Heavy metal tolerance in common fern species

Abstract

The effects of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) on the growth and uptake of 10 fern species was investigated under a controlled environment in order to evaluate their suitability for phytoremediation. Fern species included Adiantum aethiopicum, Blechnum cartilagineum, Blechnum nudum, Calochlaena dubia, Dennstaedtia davallioides, Doodia aspera, Hypolepis muelleri, Nephrolepis cordifolia, Pellaea falcata and the arsenic (As) hyperaccumulating Pteris vittata. Ferns were exposed to four levels of metals at concentrations of 0, 50, 100 and 500 mg kg–1 for a period of 20 weeks. The response of ferns significantly varied among species and metals. In general, heavy-metal translocation was limited, with metals being absorbed and held in roots, suggesting an exclusion mechanism as part of the ferns’ tolerance to the applied metals. Similar metal-accumulation patterns were observed for all species in that accumulation generally increased with increasing metal treatments; in most cases a sharp increase in metal accumulation was observed between 100 and 500 mg kg–1 treatments, suggesting a breakdown in tolerance mechanisms and unrestricted metal transport. This was corroborated by enhanced visual toxicity symptoms and a reduction in survival rates among ferns when exposed to 500 mg kg–1 metal treatments; and to a lesser extent 100 mg kg–1 metal treatments. Of the species investigated, N. cordifolia and H. muelleri were identified as possible candidates in phytostabilisation of Cu, Pb, Ni or Zn contaminated soils. Similarly, D. davallioides appeared favourable for use in phytostabilisation of Cu and Zn contaminated soils. These species had high survival rates and accumulated high levels of the aforementioned metals relative to other ferns investigated. Ferns belonging to the family Blechnaceae (B. nudum, B. cartilagineum and D. aspera) and C. dubia (Family Dicksoniaceae) were least tolerant to most metals, had a low survival rate and were classified as being unsuitable for phytoremediation purposes. Metal tolerance was also observed in P. vittata when exposed to Cd, Cr and Cu; however, no hyperaccumulation was observed.