By C. Kohn, Waterford, WI
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The most obvious and important plant process
affected by light is photosynthesis, the creation
of sugar from water and carbon dioxide by
using the energy of light.
Many plant processes, particularly
transpiration, change during the course of the
day due to changing levels of light
Plant growth can also be affected by light; the
same plants grown in different types of light
will have different characteristics
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Visible white light is composed of all the colors
in the visible spectrum
Light is simply a form of energy
Light energy travels in waves
Three aspects of light affect plants –
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1. Quantity – brightness of light (or height of each
wave in a wavelength)
2. Quality – color of light (or width of each
wavelength, i.e. frequency)
3. Duration – amount of time light is present
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Visible light is only one portion of the
electromagnetic spectrum.
Visible light is comprised of energy with a
wavelength between 400 and 700 nanometers
(nm = 0.000000001 m. wide from peak to peak)
Longer wavelengths
create radio waves,
etc.
Shorter wavelengths
are much more
intense; e.g. X-rays,
UV rays
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Light intensity is determined by the size of the
waves.
The ‘taller’ the waves, the more intense the
light
The shorter the waves, the less intense the light
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In general, the more light a plant receives, the
higher the rate of photosynthesis
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This in turn should translate into more plant growth
and production
However, if a plant is not acclimated to bright
light (e.g. if it was started indoors and moved
outside too quickly), the pigments of the plant
can be ‘bleached’ by intense sunlight
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Plants started inside must be ‘hardened off’ or they
may suffer in bright light.
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Light intensity
increases in summer
because the rays of
the sun are directly
overhead
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This causes them to be
‘concentrated’ on the
area just below
In winter months, the
sunlight is spread out
over a larger area;
this causes the
intensity to decrease
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Light frequency is the number of times the
peak of a wavelength passes a point.
The smaller the wavelength, the greater the
frequency, causing more energy to be carried
by the light
The longer the wavelength, the smaller the
frequency, and less energy is carried by light
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Light frequency can be thought of as the color
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The longer the wavelengths, the redder the color
The shorter the wavelengths, the bluer the color
Light intensity can be thought of as brightness
The taller the waves, the brighter the light
 The shorter the waves, the dimmer the light
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These are independent of each other
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You can have dark (short)
red (long) wavelengths
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You can have bright (tall)
blue (short) wavelengths
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Pigments are any chemical substance that
absorb light
The color of the pigment is determined by the
light that is not used by the pigment, or by the
light that is reflected back into your eye
Plants have several kinds of pigments, each
that absorb different kinds of light
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Chlorophyll a is found in all photosynthetic plants
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Chlorophyll a absorbs light mostly in the violet-blue
range and reddish orange range
Chlorophyll b and carotenoids are secondary
pigments; not all plants have them
These pigments absorb light in the orange and green
ranges
 Because these types of light are less effective, chlorophyll
b and carotenoids tend to have less of an impact on plants
 The color of these pigments is usually evident only in fall
as the chlorophyll a pigments shut down.
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As seen to the right,
most of the light used by
a plant is in the bluerange and the red range.
Far-red and green are
the least utilized by a
plant
The blue and red ranges
of light are called the
Action Spectra for plants
because they stimulate
most plant activity.
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Receptors in plants called phytochromes enable
the plant to not only detect light, but also detect
the quality of light.
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Phytochromes detect red light
Under changing intensity and wavelength of
light, the phytochrome’s physical structure will
change
This can cause a
chain reaction inside
the cell, creating a
PPr
fr
physical response
Phytochromes
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For example, if a plant is blocked by the leaves
of another tree, the plant will receive more far
red light than red light
This will cause the phytochrome to change its
shape, creating a cascade of changes that will
cause a longer stem and more branching.
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Because smaller seeds have little in the way of
energy reserves, their germination tends to be
stimulated by light
Light will strike the phytochromes in small
seeds, causing them to change shape
The change phytochrome stimulates or inhibits
genes in the DNA of the plant that create
proteins related to germination.
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Cryptochromes detect blue light
These structures work similarly to
phytochromes.
Cryptochromes are responsible for…
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Inhibiting stem elongation
Moving the plant towards sunlight
Opening the stomata
Creating a circadian rhythm for a plant
 Without the cryptochromes and phytochromes, plants
would have a circadian rhythm that varied between 21
and 27 hours
RED LIGHT – HEIGHT
LIGHT
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Causes stem elongation
Germination
Branching
Promotion of flowering
(only with blue light)
Detection of day length
BLUE LIGHT – LEAF
LIGHT
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Inhibits stem elongation
Phototropism (moving a
plant towards light)
Opening Stomata
Circadian Rhythm
Leaf Growth
UNDER RED LIGHT ONLY
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Tall & Spindly
UNDER BLUE LIGHT ONLY
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Short & Stocky
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Plant physiologist Michael J. Kasperbauer made a
career of "seeing" light the way plants do: in
wavelengths, some of which cannot be detected by the
human eye.
In research done with Clemson University, he and ARS
soil scientist Patrick G. Hunt found that tomato plants
grown over red mulch yielded about 20 percent more
fruit than those grown over standard black mulch.
He later found that strawberries grown over red mulch
smelled better, tasted sweeter, and yielded more than
those grown over black mulch.
 http://www.ghorganics.com/New%20Findings%20on%20How%20Mulc
h%20Color%20Can%20Affect%20Food%20Plants.htm
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So far, we’ve covered 2 of the three factors –
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1. Light Intensity –
2. Light Quality – color of light
The third is light duration
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How long the light lasts each day (or more
accurately, how long the dark lasts each night)
affects crucial plant processes, particularly
flowering.
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The flowering period of many plants is controlled
by the photoperiod, or length of uninterrupted
darkness
There are three kinds of photoperiod:
1. Short-day plants – need long nights and short days to
flower; e.g. poinsettias
 2. Long-day plants – need short nights and long days to
flower; e.g. most vegetable crops
 3. Day-neutral – unaffected by length of day; e.g.
dandelions
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It is not the length of the day, but the length of the
night that determine this aspect of plants
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E.g. a greenhouse of poinsettias should not be lit up at
night!