The Plant - Agroecology: The Ecology of Sustainable Food Systems - page 36

H 2 O CO 2

AT P
NADPH

Photophosphorylation

Calvin Cycle

ADP
P
NADP +

O 2 Carbohydrates

FIGURE 3.1 Basic processes of photosynthesis . Photophosphorylation is another name for what occurs during the light reactions;
the Calvin Cycle is the basis of the dark reactions.

C3 Photosynthesis stomata must be open during the day to take in carbon

dioxide, C3 plants are subject to photosynthetic limitation
during times of heat or drought stress: the closure of the
stomata to prevent moisture loss also limits the intake of
carbon dioxide and increases photorespiration. Common
crops that use C3 photosynthesis are beans, squash, and
tomatoes.

The most widespread type of photosynthesis is known as
C3 photosynthesis. The name comes from the fact that the
first stable compound formed in the dark reactions is a
three-carbon compound. In plants that use this pathway,
carbon dioxide is taken in during the day through open
stomata and used in the dark reactions to form glucose.
C3 photosynthesis plants do well under relatively cool
conditions, since their optimum temperature for photosyn-
thesis is relatively low (Table 3.1). However, because their

C4 Photosynthesis

A more recently discovered form of photosynthesis is
known as the C4 type. In this system, CO 2 is incorporated
into four-carbon compounds before it enters the dark reac-
tions. This initial carbon-fixing process takes place in
special chlorophyll-containing cells in the leaf. The four-
carbon compound is transported to special cells known as
bundle sheaths, clustered around veins in the leaves, where
enzymes break loose the extra carbon as CO 2 . The CO 2 is
then used to form the three-carbon compounds used in the
dark reactions, just as in C3 photosynthesis.
The C4 pathway allows carbon fixation to occur at
much lower concentrations of CO 2 than does the C3 path-
way. This enables photosynthesis to take place while the
stomata are closed, with CO 2 liberated by internal respira-
tion being captured rather than CO 2 from outside air. The
C4 pathway also prevents photorespiration from occurring
because it makes it much more difficult for O 2 to compete
with CO 2 in the dark reactions. Thus, photosynthesis in C4
plants can occur under conditions of moisture and temper-
ature stress, when photosynthesis in C3 plants would be
limited. At the same time, C4 plants usually have a higher
optimum temperature for photosynthesis.
C4 plants therefore use less moisture during times of
high photosynthetic potential, and under warm and dry con-
ditions have higher net photosynthesis and higher biomass
accumulation than C3 plants (Table 3.2). C4 photosynthesis

TABLE 3.1
Comparison of the Three Photosynthetic Pathways

C3 C4 CAM

Light saturation
point (foot-candles)

3000-6000 8000-10,000 ?

Optimum
temperature (°C)

15-30 30-45 30-35

CO 2 compensation
point (ppm of CO 2 )

30-70 0-10 0-4

Maximum
photosynthetic rate
(mg CO 2 /dm 2 /h)

15-35 30-45 3-13

Maximum growth
rate (g/dm 2 /d)

1 4 0.02

Photorespiration High Low Low

Stomata behavior Open day,

closed night

Open or
closed day,
closed night

Closed day,
open night

Source: Larcher, W. 1980. Physiological Plant Ecology . Springer-Ver-
lag: New York; Larcher, W. 1995. Physiological Plant Ecology . 3rd ed.
Springer: New York; Etherington, J. R. 1995. Environment and Plant
Ecology . 3rd ed. John Wiley and Sons: New York.