Quantitative Determination of Changes in the Number and Size of Chloroplasts in Naturally Senescing Leaves of Rice Seedlings

Abstract

Changes in the number and size of chloroplasts in senescing leaves of rice seedlings were determined. The method employed involves electron microscopic examination of large numbers of cells and chloroplasts in the mesophyll of leaves at different stages of senescence with the aid of a microcomputer. Analysis showed that, once leaves had been fully expanded, the number and size of the mesophyll cells remained unaltered throughout the course of senescence. By contrast, the quantity of chloroplasts present in leaves decreased with advancing senescence. When compared with the newly expanded 6th leaves, the chloroplast number per unit area of mesophyll section was reduced by 40% and the mean cross section area of chloroplasts by 23% in the oldest leaves (3rd leaves) of seedlings. Chloroplasts occupied 33% of the mesophyll section area in the 6th leaves and the percentage decreased slightly in the 5th leaves and markedly in lower leaves to reach 17% in the 3rd leaves. The rate of oxygen evolution decreased approximately in parallel to the decline in the chloroplast content. Thus, sequential decrease in the amount of chloroplasts is a main cause of loss of photosynthesis during foliar senescence of rice seedlings.