Roles of paclobutrazol and potassium, nitrate in the flowering of carabao mango (Mangifera indica L.)

One corrective measure to overcome the biennial bearing habit of mango is the use of growth retardant paclobutrazol (PBZ), and the salt potassium nitrate (KNO3). In this system of flower forcing, PBZ is believed to induce flowering by blocking the synthesis of gibberellic acid (GA)inhibition of mango flowering. Thus the accumulation of carbohydrates needed for reproductive growth is promoted while vegetative growth is retarded. KNO3, on the other hand, is supposed to release resting buds for dormancy. However, KNO3 is also viewed as an inducer of mango flowering based on observations that it could as well bring about the flowering of mango trees without the use of PBZ. This study sought to clarify the roles of those two chemicals by treating 3-yr old 'Carabao' mango trees with soil-drench of PBZ followed by foliar spray of KNO3 and analyzing the carbohydrate and GA contents of the shoots. The events of floral induction and floral initiation were distinguished by microscopic observation of morpho-anatomical changes of the shoot apex. PBZ advanced the time and increased the intensity of flowering but these effects were not associated with suppression of vegetative flushing. PBZ lowered the concentration of GA but did not increase the level of carbohydrates in the shoots of treated trees. Fluctuations in the levels of GA and carbohydrates in the shoots from 1-5 months after drenching with PBZ could not precisely define an inhibitory role for GA or demonstrate the essentiality of high carbohydrate reserves in mango flowering. Floral initials were observed to be already present in the buds of shoots as young as 2 months old from both PBZ-treated and untreated trees before spraying with KNO3. This indicated that flowering in mango is autonomous and the influence of PBZ is restricted to bud development. Floral response to KNO3 was obtained only when there was heightened meristematic activity at the shoot apex and differentiation of the inflorescence elements was underway. This pointed out that KNO3 action was on bud break and not on flower induction or initiation.