The effect of girdling on carbohydrate partitioning in the growing apple fruit

Abstract

Summary In the present work the activities of key enzymes involved in carbohydrate metabolism of the growing apple are compared with the levels of sugars and starch accumulating in the fruit. After uptake of sorbitol and sucrose the growing fruit steadily accumulates fructose and sucrose, while the starch level reaches a peak about halfway through fruit development. Fructose accumulation in the fruit correlated positively with the activity of sorbitol dehydrogenase (SDH). Concomitandy with transient partitioning of carbon to starch, the activities of the enzymes sucrose synthase, UDP-glucose pyrophosphorylase, and phospho-glucose isomerase (PGI) also increased. Activities of phosphofructophosphotransferase (PFP), phosphof-ructokinase and glucose-6-phosphate dehydrogenase (G6PDH) did not correlate with the rise in starch content, but increased during the phase of starch degradation. Invertase activity did not correlate with starch formation either. Acid invertase peaked in very young fruit, while neutral invertase showed a maximum in older fruit. These results are an indication that starch is synthesized primarily through a precursor derived from the sucrose synthase pathway. When fruit-bearing wood was girdled during the period of active starch synthesis to interrupt phloem import, sucrose and sorbitol adjusted to 20–30 % lower levels. Fructose concentration was not affected and starch level decreased continuously, with a concomitant rise in glucose content. Sucrose content decreased initially but increased again during the latter part of the observation period, indicating de novo sucrose synthesis from starch. After blocking external C-supply, starch became the alternative C-source, supplying hexoses for sucrose synthesis and storage. Moreover, girdling led to an increase in the activities of hexokinase, fructokinase, PFP, and pyruvate kinase. From these findings it seems likely that girdling activates glycolysis as a means of meeting the increased energy required to synthesize sucrose or to compensate for the loss of reduction equivalents, i.e. NADH, produced in the oxidation of sorbitol by SDH. Since interruption of the assimilate supply also led to a decrease in SDH and neutral invertase activity, it is suggested that gene expression of these two enzymes might be sugar regulated.