33. Number of effective factors controlling cold tolerance at the anthesis stage

Since Sakai (1949) suggested that two independent dominant genes were involved in cold tolerance at the booting stage, considerable efforts have been made for analysing the genetic basis of cold tolerance. However, most experiments so far done were carried out under natural conditions where low temperatures affect the plants at different stages. In our study on the genetic control of cold tolerance at the anthesis stage, we carried out experiments under controlled conditions.

A diallel analysis was applied to F1 plants from crosses between seven rice varieties of Hokkaido with different degrees of cold tolerance. The plants were grown in a phytotron at 24 deg.C day and 19 deg.C night temperatures, and they were exposed to a low temperature of 15 deg.C for 10 days or 5 days at the anthesis stage. The degree of cold tolerance was judged from the reduction in spikelet fertility which was highly correlated with the tolerance degree reported proviously (Yamamoto and Nishimura 1986). Genetic analysis was carried out following the model of Hayman (1954a, b) and its modification for half diallel by Jones (1965). The Wr, Vr regression deviated from unit slope, and the heterogeneity of Wr-Vr was significant, implying the presence of non-allelic interaction. Excluding an interacting parent (Hayakogane), a 6 X 6 subset of diallel crosses under the 1O-day treatment gave more uniform Wr-Vr values and a regression of near unity, indi-

Fig. 1. Relation between Wr and Vr in 6x6 diallel subset based on the data from plants exposed to 15 deg. C for 10 days.

Additive gene action (a) predominated over dominance action (b). The value of average degree of domiance SQRT(H\1\/)D = 1.20 indicated the presence of slight overdominance, which was also confimed from the results of Wr, Vr graph analy- sis (Fig. 1). In addition, the number of effective factors was estimated by two formulas, i.e., h²/H/2 and (maximum parent value-minimum parent value)²/4D. The value obtained by the first formula was less than one and that by the second formula was 1.22. These results suggest that there are one or two dominant genes controlling the cold tolerance at the anthesis stage. Further analysis by a standardized deviation graph indicated that high cold tolerance was controlled by an excess of dominant alleles on the average (Fig. 2).

Fig. 2. Correlation of the standardized deviations, Yr and Wr+Vr, in a 6x6 diallel subset based on the data from plants exposed to 15 deg.C for 10 days.

Hayman, B. I., 1954a. The analysis of variance of diallel tables. Biometrics 10: 235-244.

Jones, R. M., 1965. Analysis of variance of the half diallel table. Heredity 20: 117-121.

Sakai, K., 1949. Cytological and thremmatological studies on sterility of rice in nothern part of Japan, with special reference to abnormal hypertrophy of tapetal cells due to low temperature. Hokkaido Agric. Exp. Stn. Report 43: 1-46.

Yamamoto, T. and M. Nishimura, 1986. Relation between the tolerance to the sterility type of cool injury and the amount of bleeding water in rice plants. Jpn. J. Breed. 36: 147-154.