Physiological and biochemical responses of arbuscular mycorrhizal fungi in symbiosis with Juglans nigra L. seedlings to alleviate salt stress

Salt stress is a significant challenge for agricultural and forestry production, which severely limits crop growth and yield. Arbuscular mycorrhizal fungi (AMF) act as natural bioregulators and have demonstrated notable efficacy in mitigating the impacts of salt stress within agricultural and forestry ecosystems. Accordingly, this study investigated the effects of inoculating black walnut seedlings with two species of AMF, Funneliformis mosseae and Rhizophagus irregularis, under varying levels of salt stress (0, 100, 200, and 300 mM NaCl). The growth performance, physiological, and biochemical responses of the seedlings were assessed. The results confirmed the severe effects of salt stress on the growth and physiology of black walnut seedlings. The seedlings inoculated with AMF exhibited superior performance in many aspects. Firstly, both species of AMF significantly increased the proline (Pro) content in the leaves and roots of the seedlings and significantly reduced the hydrogen peroxide (H₂O₂) content in the leaves. However, their impact on the activity of antioxidant enzymes and the content of malondialdehyde (MDA) in the leaves and roots was not significantly pronounced. Secondly, AMF-treated seedlings demonstrated enhanced photosynthetic performance, including a significant improvement in photosynthetic parameters (Gs, Tr, and Pn), and also elevated the efficiency of photosynthesis and energy utilization rate. Notably, F. mosseae also significantly increased the chlorophyll content and stomatal dimensions under low salt concentrations (0 and 100 mM NaCl). Furthermore, AMF inoculation promoted the accumulation of growth-related endogenous hormones (IAA, ABA, GA₃, ZR), further supporting plant development. Principal component analysis concluded that AMF primarily enhance the tolerance of black walnut to salt stress by improving photosynthetic performance and regulating the levels of endogenous hormones. F. mosseae, in particular, may be more suited to enhancing the adaptability and survival of black walnut under salt stress conditions. Overall, our study underscores the significant role of AMF in enhancing the salt tolerance of crops in saline soils and promoting sustainable agricultural development.