Polley: Crop Responses to Global Change

agriculture is to feed the world’s burgeoning population, yields of water-limited crops must be improved substanYield of water-limited crops is determined by crop water use and tially. Efforts to accomplish this have concentrated on by plant water use efficiency, each of which will be affected by the anticipated rise in atmospheric carbon dioxide (CO2) concentration increasing the fraction of available water that crops tranand concomitant increase in temperature. At the leaf level, a given spire and increasing plant water use efficiency (biomass proportional increase in CO2 concentration generally elicits a similar produced per unit of transpiration). These and other comrelative increase in transpiration efficiency (ratio of net photosynthesis ponents of crop water economy will be affected by anticito transpiration). The increase in transpiration efficiency may result pated global changes, changes that include correlated both from an increase in photosynthetic rate and a decrease in stomatal increases in both atmospheric carbon dioxide (CO2) conconductance. Feedbacks involved in scaling from leaf to crop constrain centration and mean temperature. the increase in net carbon gain and reduce the anti-transpiration Atmospheric CO2 concentration has risen by about effect of CO2 enrichment. As a result, the increase in crop water use 37% during the last two centuries to the present level efficiency at high CO2 typically is less than 75% of that measured at near 370 mol mol 1 (Keeling and Whorf, 2000). The the leaf level. By accelerating crop development and reducing harvest index, higher temperatures often erode yield benefits of improved CO2 concentration is projected to double again during water use efficiency at high CO2. The fraction of available water that the next century (Alcamo et al., 1996), and to contribute is used by crops could increase with CO2 concentration because of to a warmer climate. Also increasing are atmospheric greater root growth and faster canopy closure, but these effects have concentrations of other trace gases (CH4, N2O, NOx, received scant study. Field experiments indicate that CO2 enrichment CO) that could intensify global warming. The increase will increase crop water use efficiency mainly by increasing photosynin CO2 concentration alone is expected to warm Earth thesis and growth. Yield should be most responsive to CO2 when by 2 to 4.5 C by the middle of next century, with associtemperatures approximate the optimum for crop growth. Elevating ated changes in precipitation (Giorgi et al., 1998). WarmCO2 can ameliorate negative effects of above-optimal temperatures, ing is predicted to be greatest at high northern latitudes but temperatures near the upper limit for crops will depress yields during autumn and winter. irrespective of CO2 concentration. That atmospheric CO2 concentration is increasing is undeniable. Projections of future climate are more unC losses to water shortage may exceed those certain. Inclusion of aerosols in climatic models, for from all other causes combined (Kramer, 1980). If example, reduces anticipated changes in temperature and precipitation, and can yield regional estimates that USDA-ARS, Grassland, Soil and Water Research Laboratory, 808 differ from those obtained by simulating effects of CO2 E. Blackland Road, Temple, TX 76502; All programs and services of enrichment alone (Giorgi et al., 1998). the U.S. Department of Agriculture are offered on a nondiscriminaGlobal changes pose significant challenges to agricultory basis without regard to race, color, national origin, religion, sex, ture, but also provide opportunities to boost crop yields age, marital status, or handicap. Presented at the 1999 CSSA Symposium on Water Use Efficiency, organized by Div. C-2 chair, Dr. Tom in water-limited environments. Here, I summarize some Gerik. Received 19 Sept. 2000. *Corresponding author (polley@brc. of the challenges and opportunities of a warmer and tamus.edu). CO2-rich world for crop water economy and production. Yield of water-limited crops is determined by water Published in Crop Sci. 42:131–140 (2002). 132 CROP SCIENCE, VOL. 42, JANUARY–FEBRUARY 2002 capture, water use efficiency, and harvest index. Effects rent CO2 concentration. Morison (1985) showed that g declined more per unit increase in CO2 when g was high of anticipated changes on each of these components of crop water economy will be reviewed, but emphasis than low. Stomatal sensitivity to CO2 was linearly related to g in both C3 and C4 species. will be given global change effects on crop water use efficiency as these have been researched most extenVariation in g is also linearly correlated with A, with the result that ci/ca remains relatively constant (is consively. servative) across CO2 concentrations (Morison, 1993). Leaf Transpiration Efficiency Maintenance of a near-constant ci/ca implies that TE will increase linearly with ca, Eq. [1]. Indeed if w (or At the leaf level, instantaneous water use efficiency vpd) remains constant, TE will increase by the same or transpiration efficiency (TE) may be defined as the relative amount as does ca in both C3 and C4 species (Fig. ratio of the rate of net photosynthesis or assimilation 1). Significantly, these trends have also been observed rate (A) to transpiration (E), and approximated by over lower-than-present CO2 concentrations (Polley et al., 1993a), indicating that CO2 enrichment may already TE A