Dependency of nitrogen dioxide (NO2) fluxes to wheat (Triticum aestivum L.) leaves from NO2 concentration, light intensity, temperature and relative humidity determined from controlled dynamic chamber experiments

The dynamic chamber technique was applied to investigate the dependency of nitrogen dioxide fluxes to wheat leaves from atmospheric NO2 concentration, light intensity, air temperature and relative humidity. Experiments were performed with 4- to 5-weeks-old wheat plants under controlled environmental conditions. When exposed to NO2-free air wheat leaves emitted 3.7 ng N m−2 s−1into the atmosphere. With increasing NO2 concentrations the flux of NO2 changed from emission to deposition. Up to NO2 concentrations of 60 nl ℓ−1 the NO2 flux increased linearly; at higher concentrations of NO2 the NO2 flux further increased, but the increment declined. In the range of NO2 concentrations studied (0–90 nl ℓ−1) neither transpiration nor photosynthesis was affected by the NO2 exposure. With increasing light intensity NO2 deposition increased from 29 ng N m−2 s−1 in the dark to 120 ng N m−2 s−1 at 510 μmol m−2 s−1 PAR, when wheat plants were exposed to 30 nl ℓ−1 NO2. This effect could be attributed to the light dependent increase in stomatal aperture. With increasing air temperature from 17 to 40°C NO2 deposition decreased, most likely due to decreasing solubility of NO2 in the aqueous phase of the apoplastic space. NO2 deposition also increased with increasing relative humidity. This increase could not be explained by changes in stomatal aperture, but may at least partially be due to the formation of ultra thin water films on the surface of the wheat leaves, and the solubilization of atmospheric NO2 within these water films.