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Cold is one of the main problems for rice cultivation in Rio Grande do Sul because the vast majority of cultivars in use are of tropical origin. The occurrence of low temperatures combined with the susceptibility of the materials used can cause serious damage to the establishment of the crop reducing the plant stand. The objective of this study was to evaluate the physiological changes in different rice cultivars, when exposed to low temperature during germination. Seeds from four cultivars, BRS Agrisul, BRS Chuí, Lemont and Oro were used and evaluated regarding low temperature tolerance by the cold test. To evaluate the physiological changes, isoenzyme analyses of alcohol dehydrogenase and esterase were performed at temperatures of 25 °C (ideal for germination) and 13 °C (low temperature). The other characteristics evaluated were germination, germination speed, coleoptile length and dry weight at 25 °C and 13 °C. According to cold test results, seeds of the Lemont and Oro cultivars presented better tolerance to the low temperature. In the isoenzyme analysis, the cold caused an increase in the expression of the isoenzyme alcohol dehydrogenase and reduction in the esterase isoenzyme expression. For the other ratings, the cold caused damage to all the characteristics evaluated, especially in the BRS Agrisul and BRS Chuí cultivars. [ABSTRACT FROM AUTHOR]