Effect of leaf structure and water status on carbon isotope discrimination in field‐grown durum wheat

The relationships between leaf and kernel carbon isotope discriminations (delta) and several leaf structural parameters that are indicators of photosynthetic capacity were studied in durum wheat grown in the field under three water regimens. A set of 144 genotypes were cultivated in two rain-fed trials, and 125 of these were grown under supplementary irrigation before heading. Total chlorophyll and nitrogen (N) contents, the dry mass per unit leaf area (LDM, the reciprocal of specific leaf area) and carbon isotope discrimination (delta) were measured in penultimate leaves and delta of mature kernels was also analysed. Both LDM and N per unit area showed significant (P less than or equal to 0.001) negative correlation (r = - 0.60 and r = - 0.36, respectively) with leaf delta in the wettest trial. Little or no correlation was found for any structural parameter and leaf delta in the rain-fed trials. In contrast, in the two rain-fed trials LDM was the parameter with the strongest positive correlation (P less than or equal to 0.001) with kernel delta (r = 0.47 and 0.30) and grain yield (r = 0.43 and 0.29), whereas no correlation was found in the irrigation trial. These correlations, rather than representing a causal link between the amount of photosynthetic tissue and delta, were probably indirect associations caused by a parallel effect of water status and phenology on leaf structure, grain delta and yield. Correlations across trials (i.e. environments) between leaf structure and either delta and yield were very high, although also spurious. Our results suggest that LDM should be used to cull segregating population differences in leaf delta based on the internal photosynthetic capacity only in the absence of drought. Selecting for kernel delta and grain yield on the basis of LDM is worthwhile for rain-fed trials.