Nitrogen Dioxide-Induced Responses in Brassica campestris Seedlings: The Role of Hydrogen Peroxide in the Modulation of Antioxidative Level and Induced Resistance

Abstract

This article investigates the responses of Brassica campestris seedlings to an acute level of nitrogen dioxide (NO 2) exposure in a plant growth chamber, and examines whether pretreating plants with hydrogen peroxide (H 2O 2) will alleviate NO 2-caused injury. Twenty-eight-day-old B. campestris plants sprayed with 10 mmol L −1 H 2O 2 aqueous solution (corresponding to approximate 1.0 mg H 2O 2 per single plant) were exposed to different concentrations of NO 2 (0.25, 0.5, 1.0, and 2.0 μL L −1, respectively) for 24 h under controlled environment. To measure the plant biomass, the plants were fumigated with the same NO 2 concentrations as mentioned above for 7 h per day (8.00-15.00) for 7 days. As a control, charcoal filtered air alone was applied. Data were collected on plant biomass, total chlorophyll, photosynthetic rate, stomatal conductance, nitrate and nitrate reductase (NR), antioxidative enzymes, ascorbate (ASA), and malondialdehyde (MDA), immediately after exposure. The results showed that exposure to a moderate dose of NO 2 (e.g., 0.25 μL L −1) had a favorable effect on plants, and the dry weight of the above-ground part increased, whereas the exposure to high NO 2 concentrations (e.g., 0.5 μL L −1 or higher) caused a reduction in the plant biomass and the total chlorophyll, when compared with the control. In addition, at 0.5 μL L −1 or higher NO 2 concentrations, prominent increases in the MDA level and superoxide dismutase (SOD) and NR activities were observed. Exposure to 1 μL L −1 and higher NO 2 resulted in necroses appearing on older leaves, and an increase in catalase (CAT) activity, decrease in ASA content, increased accumulation of NO 3, and reduction in photosynthesis, when compared with the controls. No changes were detected in stomatal conductance under NO 2 fumigation. The pretreatment with 10 mmol L −1 H 2O 2 alleviated significantly NO 2-caused biomass decrease and photosynthetic inhibition when compared with H 2O 2-untreated plants. Under NO 2 fumigation, further induction in SOD and CAT activities occurred in H 2O 2 treated plants when compared with H 2O 2-untreated plants. The effect of NO 2 on the ASA and MDA contents was also absent in H 2O 2-treated plants. However, the H 2O 2 treatment did not alter the nitrate content and NR activity in plants under NO 2 fumigation. The H 2O 2 treatment caused a lower rate of stomatal conductance. Taken together, these data suggest that fumigation with an acute level of NO 2 causes oxidative damage to B. campestris seedlings. The H 2O 2 pretreatment markedly protects plants against NO 2 stress and this may be associated with inducible antioxidative level. NO 2 fumigation contributes, at least in part, to the enhanced levels of nitrate in B. campestris leaves.