Rhizobia inoculation alleviates ozone-induced foliar damage and root biomass loss for Robinia pseudoacacia L

Tropospheric ozone (O3) is an air pollutant with phytotoxic effects on plants. This research aimed to evaluate the impacts of O3 on Robinia pseudoacacia L., a tree species introduced worldwide due to its ability for symbiotic nitrogen fixation, and to assess if rhizobia root inoculation could alleviate the effects from O3. Using a free-air O3 exposure system, plants either inoculated with Mesorhizobium or left uninoculation (Inoculated vs. Control) were subjected to ambient, 1.5 ×, and 2 × ambient O3 levels over a 129-day period. Measurements were made of visible foliar injury (VFI), leaf color index (SPAD), biomass growth and stomatal O3 uptake. In the presence of elevated O3, rhizobia inoculation significantly decreased VFI and fine root biomass loss. This protective mechanism was associated with a 22 % reduction in maximum stomatal conductance (gmax), which restricted stomatal O3 uptake. Dose-response relationships showed that flux-based indices (PODy) better explained VFI and biomass development than exposure-based indices (AOT40). Accordingly, critical levels (CLs) for O3 were set at 24.5 mmol m−2 POD0 causing the first appearance of VFI, and 7.1 mmol m−2 POD4 resulting in a 4 % reduction in biomass. These CLs are higher than those for O3-sensitive species, suggesting moderate O3 tolerance of R. pseudoacacia to O3. Overall, the present results highlight that rhizobia inoculation can increase tree resistance to O3 stress, possibly by greater limitation of stomatal O3 uptake and by improving tolerance to oxidative stress. As climate change intensifies abiotic stressors, further research should investigate the combined stress mitigation potential of rhizobial symbioses.