Changes in Chloroplast Ultrastructure during the Senescence of Leaves in Rice Plants

Abstract

The flag leaves were sampled from rice plants of the paddy field at intervals of 10 days after the full expansion, and ultrastructure of the chloroplasts was observed by electron microscope. When the leaves had just fully expanded, the large starch grains accumulated in the chloroplasts, but afterwards the starch grains gradually decreased and after 30 days they almost disappeared (Figs. 1, 5, 6 and 7). The number of thylakoids per granum continued to increase for 10 days after leaf expansion (Fig. 3). The separation of thylakoid membranes, which is the first sign of destruction, was observed in some chloroplasts at 20 days after leaf expansion (Fig. 6). The size of chloroplasts and the number of grana continued to increase for 30 days after leaf expansion (Figs. 1 and 2), but they were followed by the breakdown of inner membrane system and by the decrease in amount of stroma (Figs. 7 and 8). As the inner membranes breakdown, the leaf colour changed to yellow-green and the plastoglobuli increased in size and number (Fig. 4). At the ripening stage of rice plants, ultrastructure was compared among chloroplasts in the upper five leaves (Table 1). Structural differences were not so marked among chloroplasts in the upper three leaves. However, the chloroplasts in the fourth leaf counted downwards from the flag leaf, showed a disappearance of the starch grains, a decrease in number of the grana, and a spectacular accumulation of the plastoglobuli. In the yellow area of the fifth leaf, the chloroplasts shrank because of the disappearance of grana and stroma (Fig. 9), and finally the chloroplasts envelope disappeared (Fig. 10). The similar pattern of destruction as mentioned above was observed in the chloroplasts from seedling leaves, but the destruction was more rapid than those at the ripening stage.