Effects of decreased net carbon exchange on carbohydrate metabolism in sugar beet source leaves.

The relationship between CO(2) concentration and starch synthesis and degradation was studied by measuring leaf starch content and disappearance of (14)C-starch. At a concentration of 340 microliters CO(2) per liter, starch accumulated without degradation of previously synthesized starch. Degradation of starch began when CO(2) concentration was lowered, but its synthesis continued. At 120 microliters CO(2) per liter rates of synthesis and degradation were equal. Even at the CO(2) compensation point, synthesis of starch continued. Concomitant starch synthesis and mobilization supported export from the leaf. Changes in starch metabolism that occur when photosynthesis is CO(2)-limited provide a means to study regulation of starch metabolism and carbon allocation in translocating leaves.