Exogenous salicylic acid prevents nitrogen dioxide-induced oxidative injury and nitrate accumulation in Brassica campestris L. ssp chinensis seedlings

An earlier study had shown that exposure to nitrogen dioxide (NO2) at 1.0 mu l l(-1) caused a decrease in plant biomass and an accumulation of nitrate in Brassica campestris seedlings. The present study was aimed at evaluating whether pre-treatment with salicylic acid (SA) could promote tolerance to NO2 and prevent NO2-induced nitrate accumulation in leaves, thus improving the quality of pak choi (B. campestris L. spp. chinensis). Thirty-day-old B. campestris plants grown in a 3:1 (v/v) soil:sand mix were sprayed with SA solutions at 0, 0.5, 1.0, or 1.5 mM and, after 24 h, fumigated with 1.0 mu l l(-1) NO2 for 6 h for 10 d in a growth chamber. The results showed that SA applied at 1.0 mM was most effective at reducing the NO2-induced decrease in biomass and accumulation of nitrate. The photosynthetic rate, chlorophyll content, maximum quantum efficiency, and nitrate reductase activity also achieved their highest values at this dose of SA compared to all other concentrations. Furthermore, the application of SA protected the activities of superoxide dismutase, peroxidase, and catalase in leaves, and maintained a high level of reduced glutathione and a high ratio of reduced:oxidised glutathione compared to non-SA-treated samples. This may be responsible for the decreased levels of H2O2, low malondialdehyde contents, and electrolyte leakage observed in SA-treated plant leaves. Our results suggest that exogenous SA may play an important role in plant responses to reduce NO(2)(-)induced stress and to reduce in vivo nitrate levels.