Rate of Protein Synthesis in Senescing, Detached Wheat Leaves

Abstract

In wheat leaves detached and kept turgid and in the light for 48 h, the content of free valine increased eightfold. The rate of valine metabolism in these leaves was not rapid, and the increase in free valine was 80% of the net loss of valine from leaf protein over 48 h. When L-[14C]valine was fed to detached leaves, the rate of incorporation into protein increased with time of feeding, although uptake of the L-[14C]valine proceeded linearly. Isotope dilution experiments established that the rate of incorporation of valine was independent of the size of the free valine pool, and that incoming L-[14C]valine was substantially diluted by the free valine of the leaves before incorporation into protein. Based on these findings, estimates were made of turnover of valine in protein in detached wheat leaves. In fully expanded leaves of two ages the turnover rate approached 2.0% per hour when the leaves were first detached, and, after 48 h, had fallen to 0.3 % per hour. Valine fed by the transpiration stream was readily incorporated into proteins made on chloroplast ribosomes.