Light Regulation of Sink Metabolism in Tomato Fruit : II. Carbohydrate Metabolizing Enzymes

Effects of light on carbohydrate levels and certain carbon metabolizing enzyme activities were studied during the early development of tomato (Lycopersicon esculentum) fruit. Sucrose levels were low and continued to decline during development and were unaffected by light. Starch was significantly greater in light. Invertase activity was similar in both light- and dark-grown fruit. Sucrose synthase activity was much lower than invertase and showed a slight decrease in light-grown fruit between days 21 and 28. Light-grown fruit also had higher ADP glucose pyrophosphorylase activity than dark-grown fruit, which was correlated with higher starch levels. The rapidly decreasing activity of ADP glucose pyrophosphorylase during early fruit development in the dark in conjunction with reduced starch levels and rates of accumulation indicates that ADP glucose pyrophosphorylase is crucial for carbon import and storage in tomato. The differential stimulation of ADP glucose pyrophosphorylase activity from light- and dark-grown tissue by 3-phosphoglycerate suggests that this enzyme may be allosterically altered by light.