Interaction between atmospheric CO 2 concentration and water deficit on gas exchange and crop growth: testing of ecosys with data from the Free Air CO 2 Enrichment (FACE) experiment

Soil water deficits are likely to influence the response of crop growth and yield to changes in atmospheric CO 2 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 μmol mol −1 in the Free Air CO 2 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 μmol mol −1 was reduced to about 0.75 that at C a = 370 μmol mol −1 , but that under low irrigation, g c was reduced less. Consequently when C a was increased from 370 to 550 μmol mol −1 , canopy transpiration was reduced less, and net CO 2 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 μmol 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 CO 2 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.