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CROP PHYSIOLOGY & METABOLISM

Implications of Atmospheric and Climatic Change for Crop Yield and Water Use Efficiency

USDA-ARS, Grassland, Soil and Water Research Laboratory, 808 E. Blackland Road, Temple, TX 76502

* Corresponding author (polley{at}brc.tamus.edu)

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.

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