THE EFFECT OF GIRDLING ON CARBOHYDRATE PARTITIONING IN THE GROWING APPLE FRUIT

In the present work the activities of key enzymes involved in carbohyd rate metabolism of the growing apple are compared with the levels of s ugars and starch accumulating in the fruit. After uptake of sorbitol a nd sucrose the growing fruit steadily accumulates fructose and sucrose , while the starch level reaches a peak about halfway through fruit de velopment. Fructose accumulation in the fruit correlated positively wi th the activity of sorbitol dehydrogenase (SDH). Concomitantly with tr ansient partitioning of carbon to starch, the activities of the enzyme s sucrose synthase, UDP-glucose pyrophosphorylase, and phosphoglucose isomerase (PGI) also increased. Activities of phosphofructophosphotran sferase (PFP), phosphofructokinase and glucose-6-phosphate dehydrogena se (G6PDH) did not correlate with the rise in starch content, but incr eased during the phase of starch degradation. Invertase activity did n ot correlate with starch formation either. Acid invertase peaked in ve ry young fruit, while neutral invertase showed a maximum in older frui t. These results are an indication that starch is synthesized primaril y through a precursor derived from the sucrose synthase pathway. When fruit-bearing wood was girdled during the period of active starch synt hesis to interrupt phloem import, sucrose and sorbitol adjusted to 20- 30% lower levels. Fructose concentration was not affected and starch l evel decreased continuously with a concomitant rise in glucose content . Sucrose content decreased initially but increased again during the l atter part of the observation period, indicating de novo sucrose synth esis from starch. After blocking external C-supply, starch became the alternative C-source, supplying hexoses for sucrose synthesis and stor age. Moreover, girdling led to an increase in the activities of hexoki nase, fructokinase, PFP, and pyruvate kinase. From these findings it s eems likely that girdling activates glycolysis as a means of meeting t he increased energy required to synthesize sucrose or to compensate fo r the loss of reduction equivalents, i.e. NADH, produced in the oxidat ion of sorbitol by SDH. Since interruption of the assimilate supply al so led to a decrease in SDH and neutral invertase activity, it is sugg ested that gene expression of these two enzymes might be sugar regulat ed.