Water and nitrogen conditions affect the relationships of Δ(13)C and Δ(18)O to gas exchange and growth in durum wheat

Whereas the effects of water and nitrogen (N) on plant Δ(13)C have been reported previously, these factors have scarcely been studied for Δ(18)O. Here the combined effect of different water and N regimes on Δ(13)C, Δ(18)O, gas exchange, water-use efficiency (WUE), and growth of four genotypes of durum wheat [Triticum turgidum L. ssp. durum (Desf.) Husn.] cultured in pots was studied. Water and N supply significantly increased plant growth. However, a reduction in water supply did not lead to a significant decrease in gas exchange parameters, and consequently Δ(13)C was only slightly modified by water input. Conversely, N fertilizer significantly decreased Δ(13)C. On the other hand, water supply decreased Δ(18)O values, whereas N did not affect this parameter. Δ(18)O variation was mainly determined by the amount of transpired water throughout plant growth (T(cum)), whereas Δ(13)C variation was explained in part by a combination of leaf N and stomatal conductance (g(s)). Even though the four genotypes showed significant differences in cumulative transpiration rates and biomass, this was not translated into significant differences in Δ(18)O(s). However, genotypic differences in Δ(13)C were observed. Moreover, ∼80% of the variation in biomass across growing conditions and genotypes was explained by a combination of both isotopes, with Δ(18)O alone accounting for ∼50%. This illustrates the usefulness of combining Δ(18)O and Δ(13)C in order to assess differences in plant growth and total transpiration, and also to provide a time-integrated record of the photosynthetic and evaporative performance of the plant during the course of crop growth.