Ultrastructural Studies on Rice Globular Embryos with Emphasis on Epidermis Initiation.

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Abstract
Cell ultrastructure was examined in rice embryos at the developmental stage between 42 and 72 HAA (hours after anthesis). Until 48 HAA, the cells gradually became cytoplasm-rich with a slight increase in the volume of the embryo exhibiting a globular shape. The globular embryo consisted of two parts : an embryo proper which was spherical in shape and a suspensor situated toward the micropylar end. From 48 to 66 HAA, cells of the embryo proper attained a very meristematic profile with vacuoles becoming small and few in number. On the other hand, in the suspensor region, cells and their vacuoles were larger than those of the embryo proper. Until 66 HAA, the absence of cuticular layer on the surface of globular embryo was ascertained since no materials reactive to the I2KI-H2SO4-AgP method were observed. At 72 HAA, however, a stable layer of cuticle began to appear on the surface of the embryo, extending from the ventral side and gradually covering the entire embryonal surface. Simultaneously, cell differentiation occurred in the internal tissues accompanied by ultrastructural changes. It was concluded from these results that the initiation of a cuticular layer is one of the first cytological events that lead rice embryos to organogenesis.

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