cused mainly on C 3 species. For C 4 and CAM plants, the mechanisms and the nature of interactive effects
of elevated [CO 2 ] and other adverse environmental conditions on growth and yield, and their fundamen-
tal physiology, biochemistry, and/or molecular biology, are still not well understood.
- C 3 Species
The present atmospheric [CO 2 ] limits the photosynthetic capability, growth, and yield of many agricul-
tural crop plants, among which the C 3 species show the greatest potential for response to rising [CO 2 ]
[11,14,31–33]. Current atmospheric CO 2 and O 2 levels and C 3 Rubisco specificity factors translate into
photorespiratory losses of 25% or more for C 3 species [14,34]. The projection that a rise in atmospheric
[CO 2 ] will reduce the deleterious effect of O 2 on C 3 photosynthesis but that it has a negligible effect on
C 4 photosynthesis is indeed supported by experimental growth data. Exposure of C 3 plants to elevated
[CO 2 ] generally results in stimulated photosynthesis (Figure 1) and enhanced growth and yield
[31–33,35]. A compilation of the existing data available from the literature for C 3 agricultural crops, in-
cluding agronomic, horticultural, and forest tree species, shows an average enhancement in net CO 2 ex-
change rates up to 63% and growth up to 58% with a doubling of the present atmospheric [CO 2 ]
[31,32,36–38].
Long-term exposure to elevated [CO 2 ] leads to a variety of acclimation effects, which include
changes in the photosynthetic biochemistry and stomatal physiology and alterations in the morphology,
anatomy, branching, tillering, biomass, and timing of developmental events as well as life cycle comple-
tion [14,33,39,40]. A greater number of mesophyll cells and chloroplasts have been reported for plants
grown under elevated [CO 2 ] [41,42]. With respect to leaf photosynthetic physiology and biochemistry,
acclimation occurs, ranging from species-specific changes in the A/Ci(assimilation rate versus intercel-
lular CO 2 ) curves [43–45] to alterations in dark respiration [33] and biochemical components with Ru-
bisco playing the leading role [46]. In terms of dark respiration, exposure of plants to elevated [CO 2 ] usu-
ally results in lowering the dark respiration rate, which can be explained by both indirect and direct effects
[33]. Whereas the mechanism for the indirect (acclimation) effect of elevated [CO 2 ] on dark respiration
may be related to changes in tissue composition, the direct effect appears to be an inhibition of the en-
zymes in the mitochondrial electron transport system [47,48].
Many C 3 species grown for long periods at elevated [CO 2 ] show a down-regulation of leaf photo-
synthesis [45,49,50], and carbohydrate source-sink balance is believed to have a major role in the regu-
lation of photosynthesis through feedback inhibition [51,52]. Source-sink imbalances may occur during
exposure to elevated [CO 2 ] when photosynthetic rate exceeds the export capacity or the capacity of sinks
to use photosynthates for growth, resulting in an accumulation of carbohydrates in photosynthetically ac-
tive source leaves [52–54]. Under elevated growth CO 2 , although the extent to which starch and soluble
sugars accumulate largely depends on the species, the increase of starch seems to be greater than that of
soluble sugars in many plants, and a correlation between starch accumulation and inhibition of leaf pho-
tosynthesis has been more frequently observed [54]. Also, for many plant species, longer exposure to el-
evated [CO 2 ] results in a down-regulation of Rubisco [33,44–46,55–66]. Both “coarse” control, through
lowering of the enzyme protein content, and “fine” control, through decreasing the enzyme activation
state, play a role in the down-regulation of Rubisco mediated by elevated [CO 2 ]. Coarse control suggests
a reallocation of nitrogen resources away from Rubisco [14] as well as an optimization of CO 2 acquisi-
tion with utilization of the fixed carbon [67]. Down-regulation of Rubisco at elevated [CO 2 ], however, is
not a universal phenomenon, and claims of altering the enzyme activity need careful evaluation, as the
basis on which Rubisco activity is expressed may vary or nullify the observation [14].
In addition to Rubisco, there are reports that elevated [CO 2 ] affects the regulation of sucrose phos-
phate synthase (SPS) and acid invertase. In rice, leaf SPS activity, expressed on a leaf total soluble pro-
tein basis, is up-regulated in CO 2 -enriched plants, suggesting an acclimation response to optimize the ca-
pacity for carbon utilization and export for this crop species [68]. On the other hand, activities of SPS,
expressed on a leaf fresh weight basis, are down-regulated by high [CO 2 ] in bean, cotton, cucumber, plan-
tain, and wheat but up-regulated in pea, soybean, spinach, sunflower, and tomato [69]. Under elevated
growth [CO 2 ], leaf acid invertase activities are down-regulated in cotton, cucumber, parsley, pea, radish,
soybean, spinach, tobacco, and wheat but up-regulated in bean, plantain, and sunflower [69].
Levels of soluble sugars in plant cells have been shown to influence the regulation of expression of
several genes coding for key photosynthetic enzymes [70–75]. The buildup in carbohydrates may signal
the repression, but does not directly inhibit the expression, of Rubisco and other proteins that are required
38 VU ET AL.