Selenite antagonizes the phytotoxicity of Cd in the cattail Typha angustifolia

The Phytotoxicity of and mechanism underlying selenite-mediated tolerance to Cd stress in Typha angustifolia were studied hydroponically with respect to metal uptake and translocation, photosynthesis-related parameters, contents of proline and O₂^•-, products of lipid peroxidation, cell viability, enzymatic and non-enzymatic antioxidants, glyoxalases and phytochelatins. T. angustifolia were exposed to 25, 50 and 100 μM of Cd alone and in conjunction with 5 mg L^-1 of selenite in full-strength Hoagland's nutrient solution for 30 days. Results showed that Cd contents in T. angustifolia leaves and roots increased in a dose-dependent manner and were higher in roots, but those of BAC, BCF and TF changed in a contrary pattern. Addition of selenite to Cd-containing treatments further reduced Cd levels in T. angustifolia leaves and roots, as well as BAC, BCF and TF. A diphasic effect was found in T. angustifolia for the contents of total chlorophyll, GSH, PC and GSSG, as well as activities of CAT, POD, SOD and GR, in response to Cd stress alone and in conjunction with selenite supplementation, but the same effect was not observed for Pn, Cond, Tr, Ci, Fv/Fm and ϕPSII. In contrast, exogenous selenite supplementation enhanced the contents of total chlorophyll and the non-enzymatic antioxidants, as well as activities of enzymatic antioxidants, while the values of photosynthetic fluorescence parameters were rescued. Selenite addition decreased Cd-induced cell death. Proline contents and Gly I activities in T. angustifolia leaves kept increasing in a dose-dependent manner of Cd concentrations in the growth media and selenite addition further enhanced both parameters. Addition of selenite could quench Cd-mediated generation of MDA, O₂^•- and MG in T. angustifolia leaves and reduce Cd-induced Gly II activity. A U-shaped GSH/GSSG ratio in T. angustifolia leaves suggests a possible trade-off between PC synthesis and GR activity since both share the same substrate GSH. Therefore, confined BAC, BCF and TF were a mechanism that confers T. angustifolia tolerance to Cd stress, and that exogenous selenite supplementation could depress Cd-induced stress in T. angustifolia by rescuing the photosynthetic fluorescence, enhancing non-enzymatic and enzymatic antioxidants that scavenge O₂^•- and MG, and potentiating PC synthesis that chelates Cd.