February 2022 - 50

technically speaking
BY ERIK RUNKLE
Blue Light as a PGR
Plant growth retardants (PGRs) are commonly applied to
floriculture crops, both during propagation and the growing
out (finishing) phase. When properly used, PGRs are a safe
and effective way to inhibit extension growth of a wide range
of ornamental crops. It can be challenging, though, to identify
a suitable chemical and concentration because the choice
depends on the species (and sometimes cultivar), cultural
conditions, environmental conditions, magnitude of the
desired response, and, of course, active ingredient. With a few
exceptions, PGRs
cannot be applied
to edible crops
such as herbs and
vegetables, so
growers must use
other techniques
to control
extension growth.
Blue light refers
Fluor.
21
to photons with
wavelengths
between 400 and
500 nm. While
species' responses
to different
wavebands of light
vary, blue light
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.
typically inhibits extension
growth. More specifically,
plants grown under more
blue light usually have
smaller and darker leaves
and shorter internodes,
creating a more compact
plant (Figure 1). Therefore, in
many respects, blue light can act as a PGR.
A common way to manipulate the greenhouse light
communicate when asked. Because of the effects of blue light
on growth, the percentage of blue light emitted by a lighting
fixture is one of several important metrics to consider when
selecting a high-intensity lighting system.
The effects of the photon spectrum (light quality) on plant
Seedlings grown at 68 °F (20 °C) for 4 weeks
at PPFD of 160 µmol·m-2
·s-1 for 18 hours/day
% Blue light from red±blue LEDs
6.25
12.5
25
50
100
growth are more pronounced when plants are grown indoors,
without the sun. Whether grown indoors or in greenhouses,
responses to blue light are enhanced when plants are grown
under low-light conditions, such as an average daily light
integral of less than
10 mol∙m-2
∙d-1
. Plants
Figure 1. Extension growth of salvia (top) and impatiens (bottom)
was generally suppressed as the percentage of light from blue LEDs
increased. Plants were grown indoors under fluorescent lamps (left)
or blue and/or red LEDs. PPFD = photosynthetic photon flux density
(light intensity). (Photos by Heidi Lindberg)
are often more compact
when grown under highlight
levels and, therefore,
the effects of the photon
spectrum are less when
plants are grown under a
moderate to high daily
light integral.
In addition to blue light,
other wavebands can
also influence extension
growth, especially far-red
light (700 to 800 nm).
Far-red light generally
acts antagonistically to
blue light; it typically
increases extension growth.
Therefore, depending on
the crop, the effects of high
blue light can be partly
diminished by far-red
light. The vast majority of
LED products developed for
supplemental (high intensity)
environment is with supplemental lighting. When considering
photosynthetically active radiation (400 to 700 nm), only 6% of
light emitted by high-pressure sodium (HPS) fixtures is as blue
light; the rest is as green and red light. Metal halide fixtures emit
much more blue light (24%). The blue light fraction emitted by
LED fixtures depends on the number of blue and/or white LEDs
relative to red LEDs.
Conventional and LED lighting fixtures can be subjectively
classified based on the percentage of light emitted as blue: 4 to
8% can be considered as low blue, 9 to 16% as medium blue,
17 to 24% as high blue, and 25% or higher as very high blue.
In many cases, the percentage of light emitted by a lighting
fixture as blue is not explicitly stated. However, it is a metric
that lighting companies and their distributors should readily
50 FEBRUARY 2022 GPNMAG.COM
greenhouse lighting emit very little far-red light (usually less
than 2%). In contrast, HPS fixtures emit slightly more far-red
light than blue light.
In addition to inhibiting extension growth, blue light can have
other effects on crops. Probably the most visible effect is on
increasing leaf pigmentation. Blue light increases the synthesis
of secondary metabolites, some of which cause leaves to turn
more purple in color. We have also seen decreased incidence of
intumescence when plants are grown under a high percentage
of blue light.
Blue light has relatively high energy, although our visual
perception of blue light isn't very good. In other words, a high
intensity of blue light can still appear fairly dim. Because of
this high energy yet poor visibility combination, we should not
look directly at blue LEDs; doing so can cause eye strain or
damage.
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February 2022

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