Parallel Changes in H(2)O(2) and Catalase during Thermotolerance Induced by Salicylic Acid or Heat Acclimation in Mustard Seedlings

Spraying mustard (Sinapis alba L.) seedlings with salicylic acid (SA) solutions between 10 and 500 μm significantly improved their tolerance to a subsequent heat shock at 55°C for 1.5 h. The effects of SA were concentration dependent, with higher concentrations failing to induce thermotolerance. The time course of thermotolerance induced by 100 μm SA was similar to that obtained with seedlings acclimated at 45°C for 1 h. We examined the hypothesis that induced thermotolerance involved H(2)O(2). Heat shock at 55°C caused a significant increase in endogenous H(2)O(2) and reduced catalase activity. A peak in H(2)O(2) content was observed within 5 min of either SA treatment or transfer to the 45°C acclimation temperature. Between 2 and 3 h after SA treatment or heat acclimation, both H(2)O(2) and catalase activity significantly decreased below control levels. The lowered H(2)O(2) content and catalase activity occurred in the period of maximum thermoprotection. It is suggested that thermoprotection obtained either by spraying SA or by heat acclimation may be achieved by a common signal transduction pathway involving an early increase in H(2)O(2).