Rising Nitrogen Dioxide (No2): A Growing Threat to Environment and Plant Health

Abstract

The atmospheric increase in nitrogen dioxide (NO₂) has greatly impacted plant growth and health, which have been mainly due to the burning of fossil fuels and agricultural practices. Although diatomic nitrogen N₂ constitutes more than 78% of atmospheric composition, it is less reactive and not absorbed directly by plants. Since human activity increases reactive nitrogen, NO₂ is one such highly reactive nitrogen species. NO₂ is significant in photochemical reactions due to its interesting physicochemical characteristics and a density that is 1.59 times denser than air. Under the influence of sunshine, it oscillates between NO and NO₂ to sustain photochemical equilibrium. Some major sources of NOx for developing countries are crop waste burning, industrial processes, vehicle emissions, and combustion of biomass. Such emissions deteriorate air quality. This is because they result in the formation of such secondary pollutants as aldehydes, methane, ozone, or O₃, and HNO₃. NO₂ is seasonal and weather sensitive; it is also man-affected. High concentration may even promote photosynthesis or an accelerated rate of plant growth from delivering greater availability of reactive nitrogen and enhancing protection in the plants against diseases, provided that molecular defense mechanisms apply. On the other hand, over-accumulation of NO₂ leads to the accumulation of nitrite, the formation of reactive oxygen species, acidification of cells, reduced nitrogen uptake, and acute leaf damage. NOx emissions also play a role in sudden climate changes.