Heat Stress Responses in Cultured Plant Cells : Heat Tolerance Induced by Heat Shock versus Elevated Growing Temperature.

Using cultured pear (Pyrus communis cv Bartlett) cells, heat tolerance induced by heat shock was compared to that developed during growth at high temperature. After growth at 22 degrees C, cells exposed to 38 degrees C for 20 minutes (heat shock) showed maximum increased tolerance within 6 hours. Cells grown at 30 degrees C developed maximum heat tolerance after 5 to 6 days; this maximum was well below that induced by heat shock. Heat shock-induced tolerance was fully retained at 22 degrees C for 2 days and was only partly lost after 4 days. However, pear cells acclimated at 30 degrees C lost all acquired heat tolerance 1 to 2 days after transfer to 22 degrees C. In addition, cells which had been heat-acclimated by growth at 30 degrees C showed an additional increase in heat tolerance in response to 39 degrees C heat shock. The most striking difference between heat shock and high growth temperature effects on heat tolerance was revealed when tolerance was determined using viability tests based on different cell functions. Growth at 30 degrees C produced a general hardening, i.e. increased heat tolerance was observed with all three viability tests. In contrast, significantly increased tolerance of heat-shocked cells was observed only with the culture regrowth test. The two types of treatment evoke different mechanisms of heat acclimation.