STOMATAL LIMITATION TO CARBON-DIOXIDE ASSIMILATION IN NITROGEN-STRESSED AND DROUGHT-STRESSED WHEAT

Both stomatal and metabolic factors have been shown to limit gas exchange rates in leaves. Two growth chamber experiments using two levels of N supply and two levels of water supply were conducted to determine the effect of N nutrition on the regulation of CO2 assimilation rate (A) and stomatal limitation to A under drought stress. Wheat (Triticum aestivum L.) plants were soil grown under controlled conditions and subjected to no stress, N limitation, drought stress, or both N limitation and drought stress treatments when 4 wk old. After the plants were 6 wk old, A at varying CO2 concentrations (from 0 to 550-mu-L L-1), A at 20 mL O2 L-1, stomatal conductance, intercellular CO2 concentration (C(i)), leaf water potential (LWP), leaf N concentration, stomatal limitation to A, and ribulose bisphosphate carboxylase (rubisco) activity were determined. Drought stress reduced LWP (from -0.73 to -1.65 MPa) and N stress reduced leaf N concentration (from > 30 to < 20 g N kg-1), A, and rubisco activity. The stomatal limitation to A and water use efficiency (WUE) was lower and C(i) higher in the leaves of plants exposed to both N limitation and drought stress than those exposed to N limitation or drought stress alone or to those unstressed. High C(i) in the N-limited, drought-stressed leaves did not appear to be explained by lower rubisco activity or by photorespiration. Determining other nonstomatal factors might be necessary to explain the low WUE of N-limited drought-stressed leaves compared to non-stressed leaves.