Exogenous ABA and its inhibitor regulate flower bud induction of apple cv. ‘Nagafu No. 2′ grafted on different rootstocks

Exogenous ABA increased the flowering rate and advanced the flower formation of “Fuji” apple cv. “Nagafu No.2” by promoting the accumulation of soluble sugar, glucose and sucrose, and also increased the activities of α-AMY and SPS. “Fuji” apple (Malus domestica Borkh) has a long vegetative growth phase and difficulty in flower induction, which seriously affects apple production. ABA plays important roles in the flower bud induction of fruit trees. However, the effect of exogenous ABA on flower bud induction of apple trees has rarely been reported. In this experiment, 2-year-old “Fuji” apple cv. “Nagafu No. 2” scion material was grafted on dwarf self-rooted rootstock M.26 (DSR), vigorous rootstock M. sieversii (VR) and interstock M.26/M. sieversii (IR). The control trees were sprayed with water solution, while 100 µmol·L−1 ABA and 5 mmol·L−1 sodium tungstate (ST) were applied as treatments. The results showed that exogenous ABA promoted flower formation and increased the flowering rate, but the impact on the VR was not obvious. Furthermore, the accumulation of soluble sugar, glucose and sucrose, as well as the activities of α-amylase (α-AMY) and sucrose phosphate synthase (SPS) in the leaves and buds were also significantly promoted. However, the starch concentrations and sucrose synthase (SS) activities decreased. The increase of leaf specific weight and decrease of the growth of single leaf area were supportive to the growth of flower buds. Linear relationships were discovered between starch concentration and α-AMY activity (R2 = 0.998, p < 0.05), sucrose concentration and the activities of SS and SPS (R2 = 0.854–0.999, p < 0.05). Besides, exogenous ST had negative effects compared with ABA. Exogenous ABA increased sugar metabolism, and enzyme activities of DSR, and also enhanced the absorption and utilization of sugar substances, resulting in easier flowering in the DSR, followed by the IR, except for the difficult-flowering rootstock VR. Finally, the expression levels of TPS7, PPC1 and ABR1 were up-regulated in leaves and buds after ABA application, while the floral inhibiting gene TFL1 was down-regulated, indicating that exogenous ABA may promote flower bud differentiation by inhibiting TFL1 expression in apple.