Introduction

Zinc (Zn) was acknowledged as essential nutrient for plants in 1926 and soon after for mammals in 1934 (Nielsen 2012). It is vital for many enzymes of protein synthesis, energy transfer and nitrogen metabolism. Its deficiency not only impedes plant growth and yield, but also adversely affects human health (Graham et al. 2001; Cakmak 2002) that’s why soil deficits in Zn have to be applied with Zn fertilizers to fulfill the nutritional requirements of plants (Wyszkowska et al. 2013). However, being the most mobile and bioavailable nutrient its higher concentration in soil may cause phytotoxicity in plants (Reichman 2002; Sagardoy et al. 2009).

Supra-optimal Zn concentration in plants causes leaf chlorosis, instabilities in photosynthesis and chlorophyll synthesis (Wang et al. 2009). Its excess also induces oxidative damage, disturbs the protein synthesis and development of organelles in plants (Panda et al. 2003; Wang et al. 2009).

Zinc is also a common pollutant in the area of traffic routes (Malinowska et al. 2015), most of its quantity comes from petrol, lubricant oil leaks, brake linings, by wear and tear of tyres and from galvanized parts of automobiles (Thorpe and Harrison 2008; Nazzal et al. 2013) as well as from soot and heavy metal oxides, which runoff into roadside soil after precipitation (Malinowska et al. 2015). The plants growing on such soils accumulate metals that may enter into human and animal body through consumption of these plants (Nauciene et al. 2002; Liu et al. 2007).

The contamination of roadside soil by trace metals is directly proportional to traffic density on roads (Werkenthin et al. 2014), moreover, level of vehicular released metals in roadside soil and plants got increased considerably during last few years (Onder and Dursun 2006). Celik et al. (2005) reported four times higher metal concentration in roadside vegetation than control sites vegetation. This concentration drops with distance from roadsides (Joshi et al. 2010; Mmolawa et al. 2010; Werkenthin et al. 2014). Seasonal and spatial variations also influence metal contents in soil (Pathak et al. 2015). The use of native plant species to monitor the level of metal contamination is quite economic, convenient, and aesthetically pleasing technique in which the natural ability of environment is benefited to restore itself (Hernandez-Allica et al. 2008).

Several plant species have been reported as good indicator/monitor of metal pollution. Amongst them, the role of Nerium oleander (Mignorance and Oliva 2006), Robinia pseudo-acacia (Celik et al. 2005), Eucalyptu spp., Prosopis juliflora and Dalbergia sissoo (Naveed et al. 2010), Casuarina equisetifolia (Aissa and Keloufi 2012), Ageratum conyzoides (Deepalakshmi et al. 2014) and Synedrella nodiflora and Chromolaena odorata (Okoronkwo et al. 2014) to monitor environmental metal pollution has already been studied. The present study monitored the Zn contamination in roadside soil and plants. Information obtained can contribute in identification of phytomonitors of metal pollution.

Materials and Methods

Study area description

Punjab, the most populated province of Pakistan, has a huge network of roads. “National Highway (N-5)”and “Faisalabad to Okara Road (FOR)” are two high trafficked roads in the central Punjab. “Faisalabad to Okara Road (FOR)” is 103 km long and interconnect Faisalabad to Okara by passing the Ravi River. This road has numerous human settings (villages, towns), cultivated field areas and marketplaces along it. Animal driven carts, motorbikes, rickshaw, vans and mini buses are major contributors of traffic load on this road. National Highway 5 (N-5) is 1819 km long and interconnects Karachi to Torkham. A section of it from Okara to Lahore (129 km) was selected for study. This road is quite busy and in good state having several urban settlements along it. The vehicles on N-5 includes multi wheeler loaders, air-conditioned buses, vans, mini buses, oil tankers, trucks and cars and remains busy throughout the year.

Sampling and metal analysis

Excluding the markets and residential areas, five sites were selected randomly along each (FOR and N-5) road (Fig. 1). Samples and data were collected in the mid of each of four seasons. The five commonly growing native plant species (Cenchrus ciliaris L., Cynodon dactylon L., Calotropis procera A., Ricinus communis L., and N. oleander L.) were selected for study at each site along both roads (Table 1). Leaves from each selected plant and soil samples (up to 10 cm deep) were collected at each site along roadside. Same plant species and soil samples were collected at a distance of ~ 50 m from roadside and labeled as control (Jian-Hua et al. 2009). All leaf samples were washed with deionized distilled water to get rid of Table 1: Description (botanical and vernacular names) of plant species grown in the study area

soil particles and dried in an oven (65°C) then ground to powder using Wiley Mill. Collected soil samples were sieved (2-mm) and dried in oven (65°C) for 72 h. By using HNO₃ and H₂O₂, plant and soil samples were digested on a hot block digester (Environmental Express, Mt. Pleasant, SC) by following USEPA method 3050B for metal analysis (de Oliveira et al. 2015) and analyzed by atomic absorption spectrophotometer (AAS). Standard plant leaves and soil reference materials (accuracy=100 ± 20%), Internal standards and reagent blanks were used to ensure precision and accuracy in analysis.

Statistical analyses

Statistical analyses were carried through program COSTAT computer package by Cohort software (2003) Monterey, California, U.S.A. Means were compared with LSD test (α = 0.05) (Steel and Torrie 1997). The correlation was determined by Excel.

Results

Zn contents in roadside soil

Zn contents in roadside soil were higher than the control samples (Table 2). During seasons, highest Zn concentration was recorded at “Tandaliawala” site along “FOR” and “Chung” site along “N-5”. Furthermore, seasonal variations were also noticed for soil Zn content with highest during summer and least during winter season along both roads.

Zn contents in roadside plant leaves

Zn content in plant leaves were higher than the control (Table 3). Among all plants along “FOR”, maximum Zn accumulation was found in R. communis and minimum in C. ciliaris (Fig. 2). Similarly, along “N-5”, maximum Zn concentration was also noted in R. communis and minimum in C. ciliaris (Fig. 3). The spatial comparison along “FOR” showed that Zn content in plant leaves at “Tandaliawala” site was highest among all sites while “Satghara More” site presented least Zn content along this road (Fig. 4). Along “N-5” highest Zn content in plant leaves were observed at “Chung” and lowest at “Pattoki” site (Fig. 5). Nevertheless, Zn content in all trafficked sites were significantly higher than the non-trafficked site. The seasonal comparison for accumulation of Zn in plant leaves along both roads were observed in the following order summer>autumn >spring > winter (Fig. 6, 7).

Comparison among Roads (“FOR” and “N-5”) for Zn contents in soil and plant leaves

A comparison between roads for Zn content in both plant leaves and soil showed highest Zn content along N-5 and least along ‘FOR” (Fig. 8, 9).

Correlation of traffic density with soil and plant metal

Fig. 1: Map showing sites on “FOR” and “N-5”

Khanuana, Sataiana, Tandaliawala, Bangla Gogera, Satghara More, Renala Khurd, Pattoki, Bhai Phero, Manga Manadi and Chung

Source: https://www.google.com/maps

Table 2: Zn contents (mg kg^-1) in roadside soil along FOR and N-5 during different seasons (mean ± SD)

Correlations were calculated to estimate the relationship between traffic density and metal content in soil and plant leaves. During all four seasons, a strong correlation was found between average daily traffic and metal content in soil and plant leaves along both roads (Table 4).

Discussion

The present study revealed that Zn content in roadside soil along both “Faisalabad to Okara” and “Okara to Lahore” roads was significantly higher than control (~50 m away from road) site. Many studies have described that the Zn content in roadside soil decreased exponentially with per increase in distance from road (Akbar et al. 2006; Akan et al. 2013; Jankowski et al. 2015; Rolli et al. 2016). Significant amounts of Zn in roadside soil come from wear and tear of tyres, from galvanized parts of automobiles (Hjortenkrans 2008), soot and metal oxides, which run off from the road, into soil after precipitation (Malinowska et al. 2015). Furthermore, the release of metals also varies with vehicle type and age, type of fuel used, driving speed and road structure (Smith 1976).

Table 3: Zn contents (mg kg^-1) in roadside plant leaves along FOR and N-5 during different seasons (mean ± SD)

Table 4: Pearson’s correlation coefficient between average daily traffic and Zn content in soil and plant leaves during different seasons along “FOR” and “N-5”

ns=non-significant; * , ** and ***= significant at 0.1, 0.05, 0.01 levels, respectively

Significant variations in Zn contamination existed among sites with highest Zn content at “Tandaliawala” and “Chung” site along “FOR” and “N-5” roads respectively. This was due to high vehicle traffic in these areas. Many studies have reported that sites with high vehicular density are more contaminated with metals (Apeagyei et al. 2011; Duong and Lee 2011; Popescu 2011). At “Pattoki” site minimum contamination was noted that could be due to protection by significant number of plants which include herbs shrubs and trees and the metal content in roadside soil with vegetation shield are significantly lower than without vegetation (Liu et al. 2012). Khan et al. (2011) reported that Zn content in soil ranged from 13.8 to 180 mg kg^-1 along National Highway (Hyderabad, Pakistan). The Zn content in soil found during present study were higher than reported by others studies, 90.43 µg g^-1 in Nigeria (Akan et al. 2013), 123.2 mg kg^-1 in Karak, Jordan (Al-Khashman 2004) and lower than 499.20 mg kg^-1 in Delhi (Banerjee 2003). However, the limit of Zn in soil is 100–150 mg kg^-1 (ECDGE 2010).

A comparison between roads showed higher Zn content along “N-5” than “FOR”. This might be due to high traffic density along “N-5”. The contamination of Zn along roads is always directly linked to the age of road and “FOR” is a newly constructed road so it’s not only about  the traffic density at a specific time but a lifetime traffic flow counts as well (Morse et al. 2016). The presence of Zn in the roadside soil indicated that vehicles are the key anthropogenic source of Zn pollution and the Zn contents may vary at different sites due to meteorological variations, road structure and traffic density (Khan et al. 2011).

The vehicular released metal that gets deposited on roadside soil may enter into plants and plants act as a sink of metal accumulation (Liu et al. 2012). These plants could be used as indicator of metal pollution (Berlizov et al. 2007). In present study maximum Zn contents in plants were recorded in “Tandaliawala “site along “FOR” and “Chung” site along “N-5”. The uptake of Zn by plants at different sites showed a linear relationship with Zn content in soil (Kabata-Pendias 2011).

Fig. 2: Mean of Zn content (mg kg^-1 dry wt.) in plant leaves along “FOR”

Fig. 3: Mean of Zn content (mg kg^-1 dry wt.) in plant leaves along “N-5”

Fig. 4: Spatial variations in mean Zn content (mg kg^-1 dry wt.) in plant leaves along “FOR”

Fig. 5: Spatial variations in mean Zn content (mg kg^-1 dry wt.) in plant leaves along “N-5”

Fig. 6: Temporal variations in mean Zn content (mg kg^-1 dry wt.) in plant leaves along “FOR”

Fig. 7: Temporal variations in mean Zn content (mg kg^-1 dry wt.) in plant leaves along “N-5”

During present study, the mean Zn contents in plant leaves along “FOR” and “N-5” were recorded in the following order R. communis > C. dactylon > C. procera > N. oleander > C. ciliaris. The Zn content in plant leaves varied significantly among different species (Nabulo et al. 2006) due to environmental factors and genotypes (Kabata-Pendias 2011). Hesami et al. (2018) noticed 740 mg kg^-1 Zn uptake in Roemeria hybrid. In Gillgit, Pakistan, the highest Zn content (271.0 mg kg^-1) was found in Brassica campestris and 247.0 mg kg^-1 in Malva sylvestris (Khan et al. 2010). The standard limit of Zn in plant is 100 mg kg^-1 (Allen et al. 1974). However, the safe limit of Zn for plant recommended by FAO/WHO (2011) is 60.0 mg kg^-1. The mean Zn content in all plant species along both roads under investigation were above the permissible limit except for C. ciliaris along “FOR”, while at control site were within the safety limit. Concentrations of Zn in plants at polluted sites persisted to be significantly higher than control site plants. This showed a direct relationship of plant metal content with traffic volume signifying vehicles as the major source of heavy metals. All the selected plants are good indicator of Zn toxicity however, R. communis with highest levels of Zn could be used as the best indicator among all selected plant species and can be used to monitor and ameliorate heavy metal pollution along roadsides. All the plant species and soil samples showed highest Zn contents in summer and lowest in winter season. This might be due to high traffic density and wear of tyres at high temperature (Aksoy et al. 2000; Zaidi et al. 2005; Naveed et al. 2010).

A significant amount of Zn was found in petrol, diesel, used motor oil and soot (Fig. 10). A high level of Zn in diesel and lubrication oil was detected by Betha et al. (2012). Chin-Hsiang et al. (2009) observed that the relative contents of Zn by weight were 19.9% in diesel soot. Traces of Zn (0.7%) in soot were detected by Uy et al. (2014). Metal in soot was also reported by Fino et al. (2016). Diesel soot is responsible for 1/4^th of total perilous atmospheric pollution (Omidvarborna et al. 2014). Hence, it can be concluded that a considerable amount of Zn come into the surroundings from vehicle soot (Malinowska et al. 2015).

Conclusion

Metal (Zn) contamination in the soil and plants along roadside was higher as compared to the control site (~ 50 meters from roadside). These concentrations were higher than the permissible levels set by WHO/FAO. This shows that environment along these roads is contaminated with vehicular released Zn metal. This could be hazardous for crops along roads and human residing near roads. Safety measures are requisite to overcome this toxicity problem. The results also indicated that all selected plants are reasonable indicators of vehicular related Zn pollution in the area. The maximum Zn uptake was detected in R. communis during Summer season so it could be a good choice for phytomonitoring purpose.

Fig. 8: Comparison among roads for mean Zn contents (mg kg^-1 dry wt.) in soil 

Fig. 9: Comparison among roads for mean Zn contents (mg kg^-1 dry wt.) in plant leaves 

Fig. 10: Zinc content in fuel and soot (mg kg^-1)

Acknowledgement

This research project was funded by Higher Education Commission (HEC) of Pakistan (Indigenous Ph.D. Fellowship Program).

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