Phytoextraction of Pb and Cd by the Mediterranean saltbush (Atriplex halimus L.): metal uptake in relation to salinity

Background, aim, and scope The success of phytoextraction depends upon the identification of suitable plant species that hyperaccumulate heavy metals and produce large amounts of biomass using established agricultural techniques. In this study, the Mediterranean saltbush Atriplex halimus L., which is a C4 perennial native shrub of Mediterranean basin with an excellent tolerance to drought and salinity, is investigated with the main aim to assess its phytoremediation potential for Pb and Cd removal from contaminated soils. In particular, the influence of soil salinity in metal accumulation has been studied as there is notable evidence that salinity changes the bioavailability of metals in soil and is a key factor in the translocation of metals from roots to the aerial parts of the plant. Materials and methods Three pot experiments were conducted under greenhouse conditions for a 10-week period with A. halimus grown in soil artificially polluted with 20 ppm of Cd and/or 800 ppm of Pb and irrigated with three different salt solutions (0.0%, 0.5%, and 3.0% NaCl). Soil measurements for soil characterization were performed with the expiration of the first week of plant exposure to metals and NaCl, and at the end of the experimental period, chlorophyll content, leaf protein content, leaf specific activity of guaiacol peroxidase (EC 1.11.1.7), shoot water content, biomass, and Cd and Pb content in the plant tissues were determined. Additionally, any symptoms of metal or salt toxicity exhibited by the plants were visually noted during the whole experimental period. Results The experimental data suggest that increasing salinity increases cadmium uptake by A. halimus L. while in the case of lead there was not a clear effect of the presence of salt on lead accumulation in plant tissues. A. halimus developed no visible signs of metal toxicity; only salt toxicity symptoms were observed in plants irrigated with 3% NaCl solutions. Chlorophyll content, leaf protein content, shoot water content, and biomass were not negatively affected by the metals; instead, there was even an increase in the amount of photosynthetic pigments in plants treated with both metals and salinity. The specific activity of guaiacol peroxidase seems to have a general tendency for increase in plants treated with the metals in comparison with the respective controls but a statistically significant difference exists only in plants treated with the metal mixture and saline conditions. Discussion The data revealed that lead and cadmium accumulation in plant tissues was kept generally at low levels. Salinity was found to have a positive effect on cadmium uptake by the plant and this may be related to a higher bioavailability of the metal in soil due to decreased Cd sorption on soil particles. On the other hand, salinity did not influence in a clear way the uptake of Pb by the plant probably because of lead's limited mobility in soils and plant tissues. Cd and Pd usually decrease the chlorophyll content and biomass and change water relations in plants; however, A. halimus was found not to be affected indicating that it is a Cd- and Pb-tolerant plant. Guaiacol peroxidase activity as one of the parameters expressing oxidative damage and extent of stress in plants was not generally found to be significantly affected under the presence of metals in most plants suggesting that the extent of stress in plants was minimal, while only for plants treated with the metal mixture and low salinity the enzyme activity was elevated confirming that this enzyme serves as an antioxidative tool against the reactive oxygen species produced by the metals. Conclusions Atriplex halimus L. is a Pb- and Cd-tolerant plant but metal concentrations achieved in plant tissues were kept generally at low levels; however, metal accumulation in shoots, especially for Cd, considered together with its high biomass production, rapid growth, and deep root system able to cope with poor structure and xeric characteristics of several polluted soils suggest that this plant deserves further investigation. Recommendations and perspectives Phytoextraction by halophytes is a promising alternative for the remediation of heavy metal contaminated sites affected by salinity since saline depressions often indicate sites of industrial effluents accumulation, contaminated by heavy metals, including Pb and Cd. Halophytes are also promising candidates for the removal of heavy metals from non-saline soils. Furthermore, the use of such plants can be potentially viewed as an alternative method for soil desalination where salt is removed from the soil instead of being washed downwards by water or other solutions.