The Ratio of Radiant Energy to Thermal Energy: A Key to Production of High Quality Plants 1996 Proposal
Key to Production of High-Quality Plants
MI 48824
Plant growth is an energetically uphill process. The energy used is
in two forms: light, or radiant energy, and temperature, or thermal energy.
Growth in the form of dry-weight accumulation is driven by the amount of
light that a plant receives during development. Growth defined as the maturation
rate of leaf, stem, and flower cells depends on the rate of biochemical
reactions, which is controlled primarily by temperature. The relationship
between average temperature and light intensity greatly influences plant
quality. However, there is no widely used system for quantification of
temperature and light combinations’ effect on plant quality. Our hypothesis
is that the quality (stem strength and thickness) of floricultural plants
is related to the ratio of radiant energy to thermal energy. This project
is designed to quantify the effects of the radiant- to thermal-energy ratio
on growth, development, and quality of poinsettia.
Specific project objectives are 1) to quantify poinsettia lateral-shoot
development responses to a range of radiant- to thermal-energy ratios,
2) to determine how the radiant- to thermal-energy ratio affects poinsettia
quality as defined above, and 33) to determine if the ratio of radiant
to thermal energy can be used in a decision-support system of greenhouse
climate control to improve plant quality. In the broadest sense, the project
may lead to a new concept of plant quality control: the use of the radiant-
to thermal-energy ratio.
The anticipated benefits to the floricultural industry are to improve
growers’ ability to meet quality standards and market-date specifications
through 1) more-accurately manipulating combinations of light intensity
and temperature for high-quality plants, 2) improving the prediction accuracy
of poinsettia growth and development, and 3) facilitating growers’ using
their resources more efficiently by understanding plant growth and development
better.
Introduction and Literature Review
Production of high-quality plants requires a combination of proper
genetics, cultural procedures, and environmental conditions. Of the five
environmental factors of plant growth, light, temperature, water, nutrients,
and gases, the first two are different forms of energy: i.e., radiant and
thermal. Plant growth is an energetically uphill process. Light is the
primary input, promoting plant growth through photosynthesis. Total dry-matter
yield of crops is affected by the irradiance absorbed and the efficiency
of its use for CO2 fixation. In this proposal, plant quality is defined
as plant caliber, dry weight, stem strength, flower number, etc. and directly
depends on the amount of radiant energy a plant receives. In contrast,
the maturation rate of leaf, stem, and flower cells depends on the rate
of biochemical reactions, which is controlled primarily by temperature.
During maturation, dry-weight gain is limited by the amount of radiant
energy to which the plant is exposed. Assuming similar light levels, plant
dry weight (and, to a great extent, plant quality) decreases as maturation
time decreases (i.e., as plants flower faster). The market tolerates, to
a certain extent, a range of plant qualities characterized by height and
diameter, stem strength, the number and size of flowers and leaves, flower
and leaf color, etc. At some subjective point, plants with thin weak stems
are rejected by the market unless their price is reduced, often substantially.
Several reports showed that plant quality is affected by different combinations
of radiant energy and temperature. For example, petunia growth rate increased
as temperature increased (Krizek et al, 1972); however, when the light
level was constant, the increased growth rate resulted in lower plant quality
because of increased plant height and less lateral branching (Kaczperski
et al., 1991; Merritt and Kohl, 1982; Piringer and Cathey, 1960). Low light
intensities, high night temperatures, or both resulted in low carbohydrate
reserves, which led to poor rose color (Post and Howland, 1946).
During the first three weeks after potting, reduction of light by shading
(Hagen and Moe, 1981; Kristoffersen, 1969), closer spacing (Hagen, 1980),
or a later potting date (Hagen and Moe, 1981) caused a reduction in the
number and growth rate of lateral breaks and flowering side shoots of poinsettia.
Increasing temperature at low light intensity reduced the number of laterals
and promoted their excessive elongation, which resulted in poor-quality
poinsettias (Kristoffersen, 1994). Therefore, reducing night and day temperatures
to conserve carbohydrates during cloudy weather is reasonable, as is increasing
night temperatures after sunny weather to help plants use manufactured
food for growth (Miller, 1959).
The preceding conclusions imply that temperatures should be reduced
when light intensity is low. Lower temperatures reduce respiration and,
consequently, carbohydrate loss. This strategy is especially important
if low light intensity severely limits photosynthesis. Under lower temperatures.
plants harvest more radiant energy between planting and flowering because
a slower development rate extends the maturation duration. Growers adjust
cultural practices, mainly temperature and time, throughout the year to
maintain acceptable plant quality. For example, chrysanthemum plants grown
in the winter often are given at least one additional week of long days
before short-day-induced flower initiation so that they receive more radiant
energy. Harris and Scott (1968) showed that a 20-day delay in anthesis
allowed the dry weight of carnation flowers in a low-temperature shaded
treatment to approximate that of unshaded carnations under higher temperatures.
There is no question that temperature should be adjusted to match light
intensity to enhance plant quality and growth (Whealy, 1992).
Our first experiment to quantify this relationship showed that under
the same high temperature, plant quality of poinsettia was increased greatly
as light level and spacing increased (Fig. 1). A similar plant quality
can be achieved under either high temperature with high light intensity
or low temperature with low light intensity. It is possible to produce
high-quality plants with a reasonable growth duration and economical artificial
energy input through adjustment of temperature and light combinations.
However, there is no system for quantification of temperature and light
combinations’ effect on plant quality. Our hypothesis is that the ratio
of the accumulated radiant energy (mol*m-1) to thermal energy (degree-days),
i.e., mol*degree-days-1*m-2 or mol*degree*days-1 per plant, is an important
parameter for plant quality control. As this ratio becomes larger (within
limits), plant quality increases. Below a certain value, plant quality
is unacceptable. If the critical ratio for a crop is known, light, temperature,
or both can be adjusted dynamically to maintain sufficient plant quality
and meet required marketing dates, an easy process for the environmental
computers currently controlling greenhouses.
This hypothesis was tested initially on poinsettias in 1995, specifically,
during the vegetative phase (i.e., from pinching to the onset of short
days). The preliminary results showed that the ratio of radiant to thermal
energy was a critical index for poinsettia quality control. Plants exposed
to low light levels and high temperatures developed thin, elongated shoots
and weak stems (Fig. 2). Higher radiant- to thermal-energy ratios produced
thicker stronger stems (Fig. 3). Fig. 3. Relationship between stem diameter
and ratio of radiant to thermal energy five weeks after pinch.
Objectives and Anticipated Benefits
The major goal of this research project is to determine how the quality
of floricultural plants is related to the ratio of radiant to thermal energy.
Specific project objectives are as follows:
1) Quantify poinsettia lateral-shoot development responses to a range
of radiant- to thermal- energy ratios (1995).
2) Quantify how the radiant- to thermal-energy ratio affects poinsettia
quality (1995 and 1996).
3) Determine if the ratio of radiant to thermal energy can be used
in a decision-support system of greenhouse climate control to improve poinsettia
plant quality (1997).
This project is designed to reveal quantitatively the interactive effects
of temperature and daily light integral on growth, development, and quality
of poinsettia. In the broadest sense, the project may lead to a new concept
of plant quality control via the radiant- to thermal-energy ratio.
The anticipated benefits to the floricultural industry are to improve
growers’ ability to meet quality standards and market- date specifications
through 1) more-accurately manipulating combinations of light intensity
and temperature for high-quality plants, 2) improving the prediction accuracy
of poinsettia growth and development and 3) facilitating growers’ using
their resources more efficiently by understanding plant growth and development
better.
Materials and Methods
This project was initiated in the fall of 1995. Initial results are
very promising, data collection and analysis continue as this proposal
is written, and we are requesting funding to complete the final two years
of the project.
Methods here describe our protocol from 1995 and expected methods during
1996.
1) Plant materials and treatments The cultivar Freedom was used. Three
factors were applied immediately after pinch to influence the thermal-
to radiant-energy ratio: temperature, light, and plant spacing. Plant spacing
became an important issue only after lateral shoots of adjacent plants
developed sufficiently to compete for light. Plants were grown under 27
combinations of temperature, light, and spacing; i.e., three levels of
constant temperature (19, 23, or 27C), three levels of daily light integral
(5, 10, or 20 mol*m-2*day-1, equivalent to 100, 200, or 400 umol*m-2*s-1
for a 14-h photoperiod), and three levels of plant spacing (very tight
[6 x 6"], medium [8.5 x 8.5"], and open [12 x 12"]). To achieve the desired
average daily temperatures, the night temperatures were calculated at sunset
each day and adjusted so that the 24-h average (sunrise to sunrise) was
close to the target values. Rooted cuttings were planted in six-inch pots
on Aug. 15 and grown under natural photoperiods until Sept. 5, when plants
were pinched. Plants then were divided among the 27 treatment combinations
and grown until Oct. 10, when they were exposed to natural short days.
From Sept. 5 to Oct 10, long days were provided by night-interruption lighting
from 2200 to 0200 HR. From flower induction until anthesis, plants were
grown under natural light conditions in a greenhouse at a mean temperature
of 20C.
2) Environmental control Temperatures were controlled by a greenhouse
climate-control computer (Priva, Model CD750, De Lier, Holland) and photosynthetic
photon flux was measured at canopy level with quantum sensors linked to
Cambell Scientific CR-10 data loggers. Different light levels were obtained
with internal greenhouse shading (sunny days) to reduce the daily light
integral and with supplemental high-pressure sodium lighting (cloudy days)
delivered during the day and after sunset if the daily light integral was
lower than the treatment setting. Five, 10, and 20 mol were achieved within
0.5 mol*m-2*day-1.
3) Experimental design A split-plot design was used with temperature
as the main plot daily light integral as the split plot, and plant spacing
as the split split plot.
4) Observations The following data were collected weekly on five plants
from each treatment: 1) stem and leaf dry weight, 2) leaf number, 3) plant
height, 4) plant diameter, 5) leaf area, 6) stem length of each lateral
shoot, and 7) stem diameter. Temperature and light data were collected
in each greenhouse section continuously.
The following data were collected at anthesis: 1) number of lateral
shoots, 2) days to flower, 3 )) plant height, 4) plant diameter, 5) stem
diameter 2, 4, 6, 8, and 10 cm from the base of the lateral shoot. 6) stem
attachment angle to the mother shoot, and 7) estimate of stem attachment
strength (plants were dropped vertically from 5, 10, 15, and 20 cm, and
the number of broken lateral shoots was recorded). Leaf unfolding number
was checked daily and the dates of visible bud, first color, and anthesis
were recorded. 1996 The experiment in 1995 resulted in large differences
in final plant size for plants exposed to similar radiant- to thermal-energy
ratios confounding comparisons.
The differences were due to large differences in thermal time (degree-days)
delivered to plants before short days started. We propose to conduct an
experiment in 1996 similar to that in 1995, except that plant development
stage at the onset of short-day induction will be the same for all treatments.
This uniformity will be achieved by transferring plants from different
treatments to short days when they reach the same thermal-time stage of
development. The base temperature used for thermal-time calculations will
be 5C. Sampling during the experiment will be based on thermal time instead
of calendar time.
This experiment will prove or disprove our hypothesis that the quality
(stem strength, huskiness, and stoutness) of floricultural plants is related
to the ratio of radiant energy to thermal energy. 1997 Based on analysis
of two years’ experimental data, a decision-support model will be constructed
for regulating light and temperature in a greenhouse to achieve a desired
plant quality. The goal is to facilitate growers’ matching their multiobjective
requirements in floricultural crop production, e.g., acceptable plant quality,
proper marketing time, high marginal return, etc., with environmental control.
This model will be tested in our greenhouses by defining minimum plant
quality characteristics, then controlling the greenhouse environment to
achieve these characteristics based on radiant and thermal energy. The
specific experimental design will be based on capabilities determined after
analysis of 1995 and 1996 data and will be presented in the request for
1997 funding.
Literature Cited
Hagen, P. 1980. Effects of plant distances the first three weeks after
potting on growth in potted poinsettia plant. Gartnervrket 70:758-760.
Hagen, P. and R. Moe. 1981. Effect of temperature and light on lateral
branching in poinsettia (Euphorbia pulcherrima Willd.) Acta Hort. 128:47-51.
Harris, G.P. and Scott, M.A. 1968. Studies on the glasshouse carnation:
Effects of light and temperature on the growth and development of the flower.
Ann. Bot. 33:143-152.
Kaczperski, M.P., W.H. Carlson. and M.G. Karlsson. 1991. Growth and
development of Petunia x hybrida as a function of temperature and irradiance.
J. Amer. Soc. Hort. Sci. II 6(2):232-237. Kristoffersen, T. 1969. Influence
of daylength and temperature on growth and development in poinsettia (Euphorbia
pulcherrima Willd.)
Acta Hort. 14:73-89. Kristoffersen, T. 1994. Early Norwegian studies
of growth and development in poinsettia p15-21. In: E. Stromme (ed.) The
scientific basis of poinsettia production. NLH Agricultural University
of Norway, N-1432 AAS, Norway.
Krizek, D.T., H.H. Klueter, and W.A. Bailey. 1972. Effects of day and
night temperature and type of container on the growth of F1 hybrid annuals
in controlled environments. Amer. J. Bot. 59(3):284-289.
Merritt, R.H. and J.C. Kohl, Jr. 1982. Effect of root temperature and
photoperiod on growth and crop productivity efficiency of petunia. J. Amer.
Soc. Hort. Sci. 107(6):997-1000.
Miller, R.O. 1959. Growth and flowering of snapdragons as affected by
night temperatures adjusted in relation to light intensity. J. Amer. Soc.
Hort. Sci. 75:761-767.
Moe, R. 1974. Nymetode for opal av roser pa egen rot. Gartneryrket 64:828-832.
Piringer, A.A. and H.M. Cathey. 1960. Effect of photoperiod, kind of
supplemental light and temperature on the growth and flowering of petunia
plants. Proc. Amer. Soc. Hort. Sci. 76:649-660.
Post, K. and J.E. Howland. 1946. The influence of nitrate level and
light intensity on the growth and production of greenhouse roses. Proc.
Amer. Soc. Hort. Sci. 47:446-450.
Schrock, D. and J.J. Hanan. 1981. The effect of low temperature on yield
and renewal cane production in relation to carbohydrate levels in roses.
Scientia Hort. 14:69-76.
Watson, D.J. 1956. Leaf growth in relation to crop yield, p. 178-191.
In: F.L. Milthorpe (ed.). The growth of leaves. Scientific Publications,
Butterworths, London.
Whealy, C.A. 1992. Introduction to floriculture, p. 52. In: R.A. Larson
(ed.). Carnations. 2nd ed. Academic Press, San Diego.
Budget
Graduate research assistant (student assistantship plus required benefits)
$17,700
Undergraduate labor to assist with data collection (500 hours at $5.00/hr)
2,500
Supplies (pots, media, labels, film, computer supplies, etc.) 2,500
Total $22,700
Qualifications of the Applicants
Dr. Heins has extensive experience studying and modeling plant
response to light and temperature under greenhouse conditions. Students
under his direction have investigated and modeled many species’ response
to light and temperature, including chrysanthemums, poinsettias, Easter
lilies, hibiscuses, Christmas cacti, African violets, and oriental lilies.
Results from these projects led to the DIF concept of height control and
the use of graphical tracking for decision support in making height-control
management decisions now in use by growers throughout the world. The Greenhouse
CARE System, a computerized graphical tracking and decision-support program,
is now marketed to American growers.
Dr. Heins has published 75 refereed scientific manuscripts and over
185 grower articles. He also has given over 145 presentations at grower
meetings.
Bin Liu is a Ph.D. student who completed her M.S. degree in China
in 1985. Before coming to the United States, she worked at the Institute
of Agrometeorology, Chinese Academy of Agricultural Sciences, Beijing,
for eight years. Her research focused on crop simulation models and the
relationship between plant growth and development and environmental factors.
