Sorbitol and sucrose partitioning in the growing apple fruit

Growing apple fruit is supplied with the C-assimilates sorbitol and sucrose produced in leaf photosynthesis. These C-sources enter fruit metabolism and accumulate as fructose, sucrose, malic acid, and starch. In the present study the contribution of sucrose and sorbitol in carbon partitioning was investigated. After incorporation of [C-14]sorbitol and [C-14]sucrose, labeled either in the grucosyl or the fructosyl part of the molecule, into discs of parenchyma tissue, the distribution of the label among the metabolic products was compared. With [Glc-C-14]sucrose as the C-source, starch and other major metabolic products contained 2-3 times more radioactivity than when [Fru-C-14]sucrose was used. When [C-14] sorbitol was fed, fructose was preferentially labeled, but incorporation into starch and other metabolites was less compared with [Fru-C-14] sucrose. The results show that glucose derived from sucrose more readily enters into the hexose phosphate pool than fructose derived either from sucrose or sorbitol. The selective utilization of glucose is therefore dependent on the cleavage of sucrose by sucrose synthase. This reaction favors diversion of glucose to starch while fructose is accumulated in the vacuole of the parenchyma cell. The hexose phosphates produced in the sucrose synthase pathway are more liable to be used as precursors for starch synthesis than triose phosphates. This conclusion is reached from experiments in which [1-C-13]glucose and [6-C-13]glucose were incorporated into starch of excised tissue discs. After hydrolysis of labeled starch, redistribution of C-13 between carbon C1 and carbon C6 of glucose was measured by nuclear magnetic resonance (NMR). The extent of redistribution was 15-31%, indicating that a hexose or hexose monophosphate is incorporated into starch without substantial previous cleavage to triose phosphates. The partitioning of sucrose between soluble sugars and starch was also dependent on the cell turgor. In the presence of increasing external mannitol concentrations, which reduced cell turgor, asymmetrically labeled sucrose was metabolized in excised tissue discs in such a way that the glucose moiety was preferentially partitioned into starch, while fructose was favored for accumulation.