Abstract
Seventy seven Japanese varieties of rice were grown from sowing to heading using the water culture method in growth cabinets with artificial light under a 12 hr. day-length and 25°C air temperature regime. These varieties were exposed to 6 different levels of cool tem-perature (17.5°C to 22.5°C) at 1°C intervals under low radiation conditions (5.5 MJ) for 20 days from heading. The critical low temperature(CLT), at which the varietles were able to ripen with 80% of perfectly mature grains owing the 20 day treatment from heading, was determined by the percentage of perfectly mature grains at 6 Ievels of cool treatment. It was recognized that the CLT ranged from 17.5°C to 22.5°C, and higher among 77 varieties. The varieties with lower CLT values displayed a stronger degree of cool tolerance at the heading and flowering stages than the varieties with higher CLT values. On the basis of the CLT values of the varieties, 77 varieties were classified into 7 groups corresponding to the degree of cool tolerance at the headlng and flowering stages.
