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

Carbon (δ 13 C) and oxygen (δ 18 O) isotope compositions are considered indicators of the effect of water conditions on plant photosynthesis (δ 13 C) and transpiration (δ 18 O). Hydrogen isotope composition (δ 2 H), tracks transpiration like δ 18 O, while also affected by the organ trophism. Such dual behaviour, together with its highly exchangeable nature have hindered the use of δ 2 H 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 δ 2 H and δ 18 O-enriched) and isotopic signatures analysed at mid-grain filling. Higher VPD increased plant-matter δ 13 C, δ 2 H and δ 18 O, in accordance with atmospheric drought decreasing stomatal conductance and transpiration. Moreover, positive correlations within and across organs between δ 2 H and δ 18 O of organic-matter and water further supported a similar source of variation related to evaporation. However, δ 2 H 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 δ 2 H, δ 18 O and δ 13 C through the interactions of these isotopes with VPD and plant organs.