Physiological and ecological adaptation mechanisms of tobacco under combined stress of acid rain and cadmium

This study investigated the physiological and metabolic responses of flue-cured tobacco (cv. K326) to combined acid rain and cadmium (Cd) stress. A pot experiment was conducted with three acidity levels (pH 6.5, 4.5, and 2.5) and three Cd concentrations (0, 5, and 10 mg·kg –1 ) to assess changes in photosynthetic pigments, chloroplast ultrastructure, antioxidant enzyme activities, and osmotic regulators. Targeted metabolomic analysis was performed to identify key metabolites and pathways involved in stress adaptation. As acidity increased and Cd concentration rose, activities of peroxidase (POD) and superoxide dismutase (SOD) decreased by 27.4% and 34.8%, respectively, while malondialdehyde (MDA) levels significantly increased, indicating heightened oxidative damage. Under strong acidity (pH 2.5) and high Cd stress, chlorophyll a and b contents declined by 21.3% and 18.9%, respectively, alongside severe chloroplast disorganization and disrupted thylakoid stacking. Metabolomic profiling identified differential metabolites predominantly enriched in the ABC transporter and starch–sucrose metabolism pathways, suggesting that tobacco plants adapt to stress through metabolic reprogramming. Overall, combined acid rain and Cd stress severely compromised the physiological and metabolic integrity of flue-cured tobacco, with the synergistic effects of high acidity and Cd exposure significantly impairing the plant's adaptive capacity. • Acid rain–Cd co-stress impaired tobacco growth and photosynthesis. • Combined stress damaged chloroplasts and increased oxidative injury. • Moderate stress enhanced antioxidants, while severe stress inhibited enzymes. • ABC transporters and sugar metabolism were key reprogrammed pathways. • Acid–Cd synergy caused metabolic collapse beyond adaptive capacity.