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 growt h and yield to changes in atmospheric CO2 concentrations (C-a), but th e extent of this influence is uncertain. To study the interaction of w ater deficits and C-a on crop growth, the ecosystem simulation model e cosys was tested with data for diurnal gas exchange and seasonal wheat growth measured during 1993 under high and low irrigation at C-a = 37 0 and 550 mu mol mol(-1) in the Free Air CO2 Enrichment (FACE) experim ent near Phoenix, AZ. The model, supported by the data from canopy gas exchange enclosures, indicated that under high irrigation canopy cond uctance (g(c)) at C-a = 550 mu mol mol(-1) was reduced to about 0.75 t hat at C-a = 370 mu mol mol(-1), but that under low irrigation, g(c) w as reduced less. Consequently when C-a was increased from 370 to 550 m u mol mol(-1), canopy transpiration was reduced less, and net CO2 fixa tion was increased more, under low irrigation than under high irrigati on. The simulated effects of C-a and irrigation on diurnal gas exchang e were also apparent on seasonal water use and grain yield. Simulated vs. measured seasonal water use by wheat under high irrigation was red uced 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 grai n yield of wheat under high irrigation was increased by 16% vs. 8%, bu t that under low irrigation was increased by 38% vs. 21%. In ecosys, t he interaction between C-a and irrigation on diurnal gas exchange, and hence on seasonal crop growth and water use, was attributed to a conv ergence of simulated g(c) towards common values under both C-a as cano py turgor declined. This convergence caused transpiration to decrease comparatively less, but CO2 fixation to increase comparatively more, u nder high vs. low C-a. Convergence of g(c) was in turn attributed to i mproved turgor maintenance under elevated C-a caused by greater storag e C concentrations in the leaves, and by greater rooting density in th e soil.