Catalase activity, hydrogen peroxide content and thermotolerance of pepper leaves

Activated forms of oxygen, including hydrogen peroxide (H2O2), have been implicated in plant responses to stress. Catalases (CAT) and peroxidases are the primary enzymatic detoxifiers of H2O2 in most plant tissues. Pepper (Capsicum annuum L.) leaf disks floated on 0-100 mM H2O2 solutions in the dark were not affected or showed minimal effects depending on the assay. Changes in electrolyte leakage (EL) and evolution of ethylene and methanol from H2O2-treated disks were slight compared to freeze-killed tissues, indicating that pepper leaves had considerable capacity to detoxify exogenous H2O2. H2O2 concentration in leaf tissue was not significantly affected by injurious (48 degreesC) or lethal (54 degreesC) temperature treatments. As plants aged from 6 to 10 weeks, thermotolerance increased from 45 to 50 degreesC based on calculated inflection points (T-mid) of sigmoidal EL response curves. A further increase of about 1 degreesC occurred from 10 to 14 weeks after sowing. However, CAT activity decreased as plants aged from 6 to 14 weeks old. Although 11-week-old plants had lower baseline CAT activity in controls, activity was stable to a higher temperature than in 6-week-old plants (53.1 versus 48.4 degreesC). CAT activity was more stable than membrane integrity since an increase in EL occurred at a lower temperature than a decline in CAT activity. Thermotolerance of plants exposed to the acclimating regime of 38/30 degreesC (day/night temperatures) increased from 50.7 to 53.9 degreesC. CAT activity also showed an adaptive response with the inflection point increasing from 52.6 to 56.8 degreesC. Changes in H2O2 levels do not appear to have a direct role in injury to pepper leaves exposed to high temperatures in the dark. Thermal inactivation of CAT may be a consequence, rather than a cause of high temperature injury. (C) 2002 Elsevier Science B.V All rights reserved.