

 

production was species-specific and categorized crops into three groups based on action peaks 

for anthocyanin production: UV/B, R, and FR (group I); UV/B and R (group II); and UV/B only 

(group III), respectively. Based on all of these results, and that anthocyanin concentration 

significantly decreased under FR SL (Li and Kubota, 2009; Stutte et al., 2009), we postulate that 

lettuce belongs to group I, where UV/B, R, and FR all regulate anthocyanin biosynthesis and 

degradation. 

Consumer taste preference is one of the most important aspects to assess the quality of 

edible crops and appearance, flavor, bitterness, sweetness, and aftertaste to ensure marketability. 

However, very few studies have been published on the sensory evaluation of lettuce grown under 

SL with different light qualities. Eskins et al. (1995) reported that the bitterness of lettuce grown 

under 170 µmol·m–2·s–1 of R was significantly less than under B at a similar PPFD. In addition, 

they suggested that lettuce grown under 550 µmol·m–2·s–1 white had intermediate bitterness at a 

younger stage (25 d), but became the most bitter with maturity (41 d), compared to 

monochromatic R or B. In a separate study, green-leaf Boston type lettuce grown under 210 

µmol·m–2·s–1 of R+B SL was evaluated as the least preferred by consumers and the shape, 

crispness, and sweetness was rated lower than plants grown under R+B+white LEDs or 

fluorescent lamps (Lin et al., 2013). Our study indicated that consumers prefer lettuce grown 

under natural daylight compared to with SL, although there was a trade-off with lower yield. In 

addition, we did not observe any consumer preference differences between HPS or R+B LEDs 

SL. However, we only tested two green-leaf cultivars, and consumer perception for red-leaf 

cultivars may be different and additional research is needed. These findings collectively show 

that lettuce sensory properties can be influenced by light quality, and consumer preference 

should be considered when installing SL fixtures.  

 

75 

 

 

APPENDIX

76 

 

Table III-1. Means (± SD) of greenhouse air temperature and daily light integral (DLI) during the 
two experimental replications (rep) in Expt. 1 and 3 as measured by aspirated thermocouples and 
quantum sensors. The supplemental lighting treatments were none, high-pressure sodium (HPS) 
and red+blue (R+B) light-emitting diodes (LEDs), and blue (B) LEDs. The 4-h end-of-day 
lighting provided by red+white+far-red flowering lamps at 2 to 5 μmol·m–2·s–1 in Expt. 2. 
Average DLI (mol·m–2·d–1) 

Average air temp (°C) 

Lighting 
treatment 

None (control) 

HPS 
R+B LEDs 

None (control) 
HPS 
HPS+EOD 
R+B LEDs 
B LEDs 
zData missing. 
 

Rep 1 

20.8 ± 1.1 
21.1 ± 1.1 
20.8 ± 1.2 

19.7 ± 0.5 
20.2 ± 0.7 
19.6 ± 0.5 
19.4 ± 0.5 
21.0 ± 0.6 

 

Rep 2 
Expt. 1 
21.0 ± 1.5 

20.8 ± 1.2 
20.9 ± 1.5 
Expt. 3 
21.1 ± 2.0 
20.9 ± 1.9 
20.9 ± 1.9 
20.7 ± 2.0 
21.8 ± 1.8 

Rep 1 

Rep 2 

7.5 ± 3.3 

10.2 ± 2.4 
9.7 ± 2.3 

7.6 ± 3.5 
16.3 ± 7.3 
20.1 ± 8.8 

14.3 ± 5.5 
14.8 ± 2.3 

8.2 ± 3.4 

10.6 ± 2.4 
9.8 ± 2.7 

11.9 ± 4.4 
17.7 ± 4.0 
19.8 ± 4.4 
14.1 ± 2.7 

__z 

 

77 

 

Table III-2. Sensory attributes and mean scores of lettuce grown under three different greenhouse 
light conditions in Expt. 2. Samples with different letters within each cultivar and the attribute 
column are significantly different at the 95% confidence level.  For ‘Green Incised’, n=45 (12 
male and 33 female).  For ‘Green Sweet Crisp’, n=39 (12 male and 27 female). 

Lighting 
treatment 

Overall 

acceptabilityz  Appearancez 

Overall 
flavorz  Bitternessy  Sweetnessy  Aftertastex 

None 
HPS 
LED 
Significance 

None 
HPS 
LED 

6.51 a 
5.87 ab 
5.67 b 

*** 

6.49 
5.69 
5.97 

‘Salanova Green Incised’ 
7.13 
7.69 
7.29 
NS 

6.53 a 
5.22 b 
5.07 b 

*** 

1.62 b 
2.82 a 
3.11 a 

*** 

‘Salanova Green Sweet Crisp’ 

7.18 
6.49 
6.77 

6.4 
5.56 
5.56 

1.90 b 
2.44 ab 
2.51 a 

1.58 a 
1.31 b 
1.29 b 

* 

1.31 
1.51 
1.36 

5.49 a 
4.56 b 
4.38 b 

*** 

5.46 
4.90 
4.82 

NS 

NS 

Significance 
NS 
NS, *, or *** Nonsignificant or significant at P < 0.05, or 0.001, respectively. 
zEvaluated with a 9-point scale: 1= dislike extremely; 9= like extremely. 
yEvaluated with a 5-point scale: 1= not at all bitter or sweet; 5= extremely bitter or sweet. 
xEvaluated with a 9-point scale: 1= dislike extremely; 9= like extremely with 5= none detected. 
 

NS 

* 

NS 

 

 

 

78 

 

Figure III-1. The spectral distribution of supplemental lighting treatments (top) delivered by 
high-pressure sodium (HPS) fixtures, red+blue (R+B) light-emitting diodes (LEDs) (in Expt. 1 
and 2), and B LEDs (in Expt. 2), and end-of-day lighting (bottom) provided by red+white+far-
red LEDs. The peak wavelengths (in nm) are provided for each fixture type.  
 

 

 

 

79 

 

Figure III-2. Growth characteristics of five cultivars of lettuce grown without supplemental lighting (none) or with supplemental 
lighting from high-pressure sodium (HPS) or light-emitting diodes (LED) in Expt. 1. Leaf number of ‘Salanova Green Butter’ 
included leaves ≥ 7 or 3.5 cm in length for replication 1 (Rep 1) or replication 2 (Rep 2), respectively, while leaf number for the other 
four cultivars included leaves ≥ 3 cm in both replications. Within the same replication, means with the same letter are not statistically 
different by Tukey’s honest significant difference test at P ≤ 0.05. Error bars indicate standard error. 

 

 

80 

 

Figure III-3. Leaf coloration characteristics of two cultivars of lettuce grown without 
supplemental lighting (none) or with supplemental lighting from high-pressure sodium (HPS) or 
light-emitting diodes (LED) in Expt. 1. In replication (Rep) 1, a single measurement was taken 
on one outer leaf per plant while in Rep 2, measurements were taken on three outer leaves per 
plant. Within the same replication, means with the same letter are not statistically different by 
Tukey’s honest significant difference test at P ≤ 0.05. Error bars indicate standard error. 

 

 

81 

 

 

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82 

 

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Stutte, G.W., S. Edney, and T. Skerritt. 2009. Photoregulation of bioprotectant content of red leaf 

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Szczygiel, E.J., J.B. Harte, G.M. Strasburg, and S. Cho. 2017. Consumer acceptance and aroma 

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U.S. Department of Agriculture (USDA) and Economic Research Service. 2018. Per capita 

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Wojciechowska, R., O. Długosz-Grochowska, A. Kołton, and M. Żupnik. 2015. Effects of LED 

supplemental lighting on yield and some quality parameters of lamb's lettuce grown in 
two winter cycles. Scientia Hort. 187:80-86. 

 
Wojciechowska, R., A. Kołton, O. Długosz-Grochowska, M. Żupnik, and W. Grzesiak. 2013. 
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SECTION IV  

INVESTIGATING THE INTERACTION OF BLUE AND FAR-RED RADIATION ON 

GROWTH AND SUBSEQUENT FLOWERING OF FLORICULTURE CROPS

86 

 

Investigating the Interactions of Blue and Far-red Radiation on Growth and Subsequent 

Flowering of Floriculture Crops 

 

Mengzi Zhang1, Yujin Park, and Erik S. Runkle2 

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

48824 

 

We gratefully acknowledge the American Floral Endowment and OSRAM Innovation for 

supporting this project, Raker-Roberta’s and Syngenta Flowers for the donation of plant material, 

and Nate DuRussel for technical assistance. This work was supported by the USDA National 

Institute of Food and Agriculture, Hatch project 192266. 

 

1Graduate student. 

2Corresponding author. E-mail address: runkleer@msu.edu.

 

87 

 

Abstract 

 

Manipulating the radiation spectrum of sole-source lighting (SSL) with light-emitting 

diodes (LEDs) enables the regulation of plant growth, development, and quality attributes of 

high-value ornamental transplants. Blue (B, 400-500 nm) radiation generally inhibits extension 

growth while far-red (FR, 700-800 nm) radiation promotes it, and FR can accelerate subsequent 

flowering of some long-day plants. We quantified growth responses and subsequent flowering of 

a variety of ornamental seedlings to the addition of FR radiation to two combinations of B+red 

(R, 600-700 nm) SSL, and compared responses to plants under white SSL or greenhouse 

conditions. Seedlings were grown at 20 °C under an 18-h photoperiod either in the greenhouse or 

indoors under nine SSL treatments (subscript values indicate the photon flux density of each 

waveband, in µmol·m–2·s–1): B20R160, B20R160FR10, B20R160FR20, B20R160FR40, B60R120, 

B60R120FR10, B60R120FR20, B60R120FR40, and warm white (WW180) LEDs. Among all the species 

tested, the addition of 40 μmol·m–2·s–1 of FR only increased the seedling height of snapdragon 

(Antirrhinum majus) and zinnia (Zinnia elegans) by 64-134% and 52-96%, respectively, 

regardless of the proportion of B, compared to SSL treatments without FR or in the greenhouse. 

Similarly, WW180 promoted seedling stem elongation by 75-139% in snapdragon compared to 

SSL without FR or the greenhouse control, but the promotive effect of WW180 was not 

significant in the other species. Relative chlorophyll content of zinnia and petunia (Petunia 

×hybrida) generally decreased as the photon flux density of FR increased, and in both species 

was 13-14% greater under 60 µmol·m–2·s–1 of B compared to under 20 µmol·m–2·s–1 of B. 

However, radiation treatments did not influence leaf area and dry shoot weight in any species. 

The subsequent flowering of snapdragon was accelerated by 7-11 d with the additional 20 or 40 

µmol·m–2·s–1 of FR or WW180 compared to SSL without FR or the greenhouse control. We 

 

88 

 

conclude that when ≥20 µmol·m–2·s–1 of B is delivered to crops, the addition of 20 or 40 

µmol·m–2·s–1 of FR can accelerating the flowering of at least some long-day plants with little to 

no effect on extension growth. 

 

Keywords: Bedding plants, blue radiation, controlled-environment agriculture, floriculture, LEDs, 

light quality, phytochrome photoequilibrium, red radiation, red to far-red ratio 

 

Introduction 

Most floriculture crops including annuals, herbaceous perennials, potted flowering plants, 

and cut flowers, are produced in two distinct phases: the young plant stage and the finish plant 

stage (Lopez et al., 2017). The production of ornamental propagules (e.g., plugs and liners) is a 

significant segment of the floriculture crop industry in the U.S. In 2015, for the 15 States 

surveyed, its wholesale value accounted for $394 million, which was a 15% increase from 2014 

(USDA, 2016). To meet the largest market period during the spring (February through May), 

most young floriculture plants are produced commercially in greenhouses during the late winter 

and early spring (December through April). However, during this time, low ambient temperature 

and photosynthetic daily light integral (DLI) in temperate climates are limiting factors to 

consistently produce high-quality plants on schedule (Lopez and Runkle, 2008). Commercially, 

sole-source lighting (SSL) was most commonly used in closed systems (e.g., plant factory) for 

the production of high-value specialty crops such as leafy greens, microgreens, and herbs at a 

high density (Mitchell and Stutte, 2017). However, there is an increasing interest in growing 

uniform and reliable high-value floriculture transplants indoors, where the environment can be 

controlled with SSL from light-emitting diodes (LEDs).  

 

89 

 

Sole-source lighting from LEDs enables control of the radiation spectrum that can 

regulate plant growth, development, and quality attributes. The vast majority of research with 

LED SSL has focused on investigating the effects of different mixtures of blue (B, 400-500 nm) 

and red (R, 600-700 nm) radiation on plant growth (Massa et al., 2008). R radiation promotes 

leaf expansion, stem elongation, and biomass accumulation (Mitchell and Stutte, 2017). For 

example, the stem length of chrysanthemum (Dendranthema grandiflorum) in vitro grown under 

50 µmol·m–2·s–1 of R LEDs (peak = 650 nm) for five weeks was 67% and 108% greater than 

under B (peak = 440 nm) and B+R LEDs (Kim et al., 2004). Similarly, leaf area of impatiens 

(Impatiens walleriana), tomato (Solanum lycopersicum), salvia (Salvia splendens), and petunia 

(Petunia ×hybrida) seedlings was 54-60%, 40-46%, 34-39% and 35-49% greater, respectively, 

under monochromatic R (peaks = 634 and 664 nm) than under B (peak = 446 nm) alone or B+R 

at a photosynthetic photon flux density (PPFD) of 160 µmol·m–2·s–1 (Wollaeger and Runkle, 

2014). In addition, fresh and dry weight under R LED SSL increased by 52% and 55% in 

impatiens, 40% and 28% in tomato, 32% and 44% in salvia, and 38% and 60% in petunia, 

respectively, than under B SSL. Similarly, dry weight of spinach (Spinacia oleracea) was up to 

47% greater under R than under B, depending on the cultivar, at a PPFD of 300 µmol·m–2·s–1 (Li 

et al., 2011).  

Adding B radiation to an R-dominant spectrum inhibits extension growth and increases 

leaf chlorophyll concentration, eliciting phenotypes similar to plants grown under sunlight 

(Lobiuc et al., 2017; Mitchell et al., 2012; Wang, et al., 2016). Wollaeger and Runkle (2015) 

reported that substituting 50% of R (peaks = 634 and 664 nm) with B (peak = 446 nm) inhibited 

the stem length of impatiens, salvia, and tomato by 48 to 54% at a PPFD of 160 µmol·m–2·s–1. 

Similarly, hypocotyl length of lettuce (Lactuca sativa) seedlings decreased by 93% when R 

 

90 

 

(peak = 660 nm) LEDs was substituted with 60 µmol·m–2·s–1 of B (peaks = 435-470 nm) under a 

similar PPFD (Hoenecke et al., 1992). In cucumber (Cucumis sativus), leaf chlorophyll 

concentration increased by 38% as the proportion of B increased from 0 to 50% (Hogewoning et 

al., 2010). However, increasing B can inhibit leaf expansion and subsequent plant growth. For 

example, cucumber seedlings were grown under 54 µmol·m–2·s–1 of supplemental lighting with 

different B:R ratios under a low DLI in the greenhouse (Hernández and Kubota, 2014). Leaf area 

of seedlings decreased by 7% and 13% as the B proportion increased from 0 to 4% and 16%, 

respectively. In a separate study, the leaf area of tomato, cucumber, and pepper (Capsicum 

annum) grown indoors decreased by 24-40% when the B proportion of SSL increased from 10 to 

30%, and similarly the dry mass decreased by 14 to 26% (Snowden et al., 2016). In several 

ornamental seedlings, at least 10 µmol·m–2·s–1 of B added to R radiation produced high-quality 

transplants (Wollaeger and Runkle, 2015).  

In addition to B+R, recent studies have investigated the merits of including far-red (FR, 

700-800 nm) radiation in SSL (Craver and Lopez, 2015; Lee et al., 2016; Meng and Runkle, 

2017; Park and Runkle, 2017, 2018). Although the magnitude of plant responses varies among 

species, the inclusion of FR (peak = 731 nm) radiation to a B+R spectrum (peaks = 447 and 660 

nm) stimulated stem elongation in petunia, geranium (Pelagonium ×hortorum), snapdragon 

(Antirrhinum majus), and impatiens, and leaf expansion and subsequent dry matter accumulation 

in geranium and snapdragon (Park and Runkle, 2017). Similarly, the addition of FR (peak = 735 

nm) to B+R (peaks = 440 and 660 nm) increased leaf area and shoot fresh weight of lettuce by 

25-60% and 21-52%, respectively (Lee et al., 2016). In addition, FR radiation can promote 

subsequent flowering of some long-day plants (LDPs). Craig and Runkle (2016) reported that the 

flowering of petunia, snapdragon, black-eyed susan (Rudbeckia hirta), and fuchsia (Fuchsia 

 

91 

 

×hybrida) was accelerated when FR was added to R light as night-interruption (NI) lighting in 

greenhouses.  Similarly, the flowering of snapdragon was accelerated by 10-12 d when 160 

µmol·m–2·s–1 of B+R included 16-64 µmol·m–2·s–1 of FR during indoor production of seedlings 

(Park and Runkle, 2017). However, considering the different sensitivities of plants to FR 

radiation and interactive effects of radiation wavebands, more information is needed on how FR 

interacts with B and R radiation to elicit desired quality attributes of ornamental crops.  

Here we quantified the responses of a variety of floriculture crop seedlings to the addition 

of FR radiation to two combinations of B+R SSL lighting, and compared plants to those grown 

under white SSL, or under greenhouse conditions. We anticipated that the sensitivity of plants to 

FR radiation would be related to the shade tolerance and photoperiodic flowering response of 

crops.  Specifically, we postulated that FR would promote stem elongation, leaf expansion, and 

subsequent seedling growth more in shade-avoiding species than in shade-tolerant species, and 

inclusion of FR in a radiation spectrum would accelerate subsequent flowering of at least some 

LDP, but minimally influence subsequent flowering in short-day plants (SDPs) and day-neutral 

plants (DNP). We also postulated that an increase in B radiation would diminish the effects of 

FR radiation on stem elongation and leaf expansion while increasing leaf chlorophyll 

concentration. 

 

Materials and Methods 

Plant material 

 

LDPs dianthus (Dianthus barbatus ‘Jolt Cherry’), petunia (‘Wave Blue’), and snapdragon 

(‘Liberty Classic Yellow’), SDPs african marigold (Tagetes erecta ‘Antigua Orange’), coleus 

(Plectranthus scutellarioides ‘Wizard Golden’), and zinnia (Zinnia elegans ‘Magellan Pink’), 

 

92 

 

and DNPs geranium (‘Pinto Premium Orange Bicolor’), impatiens (‘Super Elfin XP Red’), and 

tomato (‘Micro Tom’) were selected for study based on commercial significance, photoperiodic 

flowering response, and shade tolerance. Impatiens and coleus are shade-tolerant plants while the 

others are shade-avoiding plants. Seeds of each species were sown in 128-cell plug trays by a 

commercial young plant producer (Raker-Roberta’s, Litchfield, MI). They were transferred to an 

environmentally controlled glass-glazed research greenhouse at Michigan State University (MSU) 

(East Lansing, MI) within 7 d of seed sow and grown at 20 °C until the appearance of the first 

true leaves. A 16-h photoperiod (from 0600 to 2200 HR) was delivered using ambient solar 

radiation with supplemental high-pressure sodium (HPS) fixtures (400W; LR48877; P.L. Light 

System, Beamsville, ON, Canada) that automatically turned on when the PPFD outdoors was 

<437 μmol·m–2·s–1. The PPFD at seedling height from the HPS fixtures was 67 µmol·m-2·s-1. 

For each replication (rep.), the days from seed sow to the start of the radiation treatments was 

(rep. 1, 2): african marigold (8, 10), coleus (12, 16), dianthus (9, 10), geranium (8, 10), impatiens 

(12, 14), petunia (12, 16), snapdragon (14, 16), tomato (12, 10), and zinnia (8, 10). Each plug 

tray was then divided into four sections, each with 32 seedlings, thinned to one seedling per cell, 

and placed under the SSL treatments.  

Radiation treatments and environmental conditions during the seedling stage 

The experiment was performed in one of the two compartments of the MSU Controlled 

Environment Lighting Laboratory (CELL), which had four racks in each compartment. Each 

rack (1.2 × 0.6 × 2.4 m) contained three vertically stacked shelves and the top of each shelf 

contained three LED panels (OSRAM Innovation, Beverly, MA) 0.5 m above the seedling trays. 

The panels contained B (peak = 449 nm), R (peak = 664 nm), FR (peak = 733 nm), and warm 

white (WW, peak = 639 nm) LEDs that faced down towards the plants. The photon flux density 

 

93 

 

of each LED type was independently adjusted by computer software customized by OSRAM 

Innovation. Radiation treatments were designed to determine the effects of the addition of FR 

radiation to typical B+R spectra with either a low or high B photon flux density. Three racks 

provided the following nine SSL radiation treatments with an 18-h photoperiod (subscript values 

indicate the photon flux density of each waveband, in µmol·m–2·s–1): B20R160, B20R160FR10, 

B20R160FR20, B20R160FR40, B60R120, B60R120FR10, B60R120FR20, B60R120FR40, and WW180 (Table 1). 

The WW180 treatment emitted a photon flux density for B, green (500-600 nm), R, and FR of 14, 

54, 95, and 17 µmol·m–2·s–1, respectively. To serve as another comparison, seedlings were 

grown in a greenhouse under ambient sunlight with supplemental lighting from HPS fixtures at a 

PPFD of 100 µmol·m–2·s–1 at plant height to achieve an 18-hour photoperiod and a similar DLI. 

For each radiation treatment, the R:FR ratio, which was calculated with 100-nm wavebands, 

phytochrome photoequilibrium (PPE) value, which was estimated as described by Sager et al. 

(1988), and the average DLI are reported in Table 1.  

All seedlings were grown at a constant 20 °C. The PPFD at plant height in CELL or on 

the greenhouse bench was measured by a quantum sensor (LI-190SA; LI-COR, Lincoln, NE). 

Seedling trays within each radiation treatment (shelf) were rotated every day to prevent 

positional effects. Air temperature in CELL and the greenhouse were measured near plant height 

by bare thermocouples and an aspirated thermocouple (Type E; Omega Engineering, Stamford, 

CT), respectively. Temperature and the PPFD at both locations were collected by a data logger 

(CR-1000; Campbell Scientific, Logan, UT) every ten seconds and average values were recorded 

hourly. Average air temperatures (± standard error) in two replications were 20.1 ± 0.5 and 19.9 

± 1.3 °C in CELL and 19.9 ± 0.6 and 21.2 ± 1.1 °C in the greenhouse, respectively.  

 

94 

 

In CELL, the temperature was automatically controlled by an air conditioner 

(HBH030A3C20CRS; Heat Controller, Jackson, MI) and the air conditioner was controlled by a 

thermostat (TH8321R1001; Honeywell, Morris Plains, NJ) through a gateway and wireless 

sensor (THM6000R1002, C7189R1004; Honeywell, Morris Plains, NJ). Humidity was 

monitored by a transmitter (HMDW110; Vaisala, Helsinki, Finland) and in rep. 2, increased by a 

humidifier (LV600HH; Lӗvoit, Anaheim, CA) that ran continuously. The actual average relative 

humidity in the compartment was 29% and 41% in rep. 1 and 2, respectively. In the greenhouse, 

whitewash (Kool Ray Classic; Continental Products Co., Euclid, OH) was applied to the 

greenhouse glazing to reduce the ambient light intensity to deliver a similar DLI as in the SSL 

treatments. Roof vents and fans were controlled by a greenhouse environmental control system 

(Integro 725; Priva North America, Vineland, Ontario, Canada). In both locations, seedlings 

were irrigated as necessary with deionized water supplemented with a water-soluble fertilizer 

containing (mg·L–1) 50 N, 19 P, 50 K, 23 Ca, 4 Mg, 1 Fe, 0.5 Cu, 0.5 Zn, 0.5 Mn, 0.3 B, and 0.1 

Mo (MSU Plug Special; GreenCare Fertilizers, Inc., Kankakee, IL). 

Environmental conditions during the finish stage 

At the end of seedling stage, when the seedlings had well–developed root systems, ten 

seedlings of each species from each radiation treatment and replication were selected and 

transplanted into 10-cm pots containing 70% peatmoss, 21% perlite, and 9% vermiculite potting 

media (SUREMIX; Michigan Grower Products, Inc., Galesburg, MI) and randomly placed on 

benches in the MSU research greenhouse. For each replication, the number of days under the 

radiation treatments before seedling data collection and transplanting was (rep. 1, 2): african 

marigold (27, 27), coleus (35, 32), dianthus (27, 26), geranium (27, 27), impatiens (23, 24), 

petunia (25, 24), snapdragon (29, 28), tomato (22, 24), and zinnia (21, 21). Plants were grown at 

 

95 

 

a constant 20 °C under a 16-h photoperiod provided by sunlight supplemented by 400-W HPS 

fixtures at a PPFD of 59 µmol·m–2·s–1. Photon flux density measurements were made during the 

finishing stage in the greenhouse with a spectroradiometer (PS-200; Apogee Instruments Inc., 

Logan, UT) above the benches at nine representative positions at plant height. HPS fixtures were 

automatically turned on from 0600 to 2200 HR when the PPFD was <437 μmol·m–2·s–1. Air 

temperature and PPFD were monitored and recorded as described above. The average air 

temperature and DLI during the finishing stage in the greenhouse for rep. 1/ 2 were 20.0/20.6 °C 

and 10.2/15.2 mol·m–2·d–1, respectively. Plants were irrigated as necessary with reverse osmosis 

water supplemented with a water-soluble fertilizer containing (mg·L–1) 125 N, 12 P, 100 K, 65 

Ca, 12 Mg, 1 Fe and Cu, 0.5 Mn and Zn, 0.3 B, and 0.1 Mo (MSU RO Water Special; GreenCare 

Fertilizers, Inc., Kankakee, IL). 

Data collection and analysis 

The experiment was performed twice in time. The most uniform 20 plants from each tray 

section of each species in each radiation treatment and replication were selected for data 

collection: ten for the seedling stage and ten for the finish stage. At the end of the seedling stage, 

stem length (from media level to apical meristem), leaf number, and leaf area [using a leaf area 

meter (LI-3000; LI-COR)] were measured for all nine species. Leaves with leaf length of ≥ 2 cm 

(or 3 cm for geranium, tomato and african marigold) were counted in leaf number and included 

in leaf area. The average leaf area was calculated by dividing total leaf area by leaf number. An 

index of relative chlorophyll concentration (SPAD value) was measured on the second (for 

geranium) or third (for coleus, petunia, and zinnia) leaf from the meristem using a SPAD meter 

(SPAD-502, Minolta corporation, Ltd., Osaka, Japan). Three readings per leaf were taken and 

the average was recorded. After drying in an oven at 80 °C for ≥ 5 d, shoot dry weight of all 

 

96 

 

species and root dry weight of petunia, tomato, and zinnia was measured using a balance (A&D 

Weighing GR-1000, San Jose, CA). During the finish stage, date of first open flower and visible 

bud or inflorescence number at first flowering were recorded on all species except coleus. Plant 

height at flowering (length of the primary stem from the substrate surface to the top of the 

inflorescence) was measured except on impatiens. Leaf number on the primary stem at flowering 

was recorded on snapdragon, tomato, and zinnia. 

The experiment used a randomized complete block design. Two replications were 

considered as two blocks and each shelf in CELL or the bench in the greenhouse were regarded 

as the experimental unit for the radiation treatment. Within the experimental unit, ten individual 

seedlings and ten individual plants at the finish stage per species were the subsamples. Data were 

pooled from two replications and were analyzed with the SAS (version 9.4; SAS Institute, Inc., 

Cary, NC) mixed-model (PROC MIXED) and glimmix-model (PROC GLIMMIX) procedures. 

Pairwise comparisons between treatments were performed using Tukey’s honest significant 

difference test at P ≤ 0.05. 

 

Results  

Seedling stage 

Seedling stem length of all species was similar among the ten radiation treatments except 

for snapdragon, petunia, and zinnia (Fig. 2). Stem length of snapdragon and zinnia generally 

increased as FR was added to either B20R160 or B60R120. Seedlings were 64-134% (for 

snapdragon) and 52-96% (for zinnia) taller with the addition of 40 μmol·m–2·s–1 of FR compared 

to that in the greenhouse or indoor treatments without FR. In snapdragon, stem length under 

WW180 was similar to that under B20R160 FR20, B20R160 FR40, and B60R120FR40. In contrast, zinnia 

 

97 

 

stem length under WW180 was statistically similar to that under the other treatments. In petunia, 

stem length was 92% and 108% taller in the greenhouse than the indoor treatments without FR 

(i.e., B20R160 and B60R120), respectively, while stem length under these three treatments was 

similar to that in the other radiation treatments. 

Under B60R120, the SPAD value of zinnia decreased by 17% with the addition of 40 

μmol·m–2·s–1 of FR (Fig. 3). Similarly, in petunia, the SPAD value under B60R120 decreased by 

16-23% with additional FR at ≥20 μmol·m–2·s–1. Under B20R160, SPAD value in petunia 

decreased with the addition of 40 μmol·m–2·s–1 of FR. SPAD value of zinnia and petunia was 14% 

or 13% higher, respectively, under B60R120 than B20R160, while SPAD value was similar under 

B60R120 and B20R160 when the same amount of FR was added. In geranium and coleus, there were 

no differences in SPAD value among treatments (data not shown). 

Radiation treatment did not influence average leaf area or dry shoot weight in any species 

(Table 2). Similarly, dry root weight of zinnia and petunia were similar under the radiation 

treatments. In tomato, the dry root weight of seedlings under B20R160FR10 was 40-47 % greater 

than that under WW180 indoors or in the GH, but dry root weight under these three treatments 

was similar to that in the other treatments (data not shown).  

Finish stage 

Radiation treatments during the seedling stage had little to no effect on days to 

subsequent visible bud or flower, inflorescence number, plant height at flowering, or leaf number 

in all the species tested except for snapdragon (Table 3). In snapdragon, subsequent flowering 

was accelerated by 7-11 d with 20 or 40 μmol·m–2·s–1 of FR (with B20R160 or B60R120) or under 

WW180 compared to indoor treatments without FR or in the greenhouse (Fig. 4). Plant height of 

snapdragon at flowering was 17-32% shorter with the addition of 10 to 40 μmol·m–2·s–1 of FR 

 

98 

 

(with B20R160 or B60R120) or under WW180 compared to seedling treatments without FR. 

Snapdragon developed 8-29% fewer visible flower buds with the addition of 20 or 40 μmol·m–

2·s–1 of FR with B20R160, 10 to 40 μmol·m–2·s–1 of FR with B60R120, and WW180 compared to 

indoor treatments without FR or in the GH. In addition, snapdragon developed 9 or 10 fewer 

leaves before flowering with the additions of 20 or 40 μmol·m–2·s–1 FR to B20R160 or 40 μmol·m–

2·s–1 of FR to B60R120 compared to indoor treatments without FR. In geranium, plants at first 

flowering had an average of one more inflorescence when seedlings were grown under B60R120 

than under B20R160FR20, and those had a similar inflorescence number to seedlings grown under 

the other treatments (data not shown).  

 

Discussion 

In a wide range of species, stem elongation increases linearly as the R:FR and estimated 

PPE decrease (Smith, 1994; Runkle and Heins, 2001). Under vegetative shade outdoors, the 

R:FR decreases and in response, shade-avoiding species elongate in an attempt to better capture 

photosynthetic light while shade-tolerant species show a reduced shade-avoidance response 

(Franklin, 2008; Gommers et al., 2013; Smith, 1994). We postulated the addition of FR in SSL 

would promote stem elongation and leaf expansion of seedlings in shade-avoiding species and 

have less of an effect in shade-tolerant species. In the shade-avoiding snapdragon and zinnia, the 

addition of 40 μmol·m–2·s–1 of FR increased stem length by 64-71% and 93-96%, respectively, 

under both B20R160 and B60R120 (Fig. 2). In contrast, the addition of FR did not meaningfully 

increase extension growth in the other five shade-avoiding species and both shade-tolerant coleus 

and impatiens. Even among the shade-avoiding species, the sensitivity of extension growth to 

R:FR depends on species. For example, when the R:FR increased from 1:1 to 1:0 (the PPE 

 

99 

 

increased from 0.69 to 0.88), stem length of shade-avoiding geranium and snapdragon decreased 

by 41%, while that of shade-avoiding petunia decreased by 95% (Park and Runkle, 2017). 

The R:FR and estimated PPE value have been used to predict the effects of the radiation 

spectrum on phytochrome-mediated extension growth. In this study, the PPE value was a more 

accurate predictor of extension growth than the R:FR. For example, WW180, which had an R:FR 

of 5:1, had a similar PPE value as B20R160FR40. Both treatments influenced seedling growth 

similarly in all species, including snapdragon and zinnia, under the same PPFD. In addition, 

B60R120FR10

 and B60R120FR20, which had similar PPE values but different R:FRs (12:1 and 6:1, 

respectively), had similar effects on promoting stem elongation on snapdragon, zinnia, and 

petunia. Together, in the presence of B radiation, these findings suggest that the PPE value is a 

better indicator to assess crop extension growth responses compared to the R:FR.  

FR radiation is involved in regulating photoperiodic flowering responses, especially in 

some LDPs. During a long night, R is the most effective at inhibiting flowering of SDPs, while 

the flowering of a wide range of LDPs is promoted most when the night is interrupted with R and 

FR (Craig and Runkle, 2013; Downs and Thomas, 1982; Oh and Runkle, 2016). For example, in 

LDPs including snapdragon, NI lighting with an intermediate PPE value between 0.63 and 0.80 

was the most effective at promoting flowering (Craig and Runkle, 2016). In a separate study, the 

subsequent flowering of snapdragon was accelerated when SSL included ≥16 μmol·m–2·s–1 of 

FR (creating a PPE ≤0.85), and the acceleration was saturated when the PPE was between 0.69 

and 0.85 (Park and Runkle, 2017). Consistent with this response, the subsequent flowering of 

LDP snapdragon was hastened in this study when SSL included ≥20 μmol·m–2·s–1 of FR 

(creating a PPE of 0.81 to 0.85) (Fig. 4). Therefore, delivering R+FR that creates a PPE value 

between 0.63 and 0.85 is effective at accelerating the flowering of LDPs. The inclusion of FR 

 

100 

 

radiation during the seedling stage had no effect on the subsequent flowering of SDP or DNP in 

this study. Similarly, a 4-h NI with different light qualities from LEDs had little or no effect on 

flowering percentage and days to visible bud on DNP such as geranium when grown indoors 

under white SSL at a PPFD of 180 μmol·m–2·s–1 (Park et al., 2017). Based on all these results, 

we recommend SSL that includes B, R, and FR radiation that creates a PPE value between 0.81 

and 0.85 to promote subsequent flowering of ornamental seedlings without excessive extension 

growth. 

FR and B radiation have antagonistic effects on chlorophyll concentration; increasing FR 

decreases chlorophyll concentration while B increases it. For example, lettuce grown indoors had 

12% less chlorophyll concentration in leaves with supplemental lighting of 160 μmol·m–2·s–1 of 

FR than without (Li and Kubota, 2009). In geranium, petunia, and snapdragon seedlings, SPAD 

value decreased linearly as the estimated PPE decreased (as the R:FR decreased) (Park and 

Runkle, 2017). In contrast, increasing B from 0 to 90 μmol·m–2·s–1 at a total PPFD of 100 

μmol·m–2·s–1 increased the chlorophyll concentration in cucumber seedlings by 50% (Hernández 

and Kubota, 2016). Similarly, the chlorophyll concentration in lettuce increased by 23% as B 

increased from 0 to 11% at a PPFD of 200 mol·m–2·s–1 (Wang et al., 2016). In this study, the 

addition of 40 μmol·m–2·s–1 of FR decreased the SPAD value of zinnia and petunia (regardless of 

the B proportion) and ≥20 μmol·m–2·s–1 of FR decreased the SPAD value of petunia under 

B60R120 (Fig. 3). Increasing B from 20 to 60 μmol·m–2·s–1 increased the SPAD value in petunia 

and zinnia by 13-14% only without FR. However, SPAD value of SSL treatments with the same 

FR photon flux density were all similar under 20 or 60 μmol·m–2·s–1of B radiation, suggesting 

that the inclusion of at least 10 μmol·m–2·s–1 of FR can diminish the effect of increasing B by 40 

μmol·m–2·s–1.  

 

101 

 

Increasing the B photon flux density inhibits plant height, leaf area, and shoot dry weight 

in many species (Hernández and Kubota, 2016; Nanya et al., 2012). However, in this study, 

increasing B from 20 to 60 μmol·m–2·s–1 had little effect on plant height, leaf area, and shoot dry 

weight in any species tested. Similarly, in cucumber seedlings, the fresh and dry mass of 

cucumber seedlings were similar as the B proportion increased from 0 to 16% at a PPFD of 54 

μmol·m–2·s–1 (Hernández and Kubota, 2014). In addition, increasing B from 0 to 30% at a PPFD 

of 100 mol·m–2·s–1 had no effect on shoot dry mass of snapdragon, impatiens, geranium, and 

petunia (Randall and Lopez, 2014). In a separate study, increasing B from 0 to 10 μmol·m–2·s–1 

inhibited plant height of impatiens, salvia, and tomato, but plants of each species under ≥ 10 

μmol·m–2·s–1 of B were generally of similar height (Wollaeger and Runkle, 2015). Considering 

that we tested 20 or 60 μmol·m–2·s–1 of B photon flux densities, apparently 20 μmol·m–2·s–1 of B 

was sufficient to saturate the suppressive effects of B on stem elongation, leaf area, and biomass 

accumulation.  

Here, we showed that including a low photon flux density of FR (10 to 40 μmol·m–2·s–1) 

in R+B SSL generally had little effect on seedling growth of the ornamental species tested except 

for the extension growth of two shade-avoiding species with the addition of 40 μmol·m–2·s–1 of 

FR (PPE ≤0.83). The inclusion of FR to create a PPE between 0.81 and 0.85 during the seedling 

stage accelerated flowering of the LDP snapdragon but did not influence the subsequent 

flowering of SDPs and DNP. The estimated PPE value for WW was similar to, or slightly higher 

than, the two B+R radiation spectra that included 40 μmol·m–2·s–1 of FR, and they had similar 

promotive effects on extension growth and subsequent flowering. A B photon flux density of 60 

μmol·m–2·s–1 produced a greater chlorophyll concentration in zinnia and petunia than 20 

μmol·m–2·s–1 of B without FR, but generally seedling growth and subsequent flowering with or 

 

102 

 

without FR were similar under the two different B photon flux densities (20 or 60 μmol·m–2·s–1). 

Therefore, SSL that includes B and has a PPE between 0.81 and 0.85 is suggested to produce 

high-quality and relatively compact seedlings with greener leaves, while accelerating the 

subsequent flowering of at least some LDPs. 

  

 

103 

 

 

APPENDIX

104 

 

Table IV-1. Spectral characteristics and average photosynthetic daily light integral (DLI) of nine 
sole-source radiation treatments in two replications (rep) delivered indoors by blue (B, 400-500 
nm), red (R, 600-700 nm), far-red (FR, 700-800 nm), and warm white (WW) light-emitting 
diodes (LEDs) or one greenhouse treatment with sunlight supplemented by high-pressure sodium 
(HPS) fixtures. The subscript values that follow each LED type indicate their photon flux density 
in µmol·m–2·s–1. 

Radiation treatment 

R:FRa 

PPEb 

Rep 1 

Rep 2 

DLI (mol·m–2·d–1) 

B20R160 

B20R160FR10 

B20R160FR20 

B20R160FR40 

B60R120 

B60R120FR10 

B60R120FR20 

B60R120FR40 

WW180 

Greenhouse 

HPS 

Sunlight 

  1:0 

16:1 

  8:1 

  4:1 

  1:0 

12:1 

  6:1 

  3:1 

  5:1 

  5:1 

  1:1 

0.88 

0.87 

0.85 

0.83 

0.87 

0.85 

0.84 

0.81 

0.83 

0.86 

0.72 

12.0 

11.7 

11.7 

11.9 

11.7 

11.5 

11.9 

11.9 

11.6 

12.1 

11.7 

11.7 

11.4 

11.6 

11.8 

11.7 

11.5 

11.8 

11.6 

12.5 

a R:FR: Ratio of the photon flux integral of red (R, 600-700 nm) and far-red (FR, 700-800 nm) 
radiation.  
b PPE: Phytochrome photoequilibria estimated following Sager et al. (1988). 
  
 

 

 

105 

 

Table IV-2. Average leaf area, dry shoot weight, and dry root weight of nine species at the end of 
the seedling stage. Lighting treatments had no statistical differences on all nine species and data 
for each species was pooled from all lighting treatments. Data for each species represent the 
mean (±SE) of two replications with 10 subsamples (plants) per replication. 

Species 
African marigold 
Coleus 
Dianthus 
Geranium 
Impatiens 
Petunia 
Snapdragon 
Tomato 
Zinnia 
a Data not collected. 
 

 

 

Dry shoot weight (g) 

Dry root weight (g) 

0.19 ± 0.06 
0.10 ± 0.03 
0.15 ± 0.03 
0.23 ± 0.06 
0.08 ± 0.04 
0.10 ± 0.03 
0.16 ± 0.04 
0.15 ± 0.04 
0.14 ± 0.03 

– a 
– 
– 
– 
– 

0.042 ± 0.02 

– 

0.023 ± 0.01 
0.062 ± 0.02 

Average leaf area 

(cm2) 

   5.96 ± 1.45 
  4.39 ± 1.56 
  3.99 ± 0.69 
11.56 ± 2.04 
  3.11 ± 0.84 
  2.37 ± 1.03 
  3.46 ± 0.79 
   6.20 ± 1.50 
   8.02 ± 1.96 

 

106 

 

Table IV-3. Days to flower after transplant, visible bud or inflorescence number, plant height, 
and leaf number of eight species at the finishing stage. Lighting treatments had no statistical 
differences on eight of the species and data for each species was pooled from all lighting 
treatments. Data for each species represent the mean (±SE) of two replications with 10 
subsamples (plants) per replication. 

Species 

Days to flower 

African marigold 
Coleus 
Dianthus 
Geranium 
Impatiens 
Petunia 
Tomato 

Zinnia 
a Data not collected. 
 

39 ± 3 

– 

55 ± 4 
51 ± 4 
19 ± 5 
24 ± 4 
10 ± 3 

27 ± 2 

 

Visible flower bud 

or inflorescence 

number 

13.8 ± 2.1 

– 

15.7 ± 2.2 
  3.0 ± 0.7 

  26.9 ± 12.3 

18.9 ± 7.8 
  6.1 ± 2.3 

  4.2 ± 1.2 

Plant height at 
flowering (cm) 

Leaf number 
at flowering 

13.8 ± 1.5 
11.6 ± 2.5 
46.7 ± 5.3 
22.9 ± 2.2 

– 

  7.9 ± 1.8 
  6.8 ± 1.2 

15.8 ± 3.0 

–a  
– 
– 
– 
– 
– 

4.2 ± 0.7 

8.2 ± 0.7 

 

107 

 

Figure IV-1. The spectral distribution of sole-source lighting treatments delivered from mint 
white (MW), red (R), blue (B), and green (G) light-emitting diodes (LEDs) at total 
photosynthetic photon flux density (PPFD) = 160 µmol∙m−2∙s−1. The subscript values after each 
LED type indicate the percentages of the total PPFD delivered from each LED type. 

 

 

 

 

108 

 

Figure IV-2. Stem length of seedlings grown under nine sole-source lighting treatments delivered 
by blue (B, 400-500 nm), red (R, 600-700 nm), far-red (FR, 700-800 nm), and warm white (WW) 
light-emitting diodes (LEDs) indoors and one greenhouse treatment (GH) with supplemental 
lighting from high-pressure sodium fixtures. The subscript values that follow each LED type 
indicate their intensity in µmol·m–2·s–1. The top, middle, bottom row represents long-day plants, 
short-day plants, and day-neutral plants, respectively. Data for each species represent the mean 
of two replications with 10 subsamples (plants) per replication. Means sharing a letter are not 
statistically different by Tukey’s honest significant difference test at P ≤ 0.05 and NS indicates 
nonsignificance. Error bars indicate the standard error and was calculated by 20 subsamples from 
two replications. 

 

 

109 

 

Figure IV-3. SPAD index value of petunia and zinnia seedlings grown under nine sole-source 
lighting treatments delivered by blue (B, 400-500 nm), red (R, 600-700 nm), far-red (FR, 700-
800 nm), and warm white (WW) light-emitting diodes (LEDs) indoors and one greenhouse 
treatment (GH) with supplemental lighting from high-pressure sodium fixtures. The subscript 
values that follow each LED type indicate their intensity in µmol·m–2·s–1. Data for each species 
represent the mean of two replications with 10 subsamples (plants) per replication. Means 
sharing a letter are not statistically different by Tukey’s honest significant difference test at P ≤ 
0.05 and NS indicates nonsignificance. Error bars indicate the standard error and was calculated 
by 20 subsamples from two replications. 

 

 
 

 

 

110 

 

Figure IV-4. Flowering characteristics of snapdragon at finishing stage when seedlings were 
grown under nine sole-source lighting treatments delivered by blue (B, 400-500 nm), red (R, 
600-700 nm), far-red (FR, 700-800 nm), and warm white (WW) light-emitting diodes (LEDs) 
indoors and one greenhouse treatment (GH) with supplemental lighting from high-pressure 
sodium fixtures. The subscript values that follow each LED type indicate their intensity in 
µmol·m–2·s–1. Data represent the mean of two replications with 10 subsamples (plants) per 
replication. Means sharing a letter are not statistically different by Tukey’s honest significant 
difference test at P ≤ 0.05 and NS indicates nonsignificance. Error bars indicate the standard 
error and was calculated by 20 subsamples from two replications. 

 

 

111 

 

 

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