Parallel changes in H2O2 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 &mgr;m significantly improved their tolerance to a subsequent heat shock at 55 degreesC 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 &mgr;m SA was similar to that obtained with seedlings acclimated at 45 degreesC for 1 h. We examined the hypothesis that induced thermotolerance involved H2O2. Heat shock at 55 degreesC caused a significant increase in endogenous H2O2 and reduced catalase activity. A peak in H2O2 content was observed within 5 min of either SA treatment or transfer to the 45 degreesC acclimation temperature. Between 2 and 3 h after SA treatment or heat acclimation, both H2O2 and catalase activity significantly decreased below control levels. The lowered H2O2 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 H2O2.