Dark septate endophytes enhance salt tolerance of Tamarix ramosissima via root remodeling and Na⁺ homeostasis: Divergent salt tolerance strategies.

Dark septate endophytes (DSEs) can improve plant adaptation to saline environments, but their strain-specific role in salt-secreting woody halophytes remains unclear. In this study, three DSE strains, Paraphaeosphaeria hydei (Ph), Chaetomium cervicicola (Cc) and Curvularia platzii (Cp), were inoculated into Tamarix ramosissima seedlings under four NaCl concentrations. The growth, root architecture, Na⁺ accumulation and secretion, oxidative injury, antioxidant enzyme activities, osmotic adjustment, water status, chlorophyll fluorescence, pigment contents and anatomical traits were systematically evaluated. Salt stress increased Na⁺ accumulation, oxidative injury and photosystem inhibition, whereas DSE inoculation alleviated these effects in a strain- and salinity-dependent manner. Cc increased under low-to-moderate salinity because it improved root architecture, maintained root activity and increased Na⁺ secretion. Under high salinity, compared with the noninoculated control, Ph and Cp performed better, mainly because they maintained the leaf water status, restricted the accumulation of Na⁺ in the leaves and protected photosystem function. Multivariate analyses revealed that the effects of inoculation were associated mainly with root traits, ion regulation, antioxidant defense and photosystem/pigment modules, whereas the results for anatomical traits provided supplementary structural evidence. Membership function evaluation further confirmed that Cc was most effective under moderate salinity, whereas Ph and Cp showed greater adaptive potential under high salinity. These results indicate that DSE inoculation enhances salt tolerance in T. ramosissima through coordinated regulation of root remodelling, Na⁺ homeostasis, cellular defense and photosystem protection, providing a basis for targeted DSE application in saline-alkaline land restoration.