Elevated CO2 and ozone reduce nitrogen acquisition by Pinus halepensis from its mycorrhizal symbiont

The effects of 700 mu mol mol(-1) CO2 and 200 nmol mol(-1) ozone on photosy nthesis in Pinus halepensis seedlings and on N translocation from its mycor rhizal symbiont, Paxillus involutus, were studied under nutrient-poor condi tions, After 79 days of exposure, ozone reduced and elevated CO2 increased net assimilation rate, However, the effect was dependent on daily accumulat ed exposure. No statistically significant differences in total plant mass a ccumulation were observed, although ozone-treated plants tended to be small er. Changes in atmospheric gas concentrations induced changes in allocation of resources: under elevated ozone, shoots showed high priority over roots and had significantly elevated N concentrations. As a result of different shoot N concentration and net carbon assimilation rates, photosynthetic N u se efficiency was significantly increased under elevated CO2 and decreased under ozone. The differences in photosynthesis were mirrored in the growth of the fungus in symbiosis with the pine seedlings. However, exposure to CO 2 and ozone both reduced the symbiosis-mediated N uptake. The results sugge st an increased carbon cost of symbiosis-mediated N uptake under elevated C O2, while under ozone, plant N acquisition is preferentially shifted toward s increased root uptake.