Limitations to the Accumulation of Starch in the Developing Wheat Grain

The postulate that photosynthetic capacity limits the rate of accumulation of starch and dry matter in the developing wheat grain has been tested by defoliating plants growing in the field, and by culturing detached ears on solutions of sucrose. In most cases complete defoliation did not reduce the amounts of starch or dry matter produced 4 to 10 days later, or affect the amounts of sucrose in the grain. Increasing the potential supply of sucrose above normal levels by culturing did not lead to increases in either the deposition of starch or the quantity of sucrose in the grain. Removing leaves caused small depressions in the gain in fresh weight of the grain, and where defoliation did reduce the accumulation of starch there was no comparable effect on the amount of sucrose in the grain. Accordingly, it is concluded that the postulate stated above is invalid. The case in support of the concept that it is the mechanism regulating the passage of sucrose into the grain which imposes a limitation on the accumulation of starch is presented.