FLUCTUATING [CO2] DRIVES SPECIES-SPECIFIC CHANGES IN WATER-USE EFFICIENCY

We have investigated the effects of fluctuating carbon dioxide (CO2) c oncentration on water use efficiency of Zea mays L. and Phaseolus vulg aris L. We found that species-specific kinetics of stomatal movements combine with photosynthetic characteristics to influence shea-term wat er use efficiency strongly under fluctuating environmental conditions. Specifically, under oscillating [CO2], average transpiration in Z. ma ys was driven higher than that observed at steady-state at the median CO2 concentration, while average photosynthesis remained fairly consta nt. Consequently, water use efficiency was lower during the fluctuatio ns in [CO2] than it was at the steady, median [CO2]. Under similar osc illations in [CO2], stomatal conductance and transpiration of P. vulga ris were driven lower than observed at steady-state at the median [CO2 ]. A concomitant slight restriction of photosynthesis balanced this de crease in transpiration, and in this case water use efficiency under f luctuating [CO2] remained practically constant in P. vulgaris. The fre quency of oscillations in [CO2] interacted with asymmetries in stomata l opening and closing kinetics in both Z. mays and P. vulgaris to dete rmine the extent to which average transpiration (and water use efficie ncy in Z. mays) departed during fluctuations from the steady-state con dition at the median CO2 level.