November December 2023 - 10

technically speaking
BY ERIK RUNKLE
Crop Acclimation
Plants are sessile organisms, meaning they
can't relocate when growing conditions become
unfavorable. Thus, plants have developed
mechanisms to perceive and acclimate to the
environment so they can tolerate adverse
conditions (to a point, of course). These
responses are important for survival
outdoors but also when grown under
less stressful conditions inside controlled
environments such as greenhouses.
Understanding these responses can give
growers insights about how to manage
environmental conditions to regulate crop
growth and flowering.
FUNDAMENTALS FIRST
Temperature is the primary controller
of plant development, meaning the rate
at which cells divide and new leaves and
Erik Runkle is
professor and
floriculture
Extension specialist
in the Department
of Horticulture at
Michigan State
University. He
can be reached at
runkleer@msu.edu.
flowers form. Temperature also regulates plant
growth by influencing the rate of photosynthesis,
the biosynthesis of sugars, and respiration,
the metabolism of sugars. Light provides the
energy for photosynthesis, and typically plant
biomass increases with light intensity (i.e., the
daily light integral). Light quality, which refers
to the spectral distribution of light, influences
morphological responses such as the magnitude
of branching and elongation of stems. Plants
perceive these environmental conditions and
make cellular modifications to maximize growth
and better tolerate environmental stress.
ACCLIMATION TO TEMPERATURE
There are different ways that plants acclimate
to temperature. The first is the regulation of
photosynthesis. When the growing temperature
increases or decreases over time, cells are
modified so that the rate of photosynthesis is
re-optimized for the new growing temperature.
Second, as the temperature decreases, the
biosynthesis of some protective compounds
increases. This can change leaf color, such as
the purpling of leaves, and increase the ability
10 NOVEMBER/DECEMBER 2023 GPNMAG.COM
of a plant to better tolerate low temperatures.
Thus, exposing plants to a period of cold
but non-freezing temperatures increases the
tolerance to freezing temperatures and decreases
susceptibility to chilling damage.
ACCLIMATION TO LIGHT INTENSITY
Plants are remarkably able to modify their
anatomy based on the light environment. Light
is a form of energy, and plants must be able to
maximize it for photosynthesis and dissipate
it when excessively high. As light intensity
increases, the layers of chlorophyll inside a
leaf typically increase to more fully utilize the
available light. The thickness of the protective
outer leaf layer, the cuticle, also increases.
Combined, these result in thicker leaves. Leaf
size typically decreases with higher light, partly
to help prevent overheating. If plants are initially
grown under high light and then transferred to a
lower-light environment, the opposite will occur;
new leaves will be thinner - less chlorophyll and
thinner cuticle - but larger to better capture
available light.
ACCLIMATION TO LIGHT QUALITY
In addition to light intensity, plants have
receptors that perceive different wavebands
(colors) of light. The major ones that influence
plant growth primarily absorb blue/UV light
or red and far-red light. Plant responses to
these colors are complicated, but generally, UV
and blue light inhibit stem elongation and leaf
expansion as well as increase leaf pigmentation.
Far-red light does the opposite; it increases
extension growth and decreases leaf coloration.
When plants experience shading from other
plants, the intensity of blue, green and red light
dramatically decreases, whereas the intensity
of far-red light only decreases slightly. This is
because leaves absorb most blue, green, and red
light but reflect or transmit most far-red light.
A decrease in red light relative to far-red light
triggers a shade-avoidance response.
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November December 2023

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