Effects of Elevated CO2 on Cotton and Wheat As Determined from FACE Experiments

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In order to determine the effects of the increasing atmospheric CO2 concentration on future crop productivity and water requirements, free-air CO2 enrichment (PACE) experiments at 550μmol/mol CO2 have been conducted in an open field at Maricopa, Arizona, U.S.A., on cotton (Gossypium hirsutum L., cv. Deltapine 77) and wheat (Triticum aestivum L. cv. Yecora Rojo) crops at ample (Wet) and limiting (Dry) levels of water supply. With ample water, canopy net photosynthesis rates of cotton and wheat crops increased about 30-40%, with little evidence of down-regulation. Under the Dry treatment, the stimulation was about the same for cotton but much larger for wheat. Cotton growth was stimulated about 30% at both wet and Dry water levels, with lint yield being stimulated even more, up to 80% under wet and 50% under Dry. Wheat growth was stimulated less than that of cotton, about 20% at mid-season. Moreover, the final wheat grain yields increased only about 10% under Wet, whereas under Dry, the increase was about 20%. The FACE treatment had very little effect on the water use of cotton, but, for wheat, there appeared to be a small (4%) but significant reduction in water use, as determined by the soil water balance. Water use efficiency increased due to the larger growth and yields.
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