Cyto-histological studies on phytin-containing particles in developing rice grains.

Abstract

The changes in the distribution of phytin-containing particles (phytin particles) were microscopically studied in rice ovary tissues during the development of embryo and endosperm. Similarity in the phytate nature of these particles was confirmed by histochemical and ultrastructural procedures7, 25). Phytin particles appeared within the aleurone layer and scutellar parenchyma 5 or 6 days after anthesis. This is followed by an increase in number and also by the formation of relatively large particles (3 to 7 μm in diameter). In addition to their presence in these storage tissues, phytin particles were also temporarily found in different parts of ovary tissues after anthesis, as follows : (1) the peripheral regions of pericarp, for a period of 10 days, (2) parenchymatous cells in dorsal vascular bundles for about 20 days, (3) nucellar epidermis and nucellar projection and the modified aleurone cells abutting the embryo or the suspensor cells for about 10 days, (4) the outer layer of starchy endosperm from day 8 to day 20, and (5) scutellar epithelium until about 25 days. With regards to the route of flowing materials into the developing endosperm, the presence of two suggested transport pathways from dorsal vascular bundles into endosperm, i. e. 'the dorsal pathway' and the nucellar epidermal pathway'6, 8, 9, 23), was confirmed by our observations with the distribution changes of phytin particles in close association with the ovary histogenesis. The observations also suggests the routes into the developing embryo, namely, (1) from nucellar tissue and modified aleurone layer to suspensor, (2) from the modified aleurone layer and starchy endosperm to the embryo surface, and (3) from endosperm to scutellar epithelium. These processes seem to operate within 10 days after anthesis, while the scutellar epithelial process mainly play a role later. Phytin particles frequently occurred in the epidermal or peripheral regions of tissues, such as pericarp and endosperm etc., in contrast to starch grains, which appeared rather in the inner parts of tissues. Accumulation of mineral elements in aleurone cells being a peripheral zone, could not only be a result of deposition of large amounts of mirlerals, supplied from source organs through the supposed transport pathways via the nucellar projection and nucellar epidermis extending around the aleurone layer, but could also be influenced by probable interactions in deposition processes of minerals and carbohydrates. Furthermore, the sink activity of young embryos appeared superior in terms of tissue development based on higher accumulation of mineral elements when compared to the modified aleurone layer which was adjacent to the embryo. The depleted layer in endosperm near the epithelial surface of the scutellum was formed due to the disappearence of starch grains and degradation of endosperm cells during embryo development. These phenomena could be explained in terms of the 'pulling' function of embryo, based on its hypothetically young developmental age, high metabolic activities in protein synthesis, ion absorption and accumulation.