The effects of root nitrogen supplies on the absorption of atmospheric NO₂ by soybean leaves

Abstract

Twelve-day-old soybean plants were exposed to atmospheric NO₂ (0·3 μl l⁻¹) and simultaneously supplied, via the roots, with 5 mM or 1 mM of NaNO₃ or NH₄Cl. After exposure for 7 d, the amount of NO₂ absorbed per plant was greater in plants supplied with nitrate than in plants supplied with the same concentration of ammonium. The NO₂ AR (absorption rate) decreased with increasing exposure time. At the beginning of exposure, the NO₂ AR for all plants was c. 12 mg NO₂ h⁻¹ m⁻² μl⁻¹ l. On the day 7 of exposure, the NO₂ AR declined to 8·46, 8·97, 8·27, and 9·04 mg NO₂ h⁻¹ m⁻² μl⁻¹ l for plants receiving 1 mM ammonium, 1 mM nitrate, 5 mM ammonium, and 5 mM nitrate respectively. The plants supplied with nitrate had a higher concentration of leaf nitrate and a higher pH than those supplied with the equivalent concentration of ammonium. These results suggest that the NO₂ absorption rate might be attenuated by the accumulation of H⁺ produced from N uptake and assimilation.