δ²H, δ¹⁸O, and δ¹³C behave differently when assessing transpirative and photosynthetic performance of wheat under contrasting VPD conditions

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.