Agriculture Reference
In-Depth Information
2.0
1.5
1.0
0.5
Maize
Wheat
0.0
0
200
400
600
800
1000
CO
2
concentration (μmol mol
-1
)
fIguRe 16.2
Photosynthetic response of maize and wheat to increasing levels of CO
2
con-
centration. (Redrawn from Akita, S. and D.N. Moss, 1973,
Crop Science
13:234-237.)
plants or crops, a portion (e.g., seeds, fruits, tubers) is harvested for human uses.
Heterotrophic terrestrial organisms, which include humans, depend directly or indi-
rectly on plants for their growth and survival.
Plants exchange gases with the atmosphere through pores called
stomata
located underneath their leaves. The main gases exchanged through stomata are
O
2
, CO
2
, and water vapor. When plants are exposed to elevated levels of CO
2
, the
stomata partially close to regulate the entry of CO
2
and indirectly reduce the rate
of water vapor loss, which leads to an increase in
water use efficiency
(WUE),
which is defined as the amount of plant biomass produced per unit of water used.
As the CO
2
concentration of air increases, plants respond by increasing the rate
of photosynthesis, more or less linearly at sub- and above-ambient concentration
levels, up to a maximum (Figure 16.2) (Akita and Moss, 1973). Notably, maize
plants reach a saturation of photosynthetic activity at lower levels of CO
2
concen-
tration than wheat plants. This photosynthetic response to elevated CO
2
is spe-
cies specific or, more precisely, dependent on the plant's photosynthetic pathway.
Plants, in general, and crops, in particular, are classified as C
3
or C
4
according
to the photosynthetic pathway used to fix CO
2
. A plant is said to have a C
3
-type
metabolism because the first organic compound formed during photosynthesis is
a three-carbon (C) molecule (3-phosphoglycerate). Conversely, a plant is of the C
4
type when the first compounds formed contain four-carbon atoms (oxaloacetate,
malate). There are many evolutionary, morphological, and biochemical differences
between C
3
and C
4
plants, so refer to any good text of plant physiology to learn
about these mechanisms, including a third one (Crassulacean acid metabolism
or CAM) not discussed here. Wheat, rice, alfalfa (
Medicago sativa
L.), cotton
(
Gossypium hirsutum
L.), and potato (
Solanum tuberosum
L.) are examples of C
3
crops. Maize and sugarcane (
Saccharum officinarum
L.) are representatives of the
C
4
-type metabolism.
