<i>Arbuscular mycorrhizal fungi</i> enhance the saline-alkali tolerance of apple rootstock M9-T337 by regulating chlorophyll fluorescence parameters and hormone balance

<italic>Arbuscular mycorrhizal fungi</italic> (AMF) can establish symbiotic relationships with most fruit trees to promote their growth and development under adverse stresses. However, studies on the relationship between AMF and saline-alkali resistance in apple remain relatively scarce. The results showed that compared with the uninoculated AMF (<italic>Funneliformis mosseae</italic>) treatments, after inoculation with AMF, the palisade tissue thickness was all significantly increased, the spongy tissue cells were enlarged, and the stomata opened to varying degrees under different NaCl + NaHCO₃ stress concentrations (0 mM, N0; 50 mM, N50; 100 mM, N100; 150 mM, N150). Additionally, Fv/Fm, F0, Fm, qP, Y (N0), K⁺, ZT, TZR, GA₃, IAA, SA, LT, PT/ST, LE, UE, CR, CTR, Lp, Wp, Lg, Wg, SD, SS, Sa, and Gsmax of rootstock M9-T337 were increased, while Na⁺ content, Na⁺/K⁺ ratio, and ABA content were decreased. Principal component analysis indicated that AMF exhibited the most significant mitigating effect on rootstock M9-T337 under the N100 treatment. Compared with the N100 treatment, among the indicators, except for the Na⁺ content, Na⁺/K⁺ ratio and ABA content were reduced by 21%, 32%, and 7%, respectively, the rest of the indicators all showed a significant increasing trend. In conclusion, AMF can mitigate the damage of salt-alkali stress on M9-T337 by maintaining ion homeostasis, regulating hormone levels, and mediating the opening and closing of stomata.