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

在全球范围内，每年总共有30至50太克的氮转化为氮氧化物（NO_x），如一氧化氮（NO）和二氧化氮（NO_2），并释放到大气中，导致城市空气中的50-150 ppb。NOx的主要来源是化石燃料燃烧、生物质燃烧、土壤中的微生物活动和闪电。大气NO_x被认为是城市空气中提供过量营养的主要污染物。然而，与人们普遍认为的内源合成的NO_x（如NO）是一种植物信号相反，人们对外源或大气中NO_x的生物学功能了解甚少。烟草（Nicotiana plumbaginifolia）、拟南芥（Arabidopsis thaliana）、莴苣（Lactuca sativa L.）向日葵（Helinthus annuus L.），黄瓜（Cucumis sativus L.）和南瓜（Cucurbita moschata Duch.）ex Lam.）在含有50-200 ppb ~（15）N标记NO_2和含有非标记NO_3的培养基中培养4-10周，然后分析植株的生长参数、元素含量和~（15）<15>N/~（15）<14>N同位素比值。结果表明，随着空气中NO_2浓度的增加，生物量、总叶面积、碳（C）、氮（N）、硫（S）、磷（P）、钾（K）、钙（Ca）、镁（Mg）、游离氨基酸和粗蛋白含量增加近一倍。NO_2-N对植株氮素的贡献很小（0.2-14%）。因此，大气中的NO_x很可能是一种多功能的激活植物的信号，而不是一种额外的氮源（NO_2的植物激活信号效应）。我们的研究结果强烈地表明，在环境水平上，大气NO_x作为一种重要的激素信号对植物，包括作物。

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.