Seasonal Fluctuations of Carbyhydrate Metabolizing Enzyme Activities Throughout Tart Cherry Fruit Development
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Carbohydrate metabolism during tart cherry ripening remains to be investigated. However, extensive carbohydrate metabolism studies of other fruits have provided insight on potential enzymes involved in tart cherry fruit ripening. Information from the aforementioned studies and the known changes in sugar composition occurring throughout ripening have allowed for hypotheses of the enzymes that may be involved in carbohydrate metabolism throughout cherry fruit development. In order to determine the level of carbohydrate metabolizing enzyme activities throughout fruit development, activities of NAD-dependent sorbitol dehydrogenase (SDH), sorbitol oxidase (SOX), acid invertase (AI), sucrose synthase (SS) and alkaline invertase (ALK) in tart cherry fruits (Prunus cerasus, L.cv. Montmorency) were assayed during the growing season. SDH, a sorbitol metabolizing enzyme, activity was found to increase while sorbitol levels declined. Also, while sucrose levels decreased during development, the sucrose catabolizing enzyme AI activity levels increased dramatically. SOX , SS, and ALK activity levels were negligible. Therefore, AI and SDH were concluded to be the main carbohydrate metabolizing enzymes involved in tart cherry fruit development. With the aim of revealing the mode of enzyme regulation, RNA blot analysis of AI and SDH were performed to extend the investigation to the level of gene expression. The levels of AI mRNA were low during the young green stage and high in the mature dark red stage, paralleling activity levels, implying transcriptional control. The northern analysis for SDH mRNA was less clear, suggesting other modes of regulation, possibly post-transcriptional or post-translational controls.