EVALUATION OF AIR POLLUTION TOLERANCE INDEX OF TREE SPECIES OF BIKANER CITY (RAJASTHAN)

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INTRODUCTION
Air pollution has emerged as a global public health problem.It has been identified as a major environmental health hazard.There are many causes of air pollution, including industrialization [1,2], urbanization [3,4], vehicle exhaust, deforestation, mining etc. [5].The number of vehicles is increasing day by day.It was observed that India became the fourth largest automobile market in 2017 [6].India was the fourth largest emitter of greenhouse gases (GHGs) and accounts for about 5 % of total GHG emission at the global level [7].The total number of registered cars and motorized two wheelers in India was 137 million in 2012 [8].Vehicle exhaust emission depends on various factors, including fuel quality, maintenance and servicing of vehicles, quality of roads etc.It has been observed that the diesel vehicles emit more particulate matter and oxides of nitrogen compared to petrol vehicles; therefore, diesel is less eco-friendly fuel than petrol [9].Due to the government subsidy for diesel, the share of diesel cars was reached 58 % in 2012-13 [10] from 27 % in 2006 [11].
The multiple pressures on the environment lead to the release of various pollutants into the atmosphere, resulting in drastic environmental and ecological impacts.The calculation of detrimental effects of pollution in terms of monetary value is very difficult, but an estimate from OECD (Organization for Economic Co-operation and Development) suggest that ambient air pollution alone may cost India more than 0.5 trillion dollars per year [12].About 80 % of cities violate air pollution regulation standards in India [13].The plants are very sensitive to such pollutants and therefore respond accordingly.Such plant species are considered bioindicators of air pollution.They are used as a tool for prediction and recognition of various environmental stresses [14,15].Various pollutants can enter into the plant tissues through opening of stomata [16], cuticle [17], cell walls and membranes [18].
Several studies have been carried out on the physiological and biochemical responses of plants to various pollutants [19 -22].The plants exposed to air pollutants show physiological changes prior to the visible damage to leaves [23].These physiological changes include pH of leaf extract [24], relative water content [25], chlorophyll content [26 -28] and ascorbic acid content [26, 29 -31].The response of plants to various pollutants is different for different species.The analysis of individual physiological parameter is not enough to describe cumulative impact of air pollution.Therefore, an air pollution tolerance index (APTI) serves as powerful tool for assessing overall impact of pollution on plant species.APTI is a realistic method which evaluates the tolerance level of plant species towards air pollution by assessing biochemical parameters of its leaves.APTI provides reliable information for establishing green belts in industrial area and selection plant species with regard to their sensitivity to pollution [32].After reviewing the researches carried out by different researchers [33 -37], the sensitivity of various plants to different air pollutants can be classified as shown in Table 1.The sensitive species can be used as bioindicators, while tolerant species can be used as a sink for air pollution.Several studies have been carried out on APTI at national and international levels [38 -42], but vegetation of the city of Bikaner in North-Western Rajasthan has not been investigated from their air pollution tolerance perspective.Therefore, the present study was carried out to assess the APTI of selected plant species commonly found along the roadsides of the residential, commercial and industrial area of the Bikaner city.

Study area
The city of Bikaner is located in the North-Western part of Rajasthan between 28.02°N and 73.3°E (Figure 1).According to the 2011 census, the city had 644,406 inhabitants.The region has an arid type of climate with an average annual rainfall of 277.55 mm in the district (1991 -2010), while the normal rainfall (1901 -1971) is lower than average rainfall and amounts to 257.8 mm [43].
Almost 90 % of the total annual rainfall falls during the south-west monsoon, which enters the district in the first week of July and withdraws in the middle of September.The study area is located in the central area of the Thar Desert, which measures extreme values of heat in the summer and cold in the winter.The temperature varies from about 1 °C in the winter to 48 °C in the summer.Atmosphere is generally dry, except for the monsoon period.
The humidity is highest in August with average daily relative humidity of 71 % in the morning and 52 % in the evening.

Sampling and analyses
The assessment of air pollution tolerance index (APTI) of vegetation of the city started with selection of tree flora.The study was carried out in the period from January to June 2018.After conducting a random survey, top twenty tree species were selected for the present investigation.The selection of these plants was based on their common presence at road sides of the residential, commercial, and industrial area of the Bikaner city.The top twenty tree species commonly found in the city are: Acacia nilotica, Acacia senegal, Ailanthus excelsa, Albizia lebbeck, Alstonia scholaris, Azadirachta indica, Cassia siamea, Colophospermum mopane, Dalbergia sissoo, Delonix regia, Eucalyptus camaldulensis, Ficus benghalensis, Ficus religiosa, Leucaena leucocephala, Moringa oleifera, Pongamia pinnata, Prosopis cineraria, Salvadora oleoides, Salvadora persica and Tecomella undulata.Three specimens of fully matured leaves of selected trees (having girth at breast height (GBH) > 20 cm) were collected in polythene bags, labelled and immediately brought to the laboratory for analysis.Standard methods were adopted for analysis of total chlorophyll [44], pH of leaf extract [33], relative moisture content [45] and ascorbic acid content [31].The previously mentioned parameters were used for analysis of air pollution tolerance index (APTI).The APTI values for different plant species were computed by using following formula proposed by Singh and Rao [33]: where is: A -ascorbic acid (mg/g), T -total chlorophyll (mg/g), P -pH of the leaf extract, R -relative water content of a leaf (%).

RESULTS AND DISCUSSION
The average values of different parameters and APTI are shown in The moisture content of the plant maintains the turgidity of the leaves and supports the vital physiological balance under stress conditions.Any environmental stress, such as air pollutants, can increase the water content in leaves to reduce the stress.Accordingly, a higher relative water content in the leaves of a plant is directly proportional to tolerance of that plant species to the stress caused by emission of pollutants.The results of the present investigation are positively correlated with earlier findings [47 -49].Dhankar et al. [48] have found that the relative water content is an indicator of stress and that its concentration increases with the induction of stress in the form of air pollutants.Sharma et al. [49] showed that Grevillea robusta had higher relative water content with increasing air pollution.
The results of the analysis of the total chlorophyll content have shown that tolerant plant species, i.e., Leucaena leucocephala (14.12 mg/g), Eucalyptus camaldulensis (13.42 (21.39).Pongamia pinnata (9.48) showed APTI lesser than 10, therefore it belongs to the sensitive category.The remaining 16 species had the APTI values from 10 to 20, which is why they are categorized as moderately sensitive species.No tree species were found in very sensitive (APTI < 1) and very tolerant category (APTI > 30).Since roads and highways are one of the important line sources of air pollution, suitable plant species must be planted along major roadsides of the city to fight air pollution.It will not only increase the greenery, but it will also help to reduce air pollution and ultimately improve the social as well as environmental health.0% 5% 80% 15% 0% Very sensitive Sensitive Moderately sensitive Tolerant Very tolerant

Figure 1 .
Figure 1.Location of Bikaner district in Rajasthan

Table 2 .
Average values of different physiological parameters and APTI of top 20 tree species of the Bikaner city Leucaena leucocephala (23.41),Eucalytptus camaldulensis (21.87) and Cassia siamea