Genetical studies on rice plant

XVII. Seed development in tetraploid rice

Abstract

As compared with diploids the embryo-sacs of tetraploid rice plants are of a larger size with larger cells and nuclei. The fertilized ovule, on the other hand, develops at a slower pace than the diploid and the differentiation is somewhat less than in the diploid. The tissual size of the tetraploid ovule is not as large as would be expected from calculations based on nucleo-plasmic ratio and cell eutery.
While the tetraploid ovules, at the flowering stage, have an average of 60% of normally organized embryosacs, after flowering and with the lapse of time, the frequencies of their appearance decreases. In matured kernels, the frequencies of fully developed kernels are less than 40% and aside from completely undeveloped sterile kernels, a certain proportion of underdeveloped kernels, shrivelled in outward appearance, are found.
Histologically, the lack of embryo, the absence of normal egg apparatus and the post-fertilization mortality at an early stage of the development results in sterile kernels, and the belated, unproportional and parthenocarpic development of ovules are responsible for shrivelled kernels, in which the development of the endosperm is inhibited to a greater extent than that of the embryo. When nursed in artificial culture a certain number of the shrivelled kernels germinate.
Plants grown from well developed kernels generally have normal tetraploid chromosomes with 2n=48, whereas plants from shrivelled kernels are found to be both tetraploid and hypo-tetraploid. Though it is not yet possible to ascertain the presence of hyper-tetraploids, as in certain tetraploid plants, it is obvious that the tetraploid rice plant is not quite stable, cytologically speaking.
In paddy field cultivation in Hokkaido which is the northern most island of Japan, however, the tetraploid is rather stable and the author has been unable to find out any aneuploid plants which in the normal sense of thinking should expected to be present since the mother plants are tetraploids. This stablity may be due to a strong selectivity of unfavourable environmental conditions, which affects the germination of aneuploid kernels.
As to the causes of low seed fertility of the tetraploid rice plant, it is too well known to be repeated here that the primary cause is the aneuploidy resulting from irregularities in the meiosis, but in addition to the above, it is highly probable that an inherent weakness of the tetraploid mother tissue plays a certain role.