Agriculture Reference
In-Depth Information
thinner epidermis, less photosynthetic pigment, spongier
leaf structure, but more stomata than sun leaves. Interest-
ingly, shade leaves often appear to be adapted to the lower
light environment, being able to photosynthesize above
the compensation point due in part to the larger surface
area for light capture. But it is important that shade leaves
be protected from the harmful effects of too much light.
In chrysanthemums, for example, the end of the far-red
phytochrome's continual dominance in autumn triggers the
growth of flowering buds. This type of response is known
as a “short-day” response, even though the actual response
is activated by the longer nighttime hours. The importance
of the dark period is accentuated by the fact that even a
short period of artificial light in the middle of the night for
greenhouse-raised mums allows for the conversion of
enough far-red phytochrome to suppress flowering.
Strawberries have the opposite type of response. In
the spring, shorter nights allow the far-red phytochrome
to regain continual dominance, causing a shift from vege-
tative production to flower production. Plants with this
kind of response are called “long-day” plants, even though
it is shorter nights that actually trigger the change. So-
called day-neutral varieties of strawberries have been
developed to extend flowering later into the summer and
early fall when normal strawberries undergo the shift to
vegetative growth characteristic of long-day plants.
Phototropism
Light can induce a plant to synthesize chlorophyll and
anthocyanins, which stimulate growth in certain plant
parts such as the leaf petiole or the flower peduncle, causing
the phenomenon of growing toward or away from light.
In some cases, this growth pattern is triggered by a hor-
mone that is activated by blue light. Leaves can be oriented
toward the sun to capture more light, or away from the
sun in high light environments. Sunflowers receive their
name from the characteristic orientation of the disc of the
inflorescence toward the morning sun.
P RODUCTION OF THE H ARVESTABLE P ORTION
OF THE P LANT
Photoperiod
Because the earth is tilted on its axis, the relative propor-
tion of daylight and nighttime hours varies from one time
of year to another. Because of the correlation of hours of
light or dark with other climatic factors, especially tem-
perature, plants have developed adaptive responses to the
changing light/dark regimes over time. Important pro-
cesses such as flowering, seed germination, leaf drop, and
pigmentation changes are examples. A pigment in plants
known as phytochrome is the major photoreceptive agent
responsible for regulating these responses.
The phytochrome pigment has two forms; one form
has an absorption peak for red light with a wavelength of
660 nm, the other has an absorption peak for far-red light
with a wavelength of 730 nm. In daylight, the red light
form is rapidly converted to the far-red form, and in the
dark, the far-red form slowly converts back to the red form.
The far-red phytochrome is biologically active and respon-
sible for the basic responses of plants to the number of
hours of light or darkness.
In the morning, after only a few minutes of light
exposure, the far-red phytochrome becomes the dominant
form and remains so throughout the day. This dominance
is maintained into the night as well, since the conversion
back to red phytochrome during darkness is slow. There-
fore, when the length of the night is relatively short, there
is insufficient time for enough far-red phytochrome to
convert to the red form, and the far-red form stays domi-
nant. However, as the number of hours of darkness
increases, a point is reached at which night is long enough
to allow a shift of dominance to the red form. Even when
this period of red dominance is short, changes occur in
the plant's response.
The conditions of the light environment have a crucial role
in the production of the part of the plant that we intend to
harvest. In general, crop plants have been selected to shunt
a great deal of photosynthate to the portions of the plant
that are harvested. In other words, the harvested portions
are major “sinks” in carbon partitioning. Nevertheless,
the ability of the plant to produce the desired amount of
biomass in its harvested parts is dependent on the condi-
tions of its light environment. By understanding the
complex relationships between plant response and light
quantity, quality, and duration of exposure as discussed
above, the light environment can be manipulated and
plants selected in order to optimize output from the
agroecosystem.
MANAGING THE LIGHT ENVIRONMENT
IN AGROECOSYSTEMS
There are two main approaches to managing the light
environment of an agroecosystem. Where light is gener-
ally not a limiting factor, management is oriented toward
accommodating the system to the excess of light that can
occur; where light is more likely to be a limiting factor,
the focus is on how to make enough light available for all
the plants present in the system.
Regions where light is not a limiting factor are gen-
erally dry regions. In these locations, the key issue in
determining the structure of the vegetation and the orga-
nization of a cropping system is usually the availability
of water, not light. Plants are usually more separated from
each other, light relations are of less importance since
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