Light - Agroecology: The Ecology of Sustainable Food Systems - page 53

there is usually an overabundance of solar energy, and
many organisms must display adaptations for “avoidance”
of light rather than capture. Leaves are often vertically
oriented to avoid direct exposure to light, they have less
chlorophyll content so as to absorb less light energy and
thus less heat, and contain higher proportions of red pig-
ments so as to reflect the red light normally absorbed in
photosynthesis.
Light is more likely to be a limiting factor in humid
regions. Both natural vegetation and agroecosystems in
humid areas are much more layered or stratified, with
both light quantity and quality being altered as light
passes through those layers on its way to the soil surface.
In these regions, the management of light can be an
important factor in optimizing the productivity of agro-
ecosystems. The more stratified the vegetation structure,
the greater the challenges for light management. In
forestry and agroforestry systems, for example, the seed-
lings of the canopy species often do not germinate well
in the shaded environment of the forest floor, a factor that
must be taken into account in managing the diversity of
the system.

produce some of the highest net dry matter returns known
for a cropping system (up to 78 t dry matter/ha/yr).
Even within crops of the same photosynthetic path-
way, crop selections can be made. Different light compen-
sation points, for example, could determine which crops
to select for shadier environments.

C ROPPING D IVERSITY AND C ANOPY S TRUCTURE

The light environment in the interior of a cropping system
varies considerably. Cropping systems can be designed to
create regions in the system where the light environment
is most appropriate for a particular crop. In the tropics,
for example, farmers make full use of the altered light
environment under the canopy of trees to grow crops such
as coffee, cacao, and vanilla. Cacao and vanilla plants do
not tolerate direct sun for any appreciable amount of time,
and often they need to have the shade-producing canopy
in place before they can be planted. Only recently have
varieties of coffee been developed that can be planted in
direct sunlight.
In mixtures of annual crops, the light environment
within the canopy of the system changes as the crop
system matures, with LAI and light intensity at different
levels undergoing considerable variation over time. Farm-
ers have learned to take advantage of these changing
conditions. A well-known example is the traditional
corn-bean-squash intercrop of Mesoamerica. In a partic-
ular form of this multiple cropping system in southeastern
Mexico (Amador and Gliessman, 1990), all three crops
are planted at the same time, hence each encounters a
very similar light environment when they first emerge.
But the corn component of the system soon dominates
the canopy structure, casting shade on the beans and
squash below. As the corn canopy closes, beans occupy
the lower half to two thirds of the corn stalk by climbing
up the corn stalk. The squash is confined to the darker
understory, itself casting yet a deeper shade on the soil
surface and aiding in weed control within the cropping
system (Gliessman, 1988). Although both the beans and
squash receive less-than-optimal light exposure, they both
receive enough to produce adequately and do not interfere
with the very high light needs of the corn. Corn is a C4
crop, and beans and squash are C3. Such an agroecosys-
tem is evidence that crops of different photosynthetic
pathways can be combined in intercropping systems, and
research aimed in this direction could certainly come up
with more.
Diverse home garden agroforestry systems are per-
haps the most complex examples of the management of
the light environment in agroecosystems (Mendez, 2000,
Nair, 2001); they are discussed in much more detail in
Chapter 17. Their high LAI (3.5 to 5.0), diversity of
distribution of the canopy layers, high light absorbance
by the foliage (90 to 95%), and patchy horizontal structure

C ROP S ELECTION

One aspect of managing the light environment is to match
the availability of light in the system to the plants'
response to light. The light requirements of plants, as well
as their tolerances, are important factors in the crop selec-
tion process.
The type of photosynthetic pathway of the crop plants
is the most basic determinant of light requirements. As
discussed previously, plants with C4-type photosynthesis
require high light intensity and long duration of light expo-
sure to produce optimally, in addition to not being well
adapted to areas with cooler, moister conditions, espe-
cially cooler nighttime conditions. In contrast, many C3
plants will not grow well in the same light conditions
favored by C4 plants.
In central coastal California, for example, where the
adjacent cold ocean currents normally keep summer
nighttime temperatures at low to moderate levels and
produce regular morning fog, C4 crops such as sweet
corn are very slow to develop and rarely obtain the yields
or sweetness of the ears grown in plantings in the interior
valleys of the state just 50 mi to the east. In contrast,
many C3 crops such as lettuce grow very well in the
coastal climate.
Sugar cane is good example of a C4 crop requiring
high light intensity. When planted in areas with adequate
light and moisture, this C4 crop achieves one of the highest
rates of photosynthetic efficiency known for crop plants.
Variety selection, row arrangement, planting density,
fertility management, and other factors have been com-
bined with the 4% conversion rate of PAR to biomass to