Carbon and nitrogen stable isotope variations in leaves of two grapevine cultivars (Chasselas and Pinot noir): Implications for ecophysiological studies

We investigated the within- and between-leaf variability in the carbon and nitrogen isotope composition (δ 13 C and δ 15 N) and total nitrogen (TN) content in two grapevine cultivars (Vitis vinifera cv. Chasselas and Pinot noir) field-grown under rain-fed conditions. The within-leaf variability was studied in discs sampled from base-to-tip and left and right regions from the margin to midrib. The intra- and interplant variability was studied by comparing leaves at different positions along the shoot (basal, median, apical). In leaves from both cultivars, a decrease in δ 13 C from base to tip was observed, which is in line with an upward gradient of stomatal density and chlorophyll concentration. Less important, but still significant differences were observed between the right and left discs. The leaf TN and δ 15 N values differed between cultivars, showed smaller variations than the δ 13 C values, and no systematic spatial trends. The intraleaf variations in δ 13 C, δ 15 N, and TN suggest that stomatal behavior, CO 2 fixation, chlorophyll concentrations, and the chemical composition of leaf components were heterogeneous in the leaves. At the canopy scale, the apical leaves had less 13 C and more 15 N and TN than the basal leaves, indicating differences in their photosynthetic capacity and remobilizations from old, senescing leaves to younger leaves. Overall, this study demonstrates patchiness in the δ 13 C and δ 15 N values of grapevine leaves and species-specificity of the nitrogen assimilation and 15 N fractionation. These findings suggest that care must be taken not to overinterpret foliar δ 13 C and δ 15 N values in studies based on fragmented material as markers of physiological and biochemical responses to environmental factors.