14

Chapter - 2

Review of Literature

2.1. Introduction

Science of Ecology gives humans a holistic view of the phenomenon of

life; its unity, diversities, complexities, the different levels of its existence and

the major principles of its stable sustenance as the ‘biosphere’. This science

enables us to understand that nature shows no special concern for any

particular form of life unless and until a certain species proves to be a success

in keeping itself as a useful/integral part of its stable frame work. Though

humans are much different from all other species, they are no exception to the

limits of nature’s constitution. Understanding of these limits and using

technological interventions to keep themselves in these limits is the ecological

wisdom that will assure the sustainable progress and happy sustenance of

humans in the biosphere forever.

Humans are the only intelligent species capable of comprehending the

true complexities of the network of relationships in nature that sustain them as

part of the biosphere. Among various forms of life on earth, humans alone are

capable of modifying the environment for their own convenience to any extent.

However, unaware of the possibilities of true and sustainable progress by

following nature’s principles of sustenance, humans have already made

innumerable mistakes in the management of nature. Their materialistic quest

to conquer nature has caused massive destruction of land systems and water

resources, and consequently nature has lost its ecological balance to sustain

human kind within its structure of life.

The extensive deforestation, the revolutionary methods of agriculture,

the unscrupulous consumption of natural resources, the intensive industrial

activities and the criminal spreading of toxic wastes of diverse kinds have

been the major illecological activities of humans, all carried out in the name of

material progress, which ultimately leads to atavistic regrets instead of

15

sustainable progress. Modern humans find themselves in an unenviable

position. They are now bound to find out practical solutions to all sorts of

environmental degradations already made and to proceed cautiously, looking

forward to safeguarding the interests of their posterity as well. Different

alternative resources and various advanced technological means of quality

restorations are all in before us. But nature has already been destroyed to

such an extent in many regions of the earth that regaining the balance seems

quite impossible. However, ecologically sound technological advancements

provide real hope for humans to regain the balance of nature.

Phytoremediation is such a plant based eco-technology that enables humans

to decontaminate the environment from toxic wastes in a quite natural way at

a very cheap cost. Hyper accumulator plant species form the key resources

for this decontamination technology.

Roadsides are multiple contaminated places affected by diverse heavy

metals such as Pb, Zn, Cu and Ni. Vegetations on contaminated roadsides

contain highly tolerant species, some of which may accumulate toxic heavy

metals at high amounts. A systematic review of literature on general studies of

roadside pollutions, specific studies on heavy metal contamination on

roadsides, roadside vegetation studies, heavy metal accumulating plants of

roadside vegetations, development of various phytoremediation technologies,

hyper accumulator species on roadsides, and reports on the application of

tolerant and hyper accumulating species on roadsides for phytoremediation

programmes will enable us to comprehend the current problem of heavy metal

accumulations of roadside vegetation in relation to the degree of

contaminations on the roadsides.

2.2. Roadside Pollution and the Impacts

It is a common phenomenon that roads are continuously increasing at

a fast rate and roadsides cover very large areas of land in most countries.

Roadsides are line sources of pollution in all urban systems. The nature and

degree of roadside pollution has been an important subject of road ecological

studies during last few decades. Hemphill et al. (1974) found high Pb

accumulation in plants growing on roadsides contaminated from automobile

16

exhausts as well as other means. According to Hunter (1976) pollution of the

air and of the soil through roadside fallout is an increasing hazard to the

environment and to health. Harrison (1979 and 1981) reported excess of Pb,

Cd, Cr, Co, Cu, Ni and Zn on the roadside and other urban dusts. Muskett

and Jones (1981) observed that heavy metal pollutants inhibit soil microbial

activities.

It is well known that concentration of pollutants including heavy metals

such as Cd, Cu, Cr, Ni, Pb, and Zn are closer to roads than sites away from

roads (Ondov et al. 1982; Ndiokwere, 1984; Harrison and Johnston, 1985;

Monaci and Bargagli, 1997; Carlosena et al. 1998). Latimer et al. (1990)

analyzed the details of organo-toxic residues such as petroleum products,

other hydrocarbons and trace metals in the urban runoff samples from

different land areas and found that the toxins accumulate not only in roadside

soils but also in the vegetation and other urban environmental components.

Kumar et al. (1993) observed that blood Pb levels are high among populations

exposed to vehicular exhaust. Sinha (1993) made a detailed study of

automobile pollution and its human impacts. A major source of pollution in the

urban areas (60-70% of the pollution) is automobile exhausts, and pollution on

roadsides may be monitored using plants (Singh et al. 1995). It is a well

established fact that vehicular emissions are a major problem in the busy

centers of the city and heavy metals form one of the major toxic components

of the emissions (Pearson et al. 2000). Mandal and Mukherji (2001) observed

that roadside plants are exposed to high levels of oxides of nitrogen and sulfur

dioxide in automobile exhausts.

According to Joshi and Shrivastava (2003) roadside plants may be

used as bio-monitors for monitoring quality of air in polluted area.

Anthropogenic contributions of Pb to the urban environment have been

dominated by combustion of leaded gasoline (Sutherland et al. 2003).

Harrison et al. (2003) conducted a study of trace metals and polycyclic

aromatic hydrocarbons in the roadside environment and concluded that the

source of most trace metals in roadside aerosol is from vehicular wear

products rather than from exhaust emissions. Sharma and Pervez (2003)

17

observed that waste water irrigation is the source of heavy metal

accumulation in vegetables in certain urban environments, especially of the

poor.

Lenz et al. (2004) surveyed the contribution of road traffic to air

pollution and its effect on the environment. Hashisho and El-Fadel (2004)

compared the impacts of traffic-induced Pb emissions in an urban system and

found that the average Pb content in the system is 353 ppm, and the levels of

Pb in the blood of inhabitants were also relatively very high. Gokhale and Patil

(2004) observed that particulate matter below 10 µm is of great concern and

the particulates of that size form the major air pollutant in all the urban

centers. Ozaki et al. (2004a) investigated the different sources of heavy metal

pollution on roadsides and found that apart from the fuels and automobile

sources, there are other sources of heavy metals on roadsides such as road

paints and pavement materials. Ozaki et al. (2004b) assessed As, Sb, and Hg

distribution and pollution sources in the roadside soil and dust around and

reported that automobiles is responsible for the higher concentrations of

metals on roadsides. Raina and Aggarwal (2004) studied the effect of

vehicular exhaust on some of the micromorphological characteristics of leaves

of plants and observed that Ervatamia and Thevetia have some adaptive

measure to tolerate the pollutant to some extent as compared to other studied

plants.

Pb can be confirmed as the most distinctive heavy metal from road

traffic pollution, though levels of Zn, Cu and Cd in roadside soil can be also

connected with road traffic (Plesnicar and Zupancic, 2005). Indukumari et al.

(2005) determined the effect of automobile exhausts on various biochemical

parameters of the commonly occurring roadside plants such as Ficus

religiosa, Ricinus communis and Carica papaya and found that most of the

plants have the capacity to absorb Pb; the metal cause changes in the

biochemistry of these plants. Sheng-gao et al. (2005) measured the magnetic

properties and total contents of Cu, Cd, Pb and Fe in 30 automobile emission

particulate samples and found that the magnetic parameters can be used as

index of Pb and Cu pollution on roadsides. Ona et al. (2006) investigated the

18

levels of Pb in urban soils and reported that automobile exhausts are the

major source of environmental contamination by Pb. Rai and Kulshreshtha

(2006) investigated the effect of roadside particulates of automobile emission

on some common plants and found that significant modification in leaf surface

characteristics occur in them as an impact of the pollutants.

Juknevicius et al. (2007) investigated concentrations of Pb, Cd, Cu, Ni

and Zn in roadside soils and grasses on roadsides and found that the

accumulation of metals depends on distance from the roads. Li et al. (2007)

are of the view that traffic activities are increasingly becoming a great threat to

urban environmental quality and human health in many municipalities. Pereira

et al. (2007a) showed that highway run off is a potential source of heavy

metals polluting the near-shore sedimentary deposits. In urban areas vehicle

emissions and re-suspension of road side dusts becomes the most important

human made source of pollutants (Pereira et al. 2007b). Roadside pollution

originates mainly from vehicular sources (El-Hasan, 2007). Most of the

pollution of roadside soils and ground water originate from traffic emissions,

road pavements and other accessories of road traffic (Hall et al. 2008).

A general review of literature on roadside pollutions published during

the past few decades has thus revealed that studies on roadside pollution are

highly relevant to the knowledge base for better and healthy management of

urban systems in general. Moreover, pollution studies related to Indian

roadsides are quite few. It is also clear that heavy metals are one of the most

dreadful components of roadside pollutants that are a major threat to urban

life. Since the main thrust of the current research is the contaminations of the

environment by heavy metals in the urban South of our country a specific

survey of literature on heavy metal contamination of soils, plants and water in

the urban environment is very pertinent.

19

2.3. Heavy metal contamination in Urban Environment

2.3.1. Urban Environment Quality and Heavy Metals

Heavy metals form one of the basic toxins that affect urban

environment quality and the degree of pollution depends on the type and

amount of heavy metals present in various part of the urban environment.

Warren and Delavault (1962) first reported heavy metal contamination of soils

and vegetation near high ways. In the same year Cannon and Bowles (1962)

observed Tetraethyl Pb in plants growing along highways. According to Chow

(1970) Pb alkyl additives in petrol are responsible for the high concentrations

of Pb in vegetation near roads. Schuck and Locke (1970) studied Pb

accumulation in roadside crops Cauliflower, Tomatoes, Cabbage,

Strawberries and Valencia oranges.

A reading of the literature of the last 3-4 decades on heavy metal

contaminations in urban systems confirms that road traffic forms the major

reason for metal contamination in urban systems. An exhaustive list is given

hereunder: Cannon and Bowles (1962), Motto et al. (1970), Lagerwerff and

Specht (1970), Daines et al. (1970), Page and Ganje (1970), Page et al.

(1971), Davies and Holmes (1972), Vandenabeele and Wood (1972), Haney

et al. (1974), Graham and Kalman (1974), Ward et al. (1975), Day et al.

(1975), David and Williams (1975), Tjell et al. (1979), Whleer and Rolf, (1979),

Ho (1979), Ho and Tai (1979), Ward et al. (1979), Fergusson et al. (1980),

Flanagan et al. (1980), Agrawal et al. (1980), Agrawal et al. (1981), Garcia-

Mirgaya et al. (1981), Rodriguez-Flores and Rodriguez-Castellon (1982),

Ajayi and Kamson (1983), Collins (1984), Johnston and Harrison (1984),

Sahu and Warrier (1985), Albasel and Cottenie (1985), Leonzio and Pisani,

(1987), Warren and Birch (1987), Burguera et al. (1988), Madany (1990),

Swamy and Lokesh (1993), Piron-Frenet et al. (1994), Ferreti et al. (1995),

Singh et al. (1995), Hafen and Brinkmann (1996), Garcia et al. (1996),

Othman et al. (1997), Peter et al. (1997), Garcia et al. (1998), Xiong (1998),

Bhatia and Ghosh (1999), Jaradat and Momani (1999), Sutherland et al.

(2000), Al-Chalabi and Hawker (2000), Shams and Beg (2000), Arslan,

(2001a&b), Sayaka et al. (2002), Naszradi et al. (2002), Turer and Maynard

20

(2002), Oztas and Ata (2002), Turkoglu et al. (2003), Birch and Scollen

(2003), Fakayode and Olu-Owolabi (2003), Fatoki (2003), Divrikli et al. (2003),

Kylander et al. (2003), Kakulu (2003), Viard et al. (2004), Massadeh et al.

(2004), Ideriah et al. (2004), Wong and Li (2004), Hakan and Murat (2004),

Naszradi et al. (2004), Oncel et al. (2004), Howari et al.(2004), Lough et al.

(2005), Plesnicar and Zupancic (2005), Shaikh et al. (2005), Bretzel and

Calderisi (2006), Akbar et al. (2006), Ahmed and Ishiga, (2006), Wang et al.

(2006), Duzgoren-Aydin et al. (2006), Legret and Pagotto (2006), Lone et al.

(2006), Mazur et al. (2007), Yetimoglu et al. (2007), Juknevicius et al. (2007),

Voegborlo and Chirgawai (2007), Chu et al. (2008), Enayatzamir et al.

(2008), Malkoc et al. (2008), Shahar and Majid (2008), Bakirdere and Yaman

(2008), Yakupoglu et al. (2008) and Okunola et al (2008) reported that Pb and

certain other heavy metals such as Cu, Zn, Ni, Cd are common in soils or

plants or both, close to highways in urban or suburban environments. Many

observations also confirm that among the heavy metals on roadsides, the

amounts of Pb and Zn are always higher in soils or plants than other heavy

metals. Shahar and Majid (2008) noted that Pb is no more used as a fuel

additive, but still used as anti-wear agent in lubricants; Zn is an alternative

additive and Cu a major component of spark-plugs of vehicles all of which

ultimately may reach roadsides; Pb, of course, is added currently to roadsides

only in small quantities.

Some have reported that heavy metals such as Cu, Cd and Ni are

higher on roadsides than other urban places. However, Olajire and Ayodele

(1997) observed no significant difference between the mean concentrations of

these metals in the high and low traffic density areas. Soylak et al. (1999)

showed that the levels of Fe and Mn in the soil samples are independent of

the distance from the motorway while the Pb, Ni, Zn, Cu and Cd contents in

urban soils depend on nearness to roads and traffic densities.

Monaci and Bargagli (1997) recognized Ba as a valuable tracer for

vehicle emissions in the place of Pb. Monaci et al. (2000) reported that Ba and

Zn are valid tracers of automotive traffic emissions instead of Pb. Mashi et al.

(2004) concluded that grazing and cultivation practices have in general

21

caused some significant elevations in the bio-available levels of Zn, Fe, Mn,

and Cu in soils. Murray et al. (2004) confirmed elevated concentrations of Ba,

Cd, Cr, Cu, Ni, and Zn in urban surface soils. Zarcinas et al. (2004a & b)

found As, Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn in soils and plants of urban

areas; positive correlations between plant and soil content were observed for

Pb in all plants, but for other metals in certain plants only. Lin et al. (2005)

attempted to characterize heavy metals in nano, ultrafine, fine and coarse

particles collected besides a heavily trafficked road. Zaidi et al. (2005)

reported significantly high amount of Zn, Mn, Pb and Cd on leaves of trees

growing on roadsides.

This general review of literature on heavy metals in roadsides and

other regions of urban environments reveal that many of the toxic metals in

roadside environment are part of different components of the traffic system

such as automobile exhausts, wear of tyre and other vehicle parts such as

breaks, and also of road pavements. However, in their writings different

authors have expressed diverse views on the major heavy metal pollutant as

well as on the specific sources of each type of heavy metals. Therefore, a

specific inquiry into reports on the major heavy metal pollutants and their

sources in the roadside and other urban environment has been carried out.

2.3.2. Major Heavy Metals and their Sources

Authors in general agree that Pb, Zn, Cu and Ni are the major heavy

metals on roadsides. Burton and John (1977) reported that Pb, Cd, Cu and Ni

are the major heavy metal pollutants in urban environment and the primary

source of all of them is automobiles. Linton et al. (1980) confirmed that

automobile exhausts are the major source of Pb particles in urban dusts.

Dueck et al. (1987) observed that the use of heavy metal contaminated

sinters from a Zn smelter for paving roads was responsible for increased

amount of the metal in the soils. Zupancic (1999) confirmed heavy traffic as

the main source of Pb contamination in urban soils. Legret and Pagotto

(1999) monitored heavy metals such as Pb, Cu, Cd and Zn in roadside

pavement runoff and observed a reduction in Pb due to use of unleaded fuels

in vehicles. Industries also substantially contribute to heavy metal

22

contaminations in urban centers (Gargava and Aggarwal, 1999). Specific

studies on the sources of heavy metals in roadside soils are comparatively a

recent thrust area in urban pollution research. Muschack (1990) observed tyre

wear as a source of Zn, Pb, Cr and Ni in urban roadsides, and wear of break

linings as another source of Ni, Cr and Pb in urban roadsides. Viklnader

(1998) found wear of studded tyres as the source of Fe, Ni, Mo, Cr, Co, Cd, Ti

and Cu whereas corrosion of bushings, break wires, and radiators also as the

source of Cu, Fe, Ni, Cr and Co. According to Kleeman et al. (2000)

particulate matter emissions from motor vehicles are among the major

contributors to fine particle concentrations in the urban atmosphere. Root

(2000) found Pb loading of urban roadside soils also results from the

pulverization of motor vehicle Pb wheel weights and in the absence of leaded

gasoline, wheel weights are potentially a major source of urban Pb exposure.

Turer et al. (2001) showed that most of the Pb in urban soils has its

source from exhausts of vehicles, which once used leaded gasoline; 40% of

that Pb is still retained in soils. According to the author, restriction in the use

of Pb additives in gasoline is the easiest measure to reduce Pb in urban

environment. Momani et al. (2002) found the reason of high levels of Pb, Cd,

Cu, Zn, Al, Cr and Fe in street dust, soil, and in plants near a Petroleum

refinery, which according to the author is the out come of high traffic density.

Parekh et al. (2002) reported that the current Pb emission from vehicular

traffic into the atmosphere is 391 metric tons a year. Adekola et al. (2002)

concluded that Pb, V and As are metals reaching urban road sediments from

automobile exhausts, whereas Fe, Ni, Cr, Sb, and Zn are emissions from

wear and tear of vehicle parts; other sources include battery charging, vehicle

repairs, iron-bending, painting and panel beating. Sternbeck et al. (2002)

found that the major source of Cu on roadsides is the wear of break pads; Ba

and Sb, are also found originating from break pads. Lu et al. (2003)

suggested that the emission from vehicles may be the main source of Pb

contamination in urban soils and observed the mobility and bioavailability of

heavy metals in urban soils in the order of Pb>Zn>Cu>Cr. Adachi and

Tainosho (2004) reported that tyre dust is the major source of Zn in the urban

environment and other sources of heavy metals include brake lining and road

23

paint. Krishna and Govil (2004, 2005) observed significantly higher than the

normal amount of metals in soils around industrial areas and found the urban

heavy metal sources mainly as industries. Mbila and Thompson (2004) found

mine spoil as the main source of Zn and Pb in certain urban systems.

Snowdon and Birch (2004) evaluated the distribution and sources of heavy

metals such as Cu, Pb and Zn in surface soils of a highly urbanized

environment and suggested sources of these metals as roads, railway lines,

Pb painted houses and also industries.

Lee et al. (2005) found that Pb in urban environment is mainly derived

from industrial sources and rather than from leaded gasoline and the order of

mobility of heavy metals in urban systems is in the order of

Zn>Ni>Cd>Pb>Cu>Cr. Jaradat et al. (2005) observed the major sources of

heavy metals in urban systems as automobile exhausts, metal scrap yards

and also tyre incinerators. According to Kamavistar et al. (2005) in developing

countries like India, 90% of all Pb emissions into the atmosphere are due to

the use of leaded gasoline. Blok (2005) gave detailed accounts of Zn

emissions along roads originating from wearing of tires, corrosion of safety

fences and other traffic related sources. Turer (2005) explained the presence

of high amount of Pb, Zn, Cu, Cr, Sb, Ba and Ra in soils along roadways as

an example of non-point contamination from automobile exhausts and wear of

vehicle parts, quite different from point sources. Chung and Lee (2006)

reported Fe as the major non point source of contaminant followed by Zn and

Cu on roadsides.

According to Momani, (2006) leaded gasoline is the major source of

the elevated Pb levels in street dust. Ona et al. (2006) observed Pb as the

major source of emissions from vehicles. Savoskul and Drechsel (2006) found

that metallurgical industry seems to contribute substantially to heavy metal

burden of the soil in industrial zones. Al-Khashman (2007) investigated the

concentrations of Fe, Cu, Cd, Pb, Zn and Ni in the street dust samples and

according to this author all these metals reach urban environment from

automobile related sources; Zn in the street dusts is derived from traffic

sources, especially vehicle tires. According to Suzuki et al. (2009) Zn

24

emission is mainly from the abrasion of tires, Cr reaches urban environment

mainly from wear of asphalt pavements and Pb loading of urban streets by

motor vehicle wheel weight is continuous, and is potentially a major source of

human Pb exposure.

This specific survey of literature on the major sources of different heavy

metals on roadsides reveals that automobiles form the major sources of

heavy metal contaminations, especially of Pb, in urban environment.

Therefore, specific review of important publications related to the amount of

various heavy metals in urban environment has been carried out.

2.3.3. Quantitative Assessment of Heavy Metals in Urban Environments

Pb is one of the most thoroughly investigated of roadside pollutants;

however, there are plenty of reports of other heavy metals such as Zn, Cu, Ni

and Cd, in the literature. Smith (1975) observed that vehicular emission of Pb

is approximately 80mg km-1 in the atmosphere in the form of inorganic salts

ranging in size from 1 to 5 m μ, and the metal content of roadside atmosphere

may be 2-20 times higher than that of non-roadside atmosphere. Havre and

Underdal (1976) found significant correlation between motor vehicle traffic

density and the Pb concentration in above ground parts of vegetation. Little

and Wiffen (1977) calculated that less than 10% of the Pb emitted from the

motorway is deposited within 30 meter distance from the roadside. Duggan

and Williams (1977) assessed the risk from the Pb content of street dust and

found that children are the most vulnerable group. Farmer and Cross (1978)

suggested elevated Br concentration in soil as a useful qualitative indicator of

Pb pollution from automobile exhaust.

Hamilton et al. (1984) observed high monthly variations in the amounts

of Pb, Cd, Zn and Ni on roadsides (0-5 cm soil layer) and the metal in the

highest quantity is Pb at about 765 to 3276 ppm. Yassoglou et al. (1987) also

observed Pb (259 ppm) as the element in the highest quantity on roadsides

followed by Zn (241 ppm) and Ni (103 ppm). Wong (1987) published a review

on different aspects of Pb contamination of urban zones. Fergusson (1987)

found varied amounts of different heavy metals such as Pb, Cu, Ni and Zn in

25

roadside soils; Pb as the element in the highest quantity of 6310 ppm. Nriagu

and Pacyna (1988) demonstrated that human activities now have major

impacts on the global and regional cycles of most of the trace elements.

According to Hewitt and Candy (1990), heavy metals, especially Pb in soil and

street dust in cities, are usually higher than the estimated regional background

concentration of 9 ppm. Many of the heavy metals in the quantities currently in

the urban environment behave as significant pollutants of ecosystems

throughout the world (Alloway, 1990).

According to Onasanya et al. (1993), Pb in busy urban environment,

varying from 165 to 312 ppm, is basically the contribution of roadsides. Sithole

et al. (1993) observed Pb levels as 1480 ppm to 23.3 ppm in soil and

vegetation respectively of urban environment. Tripathi et al. (1993) also

studied the atmospheric deposition of trace metals like Pb, Cd, Cu, and Zn at

certain urban centers and observed these elements in varying quantities.

Chon et al. (1995) and Asami et al. (1995) found high amount of heavy metals

such as Cd, Zn, Pb and Cu in polluted urban soils. Zn was found to be the

metal in the highest quantity by both these authors. Carlosena et al. (1998)

observed Pb as the most dominant heavy metal in soils, whereas Xiong

(1998) observed Zn as the major heavy metal in urban centers. Charlesworth

and Lees (1999) investigated the distribution of heavy metals in urban dusts

and sediments and found Zn as the metal in the highest amount at the range

of 659 to 8000 ppm in different environmental components. Yousufzai et al.

(2001) found that heavy metals are being generated by automobile exhaust in

the urban areas. Among all the heavy metals in urban environment, Pb is the

element in the highest quantity at the range of 250 to 1155 ppm. Nowack et

al. (2001) observed high levels of Pb and Zn contents in remote Alpine soils,

and according to them, 75% of the metals in such remote areas are also of

anthropogenic in origin.

Li et al. (2001) and Linde et al. (2001) observed Zn or Pb as the most

abundant heavy metal in various components of urban environment, along

with other heavy metals such as Pb, Cd, Cu, Cr, Hg and Ni. Malawska and

Wilkomirski (2001) also found high contamination of urban soils with heavy

26

metals and polycyclic aromatic compounds; Zn was found in the highest

amount (45 to 1244 ppm). Onianwa et al. (2001) found excess accumulation

of heavy metals such as Pb, Zn, Cd, Cu, Cr, Co and Ni in top soils in the

vicinities of auto repair workshops, gas stations and motor parks. Here also

Zn was always found in the higest amount (200 to 720 ppm). Markus and

McBratney (2001) reviewed the multitude of data on Pb contaminations such

as its accumulation in agriculture, urban and industrial areas, as well as nation

wide surveys on soil Pb concentrations. The range of Pb content in soils

observed is 4.7 to 1275 ppm. Swaileh et al. (2001) determined

concentrations of Cu, Cd, Pb, and Zn in roadside soils, plants and land snails

samples and found that average concentration (mg g-1) of the four metals in

urban soil samples are Pb 149.9, Cu 23.8, Zn 128.3 and Cd 0.45. Smolders

and Degryse (2002) found that 10-40% of the Zn from tyre debris is

isotopically exchangeable in soil and that a significant fraction of Zn is

released from the rubber matrix within one year, but the parallel increase in

soil pH limits the mobilization of Zn in it. Sisman et al. (2002) examined Pb

and Ni in soil samples from a highway side, and found that Ni accumulations

(0.0 to 58.67ppm) are lower than the world standard, but Pb accumulations

(0.0 to 47.54 ppm) are above acceptable limit values in roadsides.

Tuzen (2003) found Pb, Ni, Cd, Zn, Mn, Cu, Co, and Cr at the levels of

149, 65, 3.0, 63, 285, 29, 20, and 30 µg g-1 respectively in roadside soils and

the concentrations of heavy metals in the samples vary in accordance with the

traffic density. Romic and Romic (2003) examined the influence of different

metals in urban and industrialized environment on top soils of cultivated fields

and observed Mn as the metal of the highest quantity in the place (average of

613 ppm). Okonkwo and Maribe (2004) examined the quantity of Pb in

different environmental media such as soil, grass leaves, water, ceramics,

pencil, paint, crayons and cosmetics and the soil content of Pb was found to

be 205 to 273 ppm.

Kumar et al. (2005) compared Pb, Zn, Cd, and Cu levels of metals in

roadside soils and observed Zn as the element in the highest quantity (19.5 to

23.6 ppm), followed by Cu (19.0 to 23.4 ppm) and Pb (15.8 to 18.9 ppm); the

27

world average of these metals as per the author is Zn (75 ppm), Cd (0.11

ppm), Pb (14 ppm), Cu (50 ppm). Krishna and Govil (2005) observed that

soils in industrial areas are enriched with Cu, Cr, Co, Ni, and Zn and the metal

in the highest quantity is Cr (average 521.3 ppm) followed by Zn ( average

191.3 ppm) and Cu (104.6 ppm). Ruiz-Cortes et al. (2005) assessed the

possible influence of land uses on their metal content and their relationship

with several soil properties; the maximum acceptable limit for in various

countries for Pb (80 to 500 ppm), Zn (150 to 500 ppm), Ni (35 to 500 ppm)

and Cu (63 to 100 ppm) are also given. Lee et al. (2006) demonstrated that

urban and suburban soils are highly enriched with metals such Pb, Cu, and

Zn.

Singh and Singh (2006) reported that petrol-lead phase-out effort in

India has reduced Pb concentrations in the various environmental

components after 2000 and observed a reduction in Pb content of natural river

waters from 18 ppm in 1998 to 3.1 ppm in 2001; the Pb on tree leaves got

reduced from 18. 1ppm in 1994 to 12.1 ppm in 2001. Krishna and Govil

(2007) observed high distribution of heavy metals such as Ba, Cu, Cr, Co, Sr,

V and Zn in the soils of urban industrial areas; the element in the highest

amount was Ba (471.7 ppm) followed by V (380.6 ppm) and Sr (317.9).

Krishna and Govil (2008) used concentrations of heavy metals such as As,

Ba, Co, Cr, Cu, Ni, Mo, Pb, Sr, V, and Zn in soils to understand metal

contamination due to industrialization and urbanization; the element observed

in highest quantity in the urbanized industrial environment is Cr (418 ppm)

followed by Cu (372 ppm) and Zn (213.6 ppm).

A critical analysis of literature on contaminations of specific metals

such as Pb and others in urban environment has showed that Pb, Zn, Cu and

Ni are the major heavy metals due to road traffic; other metals such as Cr or V

may be present in high amounts in industrial environments or industrialized

urban centers. Zn or Pb is the element in the highest quantity in the usual

urban centers. It has become also clear that one of the major impacts of metal

contaminations on roadsides is its bio accumulation in plants and other

organisms. The current research mainly aims at revealing the interrelationship

28

between vegetation and heavy metals on roadsides. Therefore, a specific

review of publications on heavy metal accumulation in plants also is

imperative.

2.3.4. Heavy Metal Accumulation in Plants and other Organisms

Plants in contaminated roadsides are exposed to heavy metals both in

the aerial and underground environments. Therefore, the impact of heavy

metals in contaminated environment is more direct on plants than on other

organisms. Plant species differ in their strategies to resist pollutions,

especially of heavy metals. Many have mechanisms to exclude the toxic

metals, but many others absorb and accumulate them in their tissues as a

measure of adaptation and tolerance or as a measure of competition and

survival in the disturbed environment. Buchauer (1973) reported that metal

aerosols may enter leaves directly through open stomata. Bazzaz et al. (1974)

observed that Pb, Cd, Ni, Ti and Zn in the environment affect photosynthetic

efficiency of plants. It has also become evident in the period that heavy metal

accumulation in plants depends on the age of plants as well as the distance

from the roadside (Stanford et al. 1975). Ward et al. (1977) determined Cd,

Cr, Cu, Pb, Ni and Zn in soils and pasture species along major motorways.

The highest accumulation has been found on white clover, whereas the

lowest on Paspalum. Koeppe (1977) examined the impact of heavy metals

such as Cd and Pb in agronomic species and found that Cd is more toxic to

Corn and Soybeans than is Pb.

Dutta and Mookerjee (1980) noted health hazards of heavy metal

contaminated soils to humans and domestic animals due to uptake of metals

by agricultural crops. According to them, the accumulation of metals by

grasses is particularly serious when considering the potential toxicity. Wade et

al. (1980) assessed the roadside gradients of Pb and Zn concentration in

surface dwelling invertebrates. Dutta and Mookerjee (1981) accounted a good

correlation between traffic volumes and total extractable soil Pb and Pb

content of roots and shoots of the grass Cynodon dactylon. Agrawal et al.

(1981) observed similar accumulation of Pb from automobile exhausts in soils

and trees growing along busy roads and found that plants close to roads have

29

higher content of the metals than those that grow away from roads. According

to Ratcliff and Beeby (1984) many environmental and plant specific

characteristics including season and duration of exposure are responsible for

the amount of Pb accumulation in plants. Bacso et al. (1984) found that Pb in

Rye grass is proportional to the level of public road traffic.

Albasel and Cottenie (1985) investigated Zn, Cu, Mn and Pb contents

in samples of soil and grass collected along highways and in fields adjoining

industrial zones. Krishnayya and Bedi (1986) carried out a study on the effect

of automobile Pb pollution on pollen germination and seed viability of roadside

weeds Cassia tora and Cassia occidentalis. Wong and Lau (1985) carried out

an ecological survey of roadside vegetation and observed Cynodon dactylon

and Eleusine indica as the two most dominant species on roadsides. Tam et

al (1987) observed that metal contamination of Cd, Pb, Ni, Zn, Cu, Mn and Fe

in leaves of Bauhina variegate, growing near urban roadsides are mainly due

to aerial deposition from motor vehicles. Landolt et al. (1989) showed that

needle analysis Picea abies is valid for the identification of various air

pollutants in industrial regions.

Tumi et al. (1990) investigated Pb contamination levels in roadside

vegetation and found accumulation of the metal in many plants. Jones (1991)

reviewed the long term changes in the contaminants of soils and crops.

Farsam and Zand (1991) carried out a preliminary study on the Pb deposition

on plant leaves and showed that contamination of plant leaves with Pb

aerosol in cities is mainly a surface deposition and is due to the motor

vehicles and low rainfall. Authors like Gratani et al. (1992) and Igwegbe et al.

(1992) evaluated Pb contamination from automotive exhausts in crops near

roadsides and found significantly higher concentration of Pb in all the crops

examined. According to Kumar et al. (1993) people differentially exposed to

vehicular exhaust contain high amount of Pb content in their blood, which may

exceed the tolerable limits. Taylor et al. (1993) experimentally showed that,

compared to many grass species, Cynodon dactylon is highly tolerant to

heavy metal contamination in soils. Tolgyessy et al. (1993) observed

accumulations of Pb, Cu, Ni, and Zn contents in Taraxacum officinale growing

30

near the highways. The heavy metal concentrations vary between different

plant species and different plant parts (Ylaranta, 1994). Chukwuma (1995)

observed that phytoavailability of trace elements like Cu, Mn, Ni, and Zn in

crops such as cassava and guinea grass generally corresponds to the soil

concentrations of the metals.

Von Schirnding and Fuggle (1996) observed 0.5 to over 2.0 µg per m3

Pb in the blood of people living in an urban area, and found that the variations

of Pb in blood is proportional to traffic density and the distance from heavily

trafficked roads. Post and Beeby (1996) observed that metal contamination

from traffic sources affect soil microbial community and as a result, the rates

of microbial decomposition is reduced in contaminated soils. Alfani et al.

(1996) examined amounts of Pb, Cu, Fe and Mn in the leaves of Quercus ilex

in urban areas in relation to those in surface soils and found high metal

deposition on trees close to roadsides. Fatoki (1997) examined the amount of

Zn and Cu in 180 grasses in relation to traffic densities. Andres (1997)

compared Cu and Zn on leaf litter of different tree species of roadsides and

showed that traffic density has greater influences on the concentration of Cu

than Zn. Peter et al. (1997) observed that metal dispersion in soils and plants

alongside roads depends on the sites. Singh et al. (1997) found that Pb

accumulation in roadside plants is more in roots than in shoots. Francek

(1997) observed Pb accumulation in the range of 90-210 ppm in fruit plants

close to roadsides. Thomas and Fernandez (1997) measured high levels of

heavy metals such as Fe, Cu, Zn and Pb in the mangrove flora and sediments

of mangrove habitats close to urban centers in the Kerala coasts. Fazeli et al.

(1998) observed less of heavy metals such as Pb, Cu, Zn, Co, Cr, and Ni in

grains compared to shoots and roots of paddy growing in contaminated soils.

Lichens on roadsides also accumulate Pb and the concentration of the metal

in lichens decreases with increasing distance from the source of pollution and

the amount of accumulation vary according to species (Dubey et al. 1999).

Clijsters et al. (1999) observed inhibition in the rate of photosynthesis in

higher plants growing on heavy metal contaminated soils. Kashem and Singh

(1999) reported positive correlation between the amounts of heavy metals

such as Cd, Cu, Mn, Ni, Pb and Zn in soils close to certain industries and the

31

plants growing there. Among the plants they examined, only grasses

accumulate metals at levels that exceed toxic limits. Lam et al. (1999)

observed decrease in Pb in vegetations with distance from roadsides, Pb

content of grasses increase with traffic density and the deposition of Pb in

plants is strongly dependent on type of the species of the roadside

vegetations. According to them, reduction in emissions will reduce

environmental damage but the damage due to Pb that is already in the

environment is underestimated. Dimopoulos et al. (1999) proposed a model

for the estimation of Pb concentration in urban environment using a grass

species, Cynodon dactylon.

Yousufzai et al. (2001) reported that almost all the pollutants emitted

from automobile exhaust occur in roadside plants. Heavy metals such as Cd,

Pb, Cu, Zn, Ni, Cr and Mn are common in topsoil and vegetation on roadsides

(Fakayode and Onianwa, 2002). Nagaraju and Karimulla (2002) examined

accumulation of heavy metals in three dominant plant species, Jatropha

curcas, Dodonea viscose and Cassia auriculata in relation to the amount of

the metals in associated soils and found that the different organs of a species

exhibit different behaviour with respect to elemental concentrations and their

mobile nature. Krolak (2003) examined Zn, Cu, Pb, and Cd accumulation in

Taraxacum officinale and observed a reduction of Zn and Cu accumulation in

the plant along with an increase of the concentration of these elements. Luilo

and Othman (2003) observed accumulation of Pb and Zn in grasses on

roadsides and recommended that pasture grasses in road vicinities must not

be used for foraging dairy cattle and goats for public health reasons. Dongarra

et al. (2003) reported that joint application of biomonitoring using species such

as Nerium oleander and analysis of road dust provide important insights to

assess the presence of anthropogenic elements, their sources and the

distribution patterns of atmospheric particulate matter in urban systems.

Bhargava et al. (2003) revealed that automobile exhausts affect roadside

sugarcane crop. According to Ghosh and Singh (2003) Ipomea cranea at 200

ppm Pb shows a bio concentration factor of 0.45 times, but produces 13.5

times more biomass than Brassica juncea, a known hyper accumulator plant.

Parkpian et al. (2003) observed the strong correlation between available Pb

32

and Cd concentrations and plant content of the metals, in a land close to a

highway. Swaileh et al. (2004) analyzed the amount of heavy metals such as

Pb, Cd, Cu, Zn, Fe, Mn, Ni and Cr in roadside surface soil and a common

roadside perennial herb Inula viscose, and found that much of the Pb and Fe

are present on plants as aerial deposition. According to them, this plant may

be used as a bio-monitor of metals on roadsides.

Finster et al. (2004) observed that Pb is primarily localized in roots

followed by a decreasing gradient of concentration up the plant shoot in many

of the edible vegetables, fruits and herbs grown in contaminated residential

soils. Aydinalp and Marinova (2004) observed positive correlations between

Pb deposits on leaves of roadside plants in urban centers and traffic density.

Deo and Deo (2004) studied the accumulation of heavy metals such as Cu,

Fe, Al, and Cr in vegetation on polluted soils, which revealed that maximum

concentrations of metals are in the leafy part of trees, shrubs and herbs

studied. Maisto et al. (2004) assessed the concentration of Cd, Cr, Cu, and

Pb in the soils and leaves of Quercus ilex growing in the contaminated soils

and found that Cu and Pb concentrations are greater in the leaves in

correlation with those in soils.

Odiyo et al. (2005) observed a linear correlation between soil and the

vegetation content of Zn, Cu, Cr, Pb, Cd, Fe, Pt and Pd. Mashi et al. (2005)

examined the amount of heavy metals such as Cu, Mn, Fe, Cd, Pb and Zn in

street dusts and its flow through the food chain from soils, which result in

health hazards. Taranath et al. (2005) examined the amount of Pb, Zn, Cu,

Mn, Ni, Cr and Cd contents in roadside grass, Cynodon dactylon, and showed

that both soil and grass contained elevated levels of the metals examined.

According to them, the increased circulation of toxic metals in soils and

grasses may result in the inevitable buildup of such toxins in food chains. Al

Jassir et al. (2005) observed high levels of Pb, Cd, Cu and Zn on green leafy

vegetables sold on roadsides, but the levels were not enough to cause

immediate health hazards. Awofolu (2005) observed the presence of Cd, Cu,

Pb and Zn in soils, grasses and different kinds of animal samples and

concluded that biomagnification of these metals in higher levels in the food

33

chain is highly possible. Bako et al. (2005) oberved heavy metal content of

some Savanna plant species in relation to air pollution and reported that

heavy metal contents of plants vary between rainy and dry seasons. Eboh

and Thomas (2005) found high amount of heavy metals (As, Cd, Cr, Fe, Ni

and Pb) in the leaf and seeds of Cannabis and the amount vary in accordance

with the area of cultivation. According to Jin et al. (2005), apart from non-

edaphic factors, automobile activity in the plantation area contributes to Pb

accumulation in tea leaves. Okoronkwo et al. (2005) explained the risk and

health implications of heavy metal contaminations of crops grown in polluted

soils. Singh et al. (2005) monitored Pb accumulation on leaves of Dalbergia

sisso and found that Pb concentrations were low in spring season, increased

in monsoon to winter seasons and the range is 2.1 to 28.2ug g-1 (dry wt.).

These authors observed a reduction in Pb in roadside dusts in urban centers

in India after the introduction of unleaded fuels. Gundi et al. (2005) observed

the medicinal plant Scoparia dulcis as a poor indicator of heavy metal

contamination on roadsides as there is no significant variation in the metal

content of the plant from different areas. The high metal content of the plant,

according to them, is due to its specific physiology. Wu et al. (2005) observed

that biomass production of weed species is not affected by Pb in the polluted

soil and total Pb accumulation increases with enhancing biomass of weed

communities.

According to Khatik et al. (2006) excess of Pb in soils causes stunted

growth, chlorosis, blackening of the root systems, and a sharp decline of

productivity in plants. Peng et al. (2006) observed that wild plants at different

contaminated sites accumulate high amount of heavy metals and the average

amount of Cd, Pb, Zn and Cu in plants is 19, 81, 637 and 8 ppm respectively.

According to Baycu et al. (2006) ecophysiological changes in the urban trees

may be used as biomarkers of heavy metal stress. Swaileh et al. (2006) found

that age and prevaling wind influence metal concentrations in leaves of plants

growing near roadsides. Champanerkar et al. (2006) observed that, in

Cynodon dactylon, geographical regions of collections do not affect heavy

metal content, and hence, the high levels of heavy metals in it can be related

to the physiology of the plant. Dong-Sheng and Peart (2006) measured high

34

levels of Cu, Ni, Zn, Pb and Cr in urban soils and found that the trees growing

in urban regions have lesser amount of heavy metals in their roots than in

their leaves, which indicates that heavy metal pollution of trees in urban

environment is mainly of atmospheric in nature. Lokeswari and Chandrappa

(2006) observed that some of the heavy metals occur in high amounts beyond

the limits of Indian standards in rice and vegetables growing in contaminated

sites. Mhaske et al. (2006) reported that Pueraria tuberosa grown in

contaminated zones does not accumulate the heavy metals, Pb, Cr, Fe, and

Ni, at levels more than normal range, and hence is not a pollution indicator.

The excess accumulations of Zn and Cu in this plant do not vary with

geographical regions and hence it is a physiological characteristic of the plant.

Nabulo et al. (2006) reported that vegetables grown near roads are

considered a potential source of heavy metal contamination to farmers and

consumers in urban areas and hence leafy vegetables should be grown a

minimum of 30 m away from roads. Singh and Kumar (2006) observed heavy

metal accumulation in vegetable crops such as Spinacia oleracea and

Albelmoschus esculentus in urban environment, which exceeds heavy metal

load of the soils, but washing of the vegetables reduces the metal pollution by

75 to 100%. Shukla and Upreti (2006) observed accumulation of Fe, Ni, Zn,

Cr, Cu and Pb in Phaeophyscia hispidula, a common foliose lichen, in urban

environment and found high accumulation rate of heavy metals in the lichen in

proportion to the content in the environment. Yoon et al. (2006) examined the

uptake of Pb, Cu and Zn in native plants and showed that native plant species

growing in contaminated sites may have the potential for phytoremediation.

Okunola et al. (2007) observed that the mean concentrations of heavy

metals in urban contaminated soils and plants follow the decreasing order of

Zn>Mn>Pb>Cu>Cd, and Zn>Mn>Pb>Cd>Cu respectively. Ano et al. (2007)

observed excessive accumulation of Pb anb Cd in Manihot esculenta growing

along major expressways passing through even remote villages and found

that the leaves and tubers of plants growing near roads are potential sources

of Cd and Pb toxicity to consumers. Dalvi et al. (2007a & b) observed the

presence of heavy metals such as Cu, Pb, Zn, Fe, Ni, Cr, As and Cd in the

leaves of medicinal plants, Leucas aspera and Mallotus philippensis, do not

35

vary significantly according to geographical zones and hence the

accumulation characteristics are quite inherent in these plants. Khan et al.

(2007) observed accumulation of heavy metals in Withania somnifera growing

in contaminated soils and found chronic or subtle health hazard for people

consuming the contaminated herbal medicine for a long period. Poszyler-

Adamska and Czerniak (2007) found that roads crossing forests influences

the natural environment with metal pollutants to a very high degree. Akinola

and Adedeji (2007) assessed the level of Pb in Panicum maximum and the

concentration of Pb in soil and the plant on both sides of the road ranged

between 3.98+0.15ug/g and 69.43+4.41 ppm (soil) and 0.13+0.03 ppm (leaf)

and 9.81+0.27 ppm (root).

El-Rjoob et al. (2008) observed varied accumulation of Pb, Cu, Zn, Cd,

Ni and Fe in different parts of the medicinal plant Rosmarinus officinalis

growing in contaminated soils. Yakupoglu et al. (2008) observed accumulation

of airborne Pb in Cichorium intybus growing along roadsides, and suggested

that the plant is a potential source of Pb toxicity to humans either directly or

indirectly through food chains. Hardiyanto and Guzman (2008) identified high

levels of Pb and Cd in white cabbage (Brassica rapa), cultivated in roadside

soil or in soils with irrigation of sewage water taken from urban sites. Sharma

et al. (2008) assessed the content of heavy metals in selected vegetables

through atmospheric deposition in an urban area and found that Zn

accumulate in the highest amount followed by Cu, Cd and Pb.

A thorough review of literature on specific studies of accumulation of

heavy metals in plants growing on contaminated lands such as roadsides

clearly reveals the significance of the current research. It may be noted that

the volume of literature has tremendously increased recently thanks to the

increasing importance of the topic. An exhaustive study of roadside

contamination is important and imperative for the reasons of its spread into all

parts of the urban and suburban environments. However, the spread of

metals depends on their mobility in the environment. In order to analyze

critically the issue, a thorough review of publications available on heavy

metals and their mobility in soil environment has also been made.

36

2.3.5. Mobility of Heavy Metals from Contaminated Sites

Revitt and Ellis (1980) studied rain water leachates of heavy metals in

road surface sediments and found roads as a source of the steady enrichment

of heavy metals in water bodies. Gibson and Farmer (1984) examined the

environmental mobility and bioavailability of Pb, Zn and Cd in polluted

roadsides and analyzed the ecological implications of these characteristics.

Wada and Miura (1984) observed that roadside pollutants are flushed out in

great quantities during rain fall and contaminate the surroundings. Balades et

al. (1985) observed that in short time, runoff waters carry as much as 80% of

annual pollution load of motorways to water bodies. Gupta et al. (1996)

observed different grades of mobility for different species of metals in polluted

soils. According to Li and Shuman (1996) heavy metal movement in soil

profiles is a major environmental concern because even slow transport

through the soil may eventually lead to deterioration of ground water quality.

Lorenz (1997) observed that Cd, Zn and other metals in contaminated sites

generally decrease during plant growth, probably because of rhizosphere

effect and the subsequent redistribution of ions on to soil exchange sites at

lower ionic strength.

Sansalone et al. (1997) also observed that storm water run off from

urban roadways often contains significant quantities of metal elements and

solids and pH influences the mobility of heavy metals in soils to a great extent.

Hares and Ward (1999) suggested that a higher level of motorway derived

heavy metal contamination such as V, Cr, Mn, Co, Ni, Cu, Zn, Mo, Cd, Sb and

Pb exists in storm water runoff from roads and compared the heavy metal

content of motorway storm water following discharge into wet bio-filtration and

dry detention ponds. Sutherland et al. (2000) found out that Pb and to a lesser

extent Zn and Cu are anthropogenically enriched in watersheds. Sutherland

et al. (2001) found that Cu, Pb and Zn are significant metals found in roadside

sediments. Turer et al. (2001) examined heavy metal contamination in soils

of urban highways and the runoff from the soils. Pagotto et al. (2001) found

that Pb and Cu are immobile heavy metals on roadsides, whereas Cd and Zn

37

are easily mobile, especially Zn, the fast mobilization of which occurs in the

case of acid pH.

Ramakrishnaiah and Somashekar (2002) found that vertical movement

and partitioning of metals such as Pb, Zn, Cd, Cu, and Mn of roadside soils

depend on pH and organic carbon, but not Ni and Cr. Backstrom et al. (2003)

examined the amount and total deposition of Cd, Co, Cu, Pb and Zn in road

runoff, and found that the concentrations of most of these elements increase

significantly during the winter, owing to more intense wearing of the pavement

during winter. Kayhanian et al. (2003) found that the annual average daily

traffic has an influence on most highways run off pollutant concentration in

conjunction with watershed characteristics. Mikulic et al. (2004) assessed the

fate of heavy metals such as Cu, Pb and Zn from their sources to the final

sink and identified the sources, the input from each source as well as

transport towards the temporary and final sink, and concluded that around

30% of the analyzed elements introduced into the water column would be

deposited in their final sink, the marine sediments. Chung and Lee (2006)

found high metal concentrations in runoff and roadside soils. Preciado and

Lee (2006) concluded that sediment is a useful indicator of current metal

loadings and a key factor influencing the quality of urban watersheds.

Polkowska et al. (2007) evaluated the pollutant loading in the runoff and

surface water near major urban highway. Pereira et al. (2007) observed the

concentration of heavy metals in urban streets sediments in highways very

high and are important sources of pollution to the receiving rivers and to Bay.

Thus it became clear that mobility of toxic metals in contaminated

environment depends on the metal species as well as certain environmental

factors. However, it is the degree and nature of mobility of the toxins that

determines how dangerous are them to the environment.

An overall analysis of the above discussed literature on heavy metal

contamination of urban environment has revealed that, roads are the major

sources of excessive amount of heavy metals in urban systems. It is also

found that the major heavy metals in urban systems include Pb, Cu, Ni and

Zn. Moreover, the literature has shown that plants in urban environment

38

accumulate heavy metals in proportion to the availability of the metals in

contaminated soils. Therefore, the current research on the status of these

heavy metals in South Indian roadsides is most timely and highly significant.

However, the second major aspect of the current problem was vegetation

characteristics of the roadsides in relation to these heavy metals in soils. Thus

a specific review of literature on the ecology of roadside vegetation has

become necessary, which follows herein after.

2.4. Ecology of Roadside Vegetation

Apart from understanding the degree of contaminations, there are

many reasons to examine the ecology of roadsides. Roadside species are

highly resistant to physico-chemical disturbances. In biodiversity-rich tropical

regions like Kerala, many resistant species are found on roadsides. Resistant

and tolerant species have many ecological applications. Native plants growing

on contaminated sites, especially in subtropical and tropical areas, may have

the potential for phytoremediation (Yoon et al 2006). Phytosociological

analysis of vegetation is significant to measure the ecological potential of

species in their natural communities. Accordingly, a critical review of literature

on all aspects of the ecology of roadside vegetations for the last three

decades has been made to assess the available information in this regard.

Ecology of roadside vegetations became an interesting topic for

researchers towards the second half of 1970s. Lane (1976) first made a

serious study on the composition and structure of vegetation on roadsides,

but the author is of the view that roadsides no longer carry stands of native

species to represent natural communities. Way (1977) considered roadsides

as ecologically special communities, ecotones or edge habitats that hold

specialized species, and recognized some of the exciting opportunities for

research on plant communities of roadsides. In order to make use of the

ecological opportunities of roadside vegetations, Katsuno (1977) first

conducted a phytosociological analysis of roadside vegetation and reported

that the method is efficient and appropriate in order to select the useful plant

species. Later, Wester and Juvik (1983) also surveyed roadside plant

communities within a zone 3 m wide along roadway, and described the

39

ecological variations of the communities in relation to altitudinal differences.

According to them, vegetation pattern in disturbed roadsides may experience

dramatic temporal fluctuations as a result of site specific events. Lausi and

Nimis (1985) observed floristic variation in roadside vegetation in relation to

environmental differences and according to them the main factor underlying

floristic variation on roadsides is climate, chiefly precipitation. Wong and Lau

(1985) carried out an ecological survey on the roadside vegetation at three

different sites and observed certain indicator species of potential uses,

biomonitoring pollution on roadsides. Ross (1986) carried out a similar

vegetation study on roadsides and characterized the species diversity as well

as the vegetation change on highway verges. Nagler et al. (1989) observed

high plant diversity along the highways and roads due to geographic and

anthropogenic reasons. Ullmann and Heindl (1989) examined the ecological

and geographical differentiation of roadside plants in accordance with the

phytosociological method of Braun-Blanquet.

Berg and Mahn (1990) explained the anthropogenic changes in

roadside vegetation over the past 30 years in grassland. Ullmann et al. (1990)

analyzed the geographical differentiation of roadside vegetation and the

occurrence of roadside conenoses specific to physiographic units. Holzapfel

and Schmidt (1990) studied roadside vegetation along transects and reported

that biomass and species diversity are distinctively higher near roads. Heindel

and Ullmann (1991) studied roadside vegetation along different categories of

roads. Stella–Maris and Cristina (1992) surveyed vegetation of roadsides and

concluded that roadside must be functioning as floristic and faunistic micro

reserves in agro ecosystems. Wilson et al. (1992) made a survey of the

distributions and climatic correlations of some exotic species along roadsides.

Ullmann et al. (1995) conducted a survey of the vegetation of roadside verges

and reported that the pattern of distribution of both native and exotic species

is strongly related to altitudinal and climatic gradients.

A major and increasing impact upon the environment is that of roads

and their associated vehicular traffic; the change in vegetation near roads is

caused by pollution from vehicles (Angold, 1997). According to Allem (1997),

40

the discovery of scores of new plant species from roadsides over recent

decades has shown that plants can survive successfully under conditions far

distant from the ideal, often through their use of adaptive mechanisms. Iqbal

et al. (1998) found Senna holosericea and Prosopis Juliflora as the leading

dominant species of roadsides because of their wide ecological tolerance

along a super highway. Spellerberg (1998) published a review on the

ecological effects of roads and traffic on vegetations. Forman and Alexander

(1998) are firmly of the opinion that a huge road network with vehicles

ramifying across the land represents a surprising frontier of ecology and

species rich roadsides are conduits for new species.

Cilliers and Bredenkamp (2000) carried out a vegetation survey of road

verges along an urban to rural gradient. Forman and Deblinger (2000)

examined the ecological road-effect zone of a suburban highway and

considered road ecology as one of the great frontiers awaiting science and

society, but noted that natural areas near roads may be impoverished in

species sensitive to road and traffic disturbance. Trombulak and Frissell

(2000) explained that the roads of all kinds have seven general effects:

mortality from road construction, mortality from collision with vehicles,

modification of animal behavior, alteration of the physical environment,

alteration of the chemical environment, increased use of the areas by humans

and the spread of exotic species. According to these authors, roads promote

the dispersal of exotic species by altering habitats, stressing native species,

and providing movement corridors. Roads also promote increased hunting,

fishing, passive harassment of animals and landscape modifications. They

also observed changes in species composition, population sizes and

hydrologic and geomorphic processes that shape aquatic and riparian

systems. Sykora et al. (2002) conducted a phytosociological and floristic

evaluation of roadside verges in the Netherlands, and the data are analyzed

for changes in number of species, rarity of species, red list species and

syntaxonomical species groups. The studies enabled them to conclude that

the total number of species almost did not change that the common species

increased while rare species decreased, and that the red list declined by 40%.

According to Li-Yuehui et al. (2003), roads are a widespread and increasing

41

feature of most landscapes, and have great ecological effects, and road

ecology presents a surprising frontier of ecology. Exotic species are more

likely to thrive near roadways and native weedy species are infrequently

encountered (Larson, 2003). Shafiq and Iqbal (2003) studied the effects of

automobile pollution on the phenology and periodicity of some roadside

plants. Akbar et al. (2003) carried out an investigation into the floristic

composition of roadside vegetation and the levels of some heavy metals in

roadside soils.

Dogan et al. (2004) observed that most of the taxa of roadside plants

belong to Asteraceae, Fabaceae and Poaceae. Ahmad et al. (2004)

presented a vegetation survey of road verges and elucidated the percentage

frequency and cover of each of the most frequent species in relation to the

fertility status of the roadside soil. Spooner et al. (2004) conducted a spatial

analysis of roadside Acacia populations on a road network using the network-

K-function and recognized the importance of the study in understanding the

underlying ecological processes. According to the authors, the network-K-

function method will become a useful statistical tool for the analysis of

ecological data along roads, field margins, streams and other networks.

Rentch et al. (2005) explained the vegetation-site relationships of

roadside plant communities and concluded that roadsides are optimal growing

sites for exotic invasive species that out-compete native vegetation. Arevalo

et al. (2005) studied roadside plant communities along two roads following

distribution of alien vs native plant species against altitudinal gradients.

According to the author, altitude was the most important factor determining

species richness and composition along both roadside transects. Shuster et

al. (2005) compared results from random roadside surveys of Alliaria

petiolata, across a 5730-ha sub watershed. The random survey included 150

ha plots; the roadside survey examined 0.1-mile increments (10m deep) along

paved roads. According to the author, roadside survey is a useful and

practical method for detecting nascent invasions and management planning.

Wrobel (2006) carried out phytosociological observations of roadside

vegetation from 2001-2003, along public roads with hardened surface running

42

across the forest grounds and observed that the vegetation on roadsides are

affected by car wheels, surface erosion processes, water run off, mowing and

road works. Poszyler-Adamska (2007) is of the view that roads influence

natural environment to a very high degree.

This review of literature on roadside vegetation emphasizes the

significance of floristic analysis as a means of identifying resistant species. It

is well known that the phytosociological analysis of vegetation can reveal the

ecological potentials of many species in a natural community with respect to

specific environmental conditions of the habitat. Plant species which are most

frequent with very high relative abundance occupying metal contaminated

roadsides are the hyper tolerant species. Species which are hyper tolerant to

heavy metal contaminations may be hyper accumulators as well. Hyper

accumulator species are ecologically useful in many ways. Naturally, a review

of the literature on hyper accumulator plants is indispensable for the current

research.

2.5. Hyper accumulator plants

Hyper accumulator species are of high relevance to Phytoremediation.

Phytoremediation is an emerging eco-technology for cleaning of metal and

other toxic contaminated environments using hyper accumulator plants.

Therefore, searching out or tailoring of suitable hyper-accumulator species for

different environment is an important ecological as well as biotechnological

task. Finding out newer hyper accumulator species for different toxic metals or

other toxins is the real challenge of our time. Tolerance to heavy metals is

expressed sometimes with high accumulation of the metals in various parts of

the plant body. Therefore, investigations of resistant species on contaminated

roadside are highly significant to provide clue to the metal hyper accumulation

capacity of species, especially in the roadsides of biodiversity-rich zones of

the world, such as Kerala. Since the current research aims at assessing the

metal accumulations in roadside species, a thorough review of research

publications on hyper accumulator species, from the very beginning of hyper

accumulator studies is attempted below.

43

Researches on heavy metal tolerance of plant species began in the

mid 1960s with the report of Gregory and Bradshaw (1965) that tolerance is a

genetically controlled mechanism and Poaceae in general is tolerant to heavy

metals. Later Graham and Kalman (1974) reported that Pb content in forage

grasses of suburban areas at varying distance from roads is sufficient to pose

a threat to grazing animals. Goldsmith et al. (1976) determined the effect of

various traffic densities on Pb concentrations in soil and vegetation collected

along roads. Wu and Antonovics (1976) observed that Pb level at roadside is

sufficiently high to impose selection pressure for the evolution of tolerance in

sensitive species, causing rapid and highly localized evolutionary changes in

plants; Cynodon dactylon is more tolerant to Pb than Plantago lanceolata

growing on roadsides. Atkins et al. (1982) reported that roadside Pb is more

phytotoxic than is previously believed and the roadside species Festuca rubra

is highly tolerant to Pb in contaminated roadsides. In the same year,

Karataglis (1982) observed tolerance of Agrostis tenuis, towards heavy metals

such as Cu, Zn, and Pb. However, the author observed that the expression of

tolerance of plants to one or more metals in a soil does not necessarily

involve tolerance to other metals not present in the soils.

Nasralla and Ali (1985) investigated accumulation of Pb in different

parts of vegetables growing around six Egyptian traffic roads and showed that

leafy vegetables such as lettuce and cabbages accumulated Pb up to 78.4

ppm in the edible portions, while the least Pb accumulators were carrots and

radish. The amount of Zn, Pb or Cu accumulated in plants from Zn and Pb

tolerant and non-tolerant populations of Anthoxanthum odoratum, varied

depending on whether the plants were tolerant of the metal and on the period

of the exposure to the solution containing the metals (Qureshi et al., 1985).

According to Baker et al. (1988), plants resist the harmful effects of

toxic metals in one of the two ways; either they prevent the metal from

entering their tissues (exclusion mechanisms) or they convert the inorganic

metal inside cells into something less harmful (tolerance mechanisms).

Gibson and Pollard (1988) found that plants of contaminated sites usually are

tolerant ecotypes in response to the level of metals in the soil. According to

44

the author, heavy metal tolerance is common in Poaceae. Baker and Proctor

(1990) studied the influence of Cd, Cu, Pb and Zn on the distribution and

evolution of metallophytes, and found constitutional differences in species

sensitivity to toxic metals and environmentally-induced tolerances. Wong and

Chui (1990) observed that Cynodon dactylon, Panicum repens and Imperata

cylindrica are species capable of withstanding high concentrations of Cu, Ni,

and Mn in the contaminated soil. The authors have especially noted the heavy

metal tolerance of Cynodon dactylon.

Brown and Brinkmann (1992) observed Festuca ovina as a tolerant

species on contaminated sites. Baker et al. (1994) found that the mettalophyte

Thlaspi caerulescens have common mechanisms of absorption and transport of

several metals. Raskin et al. (1994) discovered that certain plants concentrate

essential and non essential heavy metals in their roots and shoots to levels far

exceeding those present in the soil. According to them, the use of such hyper

accumulator plants in environmental clean up may guarantee a greener and

cleaner planet for us. Tripathi and Sahu (1995) studied the nature of metal

accumulation in tolerant plant species growing on nutrient deficient and trace

metal enriched industrial areas. Pollard and Baker (1996) observed significant

differences in Zn concentration and plant size within and between populations of

the well known metal hyper accumulator Thlaspi caerucaerulescens

(Brassicaceae), which is known to hyperaccumulate Zn to foliar concentrations

exceeding 3% (d.wt. basis).

Mazen and Maghraby (1997) reported the possible role of calcium

oxalate crystallization in the toxic heavy metal accumulation in Eichhornia and

also in its tolerance toward metal contaminations in general. McGrath et al.

(1997) assessed heavy metal uptake and chemical changes in the

rhizosphere of Thlaspi caeurulesens and Thlaspi ochroleucum grown in

contaminated soils and suggested that Thlaspi caeurulesens has the potential

for removing Zn from moderately to highly contaminated soils, but that this

ability is not related to the pH changes in the rhizosphere. Ye et al. (1997a)

examined the tolerance of four populations of Typha latifolia raised by seeds

from metal contaminated and uncontaminated sites towards Zn, Pb and Cd,

45

and discarded the hypothesis that populations from metal contaminated sites

have evolved tolerance to the metals. However, the author rather reported

that such plants shows constitutional tolerance. Ye et al. (1997b) observed

small differences in tolerance towards Zn, Pb and Cd in seed grown

populations of Phragmites australis collected from contaminated as well as

non-contaminated sites. The differences were visible when seedlings of both

populations were grown in 1.0 ug ml-1 Zn and 10.0 ug ml -1 Pb treatment

solutions.

Ohtani et al. (2001) found that even without large soil pH change, there

is the possibility of Brassica rapa accumulating Cd from soils, and the

accumulation process depends on anions in precipitation and chemical form

of Cd in the soil under conditions of heavy metal enrichment. Wen-sheng et

al. (2002) observed Ni accumulation in different populations of the two

grasses Paspalum distichum and Cynodon dactylon as an elemental defence

against herbivores and pathogens. According to the authors the level of

tolerance of plants also reflected the metal concentrations in plant tissues and

their associated substrata. Moreno et al. (2002) elucidated the effects of

growth conditions on the uptake of Zn, Cu, Cd, and Pb in Chinese cabbage.

The important fact that determines hyper accumulation, according to them, is

the relationship between what is in the soil and what is in the plant and the

ratio of concentration. Nagaraju and Karimulla (2002) examined the

accumulation of heavy metals such as Al, Fe, Be, B, Ba, Mn, Sr and Zn in

certain dominant plant species growing on contaminated soils in South India.

Citterio et al. (2003) experimentally proved metal tolerance and accumulation

of Cd, Ni and Cr in Cannabis sativa. Assuncao et al. (2003) showed the

typical hyper accumulator species, Thlaspi caerulescens as a model for heavy

metal hyperaccumulation in plants. Olivares (2003) found no usual

phytochemical change except an unusual increase in phenols, in Tithonia

diversifolia exposed to roadside automotive pollution or when grown in pots of

Pb supplemented soil. Nayaka et al. (2003) compared the distribution pattern

and heavy metal accumulation of lichens in between 18-year period and

reported that lichen flora of metal contaminated areas change significantly, as

only four species are common between the two periods of studies.

46

Archer and Caldwell (2004) identified Cynodon dactylon, Lomandra

longifolia and Juncus usitatus as plants of potential use in phytostabilisation

programs, on account of their tolerance towards acid soils, and accumulation

of Pb and Cd in their tissues. Haider et al. (2004) conducted a study to

determine the level of Cu, Cr, Mn, Ni, Zn, Cd, and Pb in some medicinal

plants and found differential accumulation of metals in them. Iqbal and Shazia

(2004) investigated the differential tolerance of Albizia lebbeck and Leucaena

leucocephala at toxic levels of Pb and Cd and found that in both Pb inhibit

seed germination. Odjeba and Fasidi (2004) reported that leaves of Pistia

stratiotes differentially accumulate and tolerate toxic metals such as Ag, Cd,

Cr, Cu, Hg, Ni, Pb, and Zn; the lowest tolerance index is for Hg, but the

highest for Zn, and identified the species as a potential one for

phytoremediation of Zn, Cr, Cu, Cd, Pb, Ag and Hg. Salvador et al. (2004)

reported the use of synchrotron radiation total reflection x-ray fluorescence

(SRTXRF) analyses as the technique for trace element determination in

plants aiming at environmental pollution control. Wen-sheng et al. (2004)

conducted a field experiment to compare the growth and metal accumulation

of Vetiveria zizanioides, Paspalum notatum, Cynodon dactylon and Imperata

cylindraca on Pb and Zn tailings and found Vetiveria zizanioides as the best

choice among the four species used for phytoremediation of metal

contaminated soils, not because of high metal content per biomass, but

because of high biomass and the consequent effective accumulation capacity.

According to Tomasevic et al. (2004), higher plants may be used as

bio-monitors for the assessment of atmospheric heavy metal pollution by

means of their bio-accumulative properties. Uijily and Kumaraguru (2004)

studied the accumulation of heavy metals in some species of lichens and

showed that lichens possibly absorb heavy metals from the atmospheric

deposition rather than from the soil on tree bark. Yang et al. (2004) observed

that Sedum alfredii has an extraordinary ability to tolerate and

hyperaccumulate Cd and it provides an important plant material for

understanding the mechanism of Cd/Zn co-hyperaccumulation and for

phytoremediation of the heavy metal contaminated soils. Yanqun et al. (2004)

conducted a field survey of higher terrestrial plants growing on Lanping lead-

47

zinc mine in China to identify the species accumulating exceptionally large

concentrations of Pb, Cd, Cu, and Zn. They found that the cocentrations of Pb

and Zn in soil and in plants are higher than that of Cu and Cd in all the 17

species studied.

Antosiewicz (2005) conducted a study on calcium-dependent Pb

tolerance on plants and reported Pb toxicity increased as calcium content

decreased. Ataikiru et al. (2005) showed that certain vegetables accumulate

considerable amount of heavy metals when grown in contaminated soils.

Grigalaviciene et al. (2005) examined the accumulation of Pb, Cu, and Cd at

roadside forest soils and showed that soil and plants near the highway are

significantly heavy metal enriched, particularly of Pb. Kara (2005) found that

Nasturtium officinale is able to accumulate both Cu and Zn at high levels, but

is able to accumulate Ni at low levels, especially when grown in solutions.

Salamon et al. (2005) using pot experiment assessed the effects of Pb, Ni

and Cd on growth and quality of Chamomile plants at high enrichment of soils

with these metals and observed that these plants accumulate the metals in

the order Cd>Ni>Pb in their tissues in the order roots>leaves>flowers.

Senthilkumar et al. (2005) observed that Prosopis juliflora has higher Cu and

Cd in their roots and shoots compared to the extractable level of these metals

in the soil. Xin et al. (2005) observed that at elevated Pb conditions,

mycorrhizae promote plant growth by increasing phosphorus uptake and

mitigate Pb toxicity by sequestrating more Pb in roots.

Nikhil (2006) observed high accumulation of heavy metals in certain

medicinal plants of contaminated Jharia coalfield area in India. Anoliefo et al.

(2006) reported that high rate of occurrence in contaminated sites of a

particular plant species in the frequency table suggests that such plants are

tolerant and may be used as a possible phytoremediating agent. They

observed the presence of Poaceae over others in the contaminated area.

Yoon et al. (2006) evaluated accumulation of Pb, Cu, and Zn in different

plants growing on a contaminated site. According to them, native plant

species growing on contaminated sites may have the potential for

phytoremediation. Peciulyte et al. (2006) examined the ability of plants to

48

grow in metal contaminated soils and the accumulation of the metals in their

biomass. Romeiro et al. (2006) assessed Pb uptake and tolerance of Ricinus

communis and showed that it is a hyper accumulator species for Pb.

Rotkittkhun et al. (2006) conducted a field survey of terrestrial plants growing

in a Pb mine area and found that 26 plants have Pb concentrations above that

of 1000 mg kg-1 in their shoots. Sao et al. (2006) investigated the

phytoremediation potentials of Cyperus rotundas and Axonopus compressus

towards Cd in solutions and soils. In them the metal accumulation was higher

in roots than in shoots. Oydele et al. (2006) observed accumulation of heavy

metals in the soil and vegetables in the order Hg>Pb>Cd after the application

of super phosphate. Butkus and Baltrenaite (2007) assessed the transport of

heavy metals from soil to Pinus sylvestris and Betula pendula trees. Marques

et al. (2007) observed positive correlations between the soil total, available

and extractable Zn fractions, and metal accumulation in the roots and leaves

of two plant species, Rubus ulmifolius and Phragmites australis. Pratt and

Lottermoser (2007) reported root accumulation of heavy metals in a roadside

grass, Melinis repens. This plant accumulates heavy metals more in their

roots than in shoots.

The fast increase in research publications related to hyper

accumulators in recent times has indicated the importance of their

applications in phytoremediation of contaminated sites. Phytoremediation is a

comparatively cheap and eco-friendly technique of cleaning the soil and water

from toxic metals and other organic substances, especially when toxic

substances are spread in the environment in a highly diffused manner. Plants

with the capacity of tolerating toxins above a critical level, and have the

capacity to absorb and accumulate them in their tissues are used for

environment cleaning. Since the current research also inquires the ecological

potentials of plant species growing on contaminated roadsides, the specific

review of literature on phytoremediation is imperative.

49

2.6. Phytoremediation

Phytoremediation became a hot topic in the second half of 1990s.

Cunningham et al. (1995) reported that plant-based remediation techniques

are showing increasing promise for use in soils contaminated with organic and

inorganic pollutants. According to Kumar et al. (1995) plants with the hyper

accumulation property may be exploited for soil reclamation, especially if the

species is an easily cultivated high biomass crop. They reported that Brassica

juncea is a suitable species capable of removing Pb from contaminated

environments. Salt et al. (1995) considered the use of specially selected and

engineered metal accumulating plants for environmental cleaning as an

emerging technology called phytoremediation. Chaney et al. (1997) proposed

phytoremediation of metal contaminated soils as an economically cheap

method for soil cleaning and the method may be utilized as a method of

extraction of costly metals of valuable purposes. Ebbs and Kochian (1997)

studied the toxicity of Zn and Cu in three species from the genus Brassica and

found that the presence of two metals together in the medium reduces the

degree of accumulation of both the metals but the accumulation increases

when the metals are used individually. Salt et al. (1998) in a review

emphasized the use of phytoremediation as a cost effective plant based

approach to remediation, taking advantage of the remarkable ability of plants

to concentrate elements and compounds from the environment and to

metabolize various molecules in their tissues. Moreover, the knowledge of the

physiological and molecular mechanisms of phytoremediation began to

emerge together with biological and engineering strategies designed to

optimize and improve phytoremediation. Qian et al. (1999) examined

phytoaccumulation of trace elements by 12 species of wetland plants to find

out the degree of hyper accumulation in them and found Polygonum

hydropiperoides the best plant species for phytoremediation.

Remarkable developments in phytoremediation technology have

occurred in the current decade. Certain definite phytoremediation protocols

were established and many hyper accumulator plants have been patented in

the West. Karenlampi et al. (2000) suggested improvement of plants by

50

genetic engineering, modifying characteristics like metal uptake, transport and

accumulation as well as metal tolerance, which open up new possibilities for

phytoremediation. Nedelkoska and Doran (2000) studied the characteristics of

heavy metal uptake by plant species with potential for phytoremediation and

phytomining and found that hairy roots are useful for screening a range of

plant species for their biosorption and long-term metal uptake capabilities.

Lombi et al. (2001) conducted pot experiments to compare two strategies of

phytoremediation: natural phytoextraction using the Zn and Cd hyper

accumulator species, Thlaspi caerulescens vs chemically enhanced

phytoextration using Zea mays treated with ethylene diamine tetraaccetic acid

(EDTA), and found that though the chemical treatment increased the solubility

of the metal in the soil, a corresponding increase of the metal in plant tissues

is not followed and the chemical treatment poses an environmental risk in the

form of ground water contamination. Lasat (2002) in a review pointed out that

plants have the genetic potential to remove many toxic metals from the soils.

But despite this potential, phytoremediation is yet to become a commercially

available technology and progress in this field is hindered by a lack of

understanding of the complex interactions in the rhizosphere and plant based

mechanisms which allow metal translocation and accumulation in plants.

According to the authors, agronomists, therefore, have to further provide

solutions to applied issues such as, how to incorporate amendments including

synthetic chelates to enhance absorption of metals into plants and when and

how to harvest.

Shen et al. (2002) observed that EDTA is the best salt in solubilizing

soil-bound Pb and enhancing Pb accumulation in cabbage shoots and the

application of EDTA in three separate doses is most effective in enhancing Pb

accumulation in cabbage shoot and to decrease mobility of Pb in soils.

Barcelo and Poschenrieder (2003) in a review described phytoremediation as

a technique that offers excellent perspectives for the development of a plant

with the potential for cleaning metal-contaminated soils, at least under certain

favourable conditions and for using adequate crop management systems.

Prasad and Freitas (2003) concluded a review on phytoremediation with the

observation that the importance of biodiversity shall be increasingly

51

considered for the cleanup of the metal contaminated and polluted

ecosystems. Bondada and Ma (2003) studied arsenic hyper-accumulation in

Chinese Brake Fern (Pteris vittata) and has generated a global interest due to

the discovery of its unique property of hyper-accumulating arsenic in the

fronds from contaminated and uncontaminated sites. According to McIntyre

(2003), phytoremediation offers owners and managers of metal contaminated

sites an innovative and cost effective option to address recalcitrant

environmental contaminants. Prasad (2003) finds phytoremediation as a

widely accepted technology, both in developed and developing nations,

because of its high potential to clean up the polluted and contaminated sites.

According to Whiting et al. (2003b) the assumpiton that

hyperaccumulation of minerals contributes significantly to osmotic

adjustments and support plant capacity for drought resitance is not acceptable

due to lack of evidence. According to Ghosh and Singh (2003),

phytoremediation depends on two factors; plant biomass and plant uptake

capacity and its applications have the potential for providing most cost

effective and resource conservative approach for remediation as far as

developing world is concerned. Singh et al. (2003) are of the view that

phytoremediation has emerged as a promising eco-remediation technology,

particularly for soil and water cleanup of large volumes of contaminated sites.

Suresh and Ravishankar (2004) also accept that phytoremediation as an eco

friendly approach for remediation of contaminated soil and water using plants

and according to them, the advancement in molecular biology and better

understanding of biochemical and genetic approaches of phytoremediation

would surely make this technique a successful one.

Lim et al. (2004) reported that Indian mustard (Brassica juncea) has

expressed an effective metal accumulating high tissue concentrations of Pb

when grown in contaminated soil with the addition of a chelating agent, such

as EDTA under daily application of varying electric potential. Odjegba and

Fasidi (2004) evaluated the toxicity of eight potentially toxic trace metals Ag,

Cd, Cr, Cu, Hg, Ni, Pb and Zn in Pistia stratiotes to determine if this plant

showed sufficient tolerance and metal accumulation to be used to

52

phytoremediate waste water and /or natural water bodies polluted with these

heavy metals. The plant showed differential accumulation and tolerance for

different metals at similar treatment conditions. According to Alkorta et al.

(2004) in general, hyperaccumulators are low biomass and slow growing

plants and high biomass non-hyper accumulator plants by themselves are not

a valid alternative for phytoextraction. They tried application of chemically

induced phytoextraction and suggested applied projects to clarify the real

potential and risks of this technology. Walker and Bernal (2004) investigated

the effects of Cu and Pb on growth and Zn accumulation of Thlaspi

caeruleseens and found that Cu treatments strongly inhibited growth of

Thlaspi caeruleseens, but the Pb treatment did not affect growth significantly.

According to Gratao et al. (2005), it is essential to investigate and

understand how plants are able to tolerate toxic metals and to identify which

metabolic pathways and genes are involved in such a process. Ghosh and

Singh (2005) observed phytoremediation as a fast developing field. Over the

last ten years lots of field applications were initiated all across the world, but

most of the studies have been done in the developed countries and

knowledge of suitable plants for phytoremediation is particularly limited in

India. Kramer (2005) considered environmental pollution with metals and

xenobiotics as a global problem and the development of phytoremediation

technologies for clean up is a novel approach. According to the author, the

performance of transgenic plants generated so far is not yet sufficient for

commercial phytoextraction and the engineering of transgenic plants suitable

for phytoextraction will probably require a change in the expression levels of

several genes. Merkl et al. (2005a) reported that for petroleum contaminated

soils, one of the main traits of the plants for phytoremediation is the peculiarity

of the root zone because roots are involved in the microbial growth

enhancement for oil degradation. Therefore, specific root length, surface area,

volume and average root diameter of plants growing in crude oil contaminated

soil are important. They found that root structure is not affected by crude oil in

Eleusine indica. Merkl et al. (2005b) assessed tropical grasses and legumes

for phytoremediation of petroleum-contaminated soils and observed that the

legumes died within 6 to 8 weeks, but the grasses showed reduced biomass

53

production under the influence of the contaminant. Pilon-Smits et al. (2005)

observed that, in the past ten years phytoremediation has become an

important technology and area of research and according to the authors,

continued phytoremediation research should benefit from a more

multidisciplinary approach, collaboration between research groups and

industries, development of new genomic technology and targeting of the bio-

available fraction of pollutants. Zhuang et al. (2005) reported that the

application of EDTA to soil is the most efficient method to enhance the

phytoavailability of Pb and Zn, but did not have significant effect on Cd.

According to Prasad (2005), 300 Ni hyper accumulating plants are

known and these plants exhibit unusual appetite for toxic metals and

elemental defense. According to the author, Ni hyper accumulation has a

protective function against fungal and insect attack in plants. An et al. (2006)

found that Pteris vittata, a potential As hyper accumulator fern, has very high

tolerance to Zn and grows normally at sites co-contaminated with high amount

of Zn and As. Padmavathiamma and Li (2007) published key aspects of

phytoremediation technology and the biological mechanisms underlying

phytoremediation. Experimental studies of Jankaite and Vasarevicius (2007)

shows that the Poaceae family members have better absorptive abilities of

heavy metals. Zhuang et al. (2007) considered phytoremediation as an in situ,

cost effective potential strategy for cleanup of sites contaminated with trace

metals. They counted certain plants of paddy fields as potential species for

phytoremediation of Zn and Cd. Abou-Shanab et al. (2007) conducted a pot

experiment to compare the growth and metal accumulation of Zea mays,

Sorghum bicolor, Helianthus annus, Conyza discoridies and Cynodon

dactylon, grown in four different soils containing moderate to high amounts of

heavy metals and found that metal translocation into shoots appears to be

restricted in cultivated plants so that harvesting of such plants will not be an

effective source of metal removal in soils. Hernandez-Allica et al. (2007)

reported that in most metal polluted soils, Pb usually appears together with

other heavy metals, such as Zn and Cd and that in phytoexraction

experiments, efficient Pb uptake is generally limited by its low

phytoavailability. According to them, proper management of EDTA

54

concentration can reduce metal phytotoxicity, maintain the free uptake of

some metals and, at the same time, increase the uptake of metals with low

phytoavailability. Sinha et al. (2007) reported the role of plants in heavy metal

contaminated sites and pointed out the need to enlighten society and the

planner on the ecological and economical value of the vetiver, the wonder

grass.

This thorough review of specific literature on phytoremediation bears

out the clearly high relevance of phytoremediation technology, the

development of which depends on the finding out of hyper accumulation

potentials of plants occupying contaminated lands. Roadsides being

contaminated places, the study of plant species in relation to metal

contamination of roadside soils is highly relevant. Since many countries have

environmental databases of the hyper accumulator plants in their territory,

India also should launch such programmes immediately. The current research

programme is a useful step in this direction.

The roadsides being physico-chemically highly disturbed and

contaminated sites, they also offer the possibilities of finding out indicator

plant species affected by different grades of disturbances. Therefore, a

specific review of literature on ecological indicators also became necessary.

2.7. Pollution Indicator Plants

Roadsides are permanently polluted places as they are the line

sources of pollution. Therefore, plants growing on roadsides may be

considered as pollution indicator species as well. Serious efforts for

identification of pollution indicator species began in the mid 1980s. Ho and

Tai (1985) reported Pteris vittata as an indicator or as a biomonitor species for

aerial deposition of metals. Nyangababo (1987) used lichens as an indicator

device to assess the nature and the degree of pollution in urban systems.

Abullah and Latiff (1988) found Axonopus compressus and Paspalum

conjugatum as appropriate indicators of roadside contaminants such as Pb,

Cu, Zn and Fe. Al-Shayeb et al. (1995) considered leaflets of Phoenix

dactylifera as suitable biomonitors for pollution from metals such as Pb, Zn,

55

Cu, Ni, Cr and Li in urban centers. Aksoy and Ozturk (1997) found Nerium

oleander as a possible biomonitor of heavy metal pollution on roadsides. Hol

et al. (1997) found Rhododentron leaves as excellent biomonitors of roadside

lead pollution. Caselles (1998) observed leaves of Citrus limon as good

indicators of Pb, Cu, Mn and Zn at roadsides. Aksoy and Sahin (1999)

reported Elaeagnus angustifolia as a useful biomonitor of heavy metals such

as Pb, Cd and Zn. Aksoy et al. (1999) found out Capsella bursa-pastoris as a

good biomonitor of heavy metals such as Pb, Cd, Zn and Cu.

Aksoy et al. (2000) observed Robinia pseudo-acacia as a biomonitor

for the heavy metasl such as Pb, Cd, Cu, and Zn. Odukoya et al. (2000)

considered tree barks and leaves of Azadirachta indica as an indicator of

atmospheric pollution from Pb, Zn and Cu in cities. Mosses also have been

used as biomonitors of atmospheric pollution (Pearson et al. 2000). According

to Caselles et al. (2002) regular periodic analysis of roadside plants at regular

intervals in areas with intense traffic, can be used as a cheap biomonitor at

present. Swaileh et al. (2004) reported that a common herb Inula viscosa is a

good biomonitor for roadside metal pollution. Raina and Aggarwal (2004)

studied the effect of vehicular exhaust on some of the micro morphological

characteristics of leaves of Ipomoea cairica, Zizyphus jujuba, Ervatamia

coronaria, Psidium guajava, Eucalyptus terticornis, Jasminum sambac and

Thevetia peruviana growing along the roadsides and considered them as

good indications of pollutions. Oliva and Rautio (2004) used the leaf samples

of Duranta repens as a passive biomonitor for atmospheric pollution in urban

areas.

Celik et al. (2005) observed Robinia pseudo-acacia as a good indicator

of atmospheric pollution from industries and traffic in cities. Taranath et al.

(2005) reported that grasses, especially Cynodon dactylon reflect the extent

of aerial contamination of the roadside environment. Yilmaz et al. (2006)

studied Aesculus hippocastanum as a possible biomonitor of the heavy metal

pollution in urban and suburban areas. AL-Khlaifat and Al-Khashman (2007)

considered leaves of Phoenix dactylifera as suitable biomonitor for aerial

deposition of heavy metals in a city region. Kalbande et al. (2008) found the

56

pollens of Cassia siamea, Cyperus rotundus, and Kigelia pinnata as good

indicator of air pollutions giving results in short time of exposure of 5 to 10

hours. According to Suzuki et al. (2009) the roadside Rhododendron pulchrum

is a useful bioindicator of heavy metal pollution from traffic sources in cities.

2.8. Conclusion

The thorough and comprehensive review of literature covering all

aspects of roadside vegetation studies in relation to heavy metal

contaminations shows that the current problem is of grievous magnitude and

is to be addressed in all seriousness. The review endorses the fact that no

exactly similar work has yet been carried out. Intensive roadside vegetation

studies or extensive studies on roadside contaminations are quite new. Unlike

the current two-year intensive monitoring studies, earlier investigations on

roadside vegetations and contaminations in India were all partial and short-

duration programmes involving few aspects of the problem.

In order to understand the significance of the current research problem

in depth, its applications are also important. Roadside vegetation provide

many ecological opportunities such as the generation of knowledge-base on

the degree of different types of roadside contaminations and the consequent

health hazards, degree of tolerance of plants towards these pollutants, the

accumulation of different heavy metal pollutants in different plant tissues, and

the utility of them as ecological indicators of pollution and hyper accumulation.

Information regarding hyper accumulator and indicator species add to the

resource-database of the country. This knowledge base has applications in

biomonitoring of pollutions as well as cleaning of contaminated environments

using phytoremediation. The current research programme is arguably, a

highly useful endeavour to provide basic information of great ecological

applications.