Arsenite reduces acid content in citrus fruit, inhibits activity of citrate synthase but induces its gene expression

Arsenic compounds generate diverse effects in all living organisms. In citr us (Citrus L. sp.), they reduce acidity and improve fruit quality by unknow n mechanisms. The major organic acid in citrus fruit is citric acid, which begins accumulating early in fruit development, reaches a peak in middle-si zed fruit and then, in most species, declines as the fruit matures. In an a ttempt to understand the basis of the effect of arsenite, it was applied to 'Minneola' tangelo (Citrus paradisi Macf. x C. reticulata Blanco) approxim ate to 6 weeks postanthesis, and a detailed analysis of total titratable ac idity and citric acid concentration was performed throughout fruit growth, Within 35 days after arsenite application, total acid content and citrate c oncentration were slightly lower compared with the controls, and this diffe rence persisted throughout fruit development. The concentrations of other o rganic acids were not reduced by the treatment. Sodium arsenite reduced the citrate concentration in 'Eurieka' lemon callus [Citrus limon (L.) Burm.] also, without affecting tissue growth, Extractable activity of citrate synt hase in treated fruit was inhibited within 1 day following arsenite spray, but recovered to a normal level a few days later. In contrast, gene express ion was remarkably induced 1 day following treatment, which might explain t he recovery in enzyme activity. Data suggest that reduction in acid accumul ation may not be related to the initial inhibition of citrate synthase acti vity.