soybean when the growth [CO 2 ] was raised to 700 ppm over the same temperature range [64]. The dif-
ference may be partially attributed to the fact that the temperature optimum of 32°C for soybean under
ambient [CO 2 ] is 7°C higher than that of the model (idealized) C 3 plant [64]. At an afternoon leaf tem-
perature of 46°C, leaf CER of sour orange trees grown at ambient [CO 2 ] declined to near zero, whereas
the CO 2 -enriched trees still maintained their CER at ~4 mol/m^2 /sec [146]. Theoretically, a 300-ppm in-
crease in atmospheric [CO 2 ] could raise the temperature optimum of light-saturated CER of C 3 plants by
5°C [130].
The interactive effects of elevated [CO 2 ] and temperature for C 4 species are not well understood. As
discussed earlier, because of their CO 2 -concentrating capability, it has been generally considered that C 4
plants would show little CO 2 stimulation irrespective of temperature [46,147]. However, with reports
showing stimulation of biomass [32,62,104,107,109,110], the response of C 4 plants to both CO 2 and tem-
perature deserves more attention. For the C 4 sugarcane, the degrees of enhancement in plant growth pa-
rameters are much greater under long-term exposure to both elevated CO 2 and temperature than to ele-
vated CO 2 alone (Figure 2A–C).
For C 3 and C 4 plants adapted to similar climates, leaves of C 4 plants generally have a higher tem-
perature optimum for photosynthesis as well as a higher overall photosynthetic rate at the temperature op-
timum [11,148–150]. At the current atmospheric [CO 2 ], CERs of C 4 plants tend to increase with temper-
ature to a greater extent than those of C 3 plants. Elevated [CO 2 ] increases the temperature optimum of C 3
plants, bringing it closer to that of C 4 photosynthesis [130]. Besides, factors such as light regime, soil
moisture, nutrient status, and plant developmental stage all modify the interactive responses to elevated
CO 2 and temperature [4,46,62,151–155].
C. Rising CO 2 and Limited Soil Water Availability
As atmospheric [CO 2 ] rises, potential shifts in regional scale precipitation patterns could result in in-
creased drought conditions in many areas of the world. Responses of plants to rising [CO 2 ] in water deficit
situations have been reviewed [156]. Despite our understanding of the responses of leaf photosynthesis
to elevated [CO 2 ] as well as to soil water deficit, the interactions of CO 2 enrichment and drought stress
are still uncertain [11]. In particular, much less is known about the effects of rising [CO 2 ] on the funda-
mental regulatory aspects of leaf photosynthesis in major agricultural crop plants subjected to drought
[14,46,157,158]. A reduction in stomatal conductance is a common response of plants to elevated growth
[CO 2 ]. Observations of a variety of C 3 and C 4 species indicate that a doubling of atmospheric [CO 2 ] can
also double the instantaneous WUE [11,156,159]. As the [CO 2 ] is increased, the improvements in WUE
are the results of increased assimilation rate and decreased water loss, with the latter being more impor-
tant under water deficit situations [46]. The increase in WUE as a result of elevated [CO 2 ] is likely to be
more important than the increase in net photosynthesis per se, and the same may be true for drought-
stressed plants grown in a CO 2 -enriched atmosphere [157].
As soil water becomes less available, the relative enhancement of photosynthesis and growth by
elevated [CO 2 ] tends to be greater, which can alleviate drought stress and delay its onset
[39,40,156,160]. A delay in the adverse effects of water deficit on leaf and canopy photosynthesis by
elevated [CO 2 ] has been reported for a number of C 3 plants, including soybean [161,162], sweet potato
[163], groundnut [164], and rice [165–167]. Studies conducted on a variety of plant species indicate
that elevated [CO 2 ] may actually prevent plants from succumbing to the rigors of environmental
stresses and enable them to maintain essential growth processes [168]. Soybean plants grown under
high [CO 2 ] transpire less and conserve more soil moisture than plants grown at ambient [CO 2 ] [161].
CO 2 enrichment also enhances water conservation and midday xylem water potentials in drought-
stressed sweet potato plants [163] and leaf water potentials of soybean [169]. For groundnut, elevated
growth [CO 2 ] has a similar beneficial effect on plants subjected to severe drought stress [164]. In rice,
elevated [CO 2 ] delays the adverse effects of severe drought on rbcStranscript abundance and activities
of Rubisco and permits photosynthesis to continue for an extra day during the drought-stress cycle
[91,166,167].
There is also evidence indicating that, under water deficit conditions, C 4 growth can respond as
strongly to elevated CO 2 as does that of C 3 species. In the tallgrass prairie ecosystems, C 4 species show
increased productivity under elevated CO 2 in dry years but not in wet years [113]. In drying soil, growth
of maize also responds strongly to CO 2 enrichment [170].
RESPONSES TO RISING CO 2 AND CLIMATE CHANGE 43