Time and Temperature Interactions in Freezing Tolerance of Winter Wheat

To survive the temperature fluctuations that occur during the winter months, winter wheat (Triticum aestivum L.) plants must tolerate episodes of freezing of varying intensity and duration. In this study, the ability of six wheat cultivars to survive exposure to -13.5 to -16.5 degrees C for 2, 3, or 4 h was measured. Tolerance was described as the percentages of plants surviving after time intervals at -10 degrees C or less. The average survival of the six cultivars significantly decreased by about 50% as length of exposure to -10 degrees C or less was increased from 325 to 525 min. Surprisingly, survival then increased with longer exposure times such that the percentages of plants surviving were similar after 675, and after 325 min at -10 degrees C or less. The greater survival after longer exposure times to -10 degrees C or less compared with shorter exposure times suggested that certain freezing tolerance mechanism(s) in wheat are activated only after sufficient exposure to subfreezing temperatures for a sufficient amount of time. Lesser exposure to either temperature or time apparently does not activate this mechanism, leading to greater plant death. Further characterization of this mechanism and its genetic control may lead to development of wheat lines with enhanced ability to survive harsh freezing conditions and thus improved winterhardiness.