Wheat leaves were excised and allowed to senesce on the surface of water in the dark; they were able to synthesize heterodisperse high molecular weight RNA even after 72 hr of senescence. Polyadenylated RNA was prepared from the senescing leaves and was translated in vitro in either wheat germ extract or a reticulocyte lysate system. The translation products were separated by SDS-polyacrylamide gel electrophoresis and visualized by fluorography. Twelve different polypeptides were found to change their intensities on the fluorogram during the progress of senescence. The mRNA's for those polypeptides were classified into three types according to their mode of change: mRNA's of the first type were present in non-senescent leaves and disappeared soon after the excision of the leaves; those of the second type appeared only after the onset of senescence; and those of the last type appeared temporarily after leaf detachment and disappeared again within the first 72 hr of senescence. The first change in the translatable mRNA was observed as early as 12 hr after the onset of senescence. Benzyladenine and ethylene applied to the senescing leaves affected some, but not all, of these mRNA's in a manner parallel to their effects on the yellowing of the leaves.
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