Production of hydrogen peroxide is commonly the first of various biochemical reactions of plant cells to stress. For monitoring this oxidative stress response of plant cells pyranine, a fluorescent dye, is a useful system, because of its rapid bleaching by plant peroxidases. Measurement of the reduction of fluorescence can provide valuable information about the stress effects of processing operations on plant foods. The effect of high pressure treatment at 50 and 90 MPa, a thermal process at 50 °C, and a freeze‐thaw cycle on tomato cell cultures as model system was examined. Thermal treatment, freezing as well as pressures of 90 MPa caused destruction and the death of the plant cells, consequently no hydrogen peroxide production occured. Pressure treatments for 10 minutes at 50 MPa and room temperature were shown to induce stress responses in the plant cells. In contrast to elicitation with chitosan the cells showed a lag‐time in their reaction after high pressure treatment. The addition of pectolytic enzymes to the cell culture system resulted in delayed stress reaction most likely due to the formation of degradation products of pectin. We suggest that chemical and physical elicitation provoke different ways of signal transduction.