The Role of Cytokinins in the Development and Metabolism of Barley Leaves III. The Effect on the RNA Metabolism in Various Cell Compartments During Senescence

Abstract

Summary The degradation of RNA in excised barley leaves in darkness mainly concerned the 25 S and 18 S rRNAs, whereas the 23 S and 16 S rRNAs were. rehtively stable. The cytokinin prevented an increase in the RNase activity and caused an increase in the RNA level and in the incorporation of 14 C-uracil into the cytoplasmic rRNAs, their precursors and the low molecular RNAs. There was no incorporation of the precursor into the 23 S and 16 S RNAs. Changes in RNA resulting from the senescence and from the cytokinin treatment occurred mainly in the cytoplasmic and nuclear cell fractions; they were relatively small or non-existent in the chloroplast fraction. It is concluded that changes in chloroplast structure during senescence of barley leaves are rather secondary results of processes occuring in other cell compartments. Also the retarding effect of cytokinin on the chloroplast senescence seems to be rather indirect and resulting inter alia from a protection of the RNA and a stimulation of its synthesis in the cytoplasm and nucleus.