Starving longan fruit sends weakened abscission-suppressing signal rather than enhanced abscission-triggering signal to the abscission zone

Abstract Fruit abscission is largely governed by carbohydrate supply, and carbohydrate shortage intensifies fruit abscission. Understanding cues from the starving fruit that activate the abscission zone (AZ) is an intriguing research subject and important for control fruit abscission. In this study, starvation stress generated by girdling and defoliating the bearing shoots was used to explore the cues from the starving fruit that triggers abscission in longan. The starving fruit abscised quickly within 5 days. However, removing the starving fruit induced even faster AZ activation with greater ethylene production in the pedicel. Abscission of the defruited pedicel was strongly suppressed by IAA, slightly by ethylene action inhibitor, silver thiosulfate (STS), but not by GA3 or zeatin. Both IAA and ABA concentrations in the starving fruit and the pedicel were significantly reduced compared with the control. IAA exudation from the starving fruit was significantly lower, while IAA oxidase activity in the fruit was higher. The activity of plasma membrane H+-ATPase (PM H+-ATPase) in the fruit and the pedicel was not significantly affected by carbohydrate starvation, while that of PM Ca2+-ATPase in both organs was significantly reduced by starvation. The results suggested that the starving fruit seems not send abscission-promoting signal e.g. ABA or ethylene, but sends weakened abscission-suppressing signal, IAA, to the AZ. The weakened capacity of starving fruit to export IAA is related to increase in IAA deactivation and reduction in PM Ca2+-ATPase activity. The reduced IAA in the AZ leads to increased sensitivity to ethylene synthesized in situ in the pedicel, which activates the AZ under starvation.