Effects of [CO2 Under Waterlimiting Conditions

Much of the global wheat production is water-limited and so the effect of GEC on water availability is crucial. While the effect of elevated [CO2] on gs is fairly consistent (even for varieties which differ in gs; Samarakoon et al., 1995), the effects on transpiration are complex (Lawlor, 1998). A lower rate of transpiration and less latent cooling increases the water vapour pressure in the intercellular spaces of the leaves, and decreases the humidity of the air in the boundary layer. Consequently, the gradient of vapour pressure increases, and concomitantly transpiration. However, short-term exposure to elevated [CO2] generally decreases transpiration. In a glasshouse experiment, doubling ambient [CO2] decreased water use by about 20% under wet conditions and slightly increased it under dry conditions. Also, in a controlled environment, transpiration decreased by only 8% at double-ambient [CO2] during vegetative growth (André and du Cloux, 1993). Thus, the magnitude of transpiration has a complex dependency on the environment. In the long term, increased leaf area and root growth result from elevated [CO2], and higher temperature means that water use may decrease much less than is suggested by short-term controlled studies (Chaudhuri et al., 1990a). Under water-limiting conditions, water loss may not decrease. In the FACE studies, a 200 |J.mol mol-1 increase in [CO2] reduced evapotranspiration (ET) of wheat by about 5% in irrigated plots. With drought, ET was increased by about 3% (Hunsaker et al., 1996, 1997). This may have resulted from the larger root system exploiting more water, greater leaf area, and/or slightly greater temperatures. Note that estimation of such small differences is difficult, because methods gave different values; a 5% reduction from water balance was obtained, 10% from latent energy exchange measurements and 7-23% from the sap flow measured with stem flow gauges (Senock et al., 1996).

Despite the relatively small effect of [CO2] on ET, elevated [CO2] increased biomass substantially, and so increased water-use efficiency (WUE) by 145% in the dry treatment and 21% in the wet treatment with FACE. Water use efficiency was increased by about 64% by doubling [CO2] under drought conditions in an Australian glasshouse experiment (Samarakoon et al., 1995). The relative increase in biomass due to doubled [CO2] was much greater under dry than wet conditions. In the FACE studies, a greater effect of CO2 enrichment on biomass occurred under drought conditions vs. well-watered crop conditions, which was reflected in a 20% increase in grain yields in drought and a 10% increase in irrigated plots (drought decreased grain yield by 28%). The problem of temperature differences induced by the system (see earlier) may have affected the results, but the effect is similar to that observed in OTC studies (Grashoff et al., 1995), if the smaller increment in [CO2] is considered.

In summary, the relatively few wheat studies on the interaction of [CO2] and drought under field-like conditions indicate that: (i) water use by wheat crops under elevated [CO2] may decrease slightly in wet conditions, but may increase slightly under dry conditions; and (ii) the stimulation of biomass and grain yield by elevated [CO2] tends to be greater under drought than in well-watered conditions.

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