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Leaf gas exchange theory indicates that carbon isotope discrimination (∆) is negatively associated with CER/g3 (leaf CO2 exchange rate/conductance). Discovery of genetic variation for ∆ in wheat (Triticum aestivum L.) and other species has generated interest in using carbon isotope discrimination in plant breeding. However, the hypothesized negative association between CER/g3 and & has sometimes not been observed in field‐grown crops. In the present study, CER/g3, & of peduncle tissue, plant productivity and yield were examined in eight winter wheat genotypes under both irrigated and nonirrigated field conditions. Water stress‐ and genotype‐induced variations in CER/g3 were more consistently correlated with variations in gs than with CER. However, low CER in ‘Bezostaya’ wheat resulted in a consistently low genotypic ranking for CER/g3. In 1988, values of CER/g3 increased with water stress, while ∆ values dropped from 18.17 to 17.48‰. These results imply a negative relationship between CER/g3 and ∆, consistent with leaf gas exchange theory; however, ∆ was both positively (1988; r = 0.83** [P ≤ 0.01]) and negatively (1989; r = ‐0.79* [P ≤ 0.05]) associated with CER/g3 in irrigated plots. Positive correlations were obtained between ∆ and both biomass productivity (1988, r = 0.54*; 1989, r = 0.45 [P = 0.08]) and grain yield (1988, r = 0.66*; 1989, r = 0.55*) when data were averaged across genotypes and irrigation treatments. Consistently low ∆ values were obtained for ‘Sturdy’, an early‐released, drought‐susceptible cultivar with low productivity. These results suggest that genotypic rankings for peduncle ∆ must be carefully interpreted, as low ∆ can be associated with low yield and stress susceptibility in wheat.

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