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 are dominated by the japonica subspecies, while indica cultivars are confined to the tropical or subtropical zones. The present study was conducted to characterize a diverse set of predominately japonica rice cultivars 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 marker. While considerable diversity existed within groups, temperate japonica cultivars had better seedling-stage cold tolerance (i.e., less chlorosis at 9 and 13 degrees C treatments), higher seedling vigor, shorter growth duration, higher panicle exsertion, and lower threshability (i.e., less shattering or seed shedding) than indica cultivars. Cultivars from the same class were grouped together based on principal components analysis using the above traits, but the distributions were continuous. Tropical japonica cultivars were usually intermediate between temperate japonicas and indicas; however, there was no appreciable difference in seedling cold tolerance between tropical and temperate japonicas. Despite their association with tropical environments, the tropical japonicas possess many of the traits relative to low temperature adaptation present in the temperate types