Maximum Yield Modern Growing | Canadian Edition | May June 2017

UVA and UVB
“
CRYPTOCHROMES,
phototropins, and
Zeitlupe (ZTL) are
the three primary
photoreceptors that
mediate the effects
of UVA. UVB light is
primarily mediated by
the UV-R8 monomer.”
Sunlight is the portion of the sun’s
electromagnetic radiation that
includes infrared, visible, and ultraviolet
(UV) light. It’s these ultraviolet light
waves that can cause harmful radiation
to our dermal tissue. But, how have our
floral friends evolved to deal with this
UV light energy?
Well, UV radiation can be harmful to
plant tissue too, but modern research is
proving there are also several distinctly
positive responses to UV radiation.
WHAT IS UV LIGHT?
Let’s take a step backwards and look
at the basics. Ultraviolet light is an
electromagnetic radiation with a
wavelength from 10-400 nanometres (nm).
This is a shorter wavelength than
visible light but longer than x-rays. This
radiation can be broken down into three
bands: UVA (320-400 nm), UVB (290-320 nm),
and UVC (100-290 nm). Radiations with
wavelengths from 10-180 nm are sometimes
referred to as vacuum, or extreme, UV.
These radiations propagate only in a
vacuum. Thanks to the ozone layer, only
UVA and UVB rays reach the Earth’s
surface. UVC rays can’t make it through,
which is lucky as they could have
catastrophic effects.
HOW DOES UV LIGHT AFFECT PLANTS?
As sunlight shines down upon a crop,
plants expose the surface of their
leaves to capture solar rays. While
some of this light energy is used in
photosynthesis, some regulates different
developmental processes—such as
advancing growth in good conditions or
causing deviations for survival during
periods of stress—to optimize the
photosynthetic processes and detect
seasonal changes. This light-mediated
development of form and structure is
known as photomorphogenesis.
The different developmental or
physiological changes are induced
by a plant’s photoreceptors, which
detect specific wavelengths of light.
Photoreceptors are also sensitive to
light quantity, quality, and duration.
For example, plants growing beneath
the canopy use phytochromes to sense
the reduced amount of light reaching
the plant and regulate such processes
as shade-avoidance, competitive
interactions, and seed germination.
However, it is extremely difficult for
scientists to match specific responses
to individual photoreceptors. Typically,
multiple photoreceptors will interact
to produce a single change. Also, some
photoreceptors like phytochromes
are sensitive to more than one light
wavelength. (Phytochromes, which
mediate many aspects of vegetative
and reproductive development, are
responsible for absorbing red and
far-red light but also absorb some blue
light and UVA radiation.)
When it comes to UV radiation, there
are several other photoreceptors
responsible for absorbing those
wavelengths. Cryptochromes,
phototropins, and Zeitlupe (ZTL) are
the three primary photoreceptors that
mediate the effects of UVA. UVB light
is primarily mediated by the UV-R8
monomer. It has been proven that UV
light influences photomorphogenic
responses including gene regulation,
flavonoid biosynthesis, leaf and
epidermal cell expansion, stomatal
density, and increased photosynthetic
efficiency. However, don’t forget that UV
radiation can also damage membranes,
DNA, and proteins. That’s why many
plants undergo photomorphogenic
changes designed to protect them from
these rays when their photoreceptors
sense the presence of radiation.
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