Horticulture Principles and Practices

5.3.1 PHOTOSYNTHESIS
Photosynthesis accounts for more than 90 percent of the dry matter yield of horticultural
plants and is the ultimate source of food and fossil fuel. Photosynthesis is the single
most important chemical reaction in nature. It impacts the environment significantly
through its effects on the oxygen content of the air. This major physiological process is
important not only because of its tremendous impact on a variety of functions in nature
but also because an understanding of the process enables scientists to maximize its rate
for higher crop productivity.
Photosynthesis is a reaction occurring in green plants whereby plants utilize water
and the energy of sunlight to fix inorganic carbon dioxide in the form of organic compounds,
releasing oxygen in the process. In other words, the sun’s energy is transformed
by plants through photosynthetic processes into chemical energy usable by other living
organisms. The importance of this process is more readily apparent when we understand
that plants are ultimately the source of all food. Plants may be used directly as food
(e.g., vegetables, fruits, grains, nuts, and tubers) or may be used by animals and then
indirectly become available through animal products (e.g., poultry, fish, meat, and dairy
products). Lest we limit the importance of plants to food, it should be made clear that
plants are also sources of materials for fuel, clothing, and medicines. They are utilized
widely in the beautification of the landscape and performance of other functional roles.
The general chemical reaction of photosynthesis is
Photosynthesis
The process by which
plants convert light to
chemical energy.
6CO 2 12H 2 O
green plant
light energy
C 6 H 12 O 6 6O 2 6H 2 O
This reaction occurs in the chloroplasts, using chlorophyll as an enzyme. Carbon dioxide
comes from the air and water from the soil.
Phases of Photosynthesis
Light-Dependent Reactions LIGHT The nature of the electromagnetic spectrum is
discussed in Chapter 4. Figure 4–4 shows that visible light is only a small portion of the
vast electromagnetic spectrum. Only certain wavelengths of light are involved in photosynthesis.
These specific wavelengths depend on the absorption spectrum (the range of
wavelengths of light absorbed) of the various pigments involved in photosynthesis. A pigment
may absorb a broad range of wavelengths, but certain ones are more effective than
others in performing specific functions. The action spectrum of a pigment describes the relative
effectiveness of different wavelengths of light for a specific light-dependent process
such as flowering or photosynthesis.
Light-Independent Reactions The light-independent reactions stage is also collectively
called the dark reaction of photosynthesis. At this stage, carbon dioxide is
reduced to carbohydrate, a process called carbon dioxide fixation. This stage does not
depend directly but rather indirectly on light, since the ATP and NADPH required are
produced by the light-dependent reactions. Further, the two chemicals do not accumulate
in the cell but are used up as fast as they are produced. The fixation of carbon dioxide
(CO 2 ) comes to a halt soon after the light supply is terminated.
Carbon dioxide fixation occurs by one of two major pathways, which are distinguished
by the first product formed. These are the three-carbon (C 3 ) pathway and the
four-carbon (C 4 ) pathway.
Carbon Dioxide
Fixation
A cyclical series of
reactions in which carbon
dioxide is reduced to
carbohydrate.
1. The Calvin cycle. Named after its discoverer, the first product of the Calvin cycle
is a three-carbon compound. Thus this pathway is also called the C 3 pathway.
Plants that photosynthesize by this pathway are called C 3 plants. Carbon dioxide
enters the cycle and becomes covalently bonded to a five-carbon sugar with two
phosphate groups called ribulose 1,5-bisphosphate (RuBP) (Figure 5–5). This
process (fixation) is catalyzed by the enzyme RuBP carboxylase, also commonly
5.3 Plant Growth Processes 161