Relation between Leaf Age and Nitrogen Incorporation in the Leaf of the Rice Plant (Oryza sativa L.)

The relation between leaf age and the incorporation of nitrogen into the leaf was examined in the 13th leaf blade of the rice plant, Oryza sativa L., after feeding it 15N-labelled ammonium sulfate. The incorporation of nitrogen into the leaf was active up to the maximum leaf nitrogen content; thereafter it decreased quickly. At the beginning of senescence, when the nitrogen content of the leaf had begun to decrease, the incorporation of 15N decreased to 28% of the value during development. At the middle stage of senescence, when the nitrogen content had decreased to half the maximum, the incorporation was 13%. The incorporation of 15N into soluble proteins was examined by gel filtration on Sephadex G-200. During development, large amounts of 15N were incorporated into the ribulose bisphosphate carboxylase-rich fraction. As the leaf aged, the incroporation of 15N into this fraction decreased more sharply than it did in other fractions. This tendency was more pronounced at the late stage of senescence. We concluded that the amounts and kinds of protein synthesized in a leaf are closely related to leaf age.