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Application Note
The Phytochrome System –
Why Use Far-Red?
ANO004 // JOHANN WALDHERR // DR. RICHARD BLAKEY
1 Introduction
The light requirement of plants is now known to be far more complex
than originally thought leading to the development of numerous LED
technologies that produce a variety of different light spectra, both
monochromatic and polychromatic. The worthwhile inclusion of
some wavelengths in light recipes are as yet experimental, but one
area of the spectrum that cannot be ignored is the far-red region.
Far-red encompasses wavelengths 700 – 800 nm, a region of light
that is on the edge of visibility in humans. However, these
wavelengths have been proven to result in faster growth, increased
biomass and better sensory characteristics (e.g. smell, taste,
texture, color). But why are wavelengths that aren’t used in
photosynthesis so influential on plant development? Unlike people
and animals, plants are unable to move. Their sessile existence
means that without any outside influence, plants will grow and live
in the same place for all of their existence. This may seem like a
simple observation but the consequence for plants is that they must
be able to tolerate and survive when their immediate surroundings
change to less favorable conditions. Responses to limited resources
such as water, nutrients and light, in addition to circadian and
circannual cycles are essential for plant survival. These responses
can be manipulated to achieve favorable growth characteristics. This
application note describes why these survival techniques evolved
and why far-red wavelengths are essential for plant luminaires. For
an introduction to the use of LEDs in horticultural applications,
please refer to ANO002 LEDs - The Future of Horticultural
Lighting.
2 Photoreceptors and the Phytochrome
System
Light is primarily needed for photosynthesis, the main energy
conversion mechanism of a plant and the main development factor
that is mainly driven by red and blue light via chlorophyll in
photosystems II and I. Three factors are important here:
 Light intensity - which is the amount of photons that the plant
can use.
 Photoperiodism - which reflects the duration of the exposure.
 Light quality - which corresponds to the wavelengths of light
plants are exposed.
But light also influences a number of other plant processes. Each
process can be linked to a photoreceptor that reacts to a specific
range of wavelengths. Cryptochromes sense blue/UVA light and are
responsible for phototropism and photomorphogenesis, whereas
photoreceptors called phytochromes detect far-red light (Figure 1).
Phytochromes are unlike cryptochrome blue light receptors, as the
phytochrome system is intrinsically reliant on the interplay between
two wavelengths. The system consists of two forms of phytochromes
that differ in their absorption wavelengths.
[1]
Pr (Phytochrome red)
has an absorption maximum at 660 nm and Pfr (Phytochrome far-
red) an absorption maximum at 730 nm. However, interestingly Pr
and Pfr can reversibly interconvert their molecular structure
depending on the ratio of red and far-red wavelengths (Figure 2).
Figure 1: Typical absorbance spectra of the principle pigments of plants
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0.2
0.4
0.6
0.8
1.0
400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm 750 nm
Relative Absorption (Arb.)
Wavelength
Chlorophyll A Chlorophyll B Beta Carotene Phytochrome (Pr) Phytochrome (Pfr)
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Application Note
The Phytochrome System –
Why Use Far-Red?
Figure 2: Functionality of phytochrome system.
Pfr photoreceptors are considered the active form being converted
from the Pr form in the presence of 660 nm red light. Pfr is
physiologically active, initiating biological responses but is unstable.
This means in cases of diminished or absent 660 nm light, it will
revert to the Pr form. The Pfr form is also converted to the inactive
Pr form in the presence of 730 nm far red light. Therefore, the ratio
(light quality) in addition to the exposure time (photoperiodism) and
total amount (light intensity) of red and far-red wavelengths a plant
is exposed to, effects the phytochrome system. Different ratios of
red/far-red wavelengths can trigger biological pathways that can
significantly affect the desirable characteristics of plants.
3 Morphological responses to red and far-
red light
The phytochrome system controls a variety of molecular processes,
which effect a number of morphological changes that allow plants
to adapt to its light environment. These responses to red and far-
red light evolved as responses to the environment and in some cases
are survival methods. Namely, the shade avoidance system for cases
where a plant is not receiving enough light. This can either be a
result of a solid object (obscuring all light) or other plants
(transmitting some wavelengths of light). Another corresponds to
changes in daily and seasonal light, which correspond to variations
in temperature and humidity and the response of the plant to these
conditions.
3.1. Light quality and the shade avoidance
syndrome
In some ecosystems, plants can grow in extremely high densities. In
this environment, plants compete for a limited amount of light, which
drives photosynthesis. This can be between plants growing at the
same time or growing in the shade of a taller plant. In order to
compete and survive in this environment, plants sense the amount
Figure 3: The reflected and transmitted light paths of red and far-red
wavelengths in plant ecosystems.
of shade they are in using the phytochrome system. The red and far
red light reaching a plant in a light competitive environment arrives
via a number of routes. The majority of light from the far-red region
is either reflected or transmitted by plant material, lowering the ratio
of red/far-red reaching plants in the immediate surroundings. This
can occur as reflection from surrounding plants or transmitted light
through a mature canopy (Figure 3). This means that direct sunlight
has a high proportion of red light while light transmitting or reflecting
from leaves is red deficient while having a relatively higher
proportion of far red light. The red to far-red ratio mediates the
control of the phytochrome system and consequently the shade
avoidance response in plants that are shade intolerant or sun loving.
Numerous studies have shown these responses to include
accelerated elongation of hypocotyls, internodes, and petioles,
elevated leaf angles to the horizontal and reduced branching in an
attempt to capture more sunlight and drive photosynthesis.
[2]
This
behavior is to ensure the survival of the plant. Additionally, the shade
avoidance response can trigger early flowering whereby the plant
reduces growth and starts the reproduction phase in an attempt to
ensure.
[3]
The practical applications of this depends on the desirable
characteristics of the plant being grown. In lettuce, germination is
inhibited by far red light,
[4]
but it is important to know that the
germination response here depend on the last light treatment.
[5]
Supplementation with far red light during the growth stage results
in increased shoot and root growth with higher shoot fresh weight,
and leaf area.
[6]
Eucalyptus cuttings have greater rooting success
under low red to far-red ratios.
[7]
Far-red light can increase yields of
green beans
[8]
and promote taller tomato plants.
[9]
Interestingly, far-
red can also be used to inhibit stem growth in plants where it is
undesirable to grow too tall, reducing or even eliminating the use of
growth inhibitors. The phytochrome system also controls carbon
allocation and the metabolic status in developing plants.
[10]