Abstract: To explore Cd tolerance mechanism and Cd accumulation characteristics of two ectomycorrhizal fungi \[ECMF; Pisolithus tinctorius(Pt) and Suillus luteus (Sl)\], we examined the physiological and ecological changes including cell morphology, extracellular secretion and antioxidant substances of Pt and Sl under three Cd²⁺ concentrations (0.1, 1.0, and 10.0 mg·L^-1). The results showed that cell wall of Pt was significantly thickened and the plasmolysis occurred. The cells of Pt were seriously deformed and a great amount of inclusions were exuded. Both Sl and Pt secreted siderophores of hydroxamic acid type which chelate Cd²⁺. The secreted quantities of different organic acids varied with different strains, with succinic acid being the main organic acid secreted by both ECMF species. In addition, the activities of antioxidant enzymes and the content of non-enzymatic antioxidant substances in Pt and Sl varied with different strains. With the increases of Cd²⁺ concentration, ascorbate peroxidase (APX) activity and ascorbic acid (AsA) content in Pt and Sl increased obviously. The activities of superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and the content of reduced glutathione (GSH) were different. GSH content and GR activity showed an opposite trend in Pt. Under low Cd concentration, SOD activity increased significantly in Pt and Sl. For Pt, CAT activity decreased first and then increased. In all, Pt and Sl showed different physiological and ecological responses to Cd²⁺ stress. They could resist Cd stress by enhancing the activities of SOD, CAT, APX, increasing the content of AsA, secreting organic acid and siderophores. In addition, cell wall thickening and chelation of GSH are important mechanisms underlying the tolerance of Sl to Cd stress.

Key words: ectomycorrhizal fungi, Pisolithus tinctorius, Suillus luteus, Cd stress, pure culture.