Sorbitol Metabolism in Nectaries From Flowers of Rosaceae

Abstract

Although sorbitol is the major soluble carbohydrate in plants of the woody Rosaceae, the floral nectars contained virtually no sorbitol. Glucose and fructose occurred in approximately equal proportions, with sucrose making up the balance. Nectar secretion was studied using excised flowers. Extensive conversion of sorbitol to other sugars occurred within the flower, at least partly within the nectary itself. Cutting the nectary tissue into slices greatly reduced its ability to transform sorbitol to other sugars. It is suggested that during nectar secretion sorbitol conversion occurs mainly during phloem unloading. Changes in sugar composition of nectars were studied. There was no evidence that sucrose was secreted then subsequently hydrolysed by invertase or by a microbial flora. There was bidirectional movement of sugars: radioactive sucrose and other sugars added to the nectar entered the nectary tissue, were metabolized there to other sugars, and were secreted again to the nectar. Composition of the nectar was therefore a product of both secretion and uptake processes.