The Role of Hexose Transporter in Sugar Accumulation of Papaya Fruit during Maturation and Ripening

Papaya hexose uptake during fruit development was studied by comparing [14C]glucose uptake by mesocarp slices from two cultivars; 'Sunset' and UH801 (a low sugar line). 'Sunset' papaya fruit though one third the weight of UH801, had higher total soluble solids (TSS), total sugar, and dry weight (DW)than UH801. Both cultivars reached the color break stage at the same time, 125 days after anthesis (DAA), but flesh color, TSS and dry weight of 'Sunset' fruit began to increase 111 DAA, one to three weeks before UH801. Glucose uptake in 'Sunset' fruit was lower than for UH801. The pattern of 14C-glucose uptake by 'Sunset' papaya flesh discs was divided into three fruit growth phases during fruit maturation. Hexose uptake was initially low during the first phase, 90-97 DAA, and last phase, after 132 DAA, and higher 97-132 DAA. The maximum hexose uptake in 'Sunset' was found at 118 DAA, 1,367 ìmol mg protein-1 h-1, and appeared to be an energy-dependent process. Hexose transporter activity was detected in both varieties but 14C-glucose uptake did not appear to correlate with papaya sugar accumulation. Hence, the role of invertase in phloem unloading may be a more important factor determining fruit sugar levels at harvest.