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).