Studies on the Developmental Physiology of the Endosperm Tissue in Rice Plants : II. Nuclear DNA content and endosperm structure in early developmental stage of caryopsis

Abstract

Rice endosperm at 4 to 5 days after flowering is presumed to be considerably heterogenous judging from the high frequency of occurerlce of abnormal mitoses and wide variation of nuclear volume. In such endosperm the differences of starch polimerization degree were frequently observed among the cells which were produced simultaneously. By using the endosperm at 3 to 5 days after flowering for investigating this phenomenon, diurnal periodicity of mitosis, nuclear DNA contents and chromosome number of endosperm cells were studied. Diurnal mitotic activities of the endosperm cells of cv. Towada (japonica) from the 3rd to the 4th day after flowering and cv. Century patna (indica) from the 4th to the 5th day after flowering, were examined by the acetocarmine squash techinique. The mitotic frequency was found to increase at 3 or 4, 17, 21 and 23 or 24 o'clock respectively, regardless of cultivar and days after flowering. Two kinds of microspectrophotometric methods, the one spot measuring method and the two wave length one, compared with each other using the intact nuclei which were prepared from homogenate of the endosperm in cv. Aichiasahi (japonica) at 5 days after flowering and stained by Feulgen reaction. DNA content of nucleus determined by the one spot measuring method was about 12 to l5 percent less than that determined by the two wave length method, but the values by the former were stable. So the estimate of nuclear DNA content in this experiment was made by the one spot measuring method with the wave length 550 nm. Relative DNA contents of intact endosperm nuclei varied from 0.4 to 9.1 in cv Aichiasahi at 5 days after flowering. The relative DNA content per nucleus was examined by use of the longisection of caryopsis of cv. Aichiasahi at 5 days after flowering in four zones of endosperm showing in Fig. 1. The comparable and standard values of DNA content were taken by the DNA content of embryo nuclei on the same section. The 2C, 3C, 4C and 5C nuclei were found in each four zones in this stage, though the rice endosperm had a common origin from the triploid fusion nucleus. The frequency of occurence of 3C nuclei was highest in aleuron layer and central zone. While the frequcncy of occurence of 5C nuclei was relatively higher in marginal and middle zones. The ranges in relative DNA contents of nuclei in middle zone were largest, from 0.2 to 9.1. Chromosome number of endosperm cells of cv. Kosen (indica) at 4 and 5 days after flowering was examined by acetocarmine squash techinique. It could not be ascertained whether the endosperm cells were aneuploidy or not, but the approximate values of 1C, 2C, 3C and 4C in chromosome number were obtained. These results mentioned above indicate that the rice endosperm was consisted of heterogenous cells which markedly differed from the DNA contents of nucleus, but these cells were almost systematically distributed from the aleuron layer towards the central zone; polyploidy level rised from aleuron layer via marginal zone to middle zone and declined in the central zone. These polyploidy was presumed to be endopolyploidy which were due to the endomitosis leading to high polyploidy. Some functional significance of endopolyploidy was envisaged with the higher rate of synthetic activity required by the specialized cells of glands, suspensors and endosperms. Therefore it was presumed that the differences of starch polimerization degree among the cells of middle zone which were produced simultaneously, might be caused by high polyploidy and the large variation of polyploidy level among the cells of middle zone.