PRODUCTIVITY AND WATER-USE OF WHEAT UNDER FREE-AIR CO2 ENRICHMENT

A free-air CO2 enrichment (FACE) experiment was conducted at Maricopa, Arizona, on wheat from December 1992 through May 1993. The FACE appar atus maintained the CO2 concentration, [CO2], at 550 mu mol mol(-1) ac ross four replicate 25-m-diameter circular plots under natural conditi ons in an open field. Four matching Control plots at ambient [CO2] (ab out 370 mu mol mol(-1)) were also installed in the field. In addition to the two levels of [CO2], there were ample (Wet) and limiting (Dry) levels of water supplied through a subsurface drip irrigation system i n a strip, split-plot design. Measurements were made of net radiation, R(n); soil heat flux, G(o); soil temperature; foliage or surface temp erature; air dry and wet bulb temperatures; and wind speed. Sensible h eat flux, H, was calculated from the wind and temperature measurements . Latent heat nux, lambda ET, and evapotranspiration, ET, were determi ned as the residual in the energy balance. The FACE treatment reduced daily total R(n) by an average 4%. Daily FACE sensible heat flux, H, w as higher in the FACE plots. Daily latent heat flux, lambda ET, and ev apotranspiration, ET, were consistently lower in the FACE plots than i n the Control plots for most of the growing season, about 8% on the av erage. Net canopy photosynthesis was stimulated by an average 19 and 4 4% in the Wet and Dry plots, respectively, by elevated [CO2] for most of the growing season. No significant acclimation or down regulation w as observed. There was little above-ground growth response to elevated [CO2] early in the season when temperatures were cool. Then, as tempe ratures warmed into spring, the FACE plants grew about 20% more than t he Control plants at ambient [CO2], as shown by above-ground biomass a ccumulation. Root biomass accumulation was also stimulated about 20%. In May the FACE plants matured and senesced about a week earlier than the Controls in the Wet plots. The FACE plants averaged 0.6 degrees C warmer than the Controls from February through April in the well-water ed plots, and we speculate that this temperature rise contributed to t he earlier maturity. Because of the acceleration of senescence, there was a shortening of the duration of grain filling, and consequently, t here was a narrowing of the final biomass and yield differences. The 2 0% mid-season growth advantage of FACE shrunk to about an 8% yield adv antage in the Wet plots, while the yield differences between FACE and Control remained at about 20% in the Dry plots.