INTERACTIONS OF ELEVATED CO2, NH3 AND O-3 ON MYCORRHIZAL INFECTION, GAS-EXCHANGE AND N-METABOLISM IN SAPLINGS OF SCOTS PINE

Four-year-old saplings of Scots pine (Pinus sylvestris L.) were expose d for 11 weeks in controlled-environment chambers to charcoal-filtered air, or to charcoal-filtered air supplemented with NH3 (40 mu g m(-3) ), O-3 (110 mu g m(-3) during day/ 40 mu g m(-3) during night) or NH3 + O-3. All treatments were carried out at ambient (350 mu L L(-1)) and at elevated CO2 concentration (700 mu L L(-1)). Total tree biomass, m ycorrhizal infection, net CO2 assimilation (P-n), stomatal conductance (g(s)), transpiration of the shoots and NH3 metabolization of the nee dles were measured. In ambient CO2 (1) gaseous NH3 decreased mycorrhiz al infection, without significantly affecting tree biomass or N concen tration and it enhanced the activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in one-year-old needles; (2) ozone decre ased mycorrhizal infection and the activity of GS in the needles, whil e it increased the activity of GDH; (3) exposure to NH3 + O-3 lessened the effects of single exposures to NH3 and O-3 on reduction of mycorr hizal infection and on increase in GDH activity. Similar lessing effec ts on mycorrhizal infection as observed in trees exposed to NH3 + O-3 at ambient CO2, were measured in trees exposed to NH3 + O-3 at elevate d CO2. Exposure to elevated CO2 without pollutants did not significant ly affect any of the parameters studied, except for a decrease in the concentration of soluble proteins in the needles. Elevated CO2 + NH3 s trongly decreased root branching and mycorrhizal infection and tempora rily stimulated P-n and g(s). The exposure to elevated CO2 + NH3 + O-3 also transiently stimulated P-n. The possible mechanisms underlying a nd integrating these effects are discussed. Elevated CO2 clearly did n ot alleviate the negative effects of NH3 and O-3 on mycorrhizal infect ion. The significant reduction of mycorrhizal infection after exposure to NH3 or O-3, observed before significant changes in gas exchange or growth occurred, suggest the use of mycorrhizal infection as an early indicator for NH3 and O-3 induced stress.