Water and nitrogen conditions affect the relationships of Δ13C and Δ18O to gas exchange and growth in durum wheat

Abstract

Whereas the effects of water and nitrogen (N) on plant Δ¹³C have been reported previously, these factors have scarcely been studied for Δ¹⁸O. Here the combined effect of different water and N regimes on Δ¹³C, Δ¹⁸O, 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 Δ¹³C was only slightly modified by water input. Conversely, N fertilizer significantly decreased Δ¹³C. On the other hand, water supply decreased Δ¹⁸O values, whereas N did not affect this parameter. Δ¹⁸O variation was mainly determined by the amount of transpired water throughout plant growth (T_cum), whereas Δ¹³C 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 Δ¹⁸O_s. However, genotypic differences in Δ¹³C were observed. Moreover, ∼80% of the variation in biomass across growing conditions and genotypes was explained by a combination of both isotopes, with Δ¹⁸O alone accounting for ∼50%. This illustrates the usefulness of combining Δ¹⁸O and Δ¹³C 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.