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 (Ψleaf), stomatal conductance (gs), photosynthesis (An) and leaf hydraulic conductance (Kleaf) 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 Ψleaf, decreasing the maximum Kleaf (Kleaf-max), a more negative Ψleaf at 50% loss of Kleaf-max (Kleaf P50), lowering the gs and An, and delayed recovery from drought when grown under increasing drought severity, displaying a significant trade-off between Kleaf-max and Kleaf P50. However, compared with non-inoculated plants, inoculated plants exhibited greater drought tolerance because of the improved Ψleaf, revealing a new trade-off relationship characterised by a greater Kleaf-max and a more negative Kleaf P50, increased gs and An, 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.