Effects of Abscisic Acid and Fluridone on Ripening of Apple Fruits

Concentrations of endogenous abscisic acid (ABA) in apple ( Malus pumila Mill.)cvs. Golden Delicious and Starkrimson fruits increased during late period of fruit development in close correlation with fruit ripening. The ABA concentration peak in seeds of Golden Delicious fruits during the late period of fruit development occurred first, followed by those in core and flesh of fruits of both varieties (Fig.1). ABA was shown to initiate system 2 ethylene synthesis in the Golden Delicious fruits 150 days after full bloom when its concentration in fruit core exceeded 1.8 μmol/kg FW (Fig.6). Treatment of fruits of both varieties with exogenous ABA, before the climacteric period during the late developmental phases, triggered efficiently the synthesis of the system 2 ethylene and activated the ACC synthase and ACC oxidase in the fruits (Figs.5,7). Treatment of fruits with fluridone significantly decreased ABA concentrations in fruit tissues, delayed the system 2 ethylene production and inhibited fruit softening (Figs.2,3,5). Treatment of fruits with dimethylsulfonide induced significant increase in ABA concentration in flesh and core and hastened fruit ripening. We concluded that ABA plays an important role in regulating the ripening of apple fruits upstream of ethylene. The mechanism of ABA action on fruit ripening was discussed.