Carbon (δ¹³C) and oxygen (δ¹⁸O) isotope compositions are considered indicators of the effect of water conditions on plant photosynthesis (δ¹³C) and transpiration (δ¹⁸O). Hydrogen isotope composition (δ²H), tracks transpiration like δ¹⁸O, while also affected by the organ trophism. Such dual behaviour, together with its highly exchangeable nature have hindered the use of δ²H to assess plant performance. We compared the effect of contrasting water pressure deficit (VPD) on the signatures of the three isotopes across different durum wheat parts. Plants were hydroponically grown under conditions, differing in VPD and the isotopic labelling of the nutrient solution (natural abundance versus δ²H and δ¹⁸O-enriched) and isotopic signatures analysed at mid-grain filling. Higher VPD increased plant-matter δ¹³C, δ²H, and δ¹⁸O, in accordance with atmospheric drought decreasing stomatal conductance and transpiration. Moreover, positive correlations within and across organs between δ²H and δ¹⁸O of organic-matter and water further supported a similar source of variation related to evaporation. However, δ²H was depleted in photoautotrophic (leaves and glumes), enriched in mixotrophic (peduncle and awns) and even more in heterotrophic (grains) organs. This study highlights the similarities and differences in mechanisms determining δ²H, δ¹⁸O, and δ¹³C through the interactions of these isotopes with VPD and plant organs.

This study evaluates the changes caused by breeding in the genotype by environment (G × E) interaction of the durum wheat varieties most widely cultivated in Spain after the Green Revolution. A set of 12 cultivars was tested in 27 environments, which are understood as the combination of different sites, years, and treatments (water regime and planting dates), representative of the durum wheat growing conditions in Spain with average grain yields (GY) ranging between 2.8 and 9.1 Mg ha⁻¹. The most important environmental factors affecting the G × E interaction for yield were themaximum and the mean temperature during the entire crop cycle. An improvement in genetic yield was observed in warm environments and under optimal water conditions that resemble those where the germplasm originated (essentially as advanced lines) before its release in Spain. Therefore, the adaptation of semi-dwarf durum in Spain has shown a tendency to specific adaptation rather than large-scale adaptation. Two different patterns of selection have been reported due the G × E interaction and changes in the ranking of genotypes: in the high yielding environments (GY > 5 Mg ha⁻¹), plants favor increased water uptake, with higher levels of transpiration and more open stomata (more negative values of carbon isotope composition, δ¹³C, and higher canopy temperature depression, CTD), whereas, in low yielding environments (GY < 5 Mg ha⁻¹) plants close their stomata and favor greater water use efficiency (less negative δ¹³C values and lower CTD values).