Photoassimilate Regulation of Sorbitol and Sucrose Metabolism in Peach Fruit

Abstract

Along with sucrose, sorbitol represents the major photosynthetic product and the main form of translocated carbon in peach. The objective of the present study was to determine whether in peach fruit, sorbitol and sucrose enzyme activities are source-regulated, and more specifically modulated by sorbitol or sucrose availability. In two separate trials, peach fruit relative growth rate (RGR), enzyme activities, and carbohydrates were measured 1) at cell division stage before and after girdling of the shoot subtending the fruit; and 2) on 14 shoots with different leaf to fruit ratio (L:F) at cell division and cell expansion stages. Fruit RGR and sorbitol dehydrogenase (SDH) activity were significantly reduced by girdling, whereas sucrose synthase (SS), acid invertase (AI), and neutral invertase (NI) where equally active in girdled and control fruits on the fourth day after girdling. All major carbohydrates (sorbitol, sucrose, glucose, fructose and starch) were reduced on the fourth day after girdling. SDH activity was the only enzyme activity proportional to L:F in both fruit developmental stages. Peach fruit incubation in sorbitol for 24 hours also resulted in SDH activities higher than those of fruits incubated in buffer and similar to those of freshly extracted samples. Overall, our data provide some evidence for regulation of sorbitol metabolism, but not sucrose metabolism, by photoassimilate availability in peach fruit. In particular, sorbitol translocated to the fruit may function as a signal for modulating SDH activity.