Comprehensive understanding on plant vitalization effect of atmospheric NO_x at an ambient level as an agricultural production signal

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Globally, a total of 30 to 50 teragrams of nitrogen per year is converted to nitrogen oxides (NO_x) such as nitric oxide (NO) and nitrogen dioxide (NO_2), and released into the atmosphere, resulting in 50-150 ppb in the urban air. The major known sources of NO_x are fossil fuel combustion, biomass burning, microbial activity in soils and lightning. Atmospheric NO_x has been regarded as a major pollutant in urban air providing excess nutrient. However, in contrast to widely-accepted view on the endogenously synthesized NO_x such as NO as a plant signal, the biological function of the exogenous or atmospheric NO_x is only poorly understood. Nicotiana plumbaginifolia, Arabidopsis thaliana, lettuce (Lactuca sativa L.), sunflower (Helinthus annuus L.), cucumber (Cucumis sativus L.) and pumpkin (Cucurbita moschata Duch. ex Lam.) were grown in presence or absence of 50-200 ppb 15N-labelled NO_2 for 4-10 weeks under irrigation with a medium containing nonlabelled nitrate, after which plants were analyzed for growth parameters, element contents and ^<15>N/^<14>N isotopic ratio. It was discovered that biomass yield, total leaf area, and contents per plant of carbon (C), nitrogen (N), sulphur (S), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), free amino acids and crude proteins were almost doubled in response to prolonged the presence of NO_2 in the air. Further, the contribution of N derived from NO_2 (NO_2-N) to total plant N appeared to be negligible (0.2-14%) in these plants. Therefore, it is very likely that the atmospheric NO_x acts as a multifunctional signal to vitalize plants rather than an additional N source (plant vitalization signal effect of NO_2). Our present findings strongly suggest that atmospheric NO_x at an ambient level acts as a vital hormonal signal on plants including crops.