PHOTOSYNTHESIS AND RESPIRATION IN RELATION TO LEAF AGE IN PERILLA FRUTESCENS (L.) BRITT

Summary The rate of photosynthesis in the third pair of leaves of Perilla frutescens declined gradually from the time of completion of expansion of the leaves until approximately I week from abscission when there was a more rapid decrease. The decrease in rate of photosynthesis first became apparent in the parts of the lamina proximal to the petiole and gradually extended to the more distal regions. The rate of respiration of the leaf remained approximately constant until I week from abscission when there was a transient rise prior to the final decrease in rate, The gradual decline in photosynthesis was accompanied by a similar decline in the RNA and protein content of the leaf although the chlorophyll content did not change during this period. Measurements of photosynthesis by means of 14CO2 incorporation showed a similar pattern of change and further revealed that the rate of incorporation of label into protein declined more rapidly than did the total protein content, The data are interpreted as suggesting the cessation of synthesis of proteins essential for photosynthesis rather than an increased rate of proteolysis as the cause of the senescence changes.