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THE EFFECT OF CUMULATIVE PHOTOSYNTHETICALLY ACTIVE RADIATION
ON THE GROWTH AND FLOWERING 0F SEEDLING GERANIUM
PELARGONIUM X HORTORUM BAILEY

 

By

Virginia Lee Erickson

A THESIS

Submitted to
Michigan State University
in partia] fuifillment of the requirements

for the degree of

MASTER OF SCIENCE

Department of Horticuiture

1979

ABSTRACT

THE EFFECT OF CUMULATIVE PHOTOSYNTHETICALLY ACTIVE RADIATION
ON THE GROWTH AND FLOWERING 0F SEEDLING GERANIUM
PELARGONIUM X HORTORUM BAILEY

 

By

Virginia Lee Erickson

Two experiments were conducted to determine the effect of cumula-
tive photosynthetically active radiation (PAR) on the growth and flow-

ering of Pelarggnium x hortorum Bailey cvs. Sprinter Scarlet, Sprinter

 

White, and Ringo.

Plants were grown under 5 light levels, with all treatments re-
ceiving natural light. Treatment 1 also included 24 hours HPS and 2
layers of Saran; treatment 2, 24 hours HPS and 1 layer Saran; treatment
3, natural light only; treatment 4, 12 hour HPS; and treatment 5, 24
hour HPS. Cumulative PAR, number of days to flower, total plant height,
vegetative plant height, number of breaks, and fresh weight were taken
for each plant when the first flower on the initial inflorescence
opened.

Within experiments and cultivars, 41-65% of the variability around
the regression line for days to flower can be accounted for by
cumulative PAR. The lower the treatment light level, the greater the
number of days to flower, and the less PAR received. For each cultivar,

plants in treatment 5 flowered in significantly fewer days than plants

Virginia Lee Erickson

in treatment 1 and treatments 2, 3, and 4 which flowered in a similar
number of days.

Vegetative and total plant height decreased as supplemental light
levels increased. The number of breaks increased as light levels in-
creased; fresh weight did not show any consistent pattern.

'Sprinter Scarlet' light saturation occurred at approximately

9OO‘FE m'zsec-1, with the light compensation point being approximately

15 PE m-zsec'l.

ACKNOWLEDGMENTS

The author wishes to express sincere appreciation to the many
people who have assisted her during graduate studies and preparation of
this manuscript, especially Dr. N. Carlson, her major professor. Grate-
ful appreciation is also due Dr. J. Flore and Dr. D. Harncke for helpful
comments and valuable suggestions throughout the duration of this
research and writing of the thesis.

Appreciation is expressed to Speedling Inc. of Sun City, Florida,
and to Mr. G. Todd and Dr. B. Thomas especially, for providing the
equipment and monetary assistance to make this research possible.

Thanks are also due Dr. P. Rasmussen for the loan of the HPS
lamps.

Appreciation is also expressed to my colleagues, Mr. A. Armitage
for great assistance, Mr. M. Leon for aid in statistical computing, and

Mr. C. Sams for the photosynthetic saturation study.

11‘

TABLE OF CONTENTS

Page

LIST OF TABLES ........................ iv
LIST OF FIGURES ....................... v
INTRODUCTION ......................... 1
REVIEW OF LITERATURE ..................... 2
Systematics ....................... 2

Seed Geraniums ..................... 4
Light .......................... 7
Supplemental Light ................... 9
Lighting ........................ 13
MATERIALS AND METHODS .................... 18
RESULTS ........................... 22
DISCUSSION .......................... 36
SUMMARY AND CONCLUSIONS ................... 41
APPENDIX ........................... 42
BIBLIOGRAPHY ......................... 43

LIST OF TABLES

Table Page

1. Five light treatments used in cumulative PAR energy exper-
ments on 3 cultivars of Pelargonium x hortorum ...... . 18

 

Correlation coefficients (CC) and R2 values for number of

days to flower, vegetative plant height, total plant height,
number of breaks, and fresh weight as a function of cumula-
tive energy for 3 seed geranium cultivars. N=24 ....... 26

[\J

3. The effect of cumulative light energy (E/mz) on the average
number of days to flower for 3 seed geranium cultivars. . . 27

4. Average light energy (E/mz) received per day for 3 seed
geranium cultivars with letters frgm Duncan's multiple
range test for cumulative PAR (E/m ) and days to flower . . 29

5. The effectzof cumulative light energy (E/mz) on the average
vegetative and totaly plant height (cm) for 3 seed geranium
cultivars ..................... . . . . 3O

6. The effect of cumglative light energy (E/mz) on the average
number of breaks per plant for 3 seed geranium cultivars. 31

7. The effect of cumulative light energy (E/mz) on the average
fresh weight (g) of 3 seed geranium cultivars. . . . . . . 33

8. Degree-days in 0C(OF) for the average number of days to
flower for 3 seed geranium cultivars. . . . . . . . . . . . 34

iv

LIST OF FIGURES

Figure Page

1. Scatter plots and regression lines for 'Sprinter Scarlet'
in experiment I and experiment 11. Cumulative PAR against
number of days to flower .............. . . . . 23

2. Scatter plots and regression lines for 'Sprinter White'
in experiment I and experiment 11. Cumulative PAR against
number of days to flower. ...... . . . . . . . . . . . 24

3. Scatter plots and regression lines for 'Ringo' in experi-
ment I and experiment II. Cumulative PAR against number
of days to flower.. . . . . . . ............. . 25

4. Photosynthetic light saturation study on 'Sprinter Scarlet'
9/29/78. ................ . . . 35

5. Converting integrator counts to total microeinsteins/meterz.
(Adapted from Appendix A4 of Li- Cor Instruction Manual
for Li- 500 Integrator.) . . . . . ...... . . . . . . 42

INTRODUCTION

Historically, geraniums have been regarded as a pot plant crop;
Larsen (1968) has indicated that the major income has come from the
sale of pot plants. This attitude, however, is changing due to the
extensive development of the seed propagated F1 hybrid geranium used
for the bedding plant industry in the past 25 years (Craig, 1976). F1
hybrid geraniums represent a new concept within the floriculture in-
dustry, allowing for the production of a "bedding plant" type of ger-
anium rather than the standard 4-inch geranium pot plant now on the
market (Lindstrom, 1967).

In 1977, 45.9 million geranium pot plants were sold at a whole-
sale value of 30.8 million dollars (Crop Reporting Board, 1978). If
seedling geraniums are added, the total geranium production for 1977
would be closer to 250 million plants (Carlson, 1978), and approaching
100 million dollars a year from growers in all states, led by Mass-
achusetts, Michigan, Ohio, and New York respectively. Until the end
of World War II, the geranium was a stable crop; since then sales have
tripled (Reilly, 1977). Yearly Bedding Plant Inc. surveys have shown
the geranium to be a consistent leader in production increase. Carlson
(1977) predicted that seed geraniums will comprise 90% of the geranium
market in 10 years.

An initial seed geranium problem was the increased number of days
to flower when compared with cutting geraniums. Earlier flowering

cultivars have been developed. Scheduling the number of days to flower

with certainty is another disadvantage of seed geraniums, as delayed
or advanced flowering can cause loss of revenue and loss of market for
the grower. Light, temperature, and use of growth regulators are
important factors affecting seedling geranium growth and flowering.
The controlling influence of daylength on flowering in many plants
was established between 1910 and 1920, and with the discovery of
photoperiodism by Garner and Allard (1920) light has become an in-
creasingly important dimension in horticulture. Craig and Walker
(1963) suggested that the number of days to flower for seed geraniums
is directly related to the cumulative solar energy received by the
plant.

This study describes the effect of cumulative light energy in the
400 to 700 nm region on the growth and flowering of three cultivars of

Pelargonium x hortorum Bailey, 'Sprinter Scarlet', 'Sprinter White',

 

and 'Ringo'.

REVIEW OF LITERATURE

Systematics. The florists' or garden geranium Pelargonium includes

 

 

the garden geranium, regal pelargonium, the ivy-leaved geranium and

all of the scented geranium types (Craig, 1971). Pelargonium L'Her.

 

ex Ait. of the family Geraniaceae is native to South Africa and con-

 

tains about 280 species of annual or perennial herbs or shrubs.

Pelargonium x hortorum L.H. Bailey, the most important species for com-

 

mercial geranium production, is commonly known as the fish geranium,
zonal geranium, house geranium, horseshoe geranium, or bedding geranium

(Hortus III, 1976). The name storksbill is sometimes applied in ref-

 

erence to the schizocarp fruit (Smith, 1977).
Intercrossing of species within the sub-genus Ciconium resulted

in the hybrid species Pelargonium x hortorum (Harney, 1976). The phy-

 

logeny of the cultigen is difficult to determine and there is consider-
able controversy among horticulturists; Craig (1971), however, postu-

lated that Pelargonium zonale L., Pelargonium inquians L., Pelargonium

 

hybridum L., Pelargonium frutetorum Dyher, and Pelargonium scandens

Ehrhart are the major contributing species. Hortus III (1976) agrees

 

with Craig in that Pelargonium zonale L., and Pelargonium inquians L.

 

were the largest contributors to Pelargonium x hortorum. Craig (1971)

also notes that the species is erroneously referred to as Pelargonium

 

zonale L. the natural species.
The presence of a spur, consisting of a tube extending downward
from the base of the uppermost sepal and fused to the pedicel or flow-

er—stalk with a slightly swollen nectary at its base distinguishes

3

Pelargonium from its relatives in the Geraniaceae. Moore (1971) de-

 

 

vised a key in which the species are arranged, in part, according to
the type of spur.

Pelargonium x hortorum contains both diploids (2N=18) and tetra-

 

ploids (2N=36). Tetraploid seed set is usually low and consequently
most are asexually propagated (as are a few diploids). Seed produced
cultivars are diploids because of their inherent fertility (Craig,
1971).

Floret type (Craig, 1971) (typically 5 petals in seedling culti-
vars) is controlled by one major gene acting without dominance. The
expression of the double character is affected by modifiers and en-
vironmental conditions. Three independent genes govern flower color

in Pelargonium x hortorum; darker colors are dominant over lighter

 

colors, and epistasis plays an important role in geranium flower color.
Plants with lighter flower colors (Craig, 1968) generally flowered
faster and at a shorter height than plants with darker flower colors.
It has been postulated that the presence of a recessive gene inhibiting
anthocyanin pigments results in white flowers. Two genes control the

3 leaf zonation types Craig studied; no zone was dominant over zoning,
and anthocyanin presence dominant over its absence. Henault and

Craig (1970) concluded that plant habit was conditioned by one major

gene acting with no dominance.

Seed Geraniums. Early in 1958, Craig (1968) found that geranium seed

 

coat scarification raised the germination percentage from 40% in 3-4
weeks to 90-100% germination in 2 weeks. Craig and Walker released

'Nittany Lion Red', the first commercial variety to come true from

seed, in 1964; this cultivar was developed specifically as a bedding
plant. Subsequent seed geranium introductions are commercially avail-
able from many seed companies (Harris, Pan American, Goldsmith,
Denholm, and Sluis and Groot), and are being used and promoted as
bedding plants. The following F1 varieties are now commercially
available: 'New Era' (1966), 'Carefree' (1968), 'Sprinter' (1973),
'Cherie' (1976), 'Firecracker' (1977), 'Scarlet Flash' (1977), “Show-
girl' (1977), 'New Early' (1977), 'Sooner' (1977), and 'Ringo' (1978)
(Adams, 1978). A semidouble seed geranium was available in 1962 but
not introduced because seed production was inefficient. In 1970 a
limited series of semidouble flowered cultivars 'Double Dip' in a seed
mixture called 'Parfait' were introduced.

Progeny uniformity is a major factor required in the production of
commercial geraniums. Geraniums propagated from cuttings are uniform,
while seedling uniformity results from the genetic homozygosity of in-
bred lines and the uniformity of F1 hybrids (Craig, 1971).

Advantages of growing geraniums from seed are: seed is free from
pathogens; germination percentage is high; many flower colors are
available (Larsen, 1968); no stock plants need be maintained; seeds
cost less than cuttings; seed geraniums outperform cutting varieties
later in the season (Adams, 1978); less bench space needed in propaga-
tion areas; growth and flowering are more uniform; outdoor performance
is good (Konjoian and Tayama, 1978). Disadvantages include: lack of
floriferousness; long flowering time required (Larsen, 1968); specific
germination temperatures must be maintained; flowers are single and
petals have a tendancy to drop off prematurely during shipping or poor

environmental conditions (Adams, 1978).

Craig (1968) felt that cultivars should be adapted through breed-
into to optimum cultural conditions. Larsen (1968) found that germina-
tion percentages were almost identical for 3 scarification sites on the
geranium seed coat, and that transplanted seedlings were generally su-
perior to direct-sown seedlings. Pinching produced a short, well
branched plant at the expense of a substantial delay in flowering
(Craig and Walker, 1963; White , 1970). Lindstrom (1967), however,
reported that pinching did not delay flowering.

A low fertility level produced an almost stunted plant (Wick and
White, 1968). Plant height, fresh weight, and total number of flowers
and buds increased with increasing quantities of Osmocote for all varie-
ties tested, with leaves being too large and succulent to be of commer-
cial value. Days to flower decreased as fertility level increased.

Some growth retardants (mostly Cycocel and A-Rest) hasten flower-
ing, reduce fresh weight, total height, vegetative height, and increase

the number of breaks (Armitage t _l., 1978; Carlson, 1976; Carpenter

and Carlson, 1970). However, Semeniuk and Taylor (1970) did not find
that Cycocel affected lower initiation or time of flowering although it
supressed vegetative growth, shortened internodes, and increased the
number of basal shoots. Jansen (1973) found that Cycocel treated
plants flowered 8-14 days earlier, and flower primordia differentiation
occurred at least one week earlier on treated plants. Miranda (1979)
found that with A-Rest and Cycocel the decrease in number of days to
flower over control is similar for varying numbers of applications and
time of applications. Miranda also found that for 'Sprinter Scarlet'

A-Rest and Cycocel treated plants produced flower primordia earlier

than controls.

Fresh weight increased with increased watering for all fertility
levels except zero (White and Wick, 1968). Increased weight resulted
mostly from increased foliage size and succulence. White (1970) report-
ed that water stress was more effective than Cycocel in height control
on seedling geraniums, but that flowering was adversely affected. Plant
height, leaf area, and dry weight increased as soil moisture increased
(Metwally, gt 21,). High moisture plants had increased petiole and stem
diameters, thicker leaves, and a greater number of xylem elements.

Geranium seeds germinate uniformly in about one week if the media
is at a nonfluctuating temperature between 70 and 75°F (Stinson, 1971).
Optimum growth temperatures for geraniums are 65°F on dull days, and
70°F on bright days (Laurie §t__l,, 1968) with night temperatures of
60-620F (Stinson, 1971). Under growth chamber conditions a 75°F light
period of 12 hours (Carpenter and Carlson, 1970) resulted in a 5 and 15
day delay in flowering when the dark period was 60°F and 50°F as com-
pared to 70°F. Night temperatures of 55, 60, and 65°F (Konjoian and
Tayama, 1978) did not alter plant quality, plant height, number of flow-

ers, or number of breaks, but did decrease days to flower by 9-10 days

for each temperature increase of 50F.

Light. Light (electromagnetic energy) provides energy for photosynthe~
sis, and the stimuli for photoperiodism, and photomorphogenesis (Bick-
ford and Dunn,1972).

It has long been observed that shade affects internode and petiole
elongation, leaf area, and flowering. Gourley (1920) found that shading
decreased flowering of geranium, tomato, and nasturtium. In the same

study, anthesis in geranium was delayed 16 days by shade. These

findings were later confirmed by Gourley and Nightingale (1921), and
Craig and Walker (1961). Craig and Walker (1961) also noted that while
geranium seedlings flowered in about 90 days in summer, under winter
conditions it took twice that long; seeds sown early in the spring also
took longer to flower than those sown later.

Supplemental light from February 13 to April 9 (Laurie and Poesch,
1932) increased the size of cutting geraniums, but did not lead to ear-
lier flowering. Post (1942) found that daylength has no effect on
geranium flowering; cutting geraniums are photoperiodically neutral in
regard to flower induction. This has also been confirmed for seedling
geraniums (Carpenter and Carlson, 1969; Craig, 1963; Larsen, 1968).

Craig and Walker (1963) hypothesized that flowering in seed ger-
aniums is directly related to cumulative solar and sky radiation and not
to chronological time. Sowing at various intervals from October to
February and pinching half of each sowing they found that as a group,
pinched plants took 37 days longer to flower, and the earlier the sow-
ing date the greater the number of days to flower. Although plantings
were significantly different in mean number of days to flower, all
plantings needed equal amounts of energy to flower, non-pinched plants
55,000 g-cal/cm2 and pinched plants 76,000 g-cal/cm2 of solar energy.
Craig and Walker concluded that cumulative solar energy was a major
environmental factor controlling flowering in Pelargonium x hortorum,
and suggested that a hypothetical substance independent of number of
days and photoperiod but dependent on cumulative solar energy and
temperature influences flower production. They also suggested that
solar energy might be used to regulate the time of geranium seedling

flowering.

In a pinching study, Lindstrom (1967) found that pinched and
unpinched plants flowered in 150-160 days when sown in November. He
felt that both groups of plants must have formed floral primordia simul-
taneously when light intensity increased, and concluded that seedling
geraniums might require high light intensity for flowering.

Based on experimental evidence, Larsen (1968) postulated that seed-
ling geranium response to solar energy followed two equations:

Seedling geraniums + low solar energy = delayed flower initiation,

longer period for vegetative growth, and taller plants.

Seedling geraniums + high solar energy = accelerated flower initia-

tion, reduced period for vegetative growth, and shorter plants.
Larsen hypothesized that precise scheduling of seedling geranium flower-
ing would require knowledge of how much solar energy was received in an
area for any given time of year, plus knowledge of cultivar response to
solar energy. This information was incorporated in seedling geranium
production schedules (White and Randolph, 1971), as in the northern
hemisphere during spring months lower latitudes have greater light
intensities than higher latitudes, permitting more rapid growth and

flowering of geraniums.

Supplemental Light. Following the invention of a practical incandescent
lamp by Edison in 1879 (Evans, 1969), experiments in 'electrohorticul-
ture' by Bailey (1893) and others showed that flowering of several hort-
icultural plants could be accelerated by extending natural daylengths
with incandescent light.

Wilkins (1968) significantly hastened maturity, when young geranium

seedlings were exposed to high intensity artificial light (1000-2000

10

ft-c) under growth chamber conditions.

Carpenter and Carlson (1969) found that adding 850 ft-c of artifi-
cial light did not result in significantly earlier flowering or greater
plant height for 'Carefree'. They concluded that low intensity artifi-
cial light used as a sunlight supplement or to lengthen the photoperiod
did not cause significantly earlier flowering.

Carpenter and Rodriguez (1971) concluded that supplemental lighting

2 for 1, 2, 3, or 4 weeks was of little value since

at 14 lamp watts/ft
it did not induce earlier flowering; the benefits of supplemental light-
ing up to 4 weeks were lost relatively quickly after lighting was dis-
continued since their days to flower were similar. Geraniums lighted 6
weeks or more flowered uniformly and appeared to be similar morpholog-
ically, indicating no value of supplemental light energy received after
a threshold level.

Stinson (1971) concluded that geraniums grow slowly in periods of
low light intensity and most rapidly when light intensity is over
5000 ft-c, if the plants don't overheat.

Maturity was hastened in geranium seedlings (White and Randolph,
1971) by about 30 days when Cool-White or Wide Spectrum Gro-lux fluores-
cent lamps were used at 1000 ft-c for the first 30 days after seedling
emergence with 2000 ppm C02. They concluded that research findings
lent support to the theory that hybrid geraniums are more responsive
to an accumulation of energy units than to low energy lighting or day-
length control.

Seed geraniums (Norton, 1973) can be flowered 3 weeks earlier than
greenhouse-grown controls if grown with high intensity lighting (high

pressure sodium and metal halide lamps) in the early seedling stage.

11

Although lighting was supplied in a growing room with daylight absent,
Norton predicted a similar response if the lighting had been supplemen-
tary to daylight. He concluded that low winter light intensity and dur-
ation appear to be the primary factors in delayed flowering of seed
propagated geraniums in northern latitudes.

Carpenter and Beck (1973) used Lucalox lamps at 58W/m2 to supple-
ment natural light on bedding plants for 1 to 4 weeks after transplant-
ing and found that lighting increased plant height, root length, and
fresh weight; differences became larger between lighted and unlighted
treatments as the lighting period increased. Plants lighted 4 weeks
flowered 9 to 23 days earlier, were slightly shorter and had larger top
fresh weight than those unlighted. Benefits from supplementary light
appeared to be cumulative.

Carpenter (1974) found that both seed and cutting propagated ger-
aniums respond well to high intensity supplementary lighting. Plants
lighted during propagation and after transplanting consistently flowered
in fewer days from propagation and had as many branches, buds, and
flowers as plants lighted only after transplanting. Lighting only
during propagation was not as effective as lighting only after trans-
planting; either, however, was more effective than using only seasonal
light.

Norton (1975) used a HPS supplementary light source from 10 AM to
10 PM at 500 ft-c for 44 days on seed geraniums after transplanting in
1974. He also used ancymidol and Cycocel in both spray and drench
applications. Results showed a significant enhancement of flowering
with growth retardants, with Cycocel spray at 1500 ppm being superior

in considering cost and effectiveness. He found that lighting alone

12

resulted in 6 days earlier flowering, and that lighting was additive to
the effect of growth regulators in enhancing bloom. In 1975, Norton
conducted another group of experiments using supplementary HPS lighting
and Cycocel at 1500 ppm. Lighting was reduced to 21 days after trans-
planting with the photoperiod being increased to 24 hours. Results
were similar to the 1974 experiment. Cycocel alone enhanced flowering
slightly, and reduced height at a low cost. Supplemental lighting
alone enhanced flowering by a week and increased branching. HPS and
Cycocel together was better than either alone. Norton concluded that
flowering in F1 geraniums is enhanced by light, and that lighted plants
are stockier, have less disease problems, more breaks, and show less
transplant shock.

Konjoian and Tayama (1978) used incandescent lights (approximately
10 ft-c) and Cycocel spray at 1500 ppm on 6 sowings of geranium seeds
spaced every 2 weeks from December 6 through February 14. Plants were
grown at 55, 60, and 65°F night temperatures. They found that days to
flower steadily decreased as sowing dates progressed which was attribu-
ted to increasing light intensity in the latter part of winter and into
spring. Later propagation dates produced later flowering plants, but
actual growing time was reduced. Incandescent dusk to dawn supplemental
lighting resulted in 4-6 days earlier flowering, with slightly more
flowers and taller plants. Lights and Cycocel together produced flowers
earliest (9 days before control), and with more flowers per plant.
Cycocel alone produced flowers 3-4 days earlier than controls. Control
plants produced the most breaks, lights and Cycocel the fewest, and
lights or Cycocel alone intermediate numbers.

In an experiment to determine the joint effect of HPS lighting

13

and Cycocel (Armitage ggHgl.,1978) on the growth and flowering of F1
hybrid seed geraniums, the combination of both treatments produced the
earliest flowering (31 days less than controls), but the plants were
dwarfed and of no commercial value. The HPS lighting treatment alone
was the most effective, reducing the number of days to flower by 26,
with the plants being of good quality. Cycocel hastened flowering more
under ambient light than under supplementary light. Plants seemed most
responsive to supplementary light during periods of low light intensity

in late February and March.

Lighting. Many types of lamps have been used for supplemental and photo-
periodic lighting; most were developed for home or industry. and then
"borrowed" by horticulturists. Various light sources have been compared
(Campbell §t_gl,, 1971; Carpenter, 1974; Norton, 1973), but controversy
over the best lamp type and how light should be measured has not been
resolved.

Incandescent lights are inefficient as most of the electrical
energy is converted to heat; visible energy is largely red which can
cause excessive plant elongation if used as night lighting. Fluorescent
lamps have a better balance in light color emisssion than incandescent
lamps and give a diffuse light with high electrical energy efficiency.
Cool-White, warm white, or daylight fluorescent lamps emit predominately
in the blue-green-yellow range. Mercury lamps are similar to fluores-
cent, they are used as supplemental lighting and have accelerated plant
growth of higher quality. Without a phosphor coating inside the glass
lamp, however, mercury lamps can cause plant damage from excessive ultra-

violet emission. Phosphor coated metal halide lamps have a better color

14

balanced light emission for plant growth than mercury lamps. Sodium
vapor lamps have a spectral emission primarily of yellow-orange-red
(550-650 nm) (Carpenter, 1974).

Meijer (1971) grew gherkin seedlings under 4 different types of
fluorescent lamps and found no reproducible differences between the ef-
fects of the lamps in fresh or dry weight. In a comparison of fluores-
cent growth lamps with emission spectra adapted to plant growth and
Cool-White fluorescent lamps on bedding plants in a growing room, Norton
(1971) found that the Cool-White illumination resulted in greater fresh
weight and dry weight, and produced a more marketable plant type. He
concluded that fluorescent lamps, used either as supplementary to
natural light, or alone, appeared efficient for commercial production of
some bedding plants, and that the standard industrial type of Cool-White
was the better light source. In another experiment, Norton compared
fluorescent supplemental lighting during the day with lighting at night
and found that 7 hours of supplemental lighting during the middle of
the night period was almost as beneficial as 14 hours during the day.

High output fluorescent lamps were used alone and in combination
with tungsten and delux mercury lamps by Leiser gt_gl, (1960) in a
series of light intensity studies for each light source to measure the
effect on growth rate of Knox wheat and red kidney bean. A growth/
light intensity curve for each source was then plotted to compare light
sources by the position and slope of the response curve. The growth
response was reported in total radiant power (watts) in the visible
spectrum, but also in ft-c as the authors felt it provides for great
convenience in reporting illumination even though it does not correct

for differences in spectral distribution. Increasing intensity of each

15

light source resulted in a highly significant increase in growth rate.
Maximum wheat growth occurred under fluorescent plus mercury vapor lamps
and maximum for bean under fluorescent and tungsten. Wheat and bean
growth rates were the most reduced under fluorescent lights alone, the
authors felt this illustrated better growth could be obtained when
longer wave-length light is added. They also felt that their method

of evaluating light sources by means of growth curves is more practical
than comparing various spectral regions at uniform illumination inten-
sities as was introduced in the mid fifties.

Cook g; _l. (1971) determined the action spectrum of cucumber and
used it to compute the photosynthetic flux of lamps investigated and in-
terpret their effects. Lamps used as supplementary light were high-
pressure mercury arc (MB/U), mercury iodide (MBI/U), low pressure sodium
arc (SOX), and high pressure arc (HPS). Results indicated no signifi-
cant difference between the lamps when they were compared in terms of
photosynthetic flux. The order of promotion of plant development was
SOX, HPS, MBI, MBF, MB, and control. The rate of development of the
plants was in the same order as the photosynthetic flux from the lamps
under which they were grown.

Bickford and Dunn (1972) found the sodium vapor lamp to be the most
efficient of the high intensity discharge lights in converting input
energy to luminosity.

Norton (1973) found that at equal intensities (750-1000 ft-c) metal
halide illumination was slightly superior to high pressure sodium in
lessening the number of days to flower for seed geranium, although HPS
is considerably more efficient in converting electricity to visible

light. He felt, however, that if the lights had been installed at

16

equivalent wattage the HPS illumination would be just as effective.

Meijer (1971) grew tomatoes and cucumbers in glasshouses with high
pressure gas discharge lamps as supplementary lighting. The SON gave
the best results (efficiency about 27%) followed by the HPI lamp
(efficiency about 22%). Meijer, taking into account the gherkin study,
concluded that light quality of the visible part of the spectrum was
much less important for plant growth than has often been stated. The
efficiency of the lamp (energy output of visible radiation/energy x
100%) is a much more important factor.

Norton (1975) compared three lamps used as supplemental light, a
400W phosphor coated metal halide, a 400W HPS, and 215W Cool-White
fluorescent with internal reflector. He found that seed geranium re—
sponse was identical in terms of flowering and vegetative growth when
the three lamps were at equivalent light intensities (500 ft-c). He
concluded that from a practical view, therefore, a ft-c meter could be
used to lay out greenhouse supplementary lighting installations instead
of using the more expensive meters that measure photosynthetically
active radiation. Norton also believed spectral energy distribution of
the light source is much less important in supplementary lighting in the
greenhouse than it would be in a growth chamber.

Cambell g3 g1. (1971) reviewed characteristics and performance of
electric lamps used in horticultural applications in the United States.
They agreed that the universal meter for light measurement is the
illumination (ft-c) meter, although they caution that the information
is based on light efficient for human vision, which is not necessarily

light efficient for plant response.

17

McCree (1972) concluded that quantum flux was clearly superior to
energy flux as a measure of photosynthetically active radiation, and
that quantum flux in the 400-700 nm waveband was an acceptable defini-
tion of photosynthetic flux. He also found that the high pressure
sodium lamp produces the most visible light per watt, as well as
producing most of its light in the most photosynthetically active part

of the spectrum.

MATERIALS AND METHODS

Two cumulative PAR (400-700 nm) energy experiments were conducted

using three Pelargonium x hortorum Bailey (seed geranium) cultivars,

 

'Sprinter Scarlet', 'Sprinter White', and 'Ringo'. The 5 light treat-

ments (Table 1) were

conducted on the north side of an east-west orient-

ed house. in the Plant Science Range at Michigan State University, with

one light treatment per bench.

Table 1. Five light
ments on 3

treatments used in cumulative PAR energy experi-
cultivars of Pelargonium x hortorum.

 

 

 

 

Treatment
1 24 hours a day HPS, natural light, 2 layers Saran
2 24 hours a day HPS, natural light, 1 layer Saran
3 natural light
4 12 hours a day HPS? natural light
5 24 hours a day HPS, natural light

 

ZHPS on from 6 AM to

Seeds were sown

Speedling, Inc., Sun

6 PM.

in Speedling Mix (a peatlite mix manufactured by

City, FL) in plastic master flats on 23 February

and 12 April, 1978, and germinated in the mist bench at 21-240c;

18

19

seedlings were removed from the mist bench two days before transplanting.

The growing medium for the first experiment, Speedling Mix plus 3
kg/m3 Osmocote (18-6-12), did not allow for frequent watering under
cloudy Michigan conditions, so a more porous l:l:l medium by volume of
soil:peat:perlite with 0.8 kg/m3 superphosphate (0-20-0) was used for
the second experiment. Three weeks after sowing, 125 seedlings in the
two leaf stage of each cultivar were selected and transplanted into 7.6
cm square plastic cells.

Three days after transplanting a dimethylaminobenzenediazo sodium
sulfonate (Dexon) - pentachloronitro benzene (Terraclor) drench was
applied at a rate of 0.6 gm of 35% wettable powder Dexon and 0.3 gm of
75% wettable powder Terraclor per liter of water. Chlormequat (Cycocel)
was applied to run—off at 1500 ppm 35 and 42 days after sowing.

Soil was sampled at the beginning of the experiment, and then at
weekly intervals until the conclusion of the experiment to maintain pH
and soluble salts. Constant liquid fertilization of 200 ppm 25-0-25
was applied to plants at each irrigation. The pH of the irrigation
water was lowered to 6.0 by addition of phosphoric acid to the fertili-
zer stock solution.

Greenhouse temperatures were set at 17°C nights and 21°C days.
Actual greenhouse temperatures were measured by thermograph (Taylor
Instruments, Asheville, NC). Degree days (Table 7) were calculated
using methods of Baskerville and Emin (1968); the daily maximum and
minimum temperatures were added, divided by 2, and the base temperature
of 16.7°C subtracted.

Twenty-five seedlings of each cultivar were randomly distributed

within each of the 5 light treatments utilizing a completely randomized

20

design. Original spacing was 97 plants/m2; an outside row of guard
plants was used in each experiment. When experiment 11 plants were
transplanted they were moved onto the center portion of each bench, with
experiment I plants respaced around them. Plants were respaced (expt I
day 69, expt II day 71) when their leaves began to overlap.

All benches received natural light. Four G.E. Duraglow luminaires
with high pressure sodium (HPS) 400 watt Lucalox lamps (General Electric
Company, Hendersonville, NC) were suspended 239 cm above each of four
benches. Blackcloth was suspended between benches and along the center
aisle to block extraneous light, and pulled at 5 PM and retracted at 8
AM. Saran cloth (50% light reduction weave) was stretched at a height
of 122 cm above two of the benches, with one bench having 2 layers. The
outer perimeters of these two benches also had a single layer of Saran
hung from 122 cm above to below the bench to equalize light entering
from the side of the bench. Light treatments began on 18 March and 3
May respectively for experiment I and II.

Quantum flux density from the HPS lamps was determined with a
Lambda LI-185 Quantum/Radiometer/Photometer with Quantum Sensor Li-19OS
(Lambda Instruments Corporation, Lincoln, NE) and seedlings were placed
where light energy was most uniform. A Lambda LI-500 Integrator was
used with Quantum Sensor LI-1903 on each bench to record total cumula-
tive light energy in the 400 to 700 nm waveband, the photosynthetically
active radiation (PAR). Mathematical conversion from integrator counts
to microeinsteins/m2 can be found in the Appendix. Quantum sensors were
leveled on inverted 7.6 cm plastic cells, and placed on the bench to
measure the average light energy received.

The following observations were recorded for each plant on the day

21

the first flower on the first inflorescence opened; cumulative
PAR, number of days to flower from sowing, vegetative plant height (cm)
measured from soil line to the uppermost leaf held parallel to the soil.
total plant height (cm) from the soil line to tip of the flower petal,
number of breaks (growing point 0.5 cm from the stem or more and 3
fully developed leaves), and fresh weight (gm) of plants above the
soil line.

A photosynthetic light saturation study was done on one 'Sprinter
Scarlet' guard plant from experiment II on 9/29/78. Photosynthetic (Pn)
and photosynthetic light responses were determined using an open gas
analysis system, similar to that described by Wolf gg_gl, (1969).
Briefly, air was pumped into whole leaf chambers (Paige Instruments,
Davis CA) which enclosed the leaf. Environment control within the
chamber was held constant at 25 :_%°C; 9 % R.H. and 346—358 ppm C02.
Light intensity was varied by raising or lowering a 400W metal halide
lamp above the chamber. Air leaving the chamber was analyzed for CO2
with a Beckman 865 C02 analyzer used in the differential mode. Leaf
area was determined immediately after removing the leaf from the
chamber. Pn was expressed as mg of COde'zh'l.

Data were analyzed using the SPSS package on the Michigan State
University CDC Computer for one way analysis of variance, regression,
and Duncan's Multiple Range Test at the 0.05 level (Duncan, 1955).

Regression was used since light is a quantitative factor (Chew, 1976).

Statistics were done on each cultivar separately.

RESULTS

Plant data taken were regressed on cumulative photosynthetic
radiation. Correlation coefficients were significant at the .01 or
.05 level with exceptions of fresh weight for all 3 cultivars in exper-
iment I, and 'Ringo' vegetative height in experiment 11. Regression for
cumulative PAR against number of days to flower for 'Sprinter Scarlet',
'Sprinter White', and 'Ringo' (Figures 1-3) were negatively correlated,
with correlation coefficients significant at the .01 level. Correlation
coefficients (Table 2) relating cumulative PAR to number of days to
flower ranged from -0.64 to -0.80; vegetative plant height from a non-
significant -0.07 in experiment 11 to a high of -0.74 for 'Ringo' in
experiment I; total height from -0.45 to -0.79; number of breaks from
0.50 to 0.87; and fresh weight from 0.01 to 0.67.

Geraniums sown 23 February (experiment I) took longer to flower
and received more cumulative PAR for corresponding treatments than
those sown 12 April (experiment II) (Table 3). In general, the lower
the treatment light level, the higher the number of days to flower and
the less amount of PAR the plants received when they first flowered.
Cumulative PAR received within each of the 3 cultivars at time of
flowering in experiment I were all significantly different from each
other except for treatments 3 and 5 for 'Sprinter White'. In experi-
ment II, treatments 4 and 5 received similar amounts of cumulative PAR
within each of the 3 cultivars while all other treatments within

cultivars received different levels of cumulative PAR.

22

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28

In experiment I, for all cultivars, the number of days to flower
for treatment 1, which flowered in the greatest number of days, was sig-
nificantly different at the 5% level from the number of days to flower
for treatment 5, which flowered in the fewest number of days; the number
of days to flower for both treatments 1 and 5 were significantly differ-
ent from the number of days to flower for treatments 2, 3, and 4
which flowered in a similar number of days. The same pattern was
followed in experiment 11 by ‘Sprinter White' and 'Ringo'. 'Sprinter
Scarlet' followed the same general trend, with the variation of treat-
ments 2 and 4 flowering in a similar number of days, and treatments
2 and 3 flowering in a similar number of days.

To obtain an equalization factor, cumulative PAR for each treat-
ment, experiment, and cultivar was divided by the respective number of
days to flower to obtain an average of E/m2 received per day (Table 4).
Letters for Duncan's multiple range test at the 5% level for cumulative
PAR and number of days to flower (from Table 3) are included in Table 4.

In general, as the supplemental light level increased, the vegeta-
tive and total plant height (Table 5) decreased significantly as heights
for treatments 1, 3, and 5 are compared for all cultivars and both ex-
periments. In experiment II 'Ringo' vegetative height was similar for
all five treatments.

For all cultivars in both experiments, plants grown under the high-
est light level, treatment 5, had significantly more breaks than those
grown under natural light, treatment 3, or the lowest light level, treat-
ment 1 (Table 6); plants receiving the lowest amount of cumulative light

energy, treatment 1, had the fewest breaks.

29

Table 4. Average light energy (E/mz) received per day for 3 seed
geranium cultivars with lettgrs from Duncan's multiple range
test for cumulative PAR (E/m ) and days to flower.

 

 

 

 

 

 

 

 

 

 

 

Treatments
1 2 3 4 5

Cultivar

Sprinter Scarlet

Experiment I E/m2 a: b c d e
Days a b b b c
Ayg 4.16 6.88 10.11 11.11 11.67

Experiment 11 E/m2 a b c d d
Days a bc b c d
Ayg 3.77 6.26 9.01 10.44 11.10

Sprinter White

Experiment I E/m2 a b c d c
Days a b b b c
Ayg 4 14 6.89 10.12 11.12 11.56

Experiment II E/m2 a b c d d
Days a b b b c
Ayg 3 79 6 27 9.03 11.43 11.05

Ringo

Experiment I E/m2 a b c d e
Dgyg a b b b c
Ayg 4 13 6 87 10.06 11.03 11.60

Experiment II E/m2 a b c d d
Days a b b b c
Ayg_ 3.69 6 24 9.02 10.32 10.94

 

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32

Plants in experiment I had greater fresh weight (Table 7) for
corresponding treatments than plants in experiment II for all 3 culti-
vars. The effect of cumulative PAR on fresh weight in these experiments
did not seem to follow any consistent pattern or trend.

Plants in experiment I had a greater number of degree-days (Table
8) for corresponding treatments and cultivars than plants in experiment
II. Degree-days increased as the number of days to flower increased;
plants in treatment 1 had a greater number of degree-days than plants
in treatment 5. Degree-days for 'Sprinter Scarlet' ranged from a high
of 423°C in treatment 1 of experiment I to a low of 294°C in treatment
5 of experiment II. The other 2 cultivars followed the same pattern;
'Sprinter White' had a high of 400°C and a low of 278°C, 'Ringo' a high
of 387°C and a low of 264°C. Day temperatures averaged 26.7-27.20C for
experiments I and 11. Night temperatures averaged 14.4-150C for exper-
iment I and 16.70C for experiment 11.

'Sprinter Scarlet' light saturation (Figure 4) occurred at approx-

2 1

imately 900 uEm' sec' , which is approximately half full sunlight out-

side on a sunny summer day; the light compensation point was approxi-

mately 15 pEm'zsec-l.

33

 

 

 

 

 

 

 

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DISCUSSION

The scatter plot, regression lines, and significant negative cor-
relation coefficients for cumulative PAR against number of days to flow-
er for 'Sprinter Scarlet', 'Sprinter White', and 'Ringo' (Figures 1-3)
indicate that 41-65% of the variability in the number of days to flower
can be accounted for by cumulative PAR. This suggests that cumulative-
light energy is the most important factor in influencing the number of
days to flower, although it is not the only factor.

Craig and Walker (1963) suggested that cumulative solar energy is
a major environmental factor controlling the flowering of seed geraniums,
and that the hypothetical substance that influences flower production
was independent of photoperiod and number of days, but dependent on
cumulative solar energy. As plants in experiment I flowered with high-
er levels of cumulative PAR than those in experiment II, and there were
only 4 instances (treatments 3 and 5 for 'Sprinter White' in experiment
I and treatments 4 and 5 for all 3 cultivars in experiment 11) where
plants flowered with similar amounts of energy and a significantly dif-
ferent number of days, the data suggests that there is not a set amount
of cumulative PAR the plant must receive before it will flower, as sug-
gested in previous work by Craig and Walker. It must be kept in mind
that Craig and Walker measured all sun and sky radiation, however, while
cumulative PAR was the l1ght energy of interest in these experiments.
The connection between these experiments and previous work may be the

effect cumulative light energy has in producing carbohydrates and other

36

37
products in the plant that influence flowering.

Light, through quality, period, and intensity, is important for
plant growth and development in photosynthesis, photoperiodism, and
photomorphogenesis. It is through photosynthesis, however, that plants
are able to obtain what they require for growth, as photosynthesis con-
verts radiant energy into chemical energy in the form of carbohydrates
and other organic compounds which are used as structural components and
as a source of energy (Mastalerz, 1977). McCree (1972) maintains that
quantum flux is the most superior measure of photosynthetically active
radiation.

Proper spectral composition and high intensity of light are neces-
sary for adequate photosynthesis to take place. This builds up a carbo-
hydrate store which is probably necessary for the proper function of the
photoperiodic mechanism as well as metabolic processes (Bickford and
Dunn, 1972). According to Bickford and Dunn, Hamner (1940) clearly dem-
onstrated what has been called the "high-intensity-light reaction"
which was verified by Liverman and Bonner (1953) when they found that
sucrose treatments could substitute for sunlight exposure. It is pos-
sible that the carbohydrate levels built up much more quickly in treat-
ment 5, the highest daily light level, and hastened flowering signifi-
cantly when compared with treatment 3, natural light, and treatment 1,
the lowest light level treatment. Mastalerz (1977) stated that the
supply of photosynthates in excess of energy requirements determines the
yield and quality of a flower crop.

In these experiments, treatments 2, 3, and 4 showed similar plant
growth patterns, although the total amount of light energy they re-

ceived was significantly different from each other. Treatments l and
f

38

5 showed marked differences as well. Larsen (1968) found that the num-
ber of days from macroscopically-visible flower buds in seedling gerani-
ums to the time the first flower expanded was approximately 25 days
regardless of the plant's sowing date. Plants receiving higher levels
of cumulative PAR may have initiated flowers sooner, thereby decreasing
vegetative growth, while plants in the lowest cumulative PAR treatments
didn't initiate flowers as quickly and thus had more time to grow veg-
etatively. Plants in the higher light level treatments had more breaks
than those in lower light level treatments; apical vegetative growth is
sometimes depressed and axial growth enhanced when apical meristems
become reproductive. The high light intensity seems to have a similar
effect to Cycocel, which decreases apical vegetative growth and produces
a shorter, better branched plant.

It is likely that plants receiving high light level treatments pro-
duce more carbohydrates and therefore became reproductive more quickly
compared with plants under low light. This is not a new idea. Accord-
ing to Sachs and Hackett (1969) Kraus and Kraybill (1918) stated that
carbohydrate levels may control floral initiation and development. They
worked on tomato, another plant that needs high light levels to produce
flowers. Allsopp (1965) in a review on morphological changes concludes
that "...sugar concentration appears to play a major role in morpho-
genesis...and...some of the effects of sugar concentration are probably
relatively direct, while others represent a genetically determined re-
sponse of the plant to a particular level of sugar concentration".
Cumming (1967) suggested there might be optimal levels of carbohydrates
for reproductive development. The lowest light level treatment, treat-

ment 1, may have been producing carbohydrates at a much slower rate

39

and it therefore took much longer for a supply to build up that would
lead to inflorescence development.

The geranium seems to be quite efficient photosynthetically, as
shown by the steep slope of the line in Figure 4 as light energy is
increased at the beginning of the study. Small increases in light
energy levels resulted in much higher photosynthetic rates. Plants
grown under high light conditions have a thicker leaf, with more
chlorophyll (Greulach, 1973). 'Sprinter Scarlet', used in the photo-
synthetic study, was from experiment 11 which was grown under higher
light energy levels. Plants in experiment I, therefore, may have had
less chlorophyll and would have saturated for photosynthesis at a
lower light intensity, allowing for less carbohydrate synthesis, which
could possibly explain why they took longer to flower when compared
with experiment 11 plants.

In Table 8 it is interesting to note that although experiment I
plants took more days to flower, and received higher levels of cumula-
tive PAR, average daily energy levels are quite similar within each
treatment to those of experiment II for all cultivars. Treatment 1
averages are much less than those for treatment 5. Treatments 2, 3,
and 4 all flowered in a similar number of days within each cultivar
and experiment, although their average daily light energy levels
vary. Averages for treatments 4 and 5 are also similar, although
treatment 4 plants took longer to flower. A possible explanation for
this is that plants in treatments 3, 4, and 5 may have saturated for
photosynthesis on extremely bright days, so some of the cumulative
PAR recorded was not being used by the plant. The amount of light

energy added to treatment 4 from the HPS lamp was approximately

40
37 xxEm‘zsec'1 from 6 AM to 6 PM, which might not have had much effect
in hastening flowering when compared with natural radiant energy.

1 during the night period.

Treatment 2, however, received 23 uEm'zsec'
According to the photosynthetic study, this is enough light energy for
photosynthesis to occur. It is possible that the effect of the l

layer of Saran was compensated for by the HPS lamp at night. Treatment
l plants received an average of IOJuEm'esec-1 from the HPS lamp, so it
is not likely that they photosynthesized during evening periods.
Treatment 5 plants received an average of 40,uEm"Zsec'1 from the HPS
lamp at night, so they should have been able to continue photosynthesis
during the night. This might account for the earlier flowering of
treatment 5 plants compared with treatment 4 plants. It is likely

that duration of light energy at a level high enough for photosynthesis

to occur can also influence the number of days to flower in seedling

geraniums.

SUMMARY AND CONCLUSIONS

The two experiments run on three cultivars of seedling geraniums
indicated that light energy in the photosynthetically active region
influences the number of days to flower. Cumulative photosyntheti-
cally active radiation accounted for 41-65% of the variaton in the
number of days to flower in these experiments, so although it may be
a major controlling factor it is not the sole controlling factor, and
other factors will have to be investigated before a precise seedling
geranium scheduling mechanism can be developed.

As cumulative PAR increased, the number of days to flowering
decreased, as did total and vegetative plant height, while the
number of breaks increased. This could be a result, perhaps, of
floral initiaton occuring sooner when daily energy levels are high.

It is possible that flowering in seedling geranium might be
connected with the amount of photosynthetically active radiation the
plant receives, and the build up of photosynthetic products in the

plant.

41

APPENDIX

APPENDIX

Given: 14 countsfiua-hr
Sensor calibration constant (SCC)
Time period in hours (TPH)
Time period in seconds (TPS)

Integrator counts (IC)

IC 1

A. =45,“Em e-_h

(14 counts/lua- hr)(SCC-}1a/1000 ,uE m 2sec

 

-2 -1
m sec -hr
8' (“E = ,uEm'Zsec'1
TPH

 

-2

C. (fi m WTPS) ff m

Figure 5. Converting integrator counts to total microeinsteins/meterz.
(Adapted from Appendix A4 of Li-Cor Instruction Manual for
Li-500 Integrator.)

42

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10.

11.

12.

13.

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Konjoian, P.S., and H.K. Tayama. 1978. Production schedules for
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Larsen, R. 1968. N.C. State research on seedling geraniums: in-
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Laurie, A., D.C. Kiplinger, and K.S. Nelson. 1968. Commercial
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Miranda, R. 1979. The effect of number and time of application of
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Norton, R.A. 1973. The present and future of artificial lighting
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DISCUSSION

Correlation coefficients for regression lines of 'Sprinter
Scarlet', 'Sprinter White', and 'Ringo' were significant for both ex-
periments (Table 2) when days to flower were regressed on cumulative
PAR. For 'Sprinter Scarlet' 61% (experiment I) and 65% (experiment II)
of the variability in the number of days to flower could be accounted
for by cumulative PAR, for 'Sprinter White' 48% (experiment I) and
58% (experiment 11), and for 'Ringo' 64% (experiment I) and 41%
(experiment II). This suggests that cumulative light energy may be an
important factor in influencing the number of days to flower.

Craig and Walker (1963) suggested cumulative solar energy as a
major environmental factor controlling the flowering of seed geraniums.
As plants in experiment I flowered with higher levels of cumulative PAR
than those in experiment II, and there were only 4 instances (treat-
ments 3 and 5 for 'Sprinter White' in experiment I and treatments 4 and
5 for all 3 cultivars in experiment II) where plants flowered with
similar amounts of energy and a significantly different number of
days, the data suggests that there is not a set amount of cumulative
PAR the plant must receive before it will flower, as suggested in
previous work by Craig and Walker. The connection between these ex-
periments and previous work may be the effect cumulative light energy
has in producing carbohydrates and other products in the plant that

influence flowering.

36

37

Light, through quality, period, and intensity, is important for
plant growth through photosynthesis, photoperiodism, and photomorpho-
genesis. It is through photosynthesis, however, that plants are able
to obtain materials necessary for growth, as photosynthesis converts
radiant energy into chemical energy in the form of carbohydrates and
other organic compounds which are used as structural components and as
a source of energy (Mastalerz, 1977).

Proper spectral composition and high intensity of light are neces-
sary for adequate photosynthesis to take place. This builds up a carbo-
hydrate store which is probably necessary for the proper function of the
photoperiodic mechanism as well as metabolic processes (Bickford and
Dunn, 1972). According to Bickford and Dunn, Hamner (1940) clearly
demonstrated what has been called the "high-intensity-light reaction"
which was verified by Liverman and Bonner (1953) when they found that
sucrose treatments could substitute for sunlight exposure. It is pos-
sible that the carbohydrate levels built up much more quickly in treat-
ment 5, the highest daily light level, and hastened flowering signifi-
cantly when compared with treatment 3, natural light, and treatment 1,
the lowest light level treatment. Mastalerz (1977) stated that the
supply of photosynthates in excess of energy requirements determines the
yield and quality of a flower crop.

In these experiments, treatments 2, 3, and 4 showed similar plant
growth patterns, although the total amount of light energy they re-
ceived was significantly different from each other. Treatments 1 and
5 showed marked differences as well. Larsen (1968) found that the num-
ber of days from macroscopically-visible flower buds in seedling gerani-

ums to the time the first flower expanded was approximately 25 days

38

regardless of the plant's sowing date. Plants receiving higher levels
of cumulative PAR may have initiated flowers sooner, thereby decreasing
vegetative growth, while plants in the lowest cumulative PAR treatments
may not have initiated flowers as quickly and thus had more time to

grow vegetatively. Plants in the higher light level treatments had

more breaks than those in lower light level treatments; apical vegeta-
tive growth is sometimes depressed and axial growth enhanced when

apical meristems become reproductive. The high light intensity response
is similar to the Cycocel response which decreases apical vegetative
growth and produces a shorter, better branched plant.

It is likely that plants receiving high light level treatments
produce more carbohydrates and therefore became reproductive more quick-
ly compared with plants under low light. This is not a new idea.
According to Sachs and Hackett (1969) Kraus and Kraybill (1918) stated
that carbohydrate levels may control floral initiation and development.
They worked on tomato, another plant that needs high light levels to
produce flowers. Allsopp (1965) in a review on morphological changes
concludes that "...sugar concentration appears to play a major role in
morphogenesis...and...some of the effects of sugar concentration are
probably relatively direct, while others represent a genetically de-
termined response of the plant to a particular level of sugar concen-
tration". Cumming (1967) suggested there might be optimal levels of
carbohydrates for reproductive development. The lowest light level
treatment, treatment 1, may have been producing carbohydrates at a
much slower rate and it therefore took much longer for a supply to

build up that would lead to inflorescence development.

39

The geranium seems to be quite efficient photosynthetically, as
shown by the steep slope of the line in Figure 4 as light energy is
increased at the beginning of the study. Small increases in light
energy levels resulted in much higher photosynthetic rates. Plants
grown under high light conditions have a thicker leaf, with more
chlorophyll (Greulach, 1973). 'Sprinter Scarlet', used in the photo-
synthetic study, was from experiment II which was grown under higher
light energy levels. Plants in experiment I, therefore, may have had
less chlorophyll and would have saturated for photosynthesis at a
lower light intensity, allowing for less carbohydrate synthesis, which
could possibly explain why they took longer to flower when compared
with experiment 11 plants.

In Table 8 it is interesting to note that although experiment I
plants took more days to flower, and received higher levels of cumula-
tive PAR, average daily energy levels are quite similar within each
treatment to those of experiment II for all cultivars. Treatment 1
averages are much less than those for treatment 5. Treatments 2, 3,
and 4 all flowered in a similar number of days within each cultivar
and experiment, although their average daily light energy levels vary.
Averages for treatments 4 and 5 are also similar, although treatment
4 plants took longer to flower. A possible explanation for this is
that plants in treatments 3, 4, and 5 may have saturated for photo-
synthesis on extremely bright days, so some of the cumulative PAR
recorded was not being used by the plant. The amount of light energy
added to treatment 4 from the HPS lamp was approximately 37,uEm'2sec'1

from 6 AM to 6 PM, which might not have had much effect in hastening

4O

flowering when compared with natural radiant energy. Treatment 2, how-
ever, received 23 LLEm"‘2sec"1 during the night period. According to the
photosynthetic study, this is enough light energy for photosynthesis to
occur. It is possible that the effect of the 1 layer of Saran was
compensated for by the HPS lamp at night. Treatment 1 plants received
an average of 10 (IEm"2sec'1 from the HPS lamp, which is probably too
low for photosynthesis. Treatment 5 plants received an average of

40 uEm'zsec'1 from the HPS lamp at night, so they should have been able
to continue photosynthesis during the night. This might account for
the earlier flowering of treatment 5 plants compared with treatment 4
plants. It is likely that duration of light energy at a level high
enough for photosynthesis to occur can also influence the number of

days to flower in seedling geraniums.

SUMMARY AND CONCLUSIONS

The two experiments run on three cultivars of seedling geraniums
suggested that light energy in the photosynthetically active region may
influence the number of days to flower. Within experiment and cultivar
41-65% of the variability around the regression line for days to flower
can be accounted for by cumulative PAR. Although cumulative PAR may
be a factor in controlling number of days to flower, other factors will
have to be investigated before a precise seedling geranium scheduling
mechanism can be developed.

As cumulative PAR increased, the number of days to flowering
decreased, as did total and vegetative plant height, while the
number of breaks increased. This could be a result, perhaps, of
floral initiation occurring sooner when daily energy levels are high.

It is possible that flowering in seedling geranium might be
connected with the amount of photosynthetically active radiation the
plant receives, and the build up of photosynthetic products in the

plant.

41