Abstract: [Objective] This paper investigated the effects of exogenous citric acid on a series of growth and physiological characteristics of Pinus massoniana seedlings under aluminum toxicityto find the optimal alleviating concentration of citric acid, and reveal the physiological mechanism how exogenous citric acid alleviates aluminum toxicity, in order to provide theoretical basis for the growth of P. massoniana in soil acidification areas.[Method]In this study,two different families of P. massoniana seedlings (aluminum-resistant FJ5, aluminum-sensitive GD20) were used to investigate the effects of exogenous citric acid on P. massoniana subjected to aluminum stressin a hydroponic experiment. The height increment contents, the form of root, as well as the activities of antioxidant enzymes (POD, SOD, CAT, APX, GR), the contents of active oxygen (H₂O₂ and O₂^·), osmotic adjustment substances (soluble sugar, soluble protein, free proline) and malondialdehyde (MDA) in the leaves of P. massoniana seedlings were measured under different concentrations of exogenous citric acid. The Principal Component Analysis was applied to explore the main physiological indexes during the process of exogenous citric acid to alleviate the aluminum poisoning of P. massoniana.[Result]Aluminum stress decreased the length, surface area, volume, diameter of P. massoniana seedling roots and the height increment, inscreasedthe activities of antioxidant enzymes in the leaves, and the contents of H₂O₂, O₂^·, MDA and osmotic adjustment substances in the leaves. After applying exogenous citric acid, the length, surface area, volume, and diameter of the root, and the seedlingheight increment increased, the activities of antioxidant enzymes in the leaves enhanced, and the contents of H₂O₂, O₂^·, MDA and osmotic adjustment substances in the leaves reduced. Compared with FJ5, GD20 had greater change range. Principal component analysis showed that the contribution rate of each index in the first principal component was MDA > H2O2 > Proline > GR.[Conclusion]Active aluminum has an obvious toxic effect on P.massoniana seedlings; Exogenous citric acid could effectively alleviate the toxicity of aluminum on P.massoniana seedlings, and it had a better effect on aluminum-sensitive variety than aluminum-resistant variety. The alleviating effect of exogenous citric acid on the aluminumtoxicityvarieddependent on the different concentrations of citric acid.The optimum concentration of citric acid was 0.02 mmol·L^-1 among all applied concentrations, and whenthe concenatrations was greater than 800 mmol·L^-1, the alleviating effects of citric acid was inhibited. MDA and H₂O₂ may be the key indexes of exogenous citric acid to alleviate aluminum toxicity.

Key words: Pinus massoniana, exogenous citric acid, aluminum stress, physiological characteristics, mitigation effect