Carbon isotope composition
Rootstock affected leaf and terminal stem carbon isotope composition. In 2018, the most dwarfing rootstocks B.9 and M.9 had more enriched values for leaves when compared to G.890 and G.210 (Table 3). Leaf δ13C for all rootstocks was approximately -25‰ and ranged by approximately 1‰ among rootstock genotypes for both 2018 and 2019 and stem δ13C values were closer to -24.5‰ and ranged by approximately 2‰. At the stem level, although there were no statistical differences, δ13C for B.9 was the most enriched and, for G.210, was the most depleted. In 2019, for both leaves and stems, B.9 was more enriched than G.210 and G.890. G.814 had the lowest δ13C for both leaves and stems. There was a positive non-linear relationship between δ13C in leaves and stems (P = 0.05, r = 0.8914) (Figure 6). The data demonstrate that the carbon isotope composition of non-photosynthetic tissues from stem tissue are more enriched compared with leaves of the same branch but converge when values are both on the high or low end of the range in variation observed in this experiment.
Carbon isotope composition in leaves was also strongly and negatively correlated with seasonal shoot growth (P <0.001, r = 0.924) (Figure 7). Rootstocks with lower vegetative vigor had lower δ13C in both stems and leaves. Net CO2assimilation, stomatal conductance and transpiration rate all correlated strongly and negatively with δ13C (P <0.01, r = 0.873; P <0.01, r = 0.807; P <0.05, r = 0.787; Figure 8). As expected, this effect was even more evident for more vigorous rootstocks. Successively, rootstock-mediated variability in stem water potential were negatively related to leaf δ13C (P <0.001, r = 0.906) (Figure 9).