Atmospheric nitrogen dioxide at ambient levels stimulates growth and development of horticultural plants

We recently discovered that atmospheric nitrogen dioxide (NO2) at ambient levels acts as a signal to Nicotiana plumbaginifolia Viviani, causing these plants to double both their biomass and all of the cell contents. Herein, we addressed whether this effect of NO₂ is also observed in various horticultural plants. Lettuce (Lactuca sativa L.), sunflower (Helianthus annuus L.), cucumber (Cucumis sativus L.), and pumpkin (Cucurbita moschata Duch. ex Lam.) were grown with 50, 200, 100, and 200 μL-L^-1, respectively, of air supplemented with stable isotope-labelled (¹⁵N) NO ₂, for 5-6 weeks. Control plants were grown without NO₂ (<5 μL-L^-1). All plants were irrigated with nonlabelled nitrate. The presence of NO₂ doubled both the aboveground and belowground biomass in sunflowers compared with their growth in the absence of NO₂, whereas lettuce, cucumber, and pumpkin doubled in aboveground biomass only. Contents per shoot of carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) were almost doubled during the "NO₂-enhanced" growth in lettuce, but not in other plants. Mass spectrometry analysis of ¹⁵N/¹⁴N indicated that only a minor proportion (0.2%-14%) of total plant N was derived from NO₂, implying that exogenous NO₂ acts as a signal rather than a significant nutrient source in horticultural plants.