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Title:   Author  item
                                            Kim, Sang Icitem
                                            KIM, DONG-MIN - Chungnam National Universityitem
                                            Tai, Thomas
  Submitted to: Chinese Journal of Rice Science
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 3/20/2012
                                    Publication  Date: 12/1/2012
                                    Citation: Kim, S., Kim, D., Tai, T. 2012. Evaluation of rice seedling tolerance to constant and intermittent low temperature stress. Chinese Journal of Rice Science. 19(3):295-308.Interpretive Summary: M202 is a California temperate japonica variety with good seedling cold tolerance. Previous analysis of a genetic mapping population derived from M202 and the susceptible indica variety IR50 resulted in the identification of two major loci in M202, qCTS12 and qCTS4, which confer tolerance to difference cold stress assays (prolonged constant and prolonged intermittent stress, respectively). Using a variety of physiological indicators of stress, we evaluated M202, IR50, and some advanced backcross lines for tolerance to constant and intermittent stress over time to examine the effects of the qCTS12 and qCTS4 loci on seedling response to cold stress. A third assay involving prolonged constant cold stress followed by a recovery period prior to evaluation was developed and also used to characterize seedling tolerance of these lines. We identified differences between two advanced backcross lines, both carrying qCTS12 and 4, under constant and constant with recovery assays. This result suggests that one of these lines contains an additional gene or genes affecting cold tolerance. Finally, a larger set of rice varieties from different geographic locations were assessed for seedling cold tolerance using all three assay conditions. Several accessions showing better tolerance than M202 using both constant and intermittent conditions were identified and may be used for germplasm enhancement. Also, some accessions which exhibited severe chilling injury due to constant cold stress showed a good ability to recover. This recovery trait may also be of significant value and warrants further investigation.
                                Technical Abstract: Low temperatures generally slow or halt the growth of rice seedlings and may result in various injuries and mortality depending on the length and severity of the cold stress. Seedling cold tolerance is complex and many quantitative trait loci (QTLs) have been identified using different conditions and evaluation methods. Two major QTLs, qCTS12 and qCTS4, were previously identified through genetic analysis of the tolerant temperate japonica M202 and the susceptible indica IR50. The M202 alleles at qCTS12 and qCTS4 are associated with tolerance to constant and intermittent cold stress, respectively. M202, IR50 and advanced backcross (BC4F6) lines carrying the cold tolerance QTLs were evaluated using three stress assays (constant, intermittent, and constant with recovery period). Under constant stress, two BC4F6 lines (MIb 4853-9 and 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 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 germplasm. 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 be useful for performance in the field and warrants further investigation.


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