Sensitivity of Seedling Radicles to Chilling and Heat-shock-induced Chilling Tolerance

The sensitivity of corn, cucumber, mung bean, and tomato seedling radicles to chilling (i.e., inhibition of subsequent elongation at 25 C after chilling at 2.5 C) was greater for radicles 5 to 7 mm long than for radicles 1 mm long. In contrast, radicles of germinating okra seeds had a similar level of chilling inhibition (i.e., 70% to 90%) at lengths of 1 to 7 mm. For seeds with 1-mm-long radicles, subsequent radicle elongation for cucumber was inhibited only 2% by 72 hours of chilling at 2.5 C, while it was inhibited about 20% for corn, mung bean, and tomato. For seeds with 5- to 7-mm-long radicles, chilling inhibition was 50% to 70% for corn, mung bean, and tomato and 80% to 90% for cucumber and okra. The degree of chilling sensitivity varied among species in relation to time required to elicit a significant response and the magnitude of the elicited response. The development of lateral roots decreased with prolonged chilling in all species. Heat shock (i.e., 4 to 10 minutes at 45 C) induced chilling tolerance in all species except okra. In okra, neither increasing the heat-shock temperature nor decreasing the severity of chilling (i.e., temperature and duration of exposure) significantly reduced chilling injury.