| Nitrogen dioxide(NO2), one of the main traffic-related air pollutants and precursors of photochemical smog and ground-level ozone, is currently under intensive investigation.Although visible injury due to NO2 is very rare in the field where NO2 is at a ppb level, growing evidence has shown that ambient NO2 has caused reduction and deterioration in crop and vegetable yield and quality in some regions on the world, especially in greenhouses where NO2 at a ppm level may show visible injury symptoms in some vegetables. To address mechanisms of plant response to NO2 exposure, fumigation experiments under controlled conditions are used intensively with NO2 concentrations up to ppm levels. Although the effects of NO2 on plant growth are not fully understood, evidence indicates that accumulations of nitrate(NO3-)and nitrite(NO2-) resulting from atmospheric NO2 in leaves results in the inhibition of photosynthesis, thereby generating reactive oxygen species(ROS), which might contribute to visible injury caused by NO2. Recent years, increasing studies have demonstrated that salicylic acid(SA) is intensively involved in plant responses to various abiotic stresses, such as drought,salt stress, low temperature, heavy metals, ozone and so on, in which SA plays a positive regulatory role in most cases with pleiotropic functions. However, to our knowledge, little information is available for SA involvement in plant response to NO2 stress. Our group’s earlier study showed that Arabidopsis plants with high accumulation of SA displayed a significant tolerance to NO2 stress relative to the control plants(basal content of SA). Based on these observations, we speculate that exogenous SA may enhance wheat plant tolerance or adaptation to NO2 exposure. In order to verify the hypothesis, we have carried out a series of studies to reveal SA regulation mechanism in plant response to NO2 exposure. The present study is a part of the project.In this study, we used wheat seedlings as experimental materials. To analyse the effect of application process of SA on its physiological functions, the SA was applied by seedling foliar spraying and seed soaking, respectively. The SA concentration tested was 0.5 m M, and NO2 was 20 μl·L-1. For NO2 exposure, 7-day-old wheat plants were transferred to a glass chamber(0.8 × 0.8 × 0.8 m) and NO2 gas was supplied directly from cylinders, into a dilution reservoir into which charcoal filtered air was drawn. Mean concentration of the chamber NO2 was monitored using an NO2 analyzer(SWG 300-1; MRU, 74172 NSU-Obereisesheim, Germany).Plants were fumigated during the light period for 3 h per day for 4 days. For the control,charcoal filtered air alone was supplied.Results showed that NO2 at the tested concentration caused acute damage to plant seedlings,with a reduction in plant growth, yellowish leaves, and leaf apex death. Under NO2 stress, chlorophyll content decreased, whereas electrolyte leakage and malondialdehyde(MDA)content dramatically increased. The contents of soluble sugar and proline were lifted. The activities of superoxide dismutase(SOD), peroxidase(POD) and catalase(CAT) were elevated.Except the chlorophyll content, all the above-mentioned parameters were effectively recovered by application of SA regardless of the application process. Based on these data, it was concluded:1. Nitrogen dioxide at 20 μl·L-1 caused acute damage to wheat seedlings.2. Exogenous SA efficiently alleviated NO2-induced damage to plants, as exhibited by a reduction in NO2-caused growth inhibition, reduction of death area in leaf apex, increase in soluble sugar and proline, and in the activities of SOD, POD and CAT, and decrease in electrolyte leakage and MDA. These observations implied that SA may be involved in the regulation of redox homeostasis and osmotic regulation during plant exposure to NO2 stress.3. The application process of SA, seedling foliar spraying or seed soaking, almost did not affect the SA physiological function. This suggested that these two methods can be haphazard selected in SA-related studies. |
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