The association of carbon isotope discrimination (Δ) with gas exchange parameters and yield traits in Chinese bread wheat cultivars under two water regimes

► We studied the relationships of CID (Δ) with gas exchange parameters, yield traits and SPAD chlorophyll on wheat. ► Two water regimes (W120, 120mm and W200, 200mm) were applied on 49-wheat genotypes in a rainout shelter. ► Δ was correlated positively with A, gs, E, Ci, Ci/Ca, and negatively with iWUE under both water regimes. ► Δ was also correlated positively with GY, BM and HI, but not with SPAD chlorophyll under two water regimes. ► Δ can be used as an indirect selection criterion for genotypic improvement in WUE of wheat under drought. Drought is a major issue affecting crop grain yield. Augmenting grain yield and crop water use efficiency (WUE) under drought is crucial for enhancing world crop production and food availability. Relationships of carbon isotope discrimination (Δ) with gas exchange parameters, yield traits and SPAD (leaf relative chlorophyll content) values were examined on a collection of 49-wheat (Triticum aestivum L.) genotypes under two levels of restricted irrigation in a rainout shelter. Water stress was applied at grain filling stage. The highest water-stress treatment received 40% less water than the milder-stress treatment. Significant and positive correlations were found between Δ and photosynthesis rate (A), stomatal conductance (gs), transpiration rate (E), and the ratio of intercellular CO2 concentration to ambient CO2 concentration (Ci/Ca), while significant and negative correlations were obtained between Δ and intrinsic water use efficiency (iWUE) under both water regimes. Strong positive correlations of Δ with grain yield (GY), biomass (BM) and harvest index (HI) were also observed in both water regimes, but no correlation was observed between Δ and SPAD values. Mean values of all other parameters were calculated for the five genotypes based on Δ value, i.e. the five genotypes which produced the highest Δ and five genotypes which produced the lowest Δ. It was found that mean values for all the parameters were increased for the high Δ genotypes and decreased for low Δ genotypes except for iWUE in both water regimes. These results suggest that Δ may be a good trait as an indirect selection criterion for genotypic improvement in drought tolerance of wheat under restricted irrigation, especially in conditions similar to those encountered here where limited water was applied during grain filling.