The role of sorbitol synthesis in photosynthesis of peach (Prunus persica)
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This dissertation examines the hypothesis that polyol synthesis enhances photosynthetic capacity in peach and related species. Members of Prunus synthesize, translocate, and utilize sorbitol as their main photosynthetic end product whereas most other plants utilize sucrose for those purposes. First, I approached this hypothesis by examining eight genetically diverse Prunus species with various sorbitol: sucrose ratios and activities of sorbitol-6-phosphate dehydrogenase (S6PDH), principal sorbitol synthesis enzyme. Leaf photosynthetic capabilities (A), in vitro activity of the S6PDH and sorbitol contents of greenhouse grown plants were measured. I found an inverse relation between A and S6PDH activity of the species. This observation does not support the working hypothesis and that sorbitol synthesis enhances A. Second, I used two peach varieties (i.e., Encore and Nemaguard) to examine the same hypothesis within a single species by source/sink manipulations (i.e., fruiting versus non- fruiting, fruit present versus fruit removed, and shoot tip removal) and existing natural variation (i.e., leaf node position). In all cases, except fruiting versus non-fruiting and fruit present, photosynthesis and S6PDH enzyme activity showed positive correlations. Finally, I analyzed the response of S6PDH gene to shoot tip removal treatment in connection with S6PDH activity and A in potted Nemaguard peach plants. To document hourly changes, leaves were sampled three times during the day (i.e., sunrise, midday, and sunset) and analyzed for S6PDH gene expression and S6PDH activity. Sorbitol-6-phosphate dehydrogenase mRNA transcript levels significantly increased while S6PDH activity decreased 24-hour following shoot tip removal. Gene transcript levels did not change in hourly manner during the day whereas enzyme activity decreased at midday. Response of the enzyme and gene mRNA transcript revealed weak correlations, suggesting that S6PDH regulation was related to posttranslational modifications of the enzyme. Downregulation of the S6PDH following sink removal did not appear to be brought about by a simple feedback inhibition of the sorbitol on enzyme activity in vitro. These studies suggested that sorbitol or sorbitol forming capacity do not necessarily enhance A across a spectrum of Prunus although within a single species, parallel changes in S6PDH activity and A were coordinated through posttranslational changes to the enzyme.