Unravelling the role of KNO3 in mango flowering

The involvement of KNO3 and ethylene in flower induction was studied in 13-year old 'Carabao' mango trees. The level of gibberellic acid (GA3) was artificially reduced through application of 1.0 gram of paclobutrazol (PBZ) per meter canopy diameter. Levels of GA3 and presence of floral initials in the terminal shoots were followed monthly. Three months after PBZ application. GA3-like content of shoots in PBZ treated trees decreased by 89 percent compared to the control. At this time, 56 percent of the terminal shoots in PBZ-treated trees had floral initials and none in the control trees. At the same time, ethylene levels declined while 1-aminocyclopropane, 1-carboxylic acid (ACC), levels increased coinciding with the time of development of floral initials. By the fourth month, shoots with floral initials in PBZ-treated trees increased to 86 percent while 42 percent of the control trees also had floral initials just before KNO3 was sprayed. The development of floral initials was accompanied by a decrease in GA3 levels suggesting an inhibitory role for GA3 in mango flowering. Induction of flower bud break by 2 percent KNO3 resulted in a flower intensity of 4 (flowers all over the canopy) and longer inflorescences in PBZ-treated trees while control trees exhibited a flower intensity index of 2 (less than 25 percent of the canopy have flowers). This study demonstrated that KNO3 was merely responsible for budbreak of existing flower buds and not for the transformation of vegetative to reproductive structures. To probe the involvement of ethylene in the KNO3-induced flowering of mango, the inhibitors amino oxyacetic acid (AOA) and silver thiosulfate (STS) were applied one hour before KNO3 spray. AOA and STS, each applied at 1mM, failed to inhibit the KNO3 induced-flowering. Flowering intensities of STS and AOA treated shoots followed by KNO3 spray, were 82 percent and 72 percent, respectively, while KNO3-sprayed trees also had 82 percent flowering. These results suggest that KNO3 does not exert its effects through the ethylene biosynthetic pathway.