Abstract: The varying levels of free and conjugated abscisic acid endogenously contained in developing and germinating embryos and endosperms of rice were determined by radioimmunoassay. The bimodal distribution curve of ABA contents in developing rice embryo was consistent with the biphasic pattern of the embryonal development. There were two peaks of ABA content, which were separately 1.4 ng per embryo (13 days after anthesis) and 2.6 ng per embryo (21 days after anthesis), at the embryo differentiation-and maturation-stage respectively. In the differentiation stage the free ABA (f-ABA), which could originate preferentially from the maternal tissue and relate with photosynthate partitioned and rapidly transported into the developing seed embryo, comprised the overwhelming majority of total ABA(t-ABA); wheras in the maturation stage the conjugated ABA(c-ABA), which might be synthesized chiefly in situ and more directly involved in the regulation of embryonic gene expression, constituted the main part of total ABA. With respect to the amount of ABA in endosperm it accounted for about 90% of that in whole seed of rice, but the ABA concentration in it, shown with ng ABA per mg tissue fresh weight, was only equal to half of that in embryo more or less, and quite constent throughout developmental stages of the endosperm except the linear filling stage during which a peak of concentration of endogenous ABA in the tissue was presented. Nevertheless, the level of endogenous ABA in embryo suddenly dropped after initiation of germination of the seed while it returned to going up when the onset of elongating growth of the germ had taken place, so that the 'V' shape curve of ABA content in the rice germ was emerged during the seed germination. In the light of above results the probable relationship among the endogenous ABA, the embryo development and germination was discussed.

Key words: ABA content (free and conjugated), Oryza sativa L., Embryo and endosperm, Development and germination