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

Whereas the effects of water and nitrogen (N) on plant Delta C-13 have been reported previously, these factors have scarcely been studied for Delta O-18. Here the combined effect of different water and N regimes on Delta C-13, Delta O-18, 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 Delta C-13 was only slightly modified by water input. Conversely, N fertilizer significantly decreased Delta C-13. On the other hand, water supply decreased Delta O-18 values, whereas N did not affect this parameter. Delta O-18 variation was mainly determined by the amount of transpired water throughout plant growth (T-cum), whereas Delta C-13 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 Delta O-18(s). However, genotypic differences in Delta C-13 were observed. Moreover, similar to 80% of the variation in biomass across growing conditions and genotypes was explained by a combination of both isotopes, with Delta O-18 alone accounting for similar to 50%. This illustrates the usefulness of combining Delta O-18 and Delta C-13 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.