Antioxidant enzymes such as superoxide dismutase (SOD), CAT and guaiacol peroxidase, aswell as ascorbate peroxidase, dehydroascorbate reductase, mon-odehydroascorbate reductase and glutathione reductase in the Halliwell–Asada cycle, are effective and efficient in responding to various oxidative stresses (Poobathy et al., 2013). Shen et al. (2009)also reported that during vegetative growth, a high H2O2 level was associated with a high content of the reduced form of l-ascorbate(AsA) in Oncidium orchids.