Abstract: M202, IR50 and advanced backcross (BC4F6) lines carrying the cold tolerance QTLs qCTS4 and qCTS12 were evaluated using three low temperature stress assays (constant, intermittent, and constant with recovery period). Under constant stress, two BC4F6 lines (MIb 4853-9 and MIb 6885-2) exhibited differences in seedling growth and accumulation of stress-related compounds although both carry the two QTLs in the IR50 background. Differences between these lines were also observed in the constant with recovery period assay, although both of them performed comparably under intermittent stress. MIb 6885-2 performed more similarly to M202 in the constant and constant with recovery period assays, suggesting that this line contains an introgressed region(s) not present in MIb 4853-9. The three assays were also applied to assess variation in cold tolerance in a set of diverse germplasms. Performance metrics and visual ratings were comparable for evaluating tolerance to constant stress. However, differences in response to intermittent stress were more evident from growth and total chlorophyll measurements than from visual ratings. With regard to the constant with recovery period assay, about 15% of the germplasm accessions did not recover. However, some lines which exhibited chilling injuries of comparable severity were able to overcome them. This recovery phenotype may improve rice performance in the field and warrant further investigation.

Key words: rice, seedling cold tolerance, quantitative trait locus, advanced backcross lines, low temperature stress