Light - Agroecology: The Ecology of Sustainable Food Systems - page 46

In general, however, because UV radiation can be
harmful to plant tissues and because the overall level of
UV energy reaching the surface is greatly reduced, plants
have not developed many adaptations for its use. Instead
UV radiation is mostly avoided: the opaque epidermis of
most plants keeps most harmful UV radiation from enter-
ing sensitive tissue or cells. Reduction of the ozone layer
of the upper atmosphere is cause for concern because of
the potentially negative effects that excess UV radiation
can cause in both plants and animals.

germination, a plant's responses to changes to daylength,
and other plant processes. In the range beyond 3000 nm,
IR light becomes heat, and different ecological impacts
are evident. (Temperature as an ecological factor is dis-
cussed in the next chapter.)

CHARACTERISTICS OF VISIBLE LIGHT
EXPOSURE

Light energy in the visible or PAR range is converted by
photosynthesis into chemical energy, and eventually into
the biomass that drives the rest of the agroecosystem,
including the part we harvest for our own use. To increase
the efficiency of this process, it is important to understand
how the light to which plants are exposed can vary.

P HOTOSYNTHETICALLY A CTIVE R ADIATION

The light energy in the visible spectrum is of greatest
importance in agroecosystems. Depending on local
climatic conditions, it forms 40 to 60% of the total
energy of solar radiation reaching the earth's surface.
Also known as photosynthetically active radiation
(PAR), this is the light with wavelengths between 400
and 760 nm. Green plants will not grow without a com-
bination of most of the wavelengths of light in the
visible spectrum.
Not all the light in this spectrum is of equal value in
photosynthesis, however. The photoreceptors in chloro-
phyll are most absorptive of violet-blue and orange-red
light; green and yellow light is not as useful. Since chlo-
rophyll cannot absorb green light very well, most of it is
reflected back, making plants appear green. Figure 4.3
shows how the absorbance of chlorophyll varies with
wavelength. The wavelengths of light that chlorophyll
absorbs best correspond roughly to the wavelengths at
which photosynthesis is most efficient.

Q UALITY

Visible light can vary in the relative amounts of the colors
that make it up — this is referred to as the light's quality.
The largest proportion of direct sunlight at the earth's sur-
face is at the center of the visible-light spectrum, dropping
off slightly at both the violet and red ends. The diffuse light
from the sky — what occurs in the shade of a building —
is relatively higher in blue and violet light. Since different
portions of the visible light spectrum can be used for pho-
tosynthesis more efficiently than others, light quality can
have an important effect on photosynthetic efficiency.
A number of factors can cause light quality to vary.
In the interior of some cropping systems, for example,
canopy species remove most of the red and blue light,
leaving primarily transmitted green and far red light. Light
quality can therefore become a limiting factor for plants
under the canopy, even though the total amount of light
may appear to be adequate.

I NFRARED L IGHT

Infrared light energy with a wavelength from 800 to 3000
nm sometimes referred to as the near IR range — has an
important role in influencing the hormones involved in

I NTENSITY

The total energy content of all the light in the PAR range
that reaches a leaf surface is the intensity of that light.
Light intensity can be expressed in a variety of energy
units, but the most common are the langley (calories per
cm 2 ), the watt (Joules per second), and the Einstein (6 ×
10 23 photons). All of these units of measure express the
amount of energy falling on a surface over some time
period. At very high light intensities, photosynthetic pig-
ments become saturated, meaning that additional light
does not effectively increase the rate of photosynthesis.
This level of light intensity is called the saturation point .
Excessive light can lead to degradation of chlorophyll
pigments and even cause harm to plant tissue. At the other
extreme, low levels of light can bring a plant to the light
compensation point , or the level of light intensity where
the amount of photosynthate produced is equal to the
amount needed for respiration. When the light intensity

Violet Green

Ye l l o w

Orange

Blue

Red

400 500 600 700

Wavelength of Light (nm)

FIGURE 4.3 Absorbance of chlorophyll in relation to the
wavelength of light. Chlorophyll absorbs mostly violet-blue and
orange-red light; thus leaves reflect green and yellow light.