Effects of O3 on alder photosynthesis and symbiosis with Frankia

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Abstract
Alnus serrulata (Aiton) Willdenow seedlings with and without root nodules formed by the nitrogen-fixing actinomycete Frankia were exposed to clean filtered air or ozone (O3) at 0.cntdot.12 .mu.l l-1 for 27 d (approximately 164 h total exposure). Gas exchange measurements on leaves and transmission electron micrographs of root nodule cells were made to detect any O3 effects on the functioning of leaves and the root symbiont. Photosynthesis, stomatal conductance, and internal CO2 concentration were calculated for all plants in clean O3- free air more than three weeks after the fumigations began. Significant positive correlations between photosynthesis and conductance were found for leaves of control nodulated and unnodulated alders and O3-treated nodulated alders. There was a weak positive correlation between photosynthesis and conductance for unnodulated O3-treated seedlings measured in clean air. When O3-treated leaves were measured during fumigation with O3, no positive correlation between photosynthesis and conductance was found for either nodulated or unnodulated seedlings. Photosynthetic rates of leaves having the highest stomatal conductance values were decreased by O3 for both nodulated and unnodulated plants. Transmission electron microscopy (TEM) revealed that after a 27 d exposure of shoots to O3, host root cells of nodules from O3-treated plants lacked organelles and showed extensive cytoplasmic breakdown. Hyphae and N2- fixing vesicles of Frankia appeared normal. The Frankia endophyte seems to be more resistant to O3 than is the host root nodule cell. These results show that ambient levels of O3 may reduce photosynthesis and bring about associated degradation in rhizosphere symbiosis.