Evolution of water use efficiency, heat tolerance, and carbon isotope discrimination among Canadian spring wheat cultivars

Climate projections predict reductions in crop-water availability and increases in the frequency of heatwaves across western Canada, posing major challenges to crop productivity and sustainability. Enhancing water use efficiency (WUE) and heat tolerance is therefore critical to improving yield stability and grain quality under future climatic conditions. In this study, 198 historical and modern Canadian spring wheat cultivars were evaluated for whole-plant and leaf-level WUE, heat tolerance, and carbon isotope discrimination (δ¹³C) to identify physiological traits associated with adaptation to water-limited environments. The δ¹³C was measured in flag leaves under water-deficient and high-temperature conditions. Leaf water potential (LWP), photosynthetically active radiation (PAR), chlorophyll fluorescence parameters (F₀, F _V /F _M , F _M , F _V ), quantum yield for heat dissipation of PSII (φDo), and relative electron transport rate (ETR) were measured across six growth stages. Significant genetic variation was observed in WUE and δ¹³C, with cultivars differing in their ability to produce biomass and grain per unit of water use. Whole-plant WUE and water use were negatively correlated with δ¹³C, indicating that genotypes with lower δ¹³C values tend to be more efficient in water use. Chlorophyll fluorescence traits varied markedly across growth stages: LWP, PAR, ETR, and F _V /F _M decreased, whereas F₀, F _M , and φDo increased, from stem elongation to booting. Overall, low to moderate correlations among WUE, δ¹³C, biomass, and water use suggest limited genetic diversity for these traits within the tested germplasm. These findings provide valuable insights for breeding climate-resilient, water-use-efficient wheat cultivars to enhance sustainability in the Canadian Prairies.