Whole-Tree Storage of Non-structural Carbohydrates in Apple and Pear Trees on Size-Controlling Rootstocks

Size-controlling rootstocks have been one of the major innovations facilitating high-efficiency fruit production; however, biological mechanisms responsible for their size-controlling effect remain unclear. In this study we investigated if apple and pear trees grafted on dwarfing and invigorating rootstocks differ in the size and dynamics of non-structural carbohydrate (NSC) storage pools. Seasonal dynamics in NSC concentrations were assessed in current-year shoots, coarse roots, trunks, and leaves. These measurements were then upscaled to whole-organ and whole-tree NSC pools and mutually compared. Because of the small variation in the relative biomass partitioning and generally similar organ-level NSC concentrations, the size of the NSC pools scaled tightly with the overall tree biomass with vigorous trees having greater absolute storage pools compared to dwarfing trees. The magnitudes of the seasonal fluctuation in NSC pools (i.e., November to May difference) were in the range from 0.55 to 3.93 kg per tree and 20 to 50 g per kg of tree’s dry weight. In absolute terms, the seasonal fluctuations in NSC pools were higher in vigorously growing trees but in relative terms and also when scaled by the tree’s biomass the differences between the low and high vigor trees became negligible, suggesting that the low and high vigor trees rely on their NSC reserves to a similar extent during their annual growth cycle. Thus, our results provide no support that the observed differences in growth vigor are driven by the availability of C-reserves.