Absorption of atmospheric NO2 by spruce (Picea abies) trees. Part 2. Parameterization of NO2 fluxes by controlled dynamic chamber experiements

The dynamic chamber technique was applied to investigate NO2 influx into Picea abies (L.) Karsten branches, and its effects on net photosynthesis and transpiration, as well as its dependency on irradiance, temperature and relative humidity. The study aimed to quantify effects of climate on atmospheric NO2 fluxes to spruce. Experiments were performed with 3- to 4-yr-old branches of 8- to 9-yr-old potted trees under controlled environmental conditions. With ambient NO2 concentrations increasing from 3.5 to 50 nl l(-1) a linear increase in the NO2 influx of up to c. 6 x 8 my mol m(-2) s(-1) was observed. From this increase a compensation point of 1.64 nl l(-1) NO2 was calculated by linear regression analysis. In the range of the NO2 concentrations studied, net photosynthesis of spruce was not affected. The responsiveness of the stomata to changes in irradiance and relative humidity was reduced at 45 nl l(-1) NO2 compared with 25 nl l(-1) NO2. With increasing irradiance up to 1000 my mol m (-2) s(-1) PAR, increasing NO2 flux to spruce branches was observed, which was attributed to a light-dependent increase in stomatal aperture. Variation of the temperature between 14 and 35 deg C did not affect the NO2 flux in light or in darkness. Higher temperatures, up to 45 deg C, resulted in an increase in NO 2 influx in the light; in darkness, changes in NO2 flux were not observed under these conditions. An increase in relative humidity from 5 to 60 per cent in the light caused an increase in NO2 influx, whereas in darkness NO2 influx was not affected by changes in relative humidity. The increase in NO2 flux in response to r.h. observed in the light could not be explained by changes in stomatal aperture. A solution of NO2 in ultra-thin water films covering the needle surface might explain this phenomenon.