A rice cultivar Shin 2 derived from a cross combination, Kameno-o 1×Kairyo Kikoku, develops notched grains with high percentage especially under low temperature condition, whereas their parental cultivars produce notched grains very little. Thus, Shin 2 canbe regalded as a transgressive type in respect of grain-notching. To clarify how the grain-notching line has been derived from the 'normal' parents is the main purpose of this study. The flrst experiment was performed using F
2 and F
3 plants of a Kameno-o 1×Kairyo Aikoku cross. After a preliminary test of F
2 and F
3 plants, a total of 918 F
2 plants, and tneir parents and Fl plants as well, were investigated for notched grain frequency and caryopsis length in 1976. About a half of the F
2 plants developed notchecl grains with a maximum frequency of 36%. The top and bottom 5% of the F
2 population were selected for grain-notching, and the F
3 lines were evaluated in 1977. Heritability value of the noched grain frequency estimatecl from the selcction response was 0.67. The hibohest mean value of notched grain frcquency was 39% in F
3 lnes evaluated (Fig 1).
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