Physiological responses and tolerance of Idesia polycarpa seedlings under simulated rhizosphere acidification

Introduction Soil acidification poses a growing threat to forest seedling establishment, yet the physiological responses and tolerance limits of many multipurpose woody species under increasing acidity remain poorly understood. Idesia polycarpa Maxim. is an emerging woody oil tree with high ecological and economic value, but its physiological responses to root-zone acidification are largely unknown. Methods We subjected one-year-old I. polycarpa seedlings to 50 d of root-zone irrigation with acidified solutions at pH 5.5, 4.5, and 3.5, with deionized water as the control, and monitored visible injury, photosynthetic pigments, malondialdehyde (MDA), osmotic adjustment substances, and the activities of peroxidase (POD) and superoxide dismutase (SOD). Results Visible injury increased with both acidity and exposure duration, and the cumulative injury index reached 85.00% under pH 3.5, 40.00% under pH 4.5, and 16.67% under pH 5.5 after 50 d, while CK remained at 0%. Under pH 3.5, photosynthetic pigment contents showed pronounced temporal fluctuations, with marked reductions at 20 and 50 d and partial recovery during the intermediate stages. MDA content also exhibited a non-monotonic response, with significantly higher values under pH 3.5 at several middle and late sampling times. Soluble sugar, proline, and soluble protein showed stage- and treatment-dependent changes rather than continuous accumulation or decline. POD activity under pH 3.5 reached approximately 3.55-fold the CK value at 10 d and subsequently declined, whereas SOD activity fluctuated markedly among treatments and sampling times. Correlation analysis revealed coordinated variation among photosynthetic pigments, MDA, osmotic adjustment substances, and the two measured antioxidant enzymes, but these associations do not establish causal relationships. Discussion Overall, seedlings under pH 5.5 and pH 4.5 showed partial physiological adjustment, whereas pH 3.5 was associated with substantially greater visible injury and physiological disturbance. The contrasting responses between the pH 4.5 and pH 3.5 irrigation treatments suggest a potential physiological tolerance boundary within this tested acidity interval under the present controlled conditions, rather than a statistically defined biological breakpoint.