Summary Carbon isotope discrimination (Δ) has been suggested as an indirect selection tool for plant water use efficiency and yield potential in wheat ( Triticum aestivum L.). Plant available soil water content (PASW) and vapor pressure deficit (VPD) are among some important factors affecting gas exchange and Δ. A two-year field experiment was conducted to (1) investigate the differences in gas exchange parameters: net CO 2 assimilation rate (An), stomatal conductance (Gs), intercellular CO 2 concentration (Ci), An/Ci, transpiration rate (E), water use efficiency (WUE), and Δ between older and newer cultivars in winter wheat; and (2) determine the relationships between Δ and gas exchange parameters, WUE and grain yield as influenced by PASW and VPD. Differences in An and An/Ci between the two years was influenced more by PASW conditions than by cultivar. In general, WUE decreased in all cultivars when PASW