Abstract: Sterile seedlings of Potamogeton crispus L. were cultured by tissue culture technique, to investigate the effects of exogenous calcium (Ca) application on antioxidase system, soluble protein, antioxidant capacity (T-AOC), non-protein thiols (NP-SH), phytochelatins (PCs) and proline metabolism of leaves of the tested plants under Hg stress. The results showed that, Hg stress resulted in a serious imbalance of antioxidase system, such as restrained activities of catalase (CAT), ascorbate peroxidase (APX), and glutathion reductase (GR), while no obvious changes were observed in superoxide dismutase (SOD) and peroxides (POD) activities. Meanwhile, 25 mmol·L^-1 exogenous application of Ca markedly retarded the degradation of antioxidases and effectively enhanced SOD, POD, APX and GR activities in P. crispus. Hg treatment severely reduced the soluble protein content and T-AOC, as well as promoted a substantial accumulation of NP-SH and PCs. Application of exogenous Ca apparently reversed this situation and decreased the levels of NP-SH and PCs. Proline is an important osmolyte in aquatic plants on exposure against abiotic stresses. Addition of exogenous Ca increased proline accumulation significantly by activating pyrroline-5-carboxylate (P5CS) and rnithineδaminotransferase (OAT) activities. Our results demonstrated that Hg disturbed the intrinsic balance of antioxidant defense system in the tested plants. The supplement of exogenous Ca relieved the phytotoxicity of Hg to P. crispus by promoting the synthesis of soluble protein, increasing the activities of antioxidant enzymes and the contents of antioxidant substances, maintaining a high total antioxidant capacity and involving in regulation of a series of physiological and biochemical mechanisms, particularly proline metabolism.

Key words: available phosphorus agronomy threshold., long-term fertilization, loessial soil, available phosphorus, soil phosphorus surplus, the Loess Plateau