Investigation of stomatal and nonstomatal parameters limiting photosynthesis(pn) under water stress condition may provide a means to understand physiological bases of drought resistance. In a greenhouse experiment leaf gas exchange, water relations, chlorophyll, soluble protein and proline content of 11 wheat cultivars were studied. Based on a two-year field trial, according to stress susceptibility index (SSI), cultivars were divided into three groups of tolerant, semitolerant and sensitive. Generally, water stress caused a significant reduction in Pn and leaf conductance (gs), while increased internal CO2 concentration (Ci) and instantaneous water use efficiency (WUE). Resistant cultivars exhibited a higher gs and transpiration rate (Tr), yet maintaining higher leaf relative water content (RWC). Under water stress condition, an osmotic adjustment of about 0.39 Mpa was attributed to proline accumulation. There was a positive correlation between proline concentration and stomatal resistance. Cultivars with higher proline content, possessed a lower Tr and specific leaf area (SLA). As water stress progressed, both soluble protein and chlorophyll concentration decreased. Cultivars with higher chlorophyll contents showed a lower Ci, accompanied by a higher mesophyle conductance. Positive and significant correlations were obtained for RWC and chlorophyll as well as RWC and protein concentration. It was concluded that inhibition of Pn under water stress conditions was due mainly to reduced mesophyl conductance (non-stomatal factor) rather than stomatal factors. Drought resistance was related to conservation of leaf water content.
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