Factors causing a decrease in spikelet fertility in the rice plants [Oryza sativa] carrying male sterile cytoplasm with Rfrf genotype

"For the effective use of cytoplasmic-genic male sterility in F1 rice seed production, the nature of the cytoplasm and restorer gene should be elucidated, because the degree of spikelet fertility can be determined based on the nature of the two components. Three developed lines, RT61C, RT98C and RT102C had the genetic background of Taichung 65, an Oryza sativa cultivar, except for the male sterile cytoplasm and one single restorer gene (Rf) derived from the three different lines of O.ntfipogon by successive backcrossing. It was found that spikelet fertility differed between the two genotypes RfRf and Rfrf in these lines, particularly in RT98C and RT102C. That is, the Rfrf plants showed a lower spikelet fertility than the RfRf plants in the two lines. It was suggested that the Rfrf plants had a larger number of deformed anthers in addition to half of degenerated pollen grains compared with the RfRf plants. In this paper, the incidence of deformed anthers, anther dehiscence and spikelet fertility were investigated in the RfRf and Rfrf plants in three lines, and attempts were made to elucidate the causes of the decrease in spikelet fertility in the Rfrf plants. In the homozygotes of each line, spikelets with all six anther deformed seldom appeared, and anther dehiscence proceeded normally as a whole. If at least one normal anther were present in a spikelet, the fertilization could be achieved normally, yielding fertile grains. Therefore, it is considered that the homozygote in each line showed a high spikelet fertility. On the other hand, although the heterozygote in RT61C showed the same expression of the characters as the homozygote, in RT98C and RT102C, the heterozygote displayed a lower spikelet fertility than the homozygote. It was considered that the expression of the low spikelet fertility in RT98C was caused by the low rate of anther dehiscence in addition to the decrease in the ability of fertilization, while in RT102C, it was mainly caused by the presence of many spikelets in which all the anthers were deformed and the rate of anther dehiscence was low."