Establishment of thermotolerance in maize by exposure to stresses other than a heat shock does not require heat shock protein synthesis.

Maize (Zea mays) seedlings were pretreated prior to heat shock with either a progressive water stress of -0.25 megapascal PEG/hour from 0 to -1.25 megapascal over a 6-hour time period, or various concentrations of copper, cadmium, or zinc for 4 days. When the subsequent heat shock of 40 or 45 degrees C was administered for 3 hours, the seedlings showed an induced thermotolerance to these temperatures, which were otherwise lethal to control (water grown) seedlings. Thermotolerance was exhibited by both the root and the shoot of pretreated seedlings, even though the water and heavy metal stresses were applied only to the roots. Neither of these pretreatments had induced the synthesis of detectable levels of heat shock proteins (Hsps) at the time of heat shock. Pretreatment of seedlings with a progressive heat shock of 2 degrees C/hour from 26 to 36 degrees C, which did induce Hsps 18, 70, and 84, resulted in tolerance of a severe water stress of -1.5, -1.75, or -2.0 megapascal for 24 hours. But these seedlings producing Hsps were no better protected against water stress than those pretreated with a progressive water stress which did not produce Hsps. Hsps appear not to act as general stress proteins and their presence is not always required for the establishment of thermotolerance.