LEAF WATER RELATIONS OF COTTON IN A FREE-AIR CO2-ENRICHED ENVIRONMENT

As part of an intensive study of crop response to CO2 enrichment in a free-air CO2 enrichment (FACE) experiment in the field, we determined aspects of the water relations of a cotton crop on selected dates in 1 991. The atmosphere was enriched from 370 mumol CO2 mol-1 (control) to about 550 mumol mol-1 in free air during daylight hours. Under full i rrigation, CO2 enrichment decreased stomatal conductance and single-le af transpiration only toward the end of the season, and these changes led to increased leaf water potentials only at that time of year. Unde r water-stressed (deficit irrigation) conditions, CO2 enrichment decre ased conductance throughout the season but there was no corresponding consistent effect on leaf water potentials. As with the fully irrigate d controls, CO2 enrichment increased leaf water potentials only at the end of the season. CO2 enrichment increased season-long biomass accum ulation 39% under full irrigation and 34% under deficit irrigation. Th ese results are consistent with previous studies of cotton in open-top chambers that found only small effects of CO2 enrichment on internal water relations of cotton, and no water stress-induced increase in cro p responsiveness to elevated CO2.