GENETICS OF COLD TOLERANCE IN RICE (ORYZA SATIVA L.): A CONCEPT OF MuLTIPLE ALLELISM

Abstract

The gene action for cold tolerance in rice (Oryza satiWl L.) cultivars was investigated In three sets of half diallel crosses, viz., a 5 x 5 Indica x Indica (I x I) set, a 5 x 5 japonlca x japonica (J x J) set, and a 7 x 7 indica x japonica (I x J) set. Minimum temperature for chloroplast biogenesis (MTCB) and spikelet fertility depression coefficient (FDC) were used for evaluating cold tolerance at the vegetative and reproductive phases, respectively. Whereas in the I x I set, general combining ability (gca) variance was predominant, In the J x J set the specific combining ability (sea) variance was more prevalent. In the I x .J set, both effects were of similar magnitude, Indicating japonlca cultivars as primary source of nonadditive effects. The covariance-variance (Wr-Vr) graph Indicated that japonica cultivars had dominant genes for MTCB but recessive genes of FDC. As these cultivars were cold tolerant at both vegetative and reproductive phases, the reversal of gene action appears to be due to Involvement of two different traits evaluated at two different growth phases, rather than reversal in gene action for cold tolerance per se. Persistent heterogeneity of Wr-Vr over the arrays In noninteractlng I x J set for MTCB and FDC and in I x I set for FDC Indicates Involvement of multiple alleles In the control of cold tolerance. In view of this, an appropriate breeding strategy has been proposed.