Accumulation and Conversion of Sugars by Developing Wheat Grains. I. Liquid Culture of Kernels Over Several Days

Wheat kernels, detached from the ear during the linear phase of grain growth and with their outer pericarps removed, took up sucrose from a liquid culture medium and sustained starch synthesis linearly for a week. Kernels cultured with their outer pericarps left on, but cut transversely in half, had more rapid starch synthesis initially but the rate was linear for only 3 days. The presence of the embryo was not necessary for the observed rates of starch synthesis. Rates of net uptake of sucrose into the kernels approximated rates of grain growth in vivo but only some was converted to starch, the rest remaining in an ethanol-soluble fraction. When the sucrose concentration in the medium was below about 300 mM, the concentration of 14C label (derived from [14C]sucrose), which had accumulated in the intracellular soluble pool after 3 days of culture, exceeded the concentration in the medium. Substrate response curves for starch synthesis showed an optimum at about 230 mM sucrose in the medium, declining at higher substrate concentrations. This optimum concentration approximated the sucrose concentration determined in the endosperm cavity directly after grain was taken from the plant.