Rice et al. 1979). The ‘javanica’ rice,

Rice grows all over the world from the sea level to the high altitude of 3000 meters stretching latitude from 50?N to 40?S (Juliano 1993), making cold tolerance a necessary trait in many of these regions. In order to cope up with the low-temperature stress, genetic improvement is an upfront and effective approach. Genetic improvement requires the existence of genetic variability in the breeding population. Fortunately, the rice species, particularly Oryza sativa L has abundant natural variability for cold tolerance at different growth stages. Among the two sub-species of Oryza sativa L, indica has been domesticated in the tropical environment and japonica in the temperate region based on their comparative fitness in the low-temperature stress. It has been shown that japonica rice genotypes have higher degree cold tolerance than the indica genotypes do have (da Cruz and Milach (2004). Although, there are indica germplasms which show a moderate level of chilling tolerance in high altitude regions (Jennings et al. 1979). The ‘javanica’ rice, which is considered as the tropical subpopulation of japonica has also variability for cold tolerance and some them were used to transfer cold tolerance gene into temperate japonica varieties (Saito et al. 2001). Successful introgression of cold tolerance into locally adapted cultivars from different sources requires a reliable phenotypic protocol. Phenotypic selection under natural cold temperature might favor false positive results due to unpredictable field conditions in terms of intensity, duration, and timing of cold stress. Rather, screening under controlled air or water temperature might help better identifying tolerant genotypes. Several methodologies of cold screening have been reported for mapping of QTLs for cold tolerance at different growth stages (Table 3). Some of these methods used controlled air or water temperature in growth chambers. Growth under controlled condition ensures to gain appropriate timing and precision cold stress exposure, but it limits space available for screening a bulk volume of germplasm evaluation. Some researchers in Japan and Korea used cold water irrigation in the field throughout the growing season allowing hundreds of thousands of lines to be screened (Snell et al. 2008). In Korea, Suh et al. (2010) have developed a reliable method of phenotyping for cold tolerance by imposing cold-water irrigation at all growth stages in the field and cold-air temperature in the glasshouse, which allowed correct measurement of the traits associated with cold tolerance. In Bangladesh, with technical assistance Korea International Cooperation Agency (KOICA), Bangladesh Rice Research Institute (BRRI) has developed a simple and quick cold screening protocol for seedling stage cold tolerance in rice using cold water irrigation in the tank (Khatun et al. 2016). 

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