The Plant - Agroecology: The Ecology of Sustainable Food Systems - page 37

community. An important crop plant using CAM photo-
synthesis is pineapple, a member of the Bromeliaceae.

TA B L E 3 . 2
Comparison of Net Photosynthetic Rates among C3
and C4 Plants

The Photosynthetic Pathways Compared

A comparison of the different photosynthetic pathways
is presented in Table 3.1. The different arrangements of
chloroplasts within the leaves of each type are correlated
with different responses to light, temperature, and water.
C3 plants tend to have their peak rate of photosynthesis
at moderate light intensities and temperatures, while
actually being inhibited by excess light exposure and
high temperatures. C4 plants are better adapted to high
light and temperature conditions, and with the ability to
close stomata during daylight hours in response to high
temperature and evaporative stress, they can use water
more efficiently under these conditions. CAM plants can
withstand the most consistently hot and dry conditions,
keeping stomata closed during daylight hours, but they
sacrifice growth and photosynthetic rates in exchange
for tolerance of extreme conditions.
Despite the greater photosynthetic efficiency of C4
plants, C3 plants such as rice and wheat are responsible
for the great bulk of world food production. The superiority
of C4 photosynthesis makes a difference only when the
ability of the crop to convert light into biomass is the sole
limiting factor, a situation that seldom occurs in the field.

Net Photosynthetic Rate (mg
CO 2 /dm 2 leaf area/h) a Crop Type

C3 plants
Spinach

16

Tobacco 16-21

Wheat 17-31

Rice 12-30

Bean 12-17

C4 plants
Corn 46-63

Sugar cane 42-49

Sorghum 55

Bermuda grass 35-43

Pigweed ( Amaranthus ) 58

a Determined under high light intensity and warm temperatures
(20-30°C).

Source: Zelitch, I. 1971. Photosynthesis, Photorespiration, and Plant
Productivity . Academic Press: New York; Larcher, W. 1980. Physio-
logical Plant Ecology . Springer-Verlag: New York.

involves an extra biochemical step, but under conditions of
intense direct sunlight, warmer temperature, and moisture
stress, it provides a distinct advantage.
Some well known crops that use C4 photosynthesis
are corn, sorghum, and sugarcane. A lesser-known C4
crop is amaranth. C4 plants are more common in tropical
areas, especially the drier tropics. Plants that originated
in drier desert regions or grassland communities of
warm temperate and tropical climates are more likely to
be C4 plants.

C ARBON P ARTITIONING

The carbon compounds produced by photosynthesis play
critical roles in plant growth and respiration because of
their dual role as an energy source and as carbon skeletons
for building other organic compounds. How a plant dis-
tributes the carbon compounds derived from photosynthe-
sis and allocates them to different physiological processes
and plant parts is described by the term carbon partition-
ing . Since we grow crops for their ability to produce
harvestable biomass, carbon partitioning is of consider-
able agricultural interest.
Although photosynthesis has an efficiency of energy
capture of about 20%, the process of converting photo-
synthate into biomass has an efficiency that rarely exceeds
2%. This efficiency is low mainly because internal respi-
ration (oxidation of photosynthate for cell maintenance)
uses up much of the photosynthate and because photores-
piration limits photosynthetic output when photosynthetic
potential is highest. Much research aimed at improving
crop yield has focused on increasing the efficiency of
photosynthetic carbon fixation, but this work has been
wholly unsuccessful. The primary basis for increasing
crop yield through plant breeding, both traditional and
modern, has been the enhancement of harvested biomass
relative to total plant biomass.
Since the ability of plants to create biomass is limited,
how they partition the fixed carbon they do create is of

Crassulacean Acid Metabolism Photosynthesis

A third type of photosynthesis is called Crassulacean acid
metabolism (CAM) photosynthesis. It is similar to C4 pho-
tosynthesis. During the night, while the stomata can be
open without causing the loss of undue amounts of mois-
ture, carbon dioxide is taken in and the four-carbon com-
pound malate is formed and stored in cellular organelles
called vacuoles. The stored malate then serves as a source
of CO 2 during the day to supply the dark reactions. Plants
using CAM photosynthesis can keep their stomata closed
during the day, taking in all the CO 2 they need during the
night. As would be expected, CAM plants are common in
hot and dry environments, such as deserts; they include
many succulents and cactus. Bromeliads that live as epi-
phytes (plants attached to other plants and not rooted in
soil) are also CAM plants; their habitat in the canopy of
rainforests is much drier than the rest of the rainforest