Floral induction in longan (Dimocarpus longan, Lour.) trees: IV. The essentiality of mature leaves for potassium chlorate induced floral induction and associated hormonal changes

This study was conducted to elucidate the function of leaves and their role in the alteration of plant hormone homeostasis during potassium chlorate (KClO3) induced “off season” floral induction (FI) of longan. KClO3 application had almost no effect on the concentration of gibberellins in receptive apical and lateral shoot meristems, whereas the auxin IAA initially decreased, but later remarkably increased. The most significant changes caused by KClO3 treatments was an increase in zeatin/zeatin-riboside (Z/ZR) type cytokinins (CKs) 8 days after treatment in the shoot apical and close by lateral buds as well as in the tissue beneath these buds. The 6–7-fold increase in CK concentrations were greatly reduced in the absence of leaves (leaves removed between the apical bud and a girdle). A significant exception was a strong accumulation of isopentenyl adenine/isopentenyl adenosine (iP/iPA) type CKs in wood and bark beneath the bud of defoliated shoots that were isolated against the rest of functional leaves of the tree by a girdle. This accumulation of iP/iPA, the immediate precursors of Z/ZR, together with the strong increase of Z/ZR in the shoot apical bud (SAB) after KClO3 application indicate that leaves may be involved in the biosynthesis of CKs. Overall, the results suggest that cytokinins play a significant role in KClO3 induced “off season” floral induction in longan. This role may not be restricted to the biosynthesis of these hormones but also to their mobilization and accumulation in the SAB of newly flushing shoots.