Metal Threshold Level of Vetiver
The tolerance level of plants to various contaminants is a major factor that affects phytoremediation. The threshold level of C. zizanioides has been demonstrated by Truong (1999) and found to be as follows: Arsenic 0.02 – 7.5mg/L, Cadmium 0.2 – 9.0mg/L, Copper 0.5 – 8.0mg/L, Chromium 0.5 – 10.0mg/L, and Nickel 0.5 – 2.0mg/L. The removal rates for Copper ranges from 4.1 % to 36.7 %, 6.9 to 15.3 % for Zn, and 6.9 to 57 % for Pb (Chen, Shiyab, Han, Monts, and Waggoner, 2009).
2.8 Adaptive potentials of Vetiver
The adaptive response of Vetiver to contaminated environments as an efficient process is based on its many physiological, molecular, genetic and ecological traits. These traits give Vetiver the ability to survive and accumulate contaminants; these contaminants are taken up by the roots system and transported to the culms and leaves without showing toxicity syndrome (Sarma, 2011).
Some investigators (Roongtanakiat et al., 2003; Marcacci, 2004) noted that heavy metal accumulation was higher in roots more than the shoots of Vetiver. This occurs mainly when the plant is mature. It has been hypothesized that the concentration of contaminants in shoots decreased due to a possible effect of dilution caused by an ever increasing plant biomass (Roongtanakiat and Chairoj, 2001).
Hyperaccumulator plants are identified by their calculated translocation factor (Yanqun, Yuan, Jianjun, Haiyan, Li, and Schvartz, 2005). If the translocation factor is more than 1, then the plant is a hyperaccumulator, if less than 1, the plant is said to be a non – hyperaccumulator (Raskin et al., 1997). The translocation factor of Vetver has been found to be less than one thus; some authors have concluded that Vetiver is a non-hyperaccumulator (Truong, 1999; Greenfield, 2002; Roongtanakiat, 2006).
2.9 Anatomical Background of Vetiver
Plants that have potentials for phytoremediation must possess the following characteristics: rapid growth rate, high biomass, tolerance to extreme conditions and deep root system (Sarma, 2011). These characteristics emanated from their physiological and anatomical composition. The anatomy of Vetiver shows a thick cuticle layer of 1.5 to 2um in the leaves, with upper epidermis having a higher stomatal density than the lower epidermis (Chaudhry and Sarwar, 2006). The leaves of Vetiver are isofacial and hypostomatic (stomata evenly distributed and only on one side of the leaf) with paracytic stomata, accompanied by triangular cells and bodies of halteriform silica. A transverse sectional view of the leaves revealed a ‘‘V’’ shaped contour with symmetrical bundles joined by a ridge of central vascular bundles (Arcana and Rodriguez 2009). According to Laviana et al. (2004) transverse sections of matured Vetiver roots contain schizogenous aerenchymatous cortex suitable for water logged conditions and well developed phloem for essential oil accumulation. In Thailand, Khnema and Thammathaworn (2011) reported the presence of large intercellular spaces in matured leaves. They further opined that the good aeration system of Vetiver is made up of aerenchyma in root cortex and air cavities in the pith; which are similar to the adaptations of aquatic plants. This led to the suggestion that Vetiver avoids hypoxia and anoxia through the intercellular spaces in its tissues. They concluded that air could flow into Vetiver via pith cavity in culms by theories called humidity induced convection or ventury induced convection.
2.9.1 Stomatal Frequency
The stoma is an anatomical structure that plays a fundamental role in controlling the most important plant processes – photosynthesis and transpiration (Zarinkamar, 2006). A study of stomatal characters can be an important systematic tool used in inferring phylogeny (Simpson, 2011). It is also used in studying changes in climatic conditions such as atmospheric CO2 and light (Lake, Quick, Beerling and Woodward, 2000). The work done by McElwain and Chaloner (1995) provided evidence that stomatal frequency declines in response to increasing CO2. Metcalfe (1961) described three types of stomata found in monocots based on the arrangement of subsidiary cells: tetracytic – when the two guard cells are surrounded by four subsidiary cells in all four directions, paracytic – when the two guard cells are accompanied by two subsidiary cells each on one side, and anomocytic – when the guard cells are not associated with any subsidiary cell but surrounded by epidermal cells. In some monocots, stomatal frequency has been found to be highest in the middle of the leaf laminar (Salisbury, 1975), but highest towards the leaf tip in maize (Heickel, 1971). Zarinkamar (2006) reported the presence of paracytic stomata in Chrysopogon gryllus (L.) Trinius in Iran with a high stomatal density on the abaxial, and a longer adaxial guard-cell.
2.10 Other Plants Used in Phytoremediation
Approximately 0.2 % of all angiosperms are known to be hyperaccumulators, especially the members of the family Brassicaceae (Kramer, 2010). Plant selection is important for remediation of contaminated sites (Sarma, 2011). Xia, Liu, and Ao (2000) in a trial conducted to compare the effectiveness and tolerance to toxicity of three plants in treating landfill leachates, concluded that the overall ranking of the three species is: C. zizanioides > Potatum notatum L. (Bahia grass) >Eichhornia crassipes (Mart.) Solms. (water hyacinth). Other plants include: Thlaspi carulescence Presl. that absorbs zinc, lead, and cadmium (Kramer, 2010), Brassica napus that remediates cadmium (Selvam and Wong, 2008), Pistia stratiotes L. that accumulates silver, cadmium, Chromium, copper, mercury, nickel, lead and zinc (Odjegba and Fasidi, 2004), Arabidiopsis thaliana (L.) Heynth. absorbs zinc and cadmium, and Crotalaria juncea L. that is capable of absorbing nickel and chromium (Saraswat and Rai, 2009).
2.11 Mechanism of Up-take and Transport of Metals in Plants
The mechanism of hyperaccumulation in plants depend on certain processes such as solubilization, uptake and sequestration (accumulation) (Mukhopadhyay, and Maiti, 2010).
2.11.1 Solubilization of Metals from Soil Matrix
Since many metals occur in insoluble forms in the soil, plants have developed mechanisms in order to dissolve these metals from the soil. These methods involve the acidification of root area through proton pumps within the plasma membrane (VanHuysen, Terry and Pilon-Smits, 2004). Plants also release ligands which may chelate the metal while in the soil. This process may lead to the release of both essential and toxic metals into the root zone (Lasat, Baker and Kochian, 1996).
2.11.2 Up-take into the Root
According to Simpson (2011), soluble substances may enter the appoplast of roots through intercellular spaces, while others pass through the xylem. The apoplastic flow may not be as efficient as the vasculature of plant (Hall, 2002). When contaminants move through the xylem, an obstacle known as “casparian strip” is often encountered. Since the casparian strip is impermeable to solutes, membrane pump mechanisms may be employed to move contaminants upwards. Some contaminants may attempt to mimic nutrients and travel through the intended channels for essential nutrients (Hall, 2002).
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