Abscission of mango fruitlets as influenced by biosynthesis of polyamines

The role of polyamines in fruitlet abscission of mango (Magnifiera indica L.) was studied by estimating the endogenous polyamines in intact and about-to-abscind fruitlets and their pedicels, and the effect of exogenous applications of polyamines and inhibitors of polyamine biosynthesis. Endogenous free polyamines (putrescine, spermidine and spermine) were estimated in intact and about-to-abscind fruitlets and their pedicels at three developmental stages in 'Kensington Pride' and 'Glen' mango. Intact fruitlets and their pedicels exhibited 276% and 341% higher total free polyamines than about-to-abscind ones in 'Kensington Pride', whilst in 'Glen'; the increase was 137% and 63%, respectively. Mean spermine concentrations were higher in intact tissues of 'Kensington Pride' (fruitlets 220 nmol g(-1) FW, pedicels 172 nmol g(-1) FW), while spermidine concentrations were greater in 'Glen' (fruitlets 244.5 nmol g(-1) FW, pedicels 256 nmol g(-1) FW). The role of spermine seems to be critical in mango fruitlet abscission as the about-to-abscind fruitlets had very low levels of spermine (10.4% and 19% of intact fruitlets) in 'Kensington Pride' and 'Glen', respectively. Exogenous applications of polyamines reduced fruitlet abscission during the initial heavy fruitlet abscission period (two weeks after fruit set) and the response was significantly influenced by type, concentration and time of application. Spermidine (0.1 mM) was the most effective of the three polyamines applied at the fruit-set stage, reducing fruitlet abscission by 6.8% compared with the control. Of the four times of application, spermine (0.1 mM) sprayed when the panicle had grown 5-8 cm showed least fruitlets abscission (91.5%) compared with controls (95.5%) after six weeks of fruit set. Higher levels of endogenous polyamines in intact fruitlets and their pedicels, compared with about-to-abscind ones, and the reduction of fruitlet abscission with exogenous application of polyamines, as well as the acceleration with inhibitors of polyamines, demonstrate the key role of polyamines biosynthesis in mango fruitlet abscission.