Effect of hormones on sucrose uptake and on ATPase activity of citrus sinensis L. Osbeck leaves

Abstract

Carbohydrate accumulation in young, fully expanded leaves of Citrus sinensis L. Osbeck is affected by the presence of the fruitlet on the shoot. Previous work gave evidence that gibberellins may be involved in this ’fruit effect’. In the present work we have studied the effect of gibberellic acid (GA3) on 14C-sucrose uptake by leaf discs and whether its action could be due to a modulation of the plasma membrane ATPase, which maintains the H+ gradient that drives H+/sucrose co-transport. The effect of GA3 on 14C-sucrose uptake depended on the osmolarity of the assay medium. At 300 mOsm a reduction in the uptake rate was observed. The inhibitory effect of the hormone disappeared after preincubating the leaf discs with para-chloromercuri-phenylsulphonic acid (PCMPS), a sulphydril binding inhibitor. ATPase activity of isolated plasma membrane vesicles was inhibited by IAA treatments, while GA3 or ABA did not affect this enzyme, even after a 3 h preincubation period. However, in the absence of a surfactant in the assay medium, GA3, together with turgor pressure, modulated plasma membrane ATPase activity, possibly through modifications of membrane permeability. The hormone effect on 14 C-sucrose uptake may involve action on the sucrose carrier. © 1994 Academic Press.