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

Tropospheric ozone (O 3 ) is an air pollutant with phytotoxic effects on plants. This research aimed to evaluate the impacts of O 3 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 O 3 . Using a free-air O 3 exposure system, plants either inoculated with Mesorhizobium or left uninoculation (Inoculated vs. Control) were subjected to ambient, 1.5 × , and 2 × ambient O 3 levels over a 129-day period. Measurements were made of visible foliar injury (VFI), leaf color index (SPAD), biomass growth and stomatal O 3 uptake. In the presence of elevated O 3 , rhizobia inoculation significantly decreased VFI and fine root biomass loss. This protective mechanism was associated with a 22 % reduction in maximum stomatal conductance ( g max ), which restricted stomatal O 3 uptake. Dose-response relationships showed that flux-based indices (POD y ) better explained VFI and biomass development than exposure-based indices (AOT40). Accordingly, critical levels (CLs) for O 3 were set at 24.5 mmol m −2 POD 0 causing the first appearance of VFI, and 7.1 mmol m −2 POD 4 resulting in a 4 % reduction in biomass. These CLs are higher than those for O 3 -sensitive species, suggesting moderate O 3 tolerance of R. pseudoacacia to O 3 . Overall, the present results highlight that rhizobia inoculation can increase tree resistance to O 3 stress, possibly by greater limitation of stomatal O 3 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. • O 3 caused visible foliar injury (VFI) and fine root biomass loss in R. pseudoacacia. • Rhizobia inoculation mitigated these O 3 damages due to reduced stomatal O 3 uptake. • R. pseudoacacia can be identified as moderately O 3 tolerant species. • Critical O3 levels are 24.5 mmol m −2 POD0 for VFI and7.1 mmol m −2 POD4 for biomass.