Implications of atmospheric and climatic change for crop yield and water use efficiency

Yield of water-limited crops is determined by crop water use and by plant water use efficiency, each of which will be affected by the anticipated rise in atmospheric carbon dioxide (CO2) concentration and concomitant increase in temperature. At the leaf level, a given proportional increase in CO2 concentration generally elicits a similar relative increase in transpiration efficiency (ratio of net photosynthesis to transpiration). The increase in transpiration efficiency may result both from an increase in photosynthetic rate and a decrease in stomatal conductance. Feedbacks involved in scaling from leaf to crop constrain the increase in net carbon gain and reduce the anti-transpiration effect of CO2 enrichment. As a result, the increase in crop water use efficiency at high CO2 typically is less than 75% of that measured at the leaf level. By accelerating crop development and reducing harvest index, higher temperatures often erode yield benefits of improved water use efficiency at high CO2. The fraction of available water that is used by crops could increase with CO2 concentration because of greater root growth and faster canopy closure, but these effects have received scant study. Field experiments indicate that CO2 enrichment will increase crop water use efficiency mainly by increasing photosynthesis and growth. Yield should be most responsive to CO2 when temperatures approximate the optimum for crop growth. Elevating CO2 can ameliorate negative effects of above-optimal temperatures, but temperatures near the upper limit for crops will depress yields irrespective of CO2 concentration.