( 2 [ XX ) ) Characterization of Viviparous Mutants in Rice

showed pre-haryest germination (vivipary) during seed development but no defects in the other traits. Vivipary was also observed in the enll mutant that lacked endosperm in the seeds. Examination of the viviparous nature of the mutants under three rain conditions, no rain, artificial rain and natural rain, revealed that the induction of yivipary in the riv mutants required a small amount of water. In contrast, precocious germination in the enll mutant was caused by the absence of endosperm in the seeds, but not by the external signal of rain. In riv mutants, the precocious germination occurred simultaneously at late stages of seed development. The germination test in the presence of exogenous ABA revealed that sensitiyity to ABA was gradually reduced from 15 through 35 days after pollination in the wild-type seeds, and was almost tost at 40 days after pollination. A similar tendency was observed in rivl and riv2 seeds. Howeyer, at 15-35 days after pollination, rivl-1 and riv2 seeds had significantly lower sensitivity to ABA than the wild-type seeds. In addition, the three riv mutants lost the sensitivity to ABA at an earlier stage of seed development than the wi]d type. These results indicate that rivl and riv2 haye a lower sensitivity and shorter period of sensitivity to ABA.