By David Kuack
Growers and researchers are studying the effects that
specific light wavelengths can have on ornamental and edible crops. Research
studies are focusing on the effect light wavelengths can have on a variety of
plant processes including growth, flowering, fruiting and postharvest quality.
Michigan State University horticulture professor Erik
Runkle and former graduate student, now floriculture/nursery production extension
educator Heidi Wollaeger studied the impact the ratio of red to blue light can
have on the production of annual bedding plant seedlings. They looked at the
effects of red and blue light on impatiens, petunia, salvia and tomato plugs.
“These four species are very common bedding plants for U.S. growers,” said Wollaeger. “They are key crops for their sales. The tomato plugs were being grown as vegetable transplants and not for production as greenhouse tomatoes for fruiting.
“These four species are very common bedding plants for U.S. growers,” said Wollaeger. “They are key crops for their sales. The tomato plugs were being grown as vegetable transplants and not for production as greenhouse tomatoes for fruiting.
“We have also used these four species in other lighting
trials that we have done recently. We wanted to be able to extrapolate from one
study to another. In previous studies we looked at green light and the ratios
of blue, green and red light.”
Seed was sown into 128-cell plug trays at a commercial
propagator and moved into a large growth chamber at the university within two
days where LED and fluorescent light treatments began immediately. The plants
were Stage 2 plugs when the light treatments began. The seedlings had
cotyledons and no true leaves. They were under the light treatments throughout
the entire duration of the study.
Light treatments
The bedding plant plugs were grown in a growth chamber
equipped with six individual LED chambers. The plugs grown under fluorescent
lamps were grown in a separate growth chamber.
For all light treatments, plants were exposed to 160 micromoles
per square meter per second (µmol·m−2·s–1) for 18 hours a
day.
“We chose 10 moles per day because that is a suggested light integral for most plants to be of at least moderate quality,” Wollaeger said. “We didn’t want to deliver a light intensity much greater because as the intensity increases so does the light installation cost as well as the energy costs to run the lamps. We were implementing a practical light level for growers.”
Plug
trays of impatiens, petunia, salvia and tomato were grown in a large growth chamber at Michigan State where they received LED or fluorescent light treatments. |
“We chose 10 moles per day because that is a suggested light integral for most plants to be of at least moderate quality,” Wollaeger said. “We didn’t want to deliver a light intensity much greater because as the intensity increases so does the light installation cost as well as the energy costs to run the lamps. We were implementing a practical light level for growers.”
Wollaeger said the study was terminated after four to five
weeks because at that time the plants were ready for a commercial grower to transplant.
“This study simulated what growers would actually do in
their facilities if they were to install a sole light source LED chamber,” she
said. “They would use this high value propagation space to produce the
propagules and then transplant them and put them into the greenhouse. These
could be used by growers who are finishing the plants themselves or by a
propagator who is selling the plugs to other growers.”
Light effects on
bedding plants
Wollaeger said all of the species grown under the red
light dominant background with at least 10 µmol·m−2·s–1
of blue light displayed desirable plant growth responses.
“These plants showed compact growth, thicker leaves and
thicker stems,” she said. “As a general rule of thumb, growers should provide at
least 10 µmol·m−2·s–1 of blue light if they are providing
a red dominant environment to increase plant quality, which results in compact,
well-branched growth.
“This treatment might reduce the need for plant growth retardants.
If the light environment is being altered to include more blue light in a sole-source
environment, stem elongation is reduced. This will depend on the crop. Every
crop has a different vigor depending on the species and cultivar. This study
only looked at four commercially important species.”
Plants grown under the fluorescent lamps usually produced the most chlorophyll, but also had the thinnest leaves. Impatiens and salvia had greater fresh shoot weight when exposed to treatments without blue light than with at least 80 µmol·m−2·s–1 of blue light.
Plants grown under the fluorescent lamps usually produced the most chlorophyll, but also had the thinnest leaves. Impatiens and salvia had greater fresh shoot weight when exposed to treatments without blue light than with at least 80 µmol·m−2·s–1 of blue light.
Impatiens grown under a high proportion of blue light
developed more flower buds. Wollaeger said whether this early flowering is a
negative or positive effect depends on the plug cell size.
Impatiens
grown under a high proportion of blue light developed more flower buds than plants provided with mostly red light. |
“If the plants are being grown in a small plug size like
a 288 cell and will be transplanted into large finished containers, it might
not be desirable for early bud development,” she said. “The formation of flower
buds could impact the rooting of the plants, but that depends on the cell pack
size. If a grower is transplanting the plugs directly into smaller containers,
he might want earlier flowering.”
Reduction of
tomato intumescences
A benefit of growing tomato plugs under high blue light
levels and fluorescent lamps was the reduced incidence of leaf intumescences
(sometimes called edema), which are small protrusions that form on leaves,
stems and petioles. Wollaeger said this physiological disorder has been
associated with a lack of ultraviolet light or blue light.
“This physiological disorder is cultivar specific,” she
said. “Some cultivars are more prone to developing this disorder compared to
others. ‘Early Girl’ is the cultivar that we used and it did develop
intumescences under treatments with small amounts of blue light.”
More greenhouse
research
This particular research study did not look at the
effects of the light treatments after plants are moved into the greenhouse.
Wollaeger said Dr. Runkle’s lab is currently conducting another study to
determine the lasting effect of light treatments on transplanted plugs.
“Whether or not a light treatment has any lasting effect
once the propagules are transplanted and placed in the greenhouse is going to
depend on the light environment in the finishing location,” she said. “If the
daily light integral is at least 10 mol·m−2·d–1 during
the plug stage, there is probably going to be some height suppression when the
plants are finished in the greenhouse. I wouldn’t expect there would be a major
lasting effect of stem elongation suppression once in the greenhouse.”
For more: Heidi
Wollaeger, Michigan State University Extension, Nazareth, MI; (269) 384-8010;
wollaege@anr.msu.edu; http://msue.anr.msu.edu/experts/heidi_wollaeger.
David Kuack is a freelance technical writer in Fort
Worth, Texas; dkuack@gmail.com.
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