Heat shocks increase the chilling tolerance of rice (Oryza sativa) seedling radicles

The growth of rice (Oryza sativa L., cv. M202) seedling radicles, initially 10 +/- 1 mm long, was linear for the 96 h it took them to grow to 150 mm at 25 degrees C. Exposure to 5 degrees C for 24 h reduced the rate of growth by about 50%, and longer exposures caused a progressive reduction in growth. Initial radicle length significantly affected chilling sensitivity: with 2 days at 5 degrees C inhibiting growth at 25 degrees C by 23% for 1-mm radicles, 63% for 10-mm radicles, and 87% for 40-mm radicles. Heat shocks of 35 degrees C for 4 min, 40 degrees C for 3 min, 45 degrees C for 2 min, or 50 degrees C for 1 min, prior to chilling, reduced the 75% inhibition of radicle growth caused by 2 days at 5 degrees C to 34%, 25%, 14%, and 13%, respectively. The length of exposure that conferred chilling tolerance increased from less than 2 min for 50 degrees C to over 8 min for 35 degrees C. This increase in effective treatment duration was accompanied by a reduction in the maximum induced chilling tolerance. Practical application of heat-shock treatments to increase the chilling tolerance of rice seedlings may sacrifice a small reduction in maximum chilling tolerance at the lower inductive temperatures for a larger margin of safety in their application.