Studies on the development of endosperm in rice : 1. Process of endosperm tissue formation

Abstract

Fertilization of polar nuclei with a male nucleus completed within 5 hours after anthesis at temperature of 28°C to 31°C. The endosperm primary nucleus thus formed began first division without rest stage, while zygote took about 10 hours' rest. By 3rd day, a multi-nucleate layer of endosperm was formed in the, periphery of embryosac cavity (Figs. 1, 9 and 10). On 4th day, cell membrane formation occurred at first from embryonic end and extended to entire of peripheral tissue. Coincidently, all cells divided inwards, thus forming two-celled layers' endosperm (Fig. 11). From 5th day onward, development of endosperm tissue was observed by increment of cell layers and their cell numbers in cross section in middle part and in longitudinal section of a caryopsis. A possible explanation of this development process was tried to be given by following GORDON's theory that the development of cereal endosperm can be attained exclusively by division of cells in outermost cell layer. Ranging over 4th to 5th day, cells of outermost layer (4-1) might have repeated periclinal divisions, thus forming 3 cell layers (5-1, -2, and -3), and at same time, half number of layer (4-1) divided in a circumferential direction, . and remaining half in a longitudinal direction. The cell layers which have been formed by The 4th day (4-2 and -3) were squeezed into central portion of embryosac cavity and folded. As a result, folded cell layers came to be arranged in numerous concentric circular cell layers (5-4, -5…-11) surrounding respective central point (Fig. 16). About 32, 000 cells were counted throughout entire tissue of 5th day's endosperm. Ranging, over 5th to 6th day, outermost layer (5-1) might have divided inward 3 times and added 3 layers (6-1, -2, and -3) to outside of 5th day's tissue, and on other hand, layer (5-1) divided once in a circumferential direction, but not in a longitudinal direction. Ranging oyer 6th to 7th day, it showed increment of 2 cell layers (7-1 and -2) in a radius direction, and half number of layer (6-1) might have divided in a circumferential direction. Total number of cells reached 95, 400 (Fig. 14). Extending over 7th to 8th day, 2 cell layers increased in a radius direction, and half of layer (7-1) divided in a longitudinal direction. Extending over 8th to 9th day, last division of layer (8-1) occurred in a radius direction. Thus 19 to 20 cell layers were formed in dorsal radius, 15 to 16 cell layers in ventral radius and 13 to 14 cell layers in lateral radius (or central point to side surface of endosperm). (Fig. 15). The endosperm cells formed amounted approximately to 180, 000 in aggregate. From 10th day on, no increase was found in number of cells. Increment in number of endosperm cells has continued after stage where embryosac cavity had been filled up with endosperm tissue. Nay, it was clarified that increase in number of cells was rather markedly in later; stages (Table 2). It was observed that development of endosperm tissue formation was actuated, from incipient stage of nucleus, in dorsal side somewhat earlier than in ventral side (Figs. 9, 10, and 11). This fact may be related to observation that antipodal tissue developed for 3 days after fertilization and then degenerated (Figs. 4, 5, 6, 7 and 10).