Agriculture Reference
In-Depth Information
lower temperatures, shading, and climatic variation.
Researchers have recently shown that when C3 and C4 crops
(oats and peas, for example) are grown together in the same
cropping system, the complementarity in light needs helps
produce a yield advantage for the mixture (Kwabiah, 2004).
Rotations of C3 and C4 crops can also respond to changing
light conditions that occur seasonally.
germination is reduced by 70% or more. The seeds of
other plants, such as those of many of the cucurbits, have
the opposite requirement: the seed must be buried fully in
order to germinate because light actually inhibits germi-
nation. In all of these cases, a light-sensitive hormone
controls the response.
G ROWTH AND D EVELOPMENT
M EASUREMENT OF P HOTOSYNTHETIC R ATE
Once a seed germinates, the newly emerged plant begins
the process of growth and development. At any stage in
the process, light intensity or duration of light exposure
can control the plant's response, either as a stimulus for
the response or as a limiting factor.
The measurement of photosynthetic rates in the field
allows us to monitor the efficiency of energy capture in
various crops. The most accurate measurement is of actual
gas exchange by the plant. An individual leaf, plant part,
or whole plant is enclosed in a transparent chamber where
conditions are monitored and maintained as close to ambient
conditions as possible. Air is passed through the chamber
and into an IR gas analyzer so that changes in carbon
dioxide content caused by the photosynthesis-respiration
balance can be determined.
The other form of measurement is based on the weight
gain in dry biomass by the whole plant or the determina-
tion of the correlation between weight gain of specific
plant parts and the whole plant over time. For an annual
plant that begins as a seed and completes its life cycle in
a single season, net photosynthetic activity is directly
related to the dry weight of the plant at harvest. For peren-
nials, some part of the plant has to be harvested, and by
using models of whole plant development and biomass
distribution, approximate values of net photosynthetic
activity can be determined. The LAI described earlier can
also be used to estimate the potential leaf area available
for photosynthesis in a crop system, and then based on
our knowledge of approximate photosynthetic rates for
individual plants or plant parts, estimates of the photosyn-
thetic rate for the whole system can be made.
Establishment
Early seedling establishment can be very much affected
by light levels, especially when seed germination or
seedling establishment takes place under the canopy of
already established plants. Some seedlings are less
shade-tolerant than others, and have more difficulty
establishing when there is a lack of sufficient light to
maintain further plant development. An example of the
importance of differences in shade tolerance is seen in
the comparison of seedlings of white pine and sugar
maple in forests of the eastern U.S. White pine seedlings
experience a photosynthetic deficit at 10% of full sun-
light and sugar maple seedling reach it at 3%. This
difference in light compensation point means that sugar
maple is more shade tolerant than white pine, so in a
dense forest with light levels consistently below 10%,
only sugar maple seedlings will reproduce. The greater
shade tolerance of sugar maple can be an important
factor in forest succession. After logging, pines estab-
lish first, but as the forest closes in and shade deepens,
sugar maples begin to establish and eventually replace
the pines.
OTHER FORMS OF RESPONSE TO LIGHT
Plant Growth
Plants respond to light in other ways besides using light
to produce energy-rich sugars. Light has an influence on
the plant from germination of the seed to its production
of new seeds.
When a plant is surrounded by other plants, the amount
of light reaching its leaves can become limiting and com-
petition for light begins to occur. Competition for light is
especially likely in same-species plant populations or in
plant communities made up of very similar species with
very similar light needs. Stem and leaf growth can be
severely limited if competition reaches the point where a
plant is completely shaded by its neighbors. If some part
of the plant is able to emerge from the shade and reach
full sunlight, photosynthesis in that part may be able to
compensate for the shading occurring over the rest of the
plant and permit adequate development.
Many plants develop anatomically different leaves
depending on the level of shading or sun. Shade leaves
are thinner and have larger surface per unit weight, a
G ERMINATION
The seeds of many plants require light to germinate; when
buried beneath the soil they do poorly. A single, brief
exposure to light, however, such as during cultivation
when a weed seed is brought to the surface but immedi-
ately buried again as the soil is turned, can be enough
to induce germination. Other seeds need repeated expo-
sure, or even constant exposure to the light in order to
germinate. Lettuce is perhaps one of the best known exam-
ples of such a crop species — without light exposure,
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