Effect of heavy metals on photosynthetic apparatus and antioxidant status of elodea

Elodea plants (Elodea (Egeria) densa Planch.) were incubated in the presence of individual and mixed 1 μM sulfate salts of Ni, Cd, Cu, Zn, and Mn to study the influence of heavy metals (HM) on shoot growth, structural-and functional parameters of the photosynthetic apparatus, lipid peroxidation, enzymatic activities of the antioxidant defense system (superoxide dismutase and catalase), and the content of non-protein and protein thiols in leaves. The accumulation of HM in leaves decreased in a row: Mn > Cu > Cd > Zn > Ni. The largest reduction in chlorophyll content was caused by Mn and Cu, whereas the strongest reduction in carotenoid content was induced by Cu. The presence of Cu produced the largest decrease in the maximal quantum efficiency of photosystem II (PSII) (Fv/Fm). These changes were paralleled by the shift of the pro-/antioxidant balance towards the dominance of oxidative processes. The presence of Cd elevated the content of chlorophyll and carotenoids without altering the photochemical efficiency of PSII; Cd retarded the shoot growth but had no appreciable effect on leaf mesostructure. The addition of the second metal to the growth medium alleviated in most treatments the detrimental action of individual ions owing to the enhanced activities of SOD and catalase and because of the significant increase in the content of non-protein thiols. It is supposed that the observed antagonism of metal ions is related to their competitive interactions restricting the entry of HM into the cell.