CHANGES IN SUGAR UPTAKE BY EXCISED DISCS AND ITS STIMULATION BY ABSCISIC AND INDOLEACETIC ACIDS DURING MELON FRUIT-DEVELOPMENT

To clarify how ABA and IAA may mediate sugar uptake during fruit devel opment, the kinetics of sugar accumulation by excised discs of melon ( Cucumis melo L. cv. Prince) was studied. Uptake kinetics for glucose a nd fructose in discs of young and premature fruit, 14 and 28 days afte r pollination, respectively, and for sucrose in young tissues have bip hasic (curvilinear) concentration curves. This indicates the presence of both carrier-mediated and simple diffusional uptake systems. The ca rrier-mediated system occurs at lower sugar concentrations for the thr ee sugars in young fruit, but for glucose and fructose in premature fr uits; 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 (-5) M enhanced the simple diffusional uptake of the three sugars, whe reas IAA at the same concentration stimulated the carrier-mediated sys tem of the three sugars but not the simple diffusional uptake system f ar sucrose, The roles of ABA and IAA on the carrier-mediated and simpl e diffusional uptakes systems for sugars seem to be very important dur ing the growth and maturation of the melon fruit.