Desmodium ramosissimum is a genus of approximately 400 species and a shrub/herb from the family of Fabaceae. It is widespread with its numerous economic importance across the regions of Senegal, Nigeria, and tropical Africa to Madagascar. Some of the species of Desmodium include: D. ramosissimum, D. intortum, D. uncinatum, D. triflorum, D. acanthocladum, D. adscendens, D. affine, D. canadense, D. canescens D. ciliare, D. concinnun, D. cuspidatun, D. dillenii, D. discolour, D. elegans, D.gangeticun, D. glabellum, D. glabrun, D. gyrans, D. heterocarpon, D. hookerianun, D. teres, and D. zonatum (Addy, 1992).
Heavy metals in soil primarily result from the weathering of parent materials and from human activities, which include mining, smelting, application of sludge, and discharge of wastewaters (Kabata- Pendias and Pendias, 2001).
Contamination of the soil environment by hydrocarbons is becoming prevalent across the globe. This is probably due to heavy dependency on petroleum as a major source of energy throughout the world, rapid industrialization, population growth, and complete disregard for environmental health (Kvenvolden and Cooper, 2003).
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The release of hydrocarbons into the environment, whether accidentally or due to human activities, is a major cause of water and soil pollution, (Holliger, et al., 1997). These several potentially harmful chemical compounds, derived from activities of urban centers, rural properties, and industries are constantly released into the terrestrial environment. In this context, the scientific community has shown increasing interest in the detection, knowledge, and control of environmental agents responsible for damages to human health and the sustainability of ecosystems.
Phytoremediation is considered to be an environmentally friendly technology that is a safe and also a cheap way to remove contaminants, in some cases doing the same as a group of engineers for one-tenth of the cost. However, such technology cannot necessarily be effective all of the time or be used in all types of contaminated sites. If the contamination runs too deep, or the concentration of toxic compounds is too high, then plants alone cannot effectively remediate the soil (Cunningham, et al., 1995).
The ideal plant to make practical use of the phytoremediation technique must necessarily have a considerable capacity of metal uptake, accumulation, and durability to reduce the length of treatment. Metal hyperaccumulator plants have been found in a wide range of families of vascular plants (Reeves and Baker, 2000; Prasad and Freitas 2003).
There are different categories of phytoremediation, including phytoextraction, phytofiltration, phytostabilization, and phytodegradation, depending on the mechanisms of remediation. Phytoextraction involves the use of plants to remove contaminants from the soil. The metal ion accumulated in aerial parts can be removed to dispose of or burnt to recover metals. Phytofiltration involves the plant’s root or seedling for the removal of metals from aqueous wastes. In phytostabilization, the plant roots absorb the pollutants from that soil and keep them in their rhizosphere rendering them harmless by preventing them from leaching. Phytovolatilization involves the use of plants to volatilize pollutants from their foliage such as Selenium (Se) and Mercury (Hg). Phytodegradation means the use of plants and associated microorganisms to degrade organic pollutants (Garbisu and Alkorta, 2001). Some plants may have one function whereas others can involve two or more functions of phytoremediation.
In the light of increasing concern over the threat posed by the accumulation of heavy metals in the environment, on human health, and natural flora and fauna, the present study on the bioremediation of toxic metals gain ample significance. Metals, being non-biodegradable, accumulate in the environment.
Contaminants such as Lead, Cadmium, Nickel, Chromium, etc, enter the environment through industrial waste and landfill runoff. The conventional technologies involve physical displacement or chemical replacement generating yet another problem of toxic sludge. The high cost of excavation and burial of the soil at hazardous waste sites was estimated at $ 1,000,000 per acre by Raskin, et al., (1997), also contributes to the use of plants (Phytoremediation) as a better alternative, and therefore, as higher plants can uptake heavy metals, phytoremediation has gained significant interest.
After doing a brief background study of D. ramosissimum, its agricultural aspects, which includes nitrogen fixation, spread root system, quick growth, and great biomass, the choice of the plant is ideal for phytoremediation and phytoindication.
Also, a careful assessment of available literature on Desmodium species has revealed that there is a scarcity of information on the species representing this family, especially that of Desmodium ramosissimum. It is noteworthy that the distribution, establishment, management, harvesting, and uses are not documented in Nigeria and other countries of origin.
Finally, it is also important to consider the safe disposal of legume edible parts after the phytoremediation process (i.e., roots, shoots, and seeds) since they could constitute an important route of Cd introduction in the food chain. For this reason as the available literature on D. ramosissimum considered it as a non-edible legume especially in this part of the country, it is therefore selectively justified as a tool for photo indication and phytoremediation.
The objectives of this study therefore are:
- Cultivating plants in control and contaminated soils.
- To properly investigate the enormous potential uses of D. ramosissimum plant for the future, especially as a phytoindicator and phytoremediator in polluted areas.
- To look at the possible accumulation of heavy metals in the vegetative parts of the plant.
- To examine the changes in the anatomy of the plant, and
- To evaluate the phytochemical composition of the plant.
2.1. Desmodium ramosissimum species- origin.
The genus Desmodium contains about 450 Species, distributed in eastern Asia, Mexico, Brazil, and some tropical African countries like Senegal, Madagascar, and Nigeria. Some species of Desmodium include D. acanthocladum, D. ramosissimum, D. affine, D. gyrans, D. adscendens, D. teres, D. triflorum, D. zonatum, D. intortum, D. uncinatum, and D. gangeticum . (Adewunmi and Sofowora, 1980).
Desmodium is a genus in the flowering plant family Fabaceae, sometimes called tick-trefoil, tick-clover, hitchhikers, or beggar lice. There are dozens of species and the delimitation of the genus has shifted much over time. These are mostly inconspicuous legumes; few have bright or large flowers. Though some can become sizeable plants, and most are herbs or small shrubs. (Adewunmi and Sofowora, 1980).
2.2. Taxonomy of the species D. ramosissimum.
Sub Kingdom : Tracheobionta (Vascular Plant)
Division: Magnoliophyta (Angiosperm)
Family: Fabaceae (Papilionoideae)
Sub Family: Faboideae
Sub Tribus: Desmodiineae
Species: Desmodium ramosissimum.
D. ramosissimum is known by several common names especially in West Africa and they include Nigeria (Hausa) Waken Zoomoo, in Igbo, it is generally called Oganana (Bean of the hare ), but in Awka, also in Igbo it is called Obi, and in Ubakala, it is called Agbirigba. In Sierra Leone, it is known in Koranko as donde, and in Loko as botugwe hokora.
D. ramosissimum among other species which may be runners is an erect, slender perennial herb, usually 0.3-2 m tall. It has several stems from a woody base with fine short hairs and long white hairs or sometimes densely covered with appressed stiff hairs. The leaves are 3- foliate with leaflets that are narrow obovate to oblong-elliptic, usually 2 – 4.8 cm long. The leaves have reticulate venation which is visible on both surfaces. The Petiole is 8 -15 mm long and the inflorescence is terminal and auxiliary, ranging from 4.5 – 20 cm long. The flowers are standard mauve or rose-colored, while the Fruit is 7-25 mm long. The leaves are green to pale green in color, depending on the variety. The leaves are alternate to sub-opposite, with three to five lobes. The plant produces fruit throughout the year, while multiple fruiting could also occur during a year on the same plant. The seeds are matured (1-3 months) when the capsule changes from the usual green color to brown. Their fruit is loments, meaning each seed is dispersed individually enclosed in its segment. This makes them tenacious plants and some species are considered weeds in places (Adewunmi and Sofowora, 1980).
2.4. Phytochemical constituents of Desmodium species
Desmodium plant extracts, as well as the active principles, have been experimentally studied for their anti-inflammatory, cytotoxic, antidiabetic, antinephrolithic, antibacterial, and nootropic activities in vitro or in vivo. So far, a total of 212 compounds have been isolated from 15 Desmodium species and were characterized mainly as flavonoids and alkaloids, followed by terpenoids, glycosides, and a number of volatile oils. The remaining unrevealed species are recorded chiefly in Asia and Africa, being used in empirical treatment for various diseases. (Addy and Awumey, 1984). Some Desmodium species have been shown to contain a high amount of tryptamine alkaloid though many tryptamine–containing Desmodium species have been transferred to other genera. (Adewunmi and Sofowora, 1980). Dimethyl tryptamine (DMT) and 5-methoxy–dimethyltryptamine (5-MeO-DMT) occur in all green parts of D. gangeticum, as well as the roots. D. triflorum roots contain DMT-N-oxide. The caterpillars of the lesser grass (Zizina otis) and the two-barred flasher (Astraptes fulgerator) feed on tick-trefoils. Deer also appear to rely on some species in certain areas, particularly during the more stressful summer months. Several Desmodium species contain potent secondary metabolites. They are used aggressively in agriculture in push-pull technology. Tick-trefoils produce high amounts of antixenotic allomone-chemicals which repel many insect pests as well as allelopathic compounds that kill weeds. For example, D. intortum and D. uncinatum are employed as ground cover in maize and sorghum fields to repel Chilopertellus, a stem-boring grass moth. They also suppress witch-weeds such as Asiatic witch-weed (Striga asiatica) and purple witch-weed (S. hermonthica) (Adewunmi and Sofowora, 1980).