Effects of Drought and Elevated CO2 on Plant Water Use Efficiency and Productivity

The biomass water use efficiency of specific crops is usually almost constant at any given ambient CO2 concentration when normalised for the evaporative demand of the environment, regardless of the severity of water stress. This conservative behaviour is rooted in two basic tenets of plant productivity, the capture of radiation and assimilation of CO2 in exchange for water lost. The dominant factor is probably radiation capture, which supplies the energy for both transpiration and photosynthesis. Radiation capture depends on the extent of canopy cover, and hence on leaf growth, a process most sensitive to water stress. When canopy cover is incomplete, even mild water stress may reduce leaf growth and radiation interception in a way that compounds with time, leading to closely linked reductions in assimilation and transpiration. If canopy cover is complete, water stress, when severe enough, may reduce photosynthesis and stomatal opening. The intercellular CO2 concentration (C i ), however, remains constant in many instances, and decreases in others. Using the equations for CO2 transport only in the gaseous phase and the equation for transpiration, and taking energy balance into account, it is shown that the photosynthetic WUE of single leaves decreases in the case of constant C i , and remains about the same or increases in the case of decreased C i , depending on the magnitude of the changes in C i -and leaf temperature. Elevated CO2 reduces stomatal aperture, steepens the CO2 gradient for assimilation and accelerates the development of leaf area, particularly in C3 plants. The first effect lowers transpiration per unit effective leaf area. The reduction, however, may not be large due to energy balance requirements, especially for closed canopies under low wind conditions. The second effect raises C i , sufficiently despite the lower stomatal conductance to effect a net increase in photosynthesis per unit effective leaf area. The combined impact of the first two effects on WUE, again evaluated using the equations for assimilation and transpiration, is that the percentage increase in photosynthetic WUE due to elevated CO2 is almost proportional to the ratio of the new to the original concentration of CO2, in agreement with some published data. The third effect of enhancing leaf area development would be to increase both transpiration and photosynthesis per unit land area prior to the completion of canopy cover, but this would have little effect on photosynthetic WUE. In contrast, consumptive WUE may improve as a consequence of soil shading and reduced soil evaporation. The compounding effect with time of elevated CO2 when the canopy is incomplete provides an explanation for the phenomenon of a disproportionately larger enhancement of biomass compared to the enhancement in photosynthesis per unit leaf area effected by elevated CO2. Effects of water stress on photosynthetic WUE under high CO2 should be minimal and similar to those observed under normal CO2 concentrations. The conceptual framework provided by the aforementioned equations offers a rational basis for the systematic evaluation of WUE, although its application at the canopy level remains to be tested.