In the case of H2O2 we used an endogenous peroxidase-dependent in situ histochemical staining, in which leaf quarters were vacuum infiltrated with 0.1 mg mL-1 3,3’-diaminobenzidine in 50 mM Tris-acetate buffer (pH 5.0) and incubated at 25 ºC in the dark for 24 hr. Controls were performed in the presence of 10 mM ascorbic acid. Leaf quarters were rinsed in 80 % (v/v) ethanol for 10 min at 70ºC, and photographed directly using an Olympus SZX 12 microscope.