Abstract: Abstract 【Objective】This paper investigated the responses of Pakchoi seedlings to acute level of nitric dioxide (NO2) exposure, examined whether pre-treating plants with hydrogen peroxide (H2O2) would alleviate NO2-induced injury and discussed the possible role of H2O2 in the modulation of some physiological and biochemical characteristics of the plants.【Method】Some indices were determined in Pakchoi seedlings sprayed with H2O2 solution and untreated control plants, both exposed to NO2 for 24 h under controlled environment. To measured the biomass, plants were pretreated with H2O2 for 1 day and then exposed to NO2 stress 7 h per day for 7 days.【Result】A low concentration of NO2 has a favorable effect on plants, and the growth was promoted. On the other hand, plants exposed to a high concentration of NO2 (1 μl•L-1 and above in this experiment) caused visible symptoms. A reduction of growth and chlorophyll content, and increase of malondialdehyde (MDA) content were occurred in plants exposed to 1 μl•L-1 or above of NO2, compared with the control. Other changes in plants responsed to NO2 stress included appeared necroses on older leaves, increased antioxidative enzymes, superoxide dismutase (SOD) and catalase (CAT) activities, decreased ascorbate (ASA) content, higher accumulated NO3-, induced nitrate reductase (NR) activity and impaired photosynthesis, but no detected change in stomatal conductance. The unfavorable influences of NO2 stress to plants were significantly alleviated on pretreatment with 10 mmol•L-1 of H2O2, as indicated by recovery of plant fresh weight, ASA and MDA contents to the control levels. The photosynthetic rate was obviously recovered. More induction of SOD and CAT activities has been observed in H2O2 – treated plants. However, H2O2 treatment did not alter the responsiveness in nitrate content and NR activity. H2O2 pretreatment resulted in a lower rate of stomatal conductance.【Conclusion】Together, these data indicate that acute stress of 1 μl•L-1 and above of NO2 caused visible injury to Pakchoi seedlings, which may be related to oxidative damage confered by impaired photosynthesis. H2O2 pretreatment alleviates NO2-generated injury to plants, which may be related to induction of antioxidative system in the plant.

Key words: Hydrogen peroxide, Nitric dioxide, Response, Antioxidative system, Oxidative stress, Pakchoi