May 2022 - 58

technically speaking
BY ERIK RUNKLE
The Shade-Avoidance Response
Most plants can tolerate a wide range of light environments.
They use different receptors, called photoreceptors, to perceive
the different dimensions of light: intensity (or quantity), quality
(or spectrum), and duration (or photoperiod). Photoreceptors
then regulate the biosynthesis of different compounds that
control plant attributes including leaf size, thickness, and
coloration; branching; stem elongation; and flowering.
Plants are innately competitive. When other plants are nearby,
their photoreceptors detect changes in the light environment
and modify their growth in an attempt to out-compete their
neighbors. This phenomenon is called the shadeavoidance
response. Understanding this response can
help growers to promote or prevent this response,
depending on the growth desired.
Shade-avoidance signals. Two major changes occur
when plants are shaded (or encroached) by others: light
intensity decreases and the light spectrum changes. The
decrease in light intensity depends on the density of the
plant canopy overhead as well as the size of adjacent
plants. Plant shading decreases the absolute amount of
light, but the magnitude of that decrease varies by the
color, or waveband, of light.
When a light particle (a photon) strikes a leaf,
it can be absorbed, transmitted or reflected. Most
photosynthetic light (blue, green, and red light)
Erik Runkle is
a professor and
floriculture
Extension specialist
in the department
of horticulture at
Michigan State
University. He can be
reached at runkleer@
msu.edu.
is absorbed by leaves, and a relatively small percentage
is reflected by or transmitted through them. In contrast,
leaves absorb less far-red light and more of it is reflected or
transmitted to other leaves. Therefore, plant shading decreases
the ratio of red light to far-red light, which is the second signal
of the shade-avoidance response.
Shade-avoidance responses. When plants perceive shade
signals, they alter their growth to increase light interception.
New leaves increase in size and are thinner, and petioles and
stems elongate. Growth of the primary stems increases and
branching is suppressed. Therefore, most plants grown under
shade (or with shade-avoidance signals) are less compact and
have thinner stems and fewer branches.
The magnitude of the shade-avoidance response increases as
the shade signal increases. In other words, extension growth
increases as the light intensity decreases and/or the red-tofar-red
ratio decreases. Virtually all types of crops have a shadeavoidance
response, but the response is stronger for plants
that grow best outdoors under high light compared with shadetolerant
crops.
Implications in controlled environments. The shadeavoidance
response is triggered by low light intensity, close
plant spacing, overhead hanging baskets, and lighting
fixtures that emit at least a modest proportion of far-red light.
Supplemental greenhouse lighting usually emits little (or
sometimes no) far-red light. In contrast, the light emitted by
Figure 1. Hanging baskets decrease the light intensity and the ratio
of red to far-red light, which trigger the shade-avoidance response
to crops below.
some low-intensity " flowering lamps " intentionally includes
the far-red spectrum. This is because the combination of red
and far-red light is most effective at regulating the flowering
of daylength-sensitive crops during light-limiting conditions.
Therefore, growth under these types of LED fixtures, or
incandescent bulbs that also emit a lot of far-red light, will
display some of the shade-avoidance responses.
Generally, shade-avoidance signals can decrease the crop
quality of full-sun ornamentals. The effect is less on shadetolerant
plants (begonia, coleus, ferns, heuchera, impatiens,
etc.) but can be noticeable when the signals are strong (e.g.,
under a dense canopy of ferns). The shade-avoidance promotion
of extension growth can be countered by applications of plant
growth retardants and/or other high-suppressing techniques,
such as a cooler day than night, also known as -DIF. However,
since light is the energy source for photosynthesis and thus
growth, nothing can be done to overcome low-light conditions
(other than supplemental lighting).
What does shade avoidance mean for the commercial
production of greenhouse crops?
* Maximize greenhouse light transmission during light-limiting
periods.
* Avoid placing hanging baskets above crops from late autumn
to early spring.
* Place shade-tolerant (not high-light) plants under baskets
whenever possible.
* More height control may be needed when plants are grown
under baskets or flowering lamps.
58 MAY 2022 GPNMAG.COM
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May 2022

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