September 2021 - 58

technically speaking
BY ERIK RUNKLE
Light Spectrum for Different Applications
There are several terms used to describe the color, or spectral
distribution of light, including light quality, spectral quality, light
spectrum and photon spectrum. These terms describe the relative
quantities of different wavebands of light, such as blue light,
red light and far-red light. With light-emitting diodes (LEDs),
the peak wavelengths are also important, since " blue light "
and " red light " are rather generic and have a broad waveband
range. This article discusses the effects of the light spectrum
on plants, and conceptually how it can be used for different
greenhouse and indoor crop production applications.
Compact growth during propagation. Whether
growing in vertical
farms or in
greenhouses, growers
strive to produce
plugs and liners that
have compact growth.
The light spectrum
and intensity regulate
extension growth and
can act as a growth
regulator. Extension
growth is typically
suppressed when
the light intensity is
moderately high, 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.
Although compact
growth is a common goal
for ornamental production,
there are some situations
by blue light. When sunlight
is a limiting factor in greenhouses (during the winter),
supplemental lighting can accelerate rooting as well as
produce more compact young plants. This is especially true
when fixtures emit at least a moderate percentage of blue light
(such as 10 to 15%) and little or no far-red light.
Increase leaf size and stem length. Although compact growth
is a common goal for ornamental production, there are some
situations where larger leaves and taller stems are desired.
A low-light environment can trigger both of these elongation
responses, but usually crop quality is negatively impacted, and
flowering can be delayed. A more effective way to increase leaf
size is to decrease the intensity of blue light that reaches plants
by delivering light that includes far-red, or both.
Some companies offer supplemental lighting products
that emit a modest proportion of far-red light. While not very
common, this type of fixture can generally produce plants that
are less compact than lighting with little or no far-red light. If
you grow crops that have undesirably small leaves and/or are
too compact, such as some begonia varieties, this may be a
worthwhile investment.
Alternatively, extension growth can be increased by delivering
daylength extension (end-of-day) lighting that includes far-red
where larger leaves and taller
stems are desired.
light. Incandescent lamps and LED fixtures that emit far-red
light (sometimes called " flowering lamps " ) are both effective
when used beginning around sunset for at least two hours. This
promotion of extension growth often increases as the duration
of lighting increases.
Increase leaf coloration. There are many different pigments
in plants, and their concentrations vary widely from one species
or cultivar to another. Pigment biosynthesis and degradation
are regulated by cultural and environmental conditions,
including by the light environment. Factors or conditions that
promote extension growth, such as far-red light or low-light
intensity, typically decrease the
concentration of pigments per
unit of leaf area. As a result,
plants appear lighter green and
lack purple pigments. Growing
practices that inhibit extension
growth, such as a high light
environment and delivery
of blue and/or ultraviolet
(UV) light, usually increase
pigmentation concentration and
thus leaf coloration.
Because blue and especially
UV photons have high energy,
pigments accumulate to protect
plants from the high energy.
This is why plants grown under high light and/or with at least
a moderate percentage of blue and/or UV have darker green
or purple leaves. Therefore, if your objective is to increase
pigmentation, supplemental lighting, especially when rich in blue
light, will produce plants with more vivid leaf and flower colors.
Accelerate flowering of long-day plants. When the natural
days are short and solar light levels are low, delivery of both red
and far-red light as day-extension or night-interruption lighting
will promote flowering of many long-day plants. Similarly, when
plants are grown indoors, inclusion of far-red light induces early
flowering of at least some long-day plants. Use of lighting fixtures
that emit far-red light generally has no positive or negative
impact on flowering time of short-day or day-neutral plants.
In summary, growers can use lighting to regulate plant
growth and pigmentation. Blue light typically inhibits
elongation of leaves and stems, but also increases
pigmentation that makes leaves a darker green or purple.
Far-red light does the opposite: it promotes extension growth
but typically decreases leaf pigmentation. The effects of these
wavebands are relatively large when the daily light integral
is low, and decrease as plants are grown under progressively
higher light levels.
58 SEPTEMBER 2021 GPNMAG.COM
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September 2021

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