Tree Jointing and Branch Bending Influence Endogenous Levels of Hormones and Flowering in Japanese Plum 'Kiyo'

The joint tree training system was developed in Japan as a horticultural intervention to promote flower induction (FI) in fruit trees. In this system, the apical tip of one tree is artificially grafted into the bent main stem of another in a row of three or more trees. The system reportedly facilitates faster establishment of the tree framework leading to faster FI and high productivity of the orchard earlier than those utilizing traditional systems. Plant hormones are putative bioregulators of flower induction and development. However, changes in phytohormone levels as a result of this cultural intervention have not been reported. To investigate this, 5-year old trees of 'Kiyo' Japanese plum (Prunus salicina) in containers were either bent and jointed into a row of three trees (Bent-Joint group), or individually bent (Bent group). Upright individual trees served as control. Young leaves were analyzed for phytohormone content by ultra-performance liquid chromatography-electrospray ionization tandem mass spectrometry (UPLC-ESI-MS/MS). Our results showed that both the Bent and Bent-Joint groups have significantly lower concentrations of the bioactive gibberellins (i.e., GA1, GA3 and GA4), but higher abscisic acid (ABA) content compared to the control. Consistent with the implicated role of GA as suppressor, and ABA as inducer of FI, both the Bent and Bent-Joint groups registered high flowering rates. Fruit-set was also observed in these groups, while the control had none. The number of new shoots did not significantly vary across treatment groups. Yet control trees, having relatively high bioactive GAs and low ABA contents, exhibited the most vigorous shoot elongation. These findings suggest that the joint tree training system appear to favor FI and fruit-set over vegetative growth that may have resulted from the observed low levels of bioactive GAs and high ABA content.