Impacts of Climate Change on Agriculture - Climate Resilient Agriculture for Ensuring Food Security - page 51

4.2.4 CO 2 Fertilization

matter production, and yield for many crops
(Acock and Allen 1985). In several cases, high
CO 2 has contributed to upward shifts in tempera-
ture optima for photosynthesis (Jurik et al. 1984 )
and to enhanced growth with higher temperatures
(Idso et al. 1987 ); other studies, however, have not
shown such benefi ts (Baker et al. 1989 ).
Temperate crops may benefi t more from
increasing CO 2 than tropical crops. In crop
species with the C3 pathway characteristic of
nontropical plants (e.g., wheat, soybean, cotton),
CO 2 enrichment has been shown to decrease
photorespiration, the rapid oxidation of recently
formed sugars in the light, a process which low-
ers the effi ciency of overall photosynthesis. C4
crops which are particularly characteristic of
tropical and warm arid regions (e.g., maize,
sorghum, and millet) are more effi cient photo-
synthetically under current CO 2 levels than C3
plants (because they fi x CO 2 into malate in their
mesophyll cells before delivering it to the RuBP
enzyme in the bundle-sheath cells). Because of
this CO 2 -concentrating and photorespiration-
avoiding mechanism, experimental data show
that C4 plants are less responsive to CO 2 enrich-
ment (Acock and Allen 1985).
The physiological effects of high levels of
atmospheric CO 2 described above have been
observed under controlled experimental condi-
tions. In the open fi eld, however, their magnitude
and signifi cance are still largely untested, and
their importance relative to the predicted large-
scale climatic effects uncertain. Greenhouse and
fi eld-chamber environments tend to be much
smaller, less variable, and more protected from
wind than fi eld conditions. Furthermore, physio-
logical feedback mechanisms such as starch
accumulation or lack of sink (i.e., growing,
storing, or metabolizing tissue) for the products
of photosynthesis may limit the extent to which
the “fertilizing” CO 2 effects may be realized.
Finally, if trace gas emissions continue to grow
unchecked, their climate warming effect is pro-
jected to continue even up to 2,000 ppm (Manabe
and Bryan 1985 ), but the benefi cial boost to pho-
tosynthesis appears to level off at about 400 ppm
for C4 crops and about 800 ppm for C3 crops
(Akita and Moss 1973 ).

Increasing atmospheric CO 2 concentrations can
also directly affect plant physiological processes
of photosynthesis and transpiration (Field et al.
1995 ). Therefore, any assessment of the impacts of
CO 2 -induced climate change on crop productivity
should account for the modifi cation of the climate
impact by the CO 2 physiological impact. The CO 2
physiological response varies between species,
and, in particular, two different pathways of photo-
synthesis (named C3 and C4) have evolved and
these affect the overall response. The difference
lies in whether ribulose-1, 5-bisphosphate carbox-
ylase-oxygenase (RuBisCO) within the plant cells
is saturated by CO 2 or not. In C3 plants, RuBisCO
is not CO 2 -saturated in present-day atmospheric
conditions, so rising CO 2 concentrations increase
net uptake of carbon and thus growth. The
RuBisCO enzyme is highly conserved in plants,
and as such it is thought that the response of all C3
crops including wheat and soybeans will be com-
parable. Theoretical estimates suggest that increas-
ing atmospheric CO 2 concentrations to 550 ppm
could increase photosynthesis in such C3 crops by
nearly 40 % (Long et al. 2004 ). The physiology of
C4 crops, such as maize, millet, sorghum, and
sugarcane, is different. In these plants, CO 2 is
concentrated to three to six time's atmospheric
concentrations, and thus, RuBisCO is already
saturated. Thus, rising CO 2 concentrations confer
no additional physiological benefi ts. These crops
may, however, become more water-use effi cient at
elevated CO 2 concentrations as stomata do not
need to stay open as long for the plant to receive
the required CO 2 . Thus, yields may increase
marginally as a result (Long et al. 2004 ).
Experiments under idealized conditions show
that a doubling of atmospheric CO 2 concentration
increases photosynthesis by 30-50 % in C3 plant
species and 10-25 % in C4 species (Ainsworth and
Long 2005 ). Crop yield increase is lower than the
photosynthetic response; increases of atmospheric
CO 2 to 550 ppm would on average increase C3 crop
yields by 10-20 % and C 4 crop yields by 0-10 %
(Long et al. 2004 ; Ainsworth and Long 2005 ).
Despite the potential positive effects on
yield quantities, elevated CO 2 may, however, be