Plant Adaptation and Phytoremediation
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Phytoremediation: Halophytes as Promising Heavy Metal Hyperaccumulators
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Your rating has been recorded. Write a review Rate this item: 1 2 3 4 5. Preview this item Preview this item. Plant Adaptation and Phytoremediation. Author: M. Ashraf, M. Ozturk, M. This book features an international set of authors and is a key reference for researchers and environmental managers, as well as anyone involved in the mining industry or landscape remediation. The book provides comprehensive coverage of current approaches to phytoremediation and begins by examining the problem. It looks at natural and human-induced toxins and their effects on natural vegetation as well as agricultural crops.
Particular attention is paid to the two largest challenges to remediation - heavy metals, and the salt stress that is impeding agricultural productivity worldwide. The text moves on to focus on the efficacy of different plant species in removing toxic pollutants from the environment. Along with analysis of a number of case studies, this section includes new and updated information on the mechanism of toxin-tolerance in plants. Read more Allow this favorite library to be seen by others Keep this favorite library private. Find a copy in the library Finding libraries that hold this item Abstract: The problems engendered by the conflicting imperatives of development and ecology show no sign of ending, and every day more locations are added to the list of landscapes poisoned by human activity.
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Consequently, the need for efficient remediation technologies becomes imperative. Phytoremediation is one of the most viable options in this regard. Hundreds of plants in laboratory experiments demonstrate the potential to remediate varying concentrations of heavy metals; however, the remediation capacity of most of these plants proved unsatisfactory under field conditions. The identification and selection of plants with higher metal uptake capacity or hyperaccumulators are one of the limitations of this technology. Additionally, the mechanism of heavy metal uptake by plants remains to be sufficiently documented.
The halophyte plants are famous for their adaptation to harsh environmental conditions, and hence could be the most suitable candidates for heavy metal hyperaccumulation. The state of Qatar in the Gulf region encompasses rich resources of halophytes that have the potential for future investment toward human and environmental health. This chapter, therefore, gives an overview of phytoremediation, with emphasis on halophytes as suitable heavy metal hyperaccumulators for improved remediation of heavy metal—contaminated areas.
Heavy Metals. Heavy metals and other organic compounds constitutes the major environmental contaminants, and the trials of phytoremediation to free pollutants from waste water and contaminated soil dates back to hundreds of years ago in plants such as the Thlaspi caerulescens and Viola calaminaria, which were reported to remediate high concentration of heavy metals [ 1 ].
Anthropogenic activities arising from industrialization largely contribute to the proliferation of these contaminants, either by direct leakage or accidents during transport of solid and liquid wastes from storage and industrial facilities [ 2 , 3 ]. Strategies to clean up environmental contaminants, both organic and inorganic are either by physical, chemical and or biological treatments [ 4 , 5 ].
However, physical and chemical methods are recognized for a number of disadvantages or limitations such as high cost and labor intensiveness.
Additionally, chemical processes create another pollution and are especially costly since they generate heaps of sludge [ 6 ]. In view of this context, new and better approaches to clean up of metal contamination were thought up and became imperative, hence the exploration of various bio-based techniques. The use of biological agents is considered cheap, safer and has limited or no negative impact to the environment [ 7 ]. Bio-based remediation methods include bio-augmentation, bioremediation, bioventing, composting and phytoremediation.
However, phytoremediation proves the most viable and useful alternative and has gain an increasing attention in recent times [ 8 , 9 ]. The adverse and negative effects associated with these elements make them targets for phytoremediation [ 10 ]. Phytoremediation offer several advantages. It is cheap, promotes biodiversity, reduces erosion, less destructive and decreased energy consumption leading to reduced carbon dioxide emission [ 11 ].
To date, about plant species were suggested to be metal hyper-accumulators [ 12 ]. However, few studies reported the toxicity of several metals combined [ 13 ], and while hyper-accumulation of nickel Ni , cadmium Cd , manganese Mn , zinc Zn and selenium Se have been well established, the same is yet be available or demonstrated beyond doubt in plant species for copper Cu , chromium Cr , lead Pb , thallium Th and cobalt Co metals. For instance, Cu is an important element for growth and general plant physiology, owing to its role as a cofactor to various types of enzymes involved in the transfer of electrons during metabolic processes, such as the tricarboxylic acid TCA cycle [ 14 , 15 ].
However, at high concentrations, it is toxic to plants signaled by stunted growth, and although there is some physiological insight to Cu stress in plants, the responses are still vague at the functional level [ 16 ]. The accumulation of heavy metals in plant tissues results in a wide range of negative effects on growth. Although it affects seed germination, growth of seedlings and photosynthetic processes, which generally leads to the inhibition of the plants important enzymatic activity [ 17 , 18 ], however, plants responds differently [ 19 ].
In dealing with the heavy metal stress, the root tissue is the first to be exposed to the associated toxins, and its cell wall has a mechanism of exchange that fixes the heavy metal ions, thereby limiting the transmission of the toxins to other plant tissues [ 20 , 21 ].
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Several studies reported many plants, including desert species as good phytoremediation agents, however, few are metal hyperaccumulators and their selection for efficient phytoremediation is still a challenge. This is demonstrated by slow growth, above ground biomass, root system and harvest [ 22 ].
Accordingly, successful heavy metal phytoremediation requirement of hyperaccumulation capacity in candidate plants position halophytes as suitable phytoremediators. This is due to their extensive stress tolerance mechanism, which enables them thrive in saline soil and in other desert conditions.
In simple terms, phytoremediation refers to a process where plants are employed to reduce or free up organic and inorganic contaminants from the environment [ 13 ] with the aid of associated microbes. The process by which contaminants are remediated differs; these may be in the form of removal, transfer, degradation and immobilization from either soil or water [ 23 ].
Plant Adaptation and Phytoremediation. (eBook, ) [gesasimosy.tk]
It is a unique approach capitalizing on plants roots ability for the initial uptake of pollutants, and eventually accumulating them onto the shoot tissue by translocation across the stem. Compared to other conventional treatment techniques, phytoremediation is new, with a great potential to providing the much-needed green technology solution to our deteriorating environment. To date, hundreds of plant species were suggested as potential phytoremediation agents [ 24 ].
During phytoremediation, plants growing on soil or water contaminated with trace or heavy metals could absorb or tolerate these elements differently, depending on the physiological means involved and the kinds of metals present [ 25 ]. According to Halder and Ghosh [ 26 ] phytoremediation techniques are categorized into five; phytoextraction, phytofiltration, phytovolatilization, phytostabilization and phytotransformation.
Phytoextraction is a technique of phytoremediation where plants take up metals by translocation, and accumulate them in a form that can be extracted on its tissue [ 27 ]. It is one of the most common types of phytoremediation and the names; phytoabsorption, phytoaccumulation and phytosequestration are often used interchangeably to refer to phytoextraction [ 28 ]. It is considered as the major phytoremediation technique among all others for the removal of metals from contaminated water, sediment and soil.
The efficiency of this remediation process depends on a number of factors from soil properties, metal bioavailability and speciation to the type of plant species. However, high concentration of absorbed metals usually ends up in the shoot biomass of the plant in harvestable form [ 12 ]. A number of recent studies reported various plant species that demonstrate phytoextraction strategy from both water and soil media [ 29 , 30 , 31 , 32 ].
Plants able to exhibit phytoextraction strategy in metal sequestration may potentially be hyper accumulators, referring to plants that consistently accumulate certain threshold of metal concentration in their shoot tissue, which varies according to the metals [ 22 ]. Generally, all hyper accumulators should possess characteristics such as high growth rate, widely branched shoot, high bioaccumulation and translocation capacity, high above ground biomass, easily cultivated and harvested [ 22 , 33 ].
However, Ali, Khan [ 28 ] demonstrated two methods or approaches for metal phytoextraction in different plants, one producing less above ground biomass but significantly accumulate metals in high concentration and vice versa in the other plant species, with final metal accumulation in agreement with those of hyper accumulators. Consequently, hyper accumulation is more important in phytoremediation than volume of biomass produced, and this suggest the use of hyper accumulators as more acceptable since it has advantages such as safe disposal, cheap process and easy handling [ 28 ].
Phytofiltration or rhizofiltration, as used interchangeably, refers to the absorption or adsorption of contaminants from surface wastewater by plant roots thereby preventing them from leaching to the underground water [ 34 ]. It is a type of phytoremediation technique that can be demonstrated in situ by directly growing plants in the polluted water body [ 24 ]. Although it is commonly applicable using aquatic plant species [ 35 ], there are suggestions that the process may be applied to terrestrial plants, which remediate metals to precipitate with the aid of microbes root bio filter [ 36 ].
Indeed, root exudates cause metal precipitation which alters the rhizosphere pH level [ 37 ]. Many terrestrial plants including grasses grown in a hydroponic culture were shown to effectively remove metals such via phytofiltration [ 38 ].
In the same study, Indian mustard was especially reported to accumulate higher fold of metal concentration far beyond the initial concentration, and the removal is by tissue specific adsorption mediated by root metal concentration. Quite a number of studies have shown many species of aquatic macrophytes that demonstrate phytofiltration potential. While experimenting for phytoremediation under different water conditions polluted with heavy metals, Liao and Chang [ 39 ] found that Eichhonia crassipes absorb and accumulates metal contaminants, it has also exhibit high growth rate and increased biomass production and thus considered a good phytofiltration agent.
This plant species absorb high concentrations of Pb, Ni, Zn and Cu which accumulates much higher in the root tissue than the shoot, suggesting the important role of fibrous and tap root system found in the plant, which is one of the key characteristics of potential phytofiltration agent. In a similar study, other aquatic plant species including Salvinia herzogii , E. Absorption of Cd in the root of all the plants relates to the added concentration. In another study by Thayaparan, Iqbal [ 41 ] also reported that Azolla pinnata have shown a great potential in the removal of high Pb concentration by phytofiltration from polluted water.
As in phytoextraction, potential phytofiltration agents should tolerate high metal concentration, exhibit fast and high growth rate as well as above ground biomass, however, in contrast to phytoextraction, they are expected to show limited translocation capacity of absorbed metals from root to shoot tissues [ 24 ].
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For efficient phytofiltration, this is an advantage over phytoextraction, since low translocation of contaminants means reduced contamination of other parts of the plant. In this technique, pollutants are converted into a less toxic or bioavailable form by the continuous precipitation of the plant rhizosphere. This is achieved either by surface run off prevention, erosion or leaching [ 27 ]. It is applicable in the stabilization of metals in contaminated soil, sediment or water environments, which ensures they are not transferred to the food chain from the soil by translocating to other parts of food crops or to the underground water.
Variation exists as to how prone a metal is to phytostabilization and is subject to its chemical character. This is evidenced in a comparative study to evaluate metal accumulation capacity of two aquatic macrophytes Phragmites australis and Typha domingensis , where both are found to stabilize As and Hg but inefficient in the phytostabilization of other metals [ 44 ].
Although phytostabilization offer some advantages over other phytoremediation techniques, it is however limited to temporary measure to deal with pollutants contamination owing to the fact that metals are only inactivated and their movement restricted, but still remains in the contaminated environmental compartment [ 45 ]. It is useful in emergencies, since it can rapidly immobilize pollutants from soil, water or sediment.
Equally important, it ensures that contaminants are not translocated to other plant tissues by trapping most of it in the plant root [ 46 ]. Considering the strategies employed in phytostabilization, plants that can appropriately fall under this mechanism is their ability to tolerate and immobilize metals and other contaminants, low translocation capacity from root to plant aerial parts and of course extensive and fibrous tap root system [ 7 ].
Among several studies that reported plants species with these characteristics [ 47 , 48 , 49 ] demonstrating the phytostabilization of Zn, Pb, Cu and Cd by different plants in soil and sediment polluted environments. Phytotransformation or phytodegradation is another technique of phytoremediation where contaminants and other nutrients are chemically modified through plant metabolism and render associated contaminants inactive in both plant root and shoot tissues [ 6 ]. Plant metabolic enzymes act on the surrounding contaminants, thereby transforming them to a less toxic form, plants rhizosphere microbes also aid in the transformation process of the compounds [ 50 ].
Although this mechanism is mostly against organic contaminants, inorganic compounds such as metals were also suggested, in which case a strategy akin to phytostabilization is employed to convert toxic metals to less toxic form [ 51 ]. However, this technique seem less efficient and reliable compared to others in that it requires longer period of time, strict soil characteristic such as depth and underground water availability and often require soil amendments. In phytovolatilization, contaminants are converted in to a volatile form and released to the air via plants leaves stomata [ 27 , 34 ].
However, this mechanism merely transfers contaminants from one environmental compartment to another, which may somehow return back to the original source soil by precipitation and hence could be less popular to other phytoremediation techniques especially phytoextraction and phytofiltration [ 34 , 52 ]. It is commonly employed when treating groups of highly volatile metals like Hg and As.
Phytovolatilization of As involves the conversion of elemental As to selenoaminoacids, such as selenomethione, which is modified by methylation to a volatile and less toxic form, dimethylselenide [ 53 ]. Several plants species are known to tolerate high concentration of toxic metals. Tolerant species are best described as excluders, where metal uptake and translocation to different tissue parts are limited. While others that are capable of accumulating higher concentrations with improved translocation from the root to shoot part of the plant, thereby significantly reducing its availability in the soil, and they do so with no visible sign of toxicity effects.
To date, heavy metals have no standard definition by recognized bodies in the area. Various researchers use different characteristics and levels in their description such as atomic mass and number, density, chemical character as well as their toxicity; however, there appears no connection between such properties [ 54 ]. According to Wang and Chen [ 55 ], three categories of heavy metals arising from both natural and artificial sources are of interest, these includes valuable metals e.
Ag, Au, Pd, Pt, harmful metals e.
The non-biodegradability and stable nature of heavy metals suggests increased exposure to living species including humans [ 54 ], periodic reviews of toxic metals effects are documented by many research groups [ 56 , 57 , 58 ]. When determining hyperaccumulators of toxic metal, the most important factor is the concentration of the metal ion threshold. Therefore, plants can be regarded as hyperaccumulators, when capable of accumulating toxic metals concentration to about 50 to times more than non-hyperaccumulator plants [ 13 , 59 ].
There is increasing interest in plant hyperaccumulators in recent times, owing to their potential use in metal contaminated soil and water detoxification [ 25 , 63 ]. The phytoremediation of heavy metals involve many physiological, biochemical and molecular activities. In this process, especially phytoextraction involves the accumulation and translocation of heavy metals to plant tissues. Plant metal chelators or phytochelatins PCs and metallothioneins MTs are the most common transporter proteins for heavy metal phytoremediation. MTs are cysteine rich proteins that are famous for metal binding and greatly assist in the process of sequestration of metals in ionic form [ 64 ].