Effect of Abscisic Acid on Sorbitol Uptake in Growing Apple Fruits

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Levels of abscisic acid (ABA) in the fruit flesh of developing apples (cv. Golden Delicious) were measured by electron capture gas chromatography. ABA content of the tissue, calculated on a fresh weight basis, decreased at a constant rate from 200 ng g_1 in young fruit to 10 fig g-1 in older fruit and then increased when the ripening process commenced. On a whole fruit basis, the ABA level increased during the initial phase of fruit growth, remained constant during the linear growth phase and increased again when fruits started to ripen. During fruit development the ABA content correlated with the rate of sorbitol uptake, when measured after discs of fruit tissue were incubated in [14C]sorbitol. Sorbitol uptake was high during the initial growth phase and declined at a constant rate during fruit development. ABA present in the incubation medium stimulated sorbitol uptake into fruit tissue at concentrations higher than 1CL8 M, whereas indolyl-3-acetic acid had no effect on uptake. When comparing sorbitol uptake in different zones of young fruit, it was found that uptake was higher in discs of outer fruit layers than in discs from inner fruit zones.