Cynara cardunculus a potentially useful plant for remediation of soils polluted with cadmium or arsenic
Graphical abstract
Highlights
► Cynara cardunculus is a high biomass energy crop. ► This species has considerable tolerance to cadmium and arsenic. ► The plant efficiently transports Cd to the leaves, while As is retained in the roots. ► This plant could be useful for phytoextraction of cadmium from polluted soils. ► C. cardunculus could be grown for biofuel on soils polluted by arsenic.
Introduction
Cadmium and arsenic are among the most dangerous trace elements for human health. Food and tobacco smoking are the main sources for increased Cd burdens in humans. Cadmium is nephrotoxic, can affect bones, may cause cancer and has estrogenic effects (Järup and Akesson, 2009). Arsenic‐rich ground water is the main source for As to humans, either directly when used as drinking water or through irrigated food crops (Bhattacharya et al., 2007). The situation is especially dramatic in certain areas of South Asia where rice, the main staple food, is accumulating considerable As levels by growing in As-contaminated water (Rahman and Hasegawa, 2011). Point sources for As are the mining and smelting industries (Bech et al., 1997, Hamilton, 2000, Vamerali et al., 2011). Elevated As levels in foods can also be due to the historical use of As-containing pesticides (Hughes et al., 2011). Arsenic can cause skin effects and neurological and vascular disorders even at subchronical exposures (Tsuji et al., 2004). Frequently both elements occur together in soils contaminated by mining or other industrial activities (Govil et al., 2008, Strosnider et al., 2011). Combined Cd and As pollution is highly relevant for food chain exposure, especially in agricultural soils due to contaminated irrigation water. Other sources for crop soils are the use of Cd and As-rich phosphate fertilizers or the application of Cd-rich amendments on soils with historical use of As-containing pesticides.
Both trace elements largely differ in their chemical properties and the uptake and transport of As and Cd in plants is governed by different mechanisms (Verbruggen et al., 2009). Cadmium enters plants via specific and unspecific transporters of essential bivalent cations such as Ca2 +, Zn2 + or Fe2 +, while arsenate is taken up through the anion transporters of the essential nutrient phosphate (Ma et al., 2008, Meharg and Hartley-Whitaker, 2002). After uptake a fast reduction of arsenate (As V) to arsenite (As III) occurs. Although different in mechanisms of uptake and toxicity, both trace elements share the importance of thiol-containing metabolites in their detoxification. Glutathione and phytochelatins seem to play a fundamental role in binding and vacuolar compartmentation of Cd and As (Verbruggen et al., 2009). Loading into the xylem to achieve long-distance transport to the shoots is operated by divalent cation transporters in the case of Cd and by phosphate transporters and aquaporin like proteins in the cases of As V and As III, respectively.
There are only a few reports comparing Cd and As sensitivity/tolerance within the same plant species in controlled environments. Even less information on species with potential for phytoremediation is available. When given separately, the relative toxicity of both trace elements is species and even variety dependent (Cao et al., 2007, Di Lonardo et al., 2011). In joint applications additive, synergistic, and antagonistic interactions, have been described (Sneller et al., 2000, Sun et al., 2008, Sun et al., 2009).
The present study investigates the response of cardoon, Cynara cardunculus, to Cd and As. C. cardunculus is a high biomass crop that recently is gaining much interest because of its use as an energy and biofuel crop (Gominho et al., 2011, Raccuia and Melilli, 2007). The plant is well-adapted to Mediterranean climate conditions and could be used on marginal land for sustainable production of renewable energy. C. cardunculus has occasionally been described as a species with relatively high Cd accumulation in shoots (Hernández-Allica et al., 2008, Papazoglou, 2011). The combination of characteristics of a high biomass industrial crop and those of a plant with possible potential for Cd extraction makes C. cardunculus an interesting species for further investigations into their phytoremediation potential. The specific objective of this investigation was to evaluate the tolerance to Cd and As in this species under controlled-environment conditions to see whether it really efficiently accumulates Cd in shoot tissues. This information will help to evaluate this species as good candidate for further studies on soils with single or combined pollution of Cd and As.
Section snippets
Plant material and growth conditions
Seeds of C. cardunculus were provided by B&T World Seeds (Aigües Vives, France). After germination at 20 °C, seedlings were grown in 16 L plastic beakers (2 plants per beaker) filled with continuously aerated modified half strength Hoagland solution (Epstein, 1972) buffered with 2 mM MES (2-(N-morpholino)ethanesulfonic acid) to pH 6.0 (Schat et al., 1996). Plants were pre-cultured during 2 weeks in a growth chamber (photoperiod 16 h light/8 h darkness; day/night temperature 24 °C/18 °C; photon fluency
Results and discussion
C. cardunculus plants exhibited considerable tolerance to Cd. Root elongation, a reliable indicator for sensitivity to Cd (Vázquez et al., 1992a, Vázquez et al., 1992b), was reduced less than 10% after 5 weeks exposure to 5 μM Cd (Fig. 1A). Cadmium had a small effect on leaf expansion; fully expanded leaves of Cd treated plants had the same length as those of controls (Fig. 1B), but leaf width was 14% lower (Fig. 1C). This was compensated by a small (13%), Cd-induced increase of leaf number (
Acknowledgments
This research was supported by the Spanish Ministry of Science and Innovation project BFU2010-14873.
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