Effect of arbuscular mycorrhizal fungi on the growth and physiology of Alhagi sparsifolia seedling under salt stress

Arbuscular mycorrhizal fungi (AMF) colonize the root systems of plant seedlings to form mycorrhizal symbionts, which improves the tolerance of seedlings to stress. To investigate the effect of AMF colonization on the salt tolerance of Alhagi sparsifolia seedlings, we conducted pot evaluations to determine several physiological indicators of seedlings under salt stress and AMF inoculation treatments. Four treatments were designed and used: control (CK), salt stress (S), AMF inoculation (A), and salt stress + AMF inoculation (SA). The following results were obtained: 1) salt stress significantly reduced the rate of mycorrhizal colonization and increased the intensity of colonization. Salt stress significantly reduced the number of AMF spores (P < 0.05). 2) Salt stress inhibited the increase in height and growth of basal stems of A. sparsifolia seedlings. However, inoculation with AMF significantly reduced the salt damage index (P < 0.05). Inoculation with AMF had a mitigating effect on leaf chlorophyll fluorescence parameters, but the effect was not significant. 3) Both salt stress and inoculation with AMF activated the antioxidant system of A. sparsifolia seedlings, and AMF inoculation under salt stress significantly increased the activity of antioxidant enzymes (P < 0.05). 4) AMF inoculation under salt stress promoted the accumulation of the osmoregulatory substances proline and soluble sugar in A. sparsifolia seedlings and significantly reduced malondialdehyde content (P < 0.05). 5) Salt stress significantly reduced strigolactone content, and indole acetic acid content in the inoculated group was significantly higher than that in the uninoculated group (P < 0.05). The content of gibberellic acid was the highest in the CK group and the lowest in the salt stress group. In contrast, the content of abscisic acid was the highest under salt stress, and inoculation with AMF significantly reduced the abscisic content, bringing each hormone into a dynamic balance. Thus, AMF colonization improved the salt tolerance of A. sparsifolia seedlings by increasing the activity of antioxidant enzymes 1- to 2-fold and accumulating osmoregulatory substances at a rate of more than 1-fold compared with the control treatment. In addition, AMF colonization of A. sparsifolia dynamically regulated and balanced the hormone levels to alleviate the damage caused by salt stress.