Altitudinal Cline in Chilling Responses in Indigenous Rice Varieties Collected from Nepal.

Abstract

Chilling injury expressed at the seedling stage and other agronomic and biochemical characters were evaluated in a total of 439 indigenous rice varieties collected from Nepal. The chilling response was tested by exposing seedlings of the third leaf stage to the temperature of 5-degrees-C for 4 days in a growth chamber. These varieties showed large variations in the response and a clear altitudinal cline was found in the character. The varieties collected at altitudes below 800 meters from sea-level were generally sensitive with Grades 4 to 6. Both sensitive and tolerant varieties were distributed together at altitudes between 801 and 1,600 meters. The varieties collected at altitudes above 1,601 meters were predominantly tolerant. Both early and late heading types were grown together in lowlands below 600 meters, and they were sensitive. The variations in heading date decreased with the increase of the altitude and the number of late maturity type varieties, which showed a chilling sensitivity, decreased. A rough altitudinal cline was found in the hull size and a weak character association was detected between the hull size and chilling response. The varieties with a hull size smaller than 15 mm2 were generally sensitive, while the varieties with a hull size larger than 27.1 mm2 were tolerant. The chilling-tolerant varieties were mostly found in the collections with a hull length/width ratio smaller than 2.4. In the collections with a hull ratio of 3.7, sensitive varieties were predominantly observed. No altitudinal cline was detected in the phenol color reaction. For the loci Est-1, Est-2 and Est-3 no relationship was observed between the allele composition and the degree of chilling injury.