GENETIC-VARIATION FOR TRAITS RELATED TO TEMPERATE ADAPTATION OF RICE CULTIVARS

Rice (Oryza sativa L.) is cultivated in diverse environments including temperate, subtropical, and tropical regions. Temperate rice areas ar e dominated by the japonica subspecies, while indica cultivars are con fined to the tropical or subtropical zones. The present study was cond ucted to characterize a diverse set of predominately japonica rice cul tivars for traits related to adaptation to temperate environments. The 117 cultivars were previously classified into indica and two japonica (temperate and tropical) groups based on RAPD markers. While consider able diversity existed within groups, temperate japonica cultivars had better seedling-stage cold tolerance (i.e., less chlorosis at 9 and 1 3 degrees C treatments), higher seedling vigor, shorter growth duratio n, higher panicle exertion, and lower threshability (i.e., less shatte ring or seed shedding) than indica cultivars. Cultivars from the same class were grouped together based on principal components analysis usi ng the above traits, but the distributions were continuous. Tropical j aponica cultivars were usually intermediate between temperate japonica s and indicas; however, there was no appreciable difference in seedlin g cold tolerance between tropical and temperate japonicas. Despite the ir association with tropical environments, the tropical japonicas poss ess many of the traits relative to low-temperature adaptation present in the temperate types.