Interaction between atmospheric CO2 concentration and water deficit on gas exchange and crop growth: Testing of ecosys with data from the Free Air CO2 Enrichment (FACE) experiment

Soil water deficits are likely to influence the response of crop growth and yield to changes in atmospheric CO2 concentrations (C-a), but the extent of this influence is uncertain. To study the interaction of water deficits and C-a on crop growth, the ecosystem simulation model ecosys was tested with data for diurnal gas exchange and seasonal wheat growth measured during 1993 under high and low irrigation at C-a = 370 and 550 mu mol mol(-1) in the Free Air CO2 Enrichment (FACE) experiment near Phoenix, AZ. The model, supported by the data from canopy gas exchange enclosures, indicated that under high irrigation canopy conductance (g(c)) at C-a = 550 mu mol mol(-1) was reduced to about 0.75 that at C-a = 370 mu mol mol(-1), but that under low irrigation, g(c) was reduced less. Consequently when C-a was increased from 370 to 550 mu mol mol(-1), canopy transpiration was reduced less, and net CO2 fixation was increased more, under low irrigation than under high irrigation. The simulated effects of C-a and irrigation on diurnal gas exchange were also apparent on seasonal water use and grain yield. Simulated vs. measured seasonal water use by wheat under high irrigation was reduced by 6% vs. 4% at C-a = 550 vs. 370 mu mol mol(-1), but that under low irrigation was increased by 3% vs. 5%. Simulated vs. measured grain yield of wheat under high irrigation was increased by 16% vs. 8%, but that under low irrigation was increased by 38% vs. 21%. In ecosys, the interaction between C-a and irrigation on diurnal gas exchange, and hence on seasonal crop growth and water use, was attributed to a convergence of simulated g(c) towards common values under both C-a as canopy turgor declined. This convergence caused transpiration to decrease comparatively less, but CO2 fixation to increase comparatively more, under high vs. low C-a. Convergence of g(c) was in turn attributed to improved turgor maintenance under elevated C-a caused by greater storage C concentrations in the leaves, and by greater rooting density in the soil.