Factors Regulating the Accumulation of Starch in Ripening Wheat Grain

Abstract

Estimates of the faculty of wheat endosperm to convert sucrose to starch were obtained by incubating dissected endosperm in solutions of [¹⁴C]sucrose and then measuring the absorption of [¹⁴C]sucrose and the amounts of ¹⁴C accumulating as insoluble material. Samples of grain were taken from plants growing in the field at each of four stages of maturity: at 17 days after anthesis, just after the synthesis of starch had begun; at 28 days when the grains were about half-grown; at 38 days when the accumulation of starch had slowed down and almost ceased; and at 49 days when the grains were fully grown.

More [¹⁴C]sugar entered the free space of the endosperm of 49- than of 28-day-old grain, and more sucrose was absorbed by the endosperm cells of the older grain. Endosperm taken from grains sampled at 28 days produced more insoluble radioactive material (mostly starch) than grains sampled at any other stage, and compared to 28-day grain, grain sampled at 38 days produced less than one-third of the quantity of [¹⁴C]starch.

At each stage, the amounts of sucrose in the free space and in the cells of the endosperm were determined on comparable samples of grain developing normally in the field. There was significantly more sucrose on day 38 than on day 28, both in the free space and in the cells of the endosperm. It is concluded that the onset of the declining phase of accumulation of starch, as the grains begin to ripen, is due to a fall in the capacity of the grains to synthesize starch, and is not attributable to postulated reductions in the supply of assimilate to ripening grain.