PROTOPLAST HEXOSE CARRIER ACTIVITY IS A DETERMINATE OF GENOTYPIC DIFFERENCE IN HEXOSE STORAGE IN TOMATO FRUIT

Post-phloem sugar transport in developing tomato (Lycopersicon esculen tum Mill. cv. Flora-Dade) fruit follows an apoplastic route during the rapid phase of sugar accumulation. The pathway is characterized by su gar retrieval by the storage parenchyma cells from the fruit apoplast. Two tomato genotypes differing in fruit hexose content were compared in terms of the transport and transfer processes controlling fruit sug ar levels. The genotypic difference in fruit sugar content was indepen dent of photoassimilate export from source leaves. Discs of pericarp t issue were cultured in a medium based on analyses of the fruit apoplas tic sap. The cultured discs maintained a composition, a relative growt h rate and a respiration rate similar to those of the pericarp tissue of intact fruit. Estimates of hexose fluxes into metabolic and storage pools suggested that membrane transport regulated the genotypic diffe rence in hexose accumulation. Short-term [C-14]hexose uptake experimen ts demonstrated a genotypic difference in V-max for glucose, fructose and 3-O-methylglucose, and this difference was abolished in the presen ce of the inhibitor p-chloromercuribenzenesulphonic acid (PCMBS). The results support the hypothesis that the activity of energized hexose c arriers on the plasma membranes of storage parenchyma cells is a signi ficant determinate of the genotypic difference in hexose accumulation.