Aluminum stress alters leaf physiology and endophytic bacterial communities in ginseng (Panax ginseng Meyer)

Aluminum (Al) stress is a significant constraint to plant growth in acidic soils, however, its effects on ginseng (Panax ginseng Meyer) remain poorly understood. This study, investigated the physiological and microbial responses of ginseng to graded Al stress. Ginseng seedlings were grown in sterilized artificial soil and exposed to six concentrations of aluminum sulfate [0 (control), 25, 50, 75, 100, and 200 mM Al₂(SO₄)₃] for 90 days. Growth phenotypes, leaf nutrient contents, photosynthetic parameters, and leaf endophytic bacterial diversity and composition were evaluated to elucidate the mechanisms of Al-induced stress. The study revealed that low Al concentrations (25 and 50 mM) increased leaf fresh weight by 5.78 %–17.33 % and dry weight by 25.93 %–64.49 %, whereas higher concentrations (75, 100, and 200 mM) reduced fresh weight by 5.80 %–18.84 % and dry weight by 20 %–65 %, respectively. As the concentration of Al treatment increased, the leaf Al and iron (Fe) contents increased by 18.00 %–83.15 % and 15.96 %–83.36 %, respectively. Meanwhile, the magnesium (Mg) and phosphorous (P) contents initially increased but then decreased. Chlorophyll content, along with variable fluorescence (Fv) and maximum fluorescence (Fm), initially increased and then decreased as Al concentration rose. The Shannon index peaked at 5.7 under 25 mM Al treatment but decreased to 1.3 under 200 mM Al treatment. At low Al levels, the leaf endophytic bacterial biomarkers were Aeromonas, Bacillus, Shewanella, and Vibrio, whereas Paenibacillus and Ralstonia were associated with high Al concentrations. PLS-PM analysis demonstrated Al showed a significant negative correlation with P, while P exhibited a significant correlation with the photosynthetic indices, as well as the diversity and structure of leaf endophytic bacterial communities. Notably, the genera Pseudomonas and Stenotrophomonas exhibited substantial positive correlations with actual photosynthetic efficiency [Y(II)], and the maximum quantum efficiency of photosystem II (Fv/Fm ratio). The findings demonstrate a concentration-dependent impact of Al on ginseng, with low levels beneficial and high levels detrimental to ginseng growth and physiology. Excessive Al stress impairs photosynthesis by disrupting the diversity of endophytic bacteria. These results underscore the need for soil Al management and the use of beneficial microbes to enhance ginseng resistance, with future efforts directed toward microbial inoculants and molecular strategies for Al-resistant cultivation.