Impacts of Climate Change on Agriculture - Climate Resilient Agriculture for Ensuring Food Security - page 48

Fig. 4.1 Effect of increasing CO 2 concentration on crops

increase the rate of plant growth. Photosynthesis
is the net accumulation of carbohydrates formed
by the uptake of CO 2 , so it increases with increas-
ing CO 2 concentration. A doubling of CO 2 may
increase the photosynthetic rate by 30-100 %,
depending on other environmental conditions
such as temperature and available moisture
(Pearch and Bjorkman 1983 ). More CO 2 enters
the leaves of plants due to the increased gradient
of CO 2 between the external atmosphere and the
air space inside the leaves. This leads to an
increase in the CO 2 available to the plant for con-
version into carbohydrate (Acock and Allen
1985 ). The difference between photosynthetic
gain and loss of carbohydrate by respiration is the
resultant growth.
There are, however, important differences
between the photosynthetic mechanisms of dif-
ferent crop plants and hence in their response to
increasing CO 2 . Plant species with the C3 photo-
synthetic pathway (the fi rst product in their bio-
chemical sequence of reactions has three carbon
atoms) use up some of the solar energy they
absorb in a process known as photorespiration, in

which a signifi cant fraction of the CO 2 initially
fi xed into carbohydrates is reoxidized back to
CO 2 (Hillel and Rosenzweig 1989 ). C3 species
tend to respond positively to increased CO 2
because it tends to suppress rates of photorespira-
tion (Fig. 4.1 ). This has major implications for
food production in a high-CO 2 world because
some of the current major food staples, such as
wheat, rice, and soybean, are C3 plants. In total,
16 of the world's 20 most important food crops
would benefi t from increased carbon dioxide lev-
els (Bianca 1976 ).
However, in C4 plants (those in which the fi rst
product has four carbon atoms), CO 2 is fi rst
trapped inside the leaf and then concentrated in
the cells which perform the photosynthesis
(Hillel and Rosenzweig 1989 ). Although more
effi cient photosynthetically under current levels
of CO 2 , these plants are less responsive to
increased CO 2 levels than C3 plants (Fig. 4.2 ).
The major C4 staples are maize, sorghum, sugar-
cane, and millet. Since these are largely tropical
crops, and most widely grown in Africa, there is
thus the suggestion that CO 2 enrichment will