Phytoremediation

Contamination of agricultural soil by heavy metals (such as Cu, Cd, Zn, Mn, Fe, Pb, Hg, As, Cr, Se, Ur, etc.) has become a serious environmental concern due to their potential impact on the ecosystems. Such toxic elements are considered as soil and water pollutants due to their widespread occurrence, and their acute and chronic toxic effect on plants grown in such soils as well as on humans living in their surrounding (Yadav 2010). Plants, as sessile organisms have developed diverse detoxification mechanisms against absorbing a diversity of natural and man-made toxic compounds. Pollutant-degrading enzymes in plants are a natural defense system against a variety of allelochemicals released by competing organisms, including microbes, insects and other plants. Therefore, plants act as natural, solar-powered pump-and-treat systems for cleaning up contaminated environments, leading to the concept of phytoremediation (Aken 2008) . A variety of plant systems have been studied for phytore-mediation practices of contaminated soil; however, each species has limitations to accumulate the toxic metals and detoxify to nontoxic compounds through the enzymatic actions. In the course of evolution from marine to freshwater habitat, halophytes are found most successful group of plants which have shown adaptations to a variety of abiotic stresses, tolerance to heavy metal stress is one of these. In recent years, more emphasis has been placed to remove the toxic metals from contaminated soil and water bodies and reclamation of such lands for sustainable agriculture. In this regard, extensive research is undertaken to exploit the use of metal hyper-accumulating plants and search for a suitable plants that can significantly accumulate heavy metals and metalloids (Zabludowska et al. 2009). However, phytoremediation constitutes a group of strategies meant not only to reduce the metal load at the contaminated site but also to stabilize the site. These strategies are referred as "phyto-extraction" or "phytostabilization" and the selection of a plant may depend on the level of contamination at the site of concern. Both strategies can be integrated into operation at highly contaminated mine sites with a plant that may not be a hyper-accumulator but can tolerate even very high concentrations of toxic metals (Lokhande et al. 2011b+ + Various halophytes have evolved distinct morphological specializations for dealing with abiotic stressed environments such as presence of "aerial stilts" in the members of families Rhizophoraceae and "pneumatophores" in the members of Avicennaceae and Sonneratiaceae which enable gaseous exchange and oxygenation for respiration in an anoxic environment (Hutchings and Saenger 1987); however, the members of Myrsinaceae, possess no aerial roots. Table 2.4 presents phytoremediation potential of some halophytes.

Numerous laboratory-based trials suggested that the concentrations of metals required to show significant negative effects on halophytes may be significantly higher when compared to their aquatic and terrestrial floral counterparts (MacFarlane et al. 2007). For example, there were no adverse effects on the growth of Rhizophora mucronata and Avicennia alba seedlings treated with Zn (10-500 mg ml-1+ and Pb (50-250 mg ml-1). In Kandelia candel seedlings, only at the highest applied metal concentrations (400 mg kg-1 Cu and Zn) inhibition of leaf and root development was observed (Chiu et al. 1995). Similarly, Pb (0-800 mg g-1) had little negative effect on Avicennia marina seedlings (MacFarlane and Burchett 2002) . Studies have demonstrated the accumulation of metals (Cu, Zn, Pb, Fe, Mn, and Cd) predominantly in root

Table 2.4 Examples of halophytic plant species used for the purpose of phytoremediation

Plant species

Phytostabilization/phytoextraction/ phytoexcretion of heavy metals

References

Sesuvium portulacastrum

Cd, Pb and As

Ghnaya et al. (2007), Nouairi et al. (2006), Zaier et al. (2010a, b), and Lokhande et al. (2011b)

Mesembryanthemum crystallinum

Cd

Ghnaya et al. (2007) and Nouairi et al. (2006)

Halimione portulacoides, Spartina maritima

Cd, Cu, Pb, and Zn

Reboreda and Ca?ador (2007, 2008)

Arthrocnemum macrostachyum, Spartina argentinensis

Cd and Cr

Redondo-Gómez et al. (2010a, b)

Triglochin maritima, Juncus maritimus, Sarcocornia perennis, Halimione portulacoides

Hg

Castro et al. (2009)

Atriplex halimus subsp. Schweinfurthii

Cd

Nedjimi and Daoud (2009) and Lefevre et al. (2009)

A. halimus

Pb and Cd

Manousaki and Kalogerakis (2009)

Spartina densiflora, S. maritima

As, Cu, Fe, Mn, Pb, and Zn

Cambrolle et al. (2008)

Aster tripolium

Cu and Pb

Fitzgerald et al. (2003)

Sarcocornia perennis

Fe, Mn, and Hg

Lilebo et al. (2010)

Halimione portulacoides

Zn, Pb, Co, Cd, Ni, and Cu

Sousa et al. (2008) and Almeida et al. (2009)

Tamarix smyrnensis

Pb and Cd

Kadukova and Kalogerakis (2007) ,

Kadukova et al. (2008), and Manousaki et al. 2008

Juncus maritimus

Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn

Almeida et al. (2006)

Sporobolus virginicus,

Spartina patens, and Atriplex nammularia

Zn, Cu, and Ni

Eid and Eisa (2010)

Salicornia europaea Cd Ozawa et al. (2009)

Salicornia europaea Cd Ozawa et al. (2009)

tissue, rather than in foliage, in numerous mangrove species grown in the field conditions, such as Avicennia sps., Rhizophora sps. and Kandelia sps. (Peters et al. 1997). It has also been observed that for some mangroves, concentrations of translocated metals are low, with bio-concentration factors (BCF; ratio of leaf metal to corresponding sediment metal concentration) ranging from <0.01 in Rhizophora mangle to 0.06 for other species such as A. marina (Lacerda 1997). However, other studies suggest that mangroves may accumulate and translocate some metals with leaf BCFs greater than 1, for example, 1.5-2.4 for A. marina (Sadiq and Zaidi 1994), 1.7 for Aegiceras corniculatum and 1.2 for Kandelia candel (Chen et al. 2003) and behaved as hyper-accumulating species. Lokhande et al. (2011b) recently demonstrated the arsenic (As) accumulation potential of Sesuvium exposed to As(V) (100-1,000 mM) for 30 days, wherein the growth of the plant was not affected even after prolonged exposure to arsenic stress with the significant As accumulation (155 mg g-1 dry weight) and a bioaccumulation factor of more than ten at each concentration. On the basis of total As accumulation, bioaccumulation factor and known biomass production capacities, the Sesuvium like other As hyper-accumulator plants has been suggested to use as potential candidates for application in arsenic removal and land re-vegetation/ reclamation projects in the As-contaminated sites of the world.

Heavy metal uptake in halophytes is generally regulated at the root endodermis through modifying uptake from predominantly apoplastic to selective symplastic transport. The contribution of each tissue type is dependent on the molecular properties of the plamsalemma (i.e., specific membrane transport proteins) and on the metal in question (MacFarlane and Burchett 2000). Some halophytic genera such as Aegiceras and Avicennia secrete excessive Na+ and K+ through specialized glands or glandular trichomes on abaxial and adaxial leaf surfaces, while such specialized structures are absent in nonsecretors, for example, Rhizophora and Sonneratia (MacFarlane and Burchett 1999). Indeed mangroves and a number of other estuarine halophytes with glandular tissue are known to excrete heavy metals concomitantly with other solutes (MacFarlane and Burchett 2000) . The variation in morphology/function of nutritive root tissue and glandular tissue to deal with the challenges of excess cations in saline environments could become significant for metal accumulation, transport, partitioning, and excretion among halophytic plant species (MacFarlane et al. 2007).

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