Rhizobium Inoculation Enhances Drought Tolerance of Legume Species by Increasing Leaf Hydraulic Efficiency and Safety and Speeding Up Post‐Drought Recovery

The trade‐off between leaf hydraulic efficiency and safety is among the principal theories explaining plant drought adaptation, but whether rhizobium inoculation modulates this trade‐off to enhance plant drought adaptation remains unclear. Here, leaf water potential ( Ψ leaf ), stomatal conductance ( g s ), photosynthesis ( A n) and leaf hydraulic conductance ( K leaf ) during and after drought were measured in rhizobium‐inoculated and non‐inoculated Glycine max and Caragana korshinskii grown under well‐watered (WW), moderate drought stress (MS) and severe drought stress (SS) conditions. Non‐inoculated plants of each species exhibited a more negative Ψ leaf , decreasing the maximum K leaf ( K leaf‐max ), a more negative Ψ leaf at 50% loss of K leaf‐max ( K leaf P 50 ), lowering the g s and A n , and delayed recovery from drought when grown under increasing drought severity, displaying a significant trade‐off between K leaf‐max and K leaf P 50 . However, compared with non‐inoculated plants, inoculated plants exhibited greater drought tolerance because of the improved Ψ leaf , revealing a new trade‐off relationship characterised by a greater K leaf‐max and a more negative K leaf P 50 , increased g s and A n , a more rapid recovery from drought, greater biomass accumulation and increased nitrogen and phosphorus contents. Our results suggest that rhizobium inoculation improves hydraulic safety and efficiency and recovery from drought, thus provides drought tolerance in legume species.