Horticulture Principles and Practices

CO 2
FIGURE 5–6 The C 4 pathway
of carbon dioxide fixation.
Phosphoenol
pyruvate (PEP)
Oxaloacetate
C 4
CYCLE
Malate
ADP
Pyruvate
CO 2
RuBP
C 3
ATP
Dark reaction
Net rate of photosynthesis
C 4 plant
C 3 plant
Net rate of photosynthesis
C 4 plant
C 3 plant
FIGURE 5–7 The relative rates
of photosynthesis in C 3 and C 4
plants as influenced by (a) temperature
and (b) light intensity.
Leaf temperature
Light intensity
(a)
(b)
lawns, where C 4 species such as crabgrass (Digitaria sanguinalis) occur among C 3
species such as Kentucky bluegrass (Poa pratensis), the crabgrass grows rapidly in
summer and tends to suppress the fine-leafed species with its broad leaves.
3. The Crassulacean acid metabolism. The Crassulacean acid metabolism (CAM)
is a photosynthetic pathway that allows certain plants to fix carbon dioxide in the
dark by the activity of PEP carboxylase. Because the stomata of leaves are closed
during the hot day, CAM plants depend on CO 2 that accumulates in the leaf
during the nighttime. This reaction provides malic acid, which accumulates in the
vacuoles of cells. During the next light period, the malic acid is decarboxylated.
The resulting CO 2 is transferred to the Calvin cycle within the same cell.
Most CAM plants inhabit environments in which moisture stress and intense light
prevail. Many are succulents such as members of the cactus (Cactaceae), stone crop
(Crassulaceae), and orchid (Orchidaceae) families. Houseplants with CAM include wax
plant (Hoya carnosa) and snake plant (Sansevieria zeylanica).
CAM plants are relatively slower growing than C 3 or C 4 plants under favorable
conditions. They grow more slowly because plants, by nature, tend to conserve moisture
and in so doing close the stomata most of the day, thus limiting the CO 2 intake needed
for fixation.
5.3 Plant Growth Processes 163