Combined use of d13C, d18O and d15N tracks nitrogen metabolism and genotypic adaptation of durum wheat to salinity and water deficit

Accurate phenotyping remains a bottleneck in breeding for salinity and drought resistance. Here the combined use of stable isotope compositions of carbon (d13C), oxygen (d18O) and nitrogen (d15N) in dry matter is aimed at assessing genotypic responses of durum wheat under different combinations of these stresses. Two tolerant and two susceptible genotypes to salinity were grown under five combinations of salinity and irrigation regimes. Plant biomass, d13C, d18O and d15N, gas-exchange parameters, ion and N concentrations, and nitrate reductase (NR) and glutamine synthetase (GS) activities were measured. Stresses significantly affected all traits studied. However, only d13C, d18O, d15N, GS and NR activities, and N concentration allowed for clear differentiation between tolerant and susceptible genotypes. Further, a conceptual model explaining differences in biomass based on such traits was developed for each growing condition. Differences in acclimation responses among durum wheat genotypes under different stress treatments were associated with d13C. However, except for the most severe stress, d13C did not have a direct (negative) relationship to biomass, being mediated through factors affecting d18O or N metabolism. Based upon these results, the key role of N metabolism in durum wheat adaptation to salinity and water stress is highlighted.