Scots pine seedlings were grown from sterilized seeds in oven sterilized sand. Half of the seedlings were inoculated with the mycorrhizal fungus Pisolithus arhizus (Pers.) Rausch (syn. Pisolithus tinctorius) while the rest were not. Seedlings were raised in a growth chamber for 61 d and supplied with a complete nutrient solution with NH4Cl as the nitrogen source. To measure uptake of NO2 by the 15N dilution method, nitrogen in the nutrient solution was enriched with 5.00 atom %15N. After 61 d pretreatment, all seedlings were transferred to cuvettes, and exposed to either 20–30 ppb NOx or carbon-filtered air for 39 d. Seedlings harvested just prior to, and after 19 and 39 d exposure were analysed for growth and concentrations of total nitrogen and atom %15N in roots and shoots and free amino acids and total protein in shoots. As a measure of mycorrhizal infection the concentration of ergosterol in roots was determined.
Exposure of seedlings; to low concentrations of NOx had no effect on growth, total nitrogen concentration, ergosterol concentration or total protein concentration. The atom %15N of seedlings increased and asymptotically approached that of the nutrient solution. Because differences in uptake of nitrogen from soil resulted in various values on atom %15N irrespective of exposure, quantification of uptake of NOx was not possible with the formula previously used for this purpose. By calculating the atom %15N of the nitrogen taken up by plants between two harvests, and comparing this value for exposed and non-exposed seedlings, uptake of nitrogen from NOx could be quantified. The uptake of nitrogen from 20–30 ppb NOx thus calculated amounted IO, on average 1.4% of the total uptake of nitrogen by seedlings during exposure. After 19 d exposure nitrogen derived from NOx was found in both shoots and roots indicating transport of nitrogen from shoots to roots. After 39 d exposure, negative values of uptake of NOx were found in roots. This might be explained as an effect of decreased transport of NO, derived nitrogen from shoots and roots and/or decreased transport of soil derived nitrogen from roots to shoots. Glutamine, arginine and GABA+ proline were the most abundant amino acids detected in the seedlings. Exposure to NOx. resulted in lower concentrations of GABA+ proline but had no significant effect on concentrations of glutamine and arginine. Higher concentrations of glycine were found in exposed seedlings but this represented only a small fraction of the total amount of amino acids in the shoots. Inoculated seedlings were smaller and had significantly higher concentrations of glutamine and arginine compared with those that were not inoculated.
Thus, 40 d exposure to 20–30 ppb NO, had no effect on seedling growth, mycorrhizal infection or total nitrogen concentration, but resulted in a significant uptake of NOx and measurable changes in amino-acid composition.