Abstract: Two different families of Pinus massoniana Lamb., namely aluminum-resistant type FJ5 and aluminum-sensitive type GD20 were used as test materials to provide theoretical basis for the regulation of aluminum toxicity in P. massoniana Lamb. in soil acidification areas. The effects of oxalic acid on root morphology and physiological characteristics of P. massoniana Lamb. seedlings under aluminum stress were studied, and the dominant physiological factors affecting alleviation effect of oxalic acid were explored. It was shown that under aluminum stress, the total root length, root surface area, root volume and root activity of P. massoniana Lamb. seedlings were reduced, while the activities of root antioxidant enzymes (superoxide dismutase, peroxidase, catalase), osmotic adjustment substances (soluble sugar, soluble protein and proline) significantly (P<0.05) increased, and membrane lipid peroxidation aggravated. The variation of these indices in GD20 was higher than that in FJ5. It indicated that active aluminum could produce toxicity to P. massoniana Lamb., and active aluminum had greater toxic effects on aluminum sensitive species. After application of exogenous oxalic acid, the root activity of P. massoniana Lamb. increased, the content of malondialdehyde and osmotic adjustment substances in roots decreased, and the antioxidant enzymes activities maintained a high level. When oxalic acid concentration did not exceed 0.2 mmol·L^-1, the aluminum content in root and root tip decreased. The variation of these indices with oxalic acid application in GD20 was greater than that in FJ5. These results indicated that exogenous oxalic acid could effectively alleviate the aluminum toxicity of P. massoniana Lamb. seedlings, and the alleviation effect on aluminum-sensitive type was better than that on aluminum-resistant type. The alleviation effect on P. massoniana Lamb. varied with different concentrations of exogenous oxalic acid, and the optimum application concentration was 0.2 mmol·L^-1. It was revealed by principal component analysis that contents of proline and soluble protein might be the dominant physiological factors to regulate alleviation effect of oxalic acid on aluminum toxicity.

Key words: Pinus massoniana Lamb., aluminum stress, oxalic acid