Although stomatal conductance is decreased under elevated CO2, the ratio of intercellular to ambient CO2 concentration usually is not modified, and stomata do not appear to limit photosynthesis more in elevated CO2 compared to ambient CO2 (Drake et al., 1997). Elevated-CO2 effects on crop evapotranspiration per unit land area (E) have been small with cotton (Dugas et al., 1994; Hunsaker et al., 1994; Kimball et al., 1994) and spring wheat (Kimball et al.,1995, 1999) crops supplied with ample nitrogen fertilizer. With rice, under field-like conditions, CO2 enrichment reduced seasonal total E by 15% at 26°C but increased E by 20% at 29.5°C (Horie et al., 2000). A larger decline (-22%) in the daily E of a C4-dominated tallgrass prairie was reported by Ham et al. (1995), and a strong reduction in water use per plant also was observed for maize (Samarakoon and Gifford, 1996), a C4 plant. The consequences of these direct effects of elevated CO2 concentrations on E are still unclear at the catchment scale (see Section 4.3.3).
Relative enhancement of growth owing to CO2 enrichment might be greater under drought conditions than in wet soil because photosynthesis would be operating in a more CO2-sensitive region of the CO2 response curve (André and Du Cloux, 1993; Samarakoon and Gifford, 1995). In the absence of water deficit, C4 photosynthesis is believed to be CO2 saturated at present atmospheric CO2 concentration (Bowes, 1993; see also Kirschbaum et al., 1996). However, as a result of stomatal closure, it can become CO2-limited under drought. Some of the literature examples in which C4 crop species, such as maize, have responded to elevated CO2 may have involved (possibly unrecognized) minor water deficits (Samarakoon and Gifford, 1996). Therefore, CO2-induced growth enhancement in C4 species (e.g., Poorter, 1993) may be caused primarily by improved water relations and WUE (Samarakoon and Gifford, 1996) and secondarily by direct photosynthetic enhancement and altered source-sink relationships (Ruget et al., 1996; Meinzer and Zhu, 1998). With rice, at the optimal temperature for growth, a doubling of CO2 increases crop WUE by about 50%. However, this increase in WUE declines sharply as temperature increases beyond the optimum (Horie et al., 2000). Although increased productivity from increased WUE is the major response to elevated CO2 in a C3 or C4 crop that is exposed frequently to water stress (Idso and Idso, 1994; Ham et al., 1995; Drake et al., 1997), changes in climatic factors (temperature, rainfall) may interact with elevated CO2 to alter soil water status, which in turn will influence hydrology and nutrient relations. Therefore, to realistically project impacts on crop yields and regional evaporation (see Chapter 4), more research is needed on the interactions of elevated CO2, high temperature, and precipitation.
An exposure-response model that linearly relates a change in gas exposure over a time period to log-scale change in biomass increment of a plant during the same period suggests that a decline in recent yields of grain crops caused by an increase in surface ozone concentrations may have reached 20% in some parts of Europe (Semenov et al., 1997, 1998, 1999). Recent research has shown that multiple changes in atmospheric chemistry can lead to compensating or synergistic effects on some crops. Heagle et al. (1999) used field studies to examine the impact of higher O3 levels on cotton growth under higher CO2 conditions. They found that higher CO2 compensates for growth suppression resulting from elevated O3 levels. With wheat, elevated CO2 fully protects against the detrimental effects of O3 on biomass but not yield (McKee et al., 1997). Similar results have been reported with soybean (Fiscus et al., 1997) and tomato (Reinert et al., 1997). Meyer et al. (1997) measured responses of spring wheat to different levels of ozone in chambers at different growth stages and found that photosynthesis and carbohydrate accumulations were strongly affected during anthesis, especially after a period of heat stress.
Mark and Tevini (1997) observed combined effects of UV-B, temperature, and CO2 in growth chambers in seedlings of sunflower and maize. They found that a 4°C rise in daily maximum temperature (from 28 to 32°C), with or without higher CO2, compensated for losses from enhanced UV-B. Teramura et al. (1990) report that yield increases with elevated CO2 are suppressed by UV-B more in cereals than in soybean; rice also loses its CO2-enhanced WUE. However, Unsworth and Hogsett (1996) assert that many research studies from the preceding decade used unrealistic UV-B exposures, and they conclude that UV-B does not pose a threat to crops alone or in combination with other stressors.
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