Light - Agroecology: The Ecology of Sustainable Food Systems - page 43

4 Light

Light from the sun is the primary source of energy for
ecosystems. It is captured by plants through photosynthe-
sis and its energy is stored in the chemical bonds of
organic compounds. Sunlight also drives the earth's
weather: light energy transformed into heat affects rainfall
patterns, surface temperature, wind, and humidity. The
way these factors of the environment are distributed
over the face of the earth determines climate and
is of considerable importance in agriculture. All these
light-related factors will be reviewed in more detail in
subsequent chapters.
This chapter focuses on the light environment as
it directly affects agroecosystems. The light environment
includes that portion of the electromagnetic spectrum from
the invisible ultraviolet (UV), through the visible light
spectrum to the invisible infrared (IR). This chapter also
discusses how the light environment can be managed to
more efficiently channel this renewable source of energy
through the system, use it to maintain the many and
diverse functions of the system, and ultimately convert
part of it into sustainable harvests.

energy. As this light interacts with the earth's atmosphere,
several things can happen to it, as shown in Figure 4.2.
Some light is dispersed or scattered — its path toward
the surface is altered due to interference from molecules
in the atmosphere, but its wavelength is not changed in
the process. Most dispersed light reaches the surface, but
in the process gives the atmosphere its unique blue color.
Some light is reflected off the atmosphere back out into
space; its wavelength is also unchanged in the process.
Finally, some light is absorbed by water, dust, smoke,
ozone, carbon dioxide, or other gases in the atmosphere.
The absorbed energy is stored for a period of time, and
then reradiated as longer-wave heat energy. Almost all
UV light with a wavelength of 300 nm or less is absorbed
by the earth's atmosphere before it strikes the surface. (UV
light with a wavelength below 200 nm is potentially lethal
to living organisms.) The light that is not reflected off the
atmosphere or absorbed is transmitted and reaches the
surface. This energy is mostly visible light, but also
includes some UV light and IR light.
At the earth's surface, this transmitted light is
absorbed by soil, water, or organisms. Some of the
absorbed energy is reflected back into the atmosphere, and
some is reradiated as heat. It is the absorption of visible
light by plants and its role in photosynthesis that concerns
us in this chapter.

SOLAR RADIATION

The energy the earth receives from the sun arrives in the
form of electromagnetic waves varying in length from less
than 0.001 nanometers (nm) to more than 1000 million
nm. This energy makes up what is known as the electro-
magnetic spectrum. The portion of the electromagnetic
spectrum between about 1 and 1 million nm is considered
to be light, although not all of it is visible. Light with a
wavelength between 1 and 390 nm is UV light. Visible
light is the next component, made up of light with wave-
lengths between 400 and 760 nm. Light with a wavelength
longer than 760 nm and shorter than 1,000,00 nm is known
as IR light, and like UV light is invisible to the eye; when
the wavelength of IR light extends beyond 3000 nm, how-
ever, it is sensed as heat. Figure 4.1 shows how the elec-
tromagnetic spectrum is divided into types of energy.

THE ECOLOGICAL SIGNIFICANCE OF
LIGHT ON EARTH

All wavelengths of light that reach the earth's surface have
significance for the living organisms that occupy the planet.
Over evolutionary time, organisms have developed different
adaptations for accommodating themselves to the various
spectra. These adaptations vary from active energy capture
to deliberate avoidance of solar energy exposure.

UV L IGHT

Despite the fact that UV light cannot be seen, it can
be very active in certain chemical reactions in plants.
Together with the shorter wavelengths of visible light,
UV tends to promote the formation of plant pigments
known as anthocyanins, and can be involved in the
inactivation of certain hormonal systems important for
stem elongation and phototropism.

THE ATMOSPHERE AS FILTER AND
REFLECTOR

When light first arrives from the sun at the outer edge of
the earth's atmosphere, it is comprised of approximately
10% UV light, 50% visible light, and 40% IR light or heat

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