Rhizobium Inoculation Enhances Drought Tolerance of Legume Species by Increasing Leaf Hydraulic Efficiency and Safety...

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 (An) 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 P₅₀), 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 P₅₀. 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 P₅₀, 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.