Rice cold tolerance at the reproductive stage in a controlled environment

Cold tolerance of rice (Oryza sativa L.) during the reproductive stage is important to guarantee high yield under low temperature environments. Field selection, however, does not allow identification of adequate tolerance sources and limits selection of segregating lines due to variable temperature. The objective of this study was to devise methods for distinguishing rice genotypes as to their cold tolerance at the reproductive stage when evaluated under controlled temperature. The effect of cold temperatures was investigated in six rice genotypes at 17 degrees C for varying length of time (three, five, seven and ten days) at two reproductive stages (microsporogenesis and anthesis). Cold tolerance was measured as the percentage of reduction in panicle exsertion and in spikelet fertility. Evaluating cold tolerance through the reduction in panicle exsertion did not allow for the distinction between cold tolerant from cold sensitive genotypes and, when the reduction in spikelet fertility was considered, a minimum of seven days was required to differentiate the genotypes for cold tolerance. Genotypes were more sensitive to cold at anthesis than at microsporogenesis and, as these stages were highly correlated, cold screening could be performed at anthesis only, since it is easier to determine. Rice cold tolerance at the reproductive stage may be characterized by the reduction in spikelet fertility due to cold temperature (17 degrees C) applied for seven days at anthesis.