Exogenous hydrogen peroxide induces chilling tolerance in Phalaenopsis seedlings through glutathione-related antioxidant system

Phalaenopsis is the most economically important flower crops wildly but hardly tolerated to low-temperature stress. Hydrogen peroxide (H2O2), a reactive oxygen species was participated in many chilling-response processes. In this study, the efficiency and mechanism of chilling tolerance induced by transient oxidative shock with H2O2 pretreatment were investigated. Phalaenopsis seedlings demonstrated chilling injury when the ambient temperature was at 6 degrees C or lower. Transient oxidative shock by pretreatment with 50 mM H2O2 induced chilling tolerance. This pretreatment reduced the leaf electrolyte leakage (EL) from 64.1% to 28.3% of plants cultivated at 6 degrees C for 6 days. Increasing the frequency of H2O2 pretreatment from 1 to 3 times strengthened the protective efficiency; however, an H2O2 concentration of >= 100 mM caused abnormal leaf morphology such as curl and necrosis. Time-course analysis findings indicated that chilled seedlings pretreated with H2O2 had lower EL and malondialdehyde (MDA) content but higher chlorophyll content and better photosynthetic performance, including a higher assimilation rate, maximum quantum yield of photosynthetic system II (Fv/Fm), quantum yield of electron transport (phi PSII), photochemical quenching (qP), and electron transport rates. The results also demonstrated that pretreatment with 50 mM exogenous H2O2 did not alter the endogenous H2O2 level only a minor elevation of approximately 2 mu mol.g(-1) was noted. Under low temperature suffering, activity of catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) were increased with fluctuant changes, but CAT and APX inhibited in the low level by H2O2 pretreatments. In addition, the significantly high GR activity was reduced the glutathione-to-oxidized glutathione ratio, with peaks occurring at 4-8 h and on day 2 during chilling. Taken together, the findings indicate that exogenous H2O2 induced chilling tolerance in Phalaenopsis seedlings, and the process may be involved the glutathione-related oxidant defense system.