SORBITOL AND SUCROSE PARTITIONING IN THE GROWING APPLE FRUIT

Growing apple fruit is supplied with the C-assimilates sorbitol and su crose produced in leaf photosynthesis. These C-sources enter fruit met abolism and accumulate as fructose, sucrose, malic acid, and starch. I n 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 par t of the molecule, into discs of parenchyma tissue, the distribution o f the label among the metabolic products was compared. With [Glc-C-14] sucrose as the C-source, starch and other major metabolic products con tained 2-3 times more radioactivity than when [Fru-C-14]sucrose was us ed. 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 s ucrose more readily enters into the hexose phosphate pool than fructos e derived either from sucrose or sorbitol. The selective utilization o f glucose is therefore dependent on the cleavage of sucrose by sucrose synthase. This reaction favors diversion of glucose to starch while f ructose is accumulated in the vacuole of the parenchyma cell. The hexo se 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 di scs. After hydrolysis of labeled starch, redistribution of C-13 betwee n carbon C1 and carbon C6 of glucose was measured by nuclear magnetic resonance (NMR). The extent of redistribution was 15-31%, indicating t hat a hexose or hexose monophosphate is incorporated into starch witho ut substantial previous cleavage to triose phosphates. The partitionin g of sucrose between soluble sugars and starch was also dependent on t he cell turgor. In the presence of increasing external mannitol concen trations, which reduced cell turgor, asymmetrically labeled sucrose wa s metabolized in excised tissue discs in such a way that the glucose m oiety was preferentially partitioned into starch, while fructose was f avored for accumulation.