Response of Frankia and Casuarina cunninghamiana Symbiotic System to Soil Pb Pollution

In order to explore the response of Frankia bacteria and Casuarina cunninghamiana symbiont to soil Pb pollution in soil, pot experiments were conducted to study the effects of Frankia wet bacteria 0 (CK), 10, 20 mg·plant⁻¹ on nodulation amount, contents of Pb, K, CA, Mg, Cu, Mn in the aboveground parts, stress physiology and root activity of Casuarina cunninghamiana under certain Pb treatment. The results of the pre-experiment showed that Casuarina cunninghamiana was stressed but not lethal under 600 mg·kg⁻¹ lead (PB), so the Pb concentration was set at 600 mg·kg⁻¹ in the formal experiment. The results showed that the application of 20 mg·plant⁻¹ Frankia wet mycelium could significantly increase the nodulation rate of Casuarina cunninghamiana. The nodulation rate of Casuarina cunninghamiana was 100% when inoculated with 10 mg·plant⁻¹ or 20 mg·plant⁻¹ Frankia wet mycelium. Compared with CK, the Pb content in aerial parts of Casuarina cunninghamiana decreased significantly with the application of Frankia wet mycelium 10 and 20 mg·plant⁻¹. When the inoculation amount was 10 mg·plant⁻¹, the K content in aerial parts of Casuarina cunninghamiana decreased significantly, while the Cu content increased significantly. The change of Ca, Mg and Mn was not significant (P < 0.05). The free proline and soluble sugar in Casuarina cunninghamiana decreased by 56.04% and 10.10% respectively, and the TTC reduction intensity of root activity increased by 87.12% when Frankia inoculation was 20 mg·plant⁻¹. In conclusion, under the same Pb stress, high inoculation amount (20 mg·plant⁻¹) of Frankia bacteria could enhance the resistance of Casuarina cunninghamiana to Pb pollution in soil.