Abstract
To clarify how ABA and IAA may mediate sugar uptake during fruit development, the kinetics of sugar accumulation by excised discs of melon (Cucumis melo L. cv. Prince) was studied. Uptake kinetics for glucose and fructose in discs of young and premature fruit, 14 and 28 days after pollination, respectively, and for sucrose in young tissues have biphasic (curvilinear) concentration curves. This indicates the presence of both carrier-mediated and simple diffusional uptake systems. The carrier-mediated system occurs at lower sugar concentrations for the three sugars in young fruit, but for glucose and fructose in premature fruits ; it was not detectable in mature fruit 42 days after pollination. The Km value for carrier-mediated sugar uptake ranged from 6 to 14 mM and did not change with fruit development and among sugars. ABA at 10-5M enhanced the simple diffusional uptake of the three sugars, whereas IAA at the same concentration stimulated the carrier-mediated system of the three sugars but not the simple diffusional uptake system for sucrose. The roles of ABA and IAA on the carrier-mediated and simple diffusional uptakes systems for sugars seem to be very important during the growth and maturation of the melon fruit.
