Changes in Protein Content and in the Structure and Number of Chloroplasts during Leaf Senescence in Rice Seedlings

Two types of experiment were carried out to examine whether or not the inactivation of photosynthesis is related to loss of chloroplasts during foliar senescence of rice seedlings. Levels of both soluble and insoluble leaf proteins decreased during senescence, the loss of the soluble proteins being faster than that of the insoluble ones. There was a good positive correlation between the rate of oxygen evolution and the level of soluble proteins. The inactivation of photosynthesis was also linearly related to the loss of a major fraction of insoluble proteins. Thus, the loss of photosynthetic ability is ascribable to the degradation of relevant proteins and enzymes during leaf senescence. Electron microscopy revealed that senescence caused the disorientation of the grana and stroma thylakoids, a decrease in the number of starch granules, and an increase in the size and number of plastoglobuli. Large grana consisting 20 to 30 thylakoids appeared in aged leaves. In addition to these changes in ultrastructure, there was a significant decrease in the size of chloroplasts. Furthermore, the number of chloroplasts in mesophyll cells was also notably reduced during senescence. Thus, the loss of leaf proteins and inactivation of photosynthesis are both related to the decrease in the total mass of chloroplasts during senescence of rice seedlings.