Redox and fungicidal properties of phthalocyanine metal complexes as related to active oxygen

Some chemical and fungicidal effects of 20 phthalocyanines of Co, Fe, Cu, a nd Al were studied. Under dark conditions, these complexes reduced nitroblu e tetrazolium in the presence of KCN, accelerated the autoxidation of ascor bate or hydroquinone and decomposed hydrogen peroxide. In the later reactio n, hydroxyl radical was generated as evidenced with the deoxyribose assay. The inhibition by superoxide dismutase and catalase of catalyzed autoxidati on of ascorbate suggests the participation of superoxide anion-radical and hydrogen peroxide in the reaction. Most complexes were toxic to the fungus Magnaporthe grisea which causes blast disease of rice. The toxicity was enh anced by light being diminished by antioxidant reagents sequestering active oxygen species. Some complexes (including nontoxic ones), after 1-day cont act with a leaf surface of the disease-susceptible rice cultivar, induced t he fungitoxicity of leaf diffusate. This toxicity was also light-activated and sensitive to antioxidant reagents. Several complexes, when added to ino cula, decreased 2-3 times the frequency of the compatible symptoms of the b last. It is suggested that in planta, the dark redox activity of phthalocya nines along with their photosensitization promote the generation of active oxygen, which damages the parasite and, therefore, favors disease resistanc e. (C) 2000 Elsevier Science S.A. All rights reserved.