Effects of high temperature on the activity of peroxidase (PRX) isozyme and leaf proteins were studied in strawberry (Fragaria x ananassa cv. Camarosa). Seedlings were grown using perlite for 3 weeks at 25/10 degreesC day/night temperature, and watered daily by 1/3 strength modified Hoagland nutrient solution. Half of the plants were transferred to a growth chamber with a constant 25 degreesC for a week to acclimate the plants. Temperature was increased stepwise (5 degreesC/48 h) to 30, 35, 40 degreesC and finally to 45 degreesC. In addition to acclimated plants, new plants were transferred from outside to the growth chamber, at each temperature step to impose a heat shock. In general, effects of gradual heat stress (GHS) and shock heat stress (SHS) on the variables studied were significant. PRX activities were high in all the samples in response to high temperature treatment. Conversely, total protein content was decreased by heat stress. GHS plants showed significantly higher activities of PRX enzyme in response to high temperature compared to those from the leaves of SHS plants. One basic PRX band (rf = 0.22) was detected in all the samples with different intensity in polyacrylamide gel electrophoresis (PAGE). In addition, plants exposed to GHS leaked less electrolytes from the leaves compared with the plants exposed to SHS. (C) 2003 Elsevier Ireland Ltd. All rights reserved.