Effects of O₃ on alder photosynthesis and symbiosis with Frankia.

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 0.12 μl 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.

Duke Scholars

Author William E. Winner DKU Faculty