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Introduction

Atmospheric particulate matter represents a complex mixture of organic and inorganic substances of varying size and may enter an organism or plant in a number of ways. Roadside vegetation communities are greatly affected by the dust from highways and roads due to changes induced in biochemical parameters (ascorbic acid content, pH, Relative Water Content, total chlorophyll content).Particulate pollutants can cause many lethal effects on plants like stomatal clogging, reduced photosynthetic activity, leaf fall and death of tissues (Shrivastava and Joshi, 2002).¹Plants play very important role in improving the ambient air quality. Morphology and internal structure of leaves is altered by heavy load of dust pollutants (Sukumaran, 2012).²Foliar surface of plants is the main receptor of dust as it is continuously exposed to the surrounding atmosphere (Rai and Panda, 2014).⁵The ability of each plant species to absorb and adsorb pollutants by their foliar surface varies greatly and depends on several biochemical, physiological and morphological characteristics (Seyyednejadet al., 2011).⁴The capacity of leaves as dust receptors depends upon their surface geometry, phyllotaxy, epidermal and cuticular features, leaf pubescence, and height and canopy of trees (Nowak, 1994⁵; Singh, 2000⁶; Singhet al.,2002⁷). Smaller plants with short petioles and rough surface accumulate more dust than larger plants with long petioles and smoother leaf surface (Thakar and Mishra, 2010)⁸. Vegetation provides a natural means of cleaning the atmosphere with large leaf area for impingement, absorption and accumulation of air pollutants level in the environment (Das and Prasad, 2010).⁹Reduction has been measured in photosynthetic pigments due to deposition of particulate matters on the upper surface of the leaves (Karmakaret al.2016).¹¹It is well evident from the previous studies that the contents of chlorophyll, ascorbic acid, protein and carbohydrate are affected due to the air pollution (Hoqueet al.,2007¹¹; Flowerset al.,2007¹²). Various biochemical studies of foliar tissues has reported decrease in chlorophyll content and the increase in ascorbic acid content as a result of air pollution.

(Joshiet al.,1997¹³; Gavaliet al.,2002¹⁴). Plants show visible changes depending on their sensitivity level which would include alteration in the biochemical processes or accumulation of certain metabolites (Agbaire and Esiefarienrhe, 2009).¹⁵The climate condition, the physico chemical properties of air pollutants have impact on surrounding plants (Waghet al., 2006).¹⁶Roadside plant leaves are in direct contact with air pollutants, and may act as stressors for pollutants, hence need to be examined for their biomonitoring potential (Sharmaet al., 2007)¹⁷. Biomonitoring of plants is an important tool to evaluate the impact of air pollution (Rai, 2011a¹⁸; Rai, 2011b¹⁹). Keeping in view the above parameters the present study was undertaken to assess the variation in leaf dust accumulation with respect to species and seasons and observe seasonal variation in biochemical parameters of plant species growing alongside the National Highway-7.

Material and Methods

Study area

Sirmaur district lies among the outer Himalayan ranges between 77^o01’12” and 77^o49’40” East longitude and 30^o22’30” North latitude. The location of the study site is presented in Fig. 1. The 40 km stretch of Markanda to Paonta Sahib National Highway (NH-7) in Sirmaur District of Himachal Pradesh was selected as the study area in the present investigations. The study area is facing a severe problem of air pollution due to large vehicular density and expansion of highway as well as increased demographic pressure.

Figure 1: Map showing sampling locations of plant species alongside Markanda to Paonta National Highway (NH-7) in Himachal Pradesh Click here to View figure

Climate

During the study period (2015-2016) in the selected stretch of National Highway (Markanda to Paonta Sahib) no rainfall was experienced during the months of October-November (post monsoon season). Whereas, a rainfall of 18.7 mm was recorded during April-May (pre monsoon season). Maximum temperature varied from 28.95 to 37.45^oC and minimum temperature varied from 8.8 to 18.5^oC. Relative Humidity varied from 53.4 to 67.34% (Fig. 2).

Figure 2:Distribution of minimum, maximum temperature ,rainfall and relative humidity during sampling seasons (2015-2016) Click here to View figure

Sampling of Dust and Leaves

The study was conducted during two seasons i.e. post-monsoon (October- November, 2015) and pre-monsoon (April-May, 2016), respectively. The 40 km stretch of the National highway from Markanda to Paonta Sahib was divided into four equal parts of ten kilometres each (four study locations). In order to study the impact of vehicular emissions on the plants, plants were selected from 0-10 m and 10-20 m horizontal distances from both sides of the road. The commonly occurring plant species namelyFicus roxburghii, Mallotus philippensis, Woodfordia fruticosaandShorea robustawere selected for the study.The morphological characteristics of the selected plant species are given in Table 1. Fully mature leaves of the selected plant species were taken for the present studies. The upper surface of the leaves was cleaned with fine brush and identification mark was put on them. These leaves were kept for 24 hours for dust accumulation and were collected in the pre weighed butter paper bags with the help of fine brush. After taking the data for dust accumulation, the leaves were cut from petiole, kept in ice box and brought to the laboratory for further analysis. The individual leaf (in m²) was measured with Leaf area meter (Model-LI-COR-3100). The samples were weighed using top pan electronic balance and the amount of dust was calculated using the equation W= (w₂- w₁) / a, Where w is dust content (mg m^-2), w₁is initial weight of butter paper bag,w₂is final weight of butter paper bag with dust and a is total area of the leaf (m²). In order to study the correlation of weather parameters of different seasons and its relationship with dust accumulation and pollution on the leaves of selected plant species the weather data was procured from (Fig.2).

Table 1: General description of selected plants species

Biochemical analysis

Ascorbic acid content was estimated by using A.O.A.C (1980)²⁰method. Total chlorophyll content (Hiscox and Israeistam, 1979)²¹and relative water content (Liu and Ding, 2008)²²was also determined. 5 g leaves was homogeneized using distilled water and pH of the filtrate was detected using digital pH meter.

Results and Discussion

Leaf dust accumulation

The seasonal variation in leaf dust accumulation of different plants under study is presented in Fig. 3. The maximum dust was accumulated inFicus roxbughii(38.30 mg m^-2) which was significantly different from all other values (Table 2). Whereas, minimum dust was noticed inWoodfordia fruticosa(16.70 mg m^-2) which was at par withMallotus philippensis(16.70). It was observed that all plants showed higher dust deposition in pre monsoon season due to construction or road making activities at the study locations (Joshiet al, 2014)。²³Further this may be attributed to higher temperature conditions in the study area as compared to post monsoon months (Fig 2).Ficus roxburghiiexhibited highest leaf dust accumulation followed byShorea robusta,Mallotus phiippensis,Woodfordia fruticosa(Table 2).Highest dust accumulation on the leaves ofFicus roxburghiimay be attributed to its broad and coriaceous leaf texture which might have accumulated more dust on leaves and prevented dust fall from leaf surface (Madan and Chauhan, 2015).²⁴Lowest amount of dust observed onWoodfordia fruticosamay be attributed to its smaller leaf size and smooth surface due to which dust settled on the leaf surface may slip down due to gravitational force or even by wind. These results are similar to those of Joshi and Bora (2011)²⁵who reported that dust deposition capacity of plants depends on their surface geometry and leaf external characteristics.The highest dust load of 49.35 mg m^-2was noticed on leaves of plant species growing at the distance of 0-10 m from the National Highway (Table 2, Fig 4) due to high dust intensity which results by the vehicles activity and capturing dust with a gentle wind (Youniset al., 2013 a).²⁶高叶表面灰尘沉积在路边with heavy vehicular traffic may be due to spray of unburnt oil residue of diesel or petrol on the leaf surface (Rahul and Jain, 2014).²⁷These results are also in conformity with the findings of Youniset al. (2013 b)²⁸who reported that dust accumulation is more in plants growing at roadsides due to high dust intensity which results by the vehicles activity and capturing dust with a gentle wind.

Figure 3: leaf dust accumulation of selected plant species durimg sampling seasons Click here to View figure

Figure 4: Leaf dust accumulation of selected plant species at different horizontal distances Click here to View figure

Table 2:Dust accumulation (mg m^-2) of the selected plant species growing alongside the Markanda to Paonta Sahib National Highway (NH-7) during sampling seasons

Ascorbic acid content

The present study reported higher ascorbic acid content during post monsoon season in all the plant species Fig. 5a. Pollution load dependent increase in ascorbic acid content of plant species may be due to more production of antioxidants (ascorbic acid) under stress conditions prevailing in the study area (road construction activities) during the post monsoon season.These results are similar to those of Bhattacharyaet al., (2013)²⁹who reported that ascorbic acid was higher in winter as the pollution load increases in this season due to meteorological conditions. The results are also in line with the findings of Garg and Kapoor (1972)³⁰who reported that dust accumulation caused alkalinity in leaves by chemical dissolution of dust particles in cell sap which results in chlorophyll damaged and in order to accommodate these stresses leaves ascorbic acid content increased.

Total Chlorophyll content

The leaf chlorophyll content of the selected plant species varied from 1.17 mg g^-1(Ficus roxburghii)to 1.64 mg g^-1inShorea robusta(表5)。结果表现出叶片氯的最低ophyll content in all plants during pre monsoon season (Fig 5b) due to maximum dust accumulation. Dust particles carry many polycyclic hydrocarbon and metals with them which inhibits the production of enzymes required for chlorophyll synthesis and results in reduction of chlorophyll content (Prajapati and Tripathi, 2008).³¹

Leaf extract pH

In the present study maximum pH during during pre monsoon season was associated with the higher leaf dust accumulation during pre monsoon months. Dust accumulation caused alkalinity in leaves by dissolution of chemicals present in dust particles i.e., metals and polycyclic hydrocarbons in the cell sap thus increasing the pH (Katiyar and Dubey (2001).³²

Relative water content

所有的植物s showed higher relative water content during post monsoon (Fig 5d) season due to high relative humidity (Fig 2) in the atmosphere during post monsoon season which might have increased relative water content due to less transpiration, whereas lowest was observed during pre monsoon season. The lower relative water content during pre monsoon season may be attributed to more dust accumulation during pre monsoon season (Table 2). Dust may absorb water through non cutinized plant surface such as leaves, stems and branches, contributing to decreased relative water content (Rai and Panda, 2014).³

Figure 5: Variation in ascorbic acid content, total chlorophyll content, pH, relative water content of selected plant species during different sampling seasons Click here to View figure

Acknowledgement

The authors acknowledge the Head, Department of Environmental Sciences, Dr. Y.S Parmar UHF, Nauni, Solan for providing necessary laboratory facilities.The authors are also thankful to the staff of the Department for providing necessary help in some of the analyses. We sincerely express our gratitude to Dean College of forestry for providing financial support to conduct the study.

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