The Hydrogen Isotope Composition δ²H Reflects Plant Performance

Abstract

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 (<i>Triticum turgidum</i> var <i>durum</i>), 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.