Effects of CEPA and 1-MCP on Flower Bud Differentiation of Apple cv. ‘Nagafu No.2’ Grafted on Different Rootstocks

The apple ( Malus domestica Borkh.) has a relatively long sapling stage which prevents fruit breeding. The understanding of the flowering system is important to improve breeding efficiency in apple. In this context, 2-year-old “Fuji” apple cv. “Nagafu No.2” trees were grafted onto dwarf self-rooted rootstock M.26, vigorous rootstock M. sieversii and interstock M.26/ M. sieversii , respectively. Trees were sprayed with clean water (as controls), 800 mg·L −1 2-chloroethylphosphonic acid (CEPA), and 2 µL·L −1 1-methylcyclopropene (1-MCP). The results showed that CEPA significantly repressed the vegetative growth attributed to the increase of ABA and ZT synthesis, and the decrease of IAA synthesis in leaves and buds. However, there was no significant difference or significant inverse effect between 1-MCP and control. Furthermore, CEPA promoted flower formation, increased the flowering rate, and advanced the blossom period for 2 days compared with the control, which was accompanied by the accumulation of soluble sugar, glucose, and sucrose; and the increase of α-amylase and sucrose phosphate synthase activities; and the decrease of starch contents and sucrose synthase activities in leaves and buds. However, the blossom period was delayed for 2 days after spraying with 1-MCP. Finally, the expression of TFL1 was significantly repressed, whereas AP1 was significantly promoted in buds from M.26 and M.26/ M. sieversii after spraying with CEPA, whereas the effect was not significant from M. sieversii . However, the expression levels of TFL1 and AP1 were not significantly different from the control after the application of 1-MCP. In spite of this, CEPA was more susceptible to easy-flowering M.26, followed by M.26/ M. sieversii , and still less susceptible to difficult-flowering rootstock M. sieversii .