Fully expanded leaves of 2- to 3-month-old tomato plants (L. esculentum cv Moneymaker) grown in the greenhouse were cut into 2 mm slices and floated on H2O overnight. For measuring the oxidative burst, active oxygen species released by tomato leaf tissue were measured by a luminol-dependent assay (Keppler et al., 1989). Slices were transferred to assay tubes (two slices corresponding to approximately 20 mg fresh weight) containing 0.1 ml of H2O supplied with 20 mM luminol and 1 mg horseradish peroxidase (Fluka). Luminescence was measured in a LKB 1250 luminometer (LKB Wallac, Turku, Finland) for 20 min after the addition of the test solution.