The stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope compositions in plant matter reflect photosynthetic and transpirative conditions in plants, respectively. However, the nature of hydrogen isotope composition (δ²H) and what it reflects of plant performance is poorly understood. Using durum wheat (Triticum turgidum var durum), this study evaluated the effect of different water and nitrogen growing field conditions on transpiration and how this effect influenced the performance of δ²H in autotrophic (flag leaf), mixotrophic (ears), and heterotrophic (grains and roots) organs. Moreover, δ²H was compared with the δ¹³C and δ¹⁸O in the same organs. Isotope compositions were analyzed in dry matter, the water-soluble fraction, and in water from different tissues of a set of genotypes. Similar to δ¹³C, the δ²H correlated negatively with stomatal conductance, whereas no correlation was observed for δ¹⁸O. Moreover, δ²H was not only affected by changes in transpiration but also by photosynthetic reactions, probably as a consequence of NADPH formation in autotrophic organs. Compared with the δ²H of stem water, plant δ²H was strongly diminished in photosynthetic organs such as the flag leaves, whereas it strongly increased in heterotrophic organs such as grains and roots. In heterotrophic organs, δ²H was associated with postphotosynthetic effects because there are several processes that lead to ²H-enrichment of carbohydrates. In summary, δ²H exhibited specific features that inform about the water conditions of the wheat crop, together with the photosynthetic characteristics of the plant part considered. Moreover, correlations of δ²H with grain yield illustrate that this isotope can be used to assess plant performance under different growing conditions.

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