Current World Environment

Introduction

Air pollution is an inevitable harmful by product of rapid industrialization and urbanization and is responsible for variety of deleterious effects on both human and plant communities. Air quality has now become a major environmental concern since the beginning of industrialisation, resulting in a greater release of gaseous and particulate pollutants into the atmosphere¹. In Asian countries including India the increasing urban population growth, industrial activities and automotive traffic have caused serious deteoriation of air quality and is considered the most dangerous, as its control is more complex than any other type of environmental problems and its amelioration is not feasible by physical and chemical methods^2,3. However, plantation with suitable species have been considered as one of the best suitable abatement technique for its control. In plants the effects of poor air quality are often apparent on the leaves which are usually the most abundant and obvious primary receptors of large number of pollutants and provide an enormous area for impingement, absorption and accumulation of pollutants, thereby decreasing the pollutant level in the environment⁴.

Plants vary in their sensitivity and tolerance to various kinds of toxins and hence their characterization helps in using them as a bio-indicator for monitoring atmospheric pollution⁵.  Consequently bio monitoring of plants is now being considered as an important tool to evaluate the impact of air pollution⁶. In order to adapt to deteriorating air quality viz. stresses plants undergo many physiological and biochemical changes before any visible change in their morphology. The ability of each plant species to absorb and adsorb pollutants on their foliar surface varies greatly and depends on several biochemical,physiological and morphological characteristics⁷. Based on these biochemical parameters such as ascorbic acid, chlorophyll, leaf extract pH and relative water content APTI is calculated to determine the tolerance and sensitivity of plants to air pollution and is effective in evaluating the effects and the ways to combat these effects^8,9. Since industrialization and urbanization has now become the way of life in the economy and Himachal Pradesh is no exception. Therefore, present study was conducted to screen out the tolerant plants in the industrial region by evaluating their APTI so that suitable plants may be screened for taking up plantations in such regions to breath quality air.

Material and Methods

The study was conducted in major industrial area of Himachal Pradesh located at Baddi in the foothills of Shiwalik range. The industrial town lies on the border of Punjab and Harayana states. It is situated in south-western side of Solan district and is about 45 km west of Solan. A detailed survey was conducted in the industrial area of Baddi to study the distribution of industries and vegetation in the region. Keeping in view the distribution of industries in the region five distances from (0-100 m), (100-200 m), (200-400 m), (400-800m), ( > 800m) from the central location of the industrial cluster were selected. To compare the effect at different distances the site falling at > 800m was considered as control. At each distance the commonly occurring tree species namely Azadirachta indica, Dalbergia sissoo and Leucaene leucocephala were selected. In total there were 45 treatment combinations which were replicated four times i.e. at each distance four replications were taken in all the directions of industrial hub. Under each replication four trees were considered. The observations were recorded during three seasons summer (March-April), monsoon (July-August) and Winter (November-December) months respectively.

Sample collection and sample analysis

In order to maintain the uniformity mature plants growing at iso-ecological conditions were selected for the study. To assess the APTI fully matured leaves of selected species were collected in the morning hours. The collected leaf samples were transported to laboratory for further analysis in ice box and were analyzed using following standard procedures for various physiological and biochemical parameters namely ascorbic acid¹⁰, total chlorophyll content¹¹, leaf extract pH¹² and relative water content¹³

Ascorbic acid

To estimate leaf ascorbic acid content of selected plant species 10g of the sample was taken. The sample was homogenized in metaphosphoric acid (3%) and filtered. The volume of the filtrate was made to 100ml by metaphosphoric acid (3%). The aliquot measuring 10ml was taken and titrated against standardized dye to an end point of pink colour as per the standard procedure outlined by Association of Official Agricultural Chemists¹⁰.The ascorbic acid content was expressed in milligrams per grams (mg g^-1.)

Total chlorophyll

For the estimation of total chlorophyll content of the leaves, 10mg of the leaf sample was homogenized with 7ml dimethyl sulphoxide and was kept in oven at a temperature of 60-65ºC for 30-35mins. The samples were filtered and volume was made to 25ml by dimethyl sulphoxide. The absorbance was measured at 663nm and 645nm in spectrophotometer and chlorophyll was estimated using the following equation given by Hiscox and Istaelstam¹¹

Where;            

V is volume of extract made

a is length of light path in cell (usually 1cm)

W is weight of sample

A₆₄₅ is absorbance at 645nm

A₆₆₃ is absorbance at 663nm

Leaf extract pH

Leaf extract pH of the sample was analyzed by the method suggested by Barrs and Weatherly¹². Fresh leaf sample (10g) was homogenized using deionised water (50ml) and the supernatant obtained after centrifugation was collected for the determination of pH using a digital pH meter.

Relative water content

Relative water content of the samples was estimated using the method proposed by Singh¹³ and was computed by using following equation

Where;

RWC is relative water content (%)

FW is fresh weight of leaf sample

DW is dry weight of leaf sample

TW is turgid weight of leaf sample

Air pollution tolerance index (APTI)

The Air pollution tolerance index based on leaf biochemical parameters was computed by using the following formulae given by Singh and Rao¹⁴.

Where;

A is ascorbic acid (mg g^-1) of leaf sample

T is total chlorophyll (mg g^-1) of leaf sample

P is leaf extract pH of leaf sample

R is relative water content (%) of leaf sample

Statistical analysis

The results were analyzed using RBD factorial by OP stat software and the treatment means were compared by means of LSD.

Results and Discussion

After performing and analyzing all the bio-chemical parameters of plant leaves and resultant APTI, following results has been concluded and the average values of the data has been presented in  tabular form.

Ascorbic acid content

The  plant species growing around industrial hub of Baddi found to exhibit significant variation in  the leaf ascorbic acid content (Table 1).Significantly, highest ascorbic acid content of 8.42 mg g^-1 was recorded in A.indica followed by L. leucocephala (6.58 mg g^-1) whereas, lowest of 5.08 mg g^-1 was noticed in D. sissoo. The ascorbic acid of this range in   A. indica has also been reported by Trapathi et al.¹⁵. The higher ascorbic acid content in the leaves of A.indica as compared to other species may be probably due to its higher adaptive capacity to tolerate air pollution and other stresses. The increase in leaf ascorbic acid may also be due to improvement in the defense mechanism of the trees which has also been reported to vary with plants¹⁶. The results are in conformity with Cocklin¹⁷ and Aguiar-Silva et al.¹⁸ who advocated that higher ascorbic acid content of the plant is a sign of its tolerance against pollution. The concentration of leaf ascorbic acid content during different seasons varied from 5.74 - 7.73 mg g^-1. Significantly, maximum ascorbic acid content of 7.73 mg g^-1 was noticed in summers followed by winters (6.61 mg g^-1) and rainy (5.74 mg g^-1) season respectively. The higher production of ascorbic acid in the leaves of selected tree species might be due to relatively more stress conditions during summers followed by winters and rainy season. The, season wise variation in the ascorbic acid content is in agreement with the findings of Jyothi and Jaya¹⁹. The leaf ascorbic acid content of tree species growing at different distances ranged from 5.98 – 7.43 mg g ^-1 which decreased with the increase in distances from the central hub of the industrial area. Significantly, highest ascorbic acid  content of 7.43 mg g^-1 was noticed in trees growing at 0-100 m and the distance wise order of ascorbic acid content in trees was 0-100 > 100-200 > 200-400 > 400-800 > 800 m (control). The higher pollution near the industries in the Baddi area (0-100 m) might have hastened the ascorbic acid content of the leaves of the selected trees as a p art of the defence mechanism of the species against the stress caused due to pollution load. The results are in line with the findings of Bhattacharya et al.²⁰ who have also recorded higher values of ascorbic acid in plant leaves at polluted sites. Pollution load dependent increase in ascorbic acid content of all the tree species may also be due to the increased rate of production of reactive oxygen species (ROS) during photo-oxidation of SO₂ to SO₃ where sulfites are generated from SO₂ as noticed by Aghajanzadeh et al.²¹. The interaction between species x season and distance was also found to be statistically significantly, highest content of 10.05 mg g^-1 was recorded in A. indica growing at a distance of 0-100 m during summer season which was followed by 9.66 and 9.55 mg g^-1 content at the same distance and during same season whereas, minimum content of 3.89 mg g^-1 was recorded in D. sissoo at the control site during rainy season.

Chlorophyll Content

The leaf chlorophyll content of selected plant species growing around industrial hub of Himachal Pradesh exhibited significant variation. The highest leaf chlorophyll content of 1.59 mg g¹ was recorded in L. leucocephala followed by D. sissoo (1.51 mg g^-1) whereas, minimum (1.36 mg g^-1) was noticed in A. indica. (Table 2).The results are in line with findings of Katiyar and Dubey²² and Ninave et al.²³ Similarly, seasons of the year also exerted significant influence on leaf chlorophyll content of the trees growing in the industrial area. The highest chlorophyll content of 2.51 mg g^-1 was recorded during rainy season followed by 1.11 mg g ^-1 content in winter and minimum (0.84 mg g ^-1) in summer season. Similar results were obtained by Jyothi and Jaya¹⁹ who also reported high chlorophyll content in plants during rainy followed by winter and summer seasons. The chlorophyll content was higher in rainy season which may be due to the washout of dust particles from the leaf surface, low level of pollution in the atmosphere and sufficient moisture in the soil. Low chlorophyll content in winter season may be due to the high pollution level, temperature stress, low sunlight intensity and short photoperiod. The trees growing at different locations from the industrial hub were found to differ significantly in their leaf chlorophyll content. The leaf chlorophyll content  of trees growing at different distances from the industrial hub ranged from 1.20 -1.76 mg g ^-1 the leaf chlorophyll was found to  increase with the increasing distance and the order was  0-100 < 100-200 < 200-400 < 400-800 < 800 m (control). The results are in line with Mir et al.²⁴ and Tripathi and Gautam²⁵ who also observed in their studies that high level of automobile pollution reduce chlorophyll pigmentation in leaves of plants near roadside. The present results are also in line with the observations of Wang and Lu²⁶ and Karmakar et al.²⁷.The three way interaction between species x season and distance had a statistically significant variation in leaf chlorophyll content the highest leaf chlorophyll content of 3.14 mg g^-1 was recorded during rainy season in D. sissoo growing away from the industrial area, i.e., > 800m (control), whereas, minimum (0.60 mg g^-1) was noticed during summer months in A. indica growing nearer to industry, i.e., at distance of 0-100 m. In all the species the leaf chlorophyll content tended to increase with the increase in distance from the industrial hub and from the stress period of summer and winter months to rainy season.

Table 1: Seasonal variation in leaf ascorbic acid content (mg g-1) of plant species growing at different distances (m) in industrial area Click here to View table

Table 2: Seasonal variation in leaf chlorophyll content (mg g-1) of plant species growing at different distances (m) in industrial area  Click here to View table

Leaf Exract pH

The significant variations in the leaf extract pH under study is presented in Table 3. The significantly highest leaf extract pH was found in A. indica (6.36) which was statistically at par with D. sissoo (6.32) and minimum (6.25) was recorded in L. leucocephala. The results are in line with Horaginamani et al.²⁸ who reported same range of leaf extract pH in D. sissoo. Significantly, highest leaf extract pH of 6.48 irrespective of the species was noticed in rainy season whereas, lowest (6.06) was in summer season and the order was rainy > winter > summer. The higher pH of 6.48 in rainy season may be due to washing effect of rains to the acidic pollutants. Jyothi and Jaya¹⁹ also reported the maximum pH during monsoon season with gradual reduction through winter and minimum in summer. The leaf pH of selected tree species growing at different distances from the industrial hub ranged from 6.20 - 6.43. The trees growing at non polluted site (control) i.e., at a distance of more than 800 m exhibited significantly highest leaf pH (6.43) which was therefore, statistically at par with (6.32) and minimum leaf pH (6.20) was recorded at a distance of 0-100 m which was however statistically at par with the trees growing at 100-200 m distance from industrial hub.The pH increased with the increasing distance and the order was 0-100 < 100-200 < 200-400 < 400-800, which may be due to more pollution load near the industries which might have induced more acidity in the leaves of the trees. The results are in line with the finding of Singare and  Talpade²⁹ and Subramani and Devaanandan³⁰ who reported that pH followed an exponential decrease with increase in the traffic pollution and drifted towards acidic range.  The interaction between species x season and distance with respect to leaf extract pH was found statistically non significant.

Leaf Relative Water Content

The relative water content of the selected tree species growing in and around the industrial area of Baddi varied significantly (Table 4). In species relative water content ranged from 65.69-67.63 %. The highest relative water of 67.63% was recorded in D. sissoo followed by A. indica (66.66 per cent) whereas, lowest of 65.69% was found in L. leucocephala. These results are similar to those of Nwadinigwe³¹ who reported that relative water content is due to the difference among plant species. The maximum relative water content of D. sissoo may be attributed to its tolerant nature towards pollution. These results are in conformity with findings of Gholami et al.³² who reported that transpiration rates are frequently high under polluted conditions therefore, maintenance of relative water content by the plant may determine its relative tolerance to pollution. Likewise, seasons also exerted a significant influence on leaf relative water content of the selected trees. The maximum relative water content of 80.10 per cent was recorded during rainy season followed by winter (64.43 per cent) and minimum of 55.45 per cent was recorded in summer season. The trend was in line with the results reported by Jyothi and Jaya¹⁹ who also observed highest relative water content in monsoon season followed by winter and rainy season. Significant variations were found in the relative water content of the leaves of trees growing at different distances in the industrial area. The leaf relative water content of plant species growing at different distances was found to decrease with the increase in distances from the central hub of the industrial area and was in the order of 0-100 > 100-200 > 200-400 > 400-800 > control. The higher relative water content near the industrial hub (0-100 m) may be ascribed to higher pollution load which might have resulted in increased water content in the leaves to maintain protoplasmic permeability under stress conditions. This is in agreement with the findings of Jyothi and Jaya¹⁹ who reported that the plants which have ability to adapt by enhancing leaf relative water content under polluted conditions. Similar results have also been reported by Tanee et al.³³ who pointed out that plants at polluted site absorbed more water, which could be a physiological mechanism of the plants to withstand the effect of pollution in its environment. The study concluded that during rainy season D. sissoo growing at 0-100 m  distance contained relative water content to the tune of 86.05 per cent  which was significantly higher as compared to other treatment combinations. The interaction between species x season and distance showed minimum relative water content of 48.45 per cent in A. indica during summer season at non polluted site (control) which was however, found to be statistically at par with D. sissoo and A. indica growing at control site and   at 400-800 m distance during summer season.

Table 3: Seasonal variation in leaf extract pH of plant species growing at different distances (m) in industrial area  Click here to View table

Table 4: Seasonal variation in leaf relative water content (%) of plant species growing at different distances (m) in industrial area  Click here to View table

Air Pollution Tolerance Index (APTI)

Among the trees growing in and around industrial area A. indica irrespective of seasons and distances registered the  highest APTI  value of 13.08 followed by L. leucocephal (11.61) and D .sissoo (10.68). The highest APTI of A. indica (13.08) indicated its higher tolerance level to air pollution (Table 5). Singh and Rao¹⁴ reported that trees which have higher index value are tolerant to air pollution and can be used as sink to control pollution, while trees with low index value show less tolerance and can be used to indicate levels of air pollution.

Similar results are also reported by Gholami et al.³² and Lohe et al.³⁴ that APTI is a species dependent plant attribute which expresses the inherent ability of plants to encounter stress arising from pollution. The variation in tolerance to the trees of a region to air pollutant has also been reported by Karthiyayini et al³⁵, Agbaire and Esiefarienrhe³⁶, Begum and Harikrishna³⁷. The Seasonal variations of the APTI of selected plants were also found to be significant. The maximum APTI value of 13.14 was recorded during rainy season followed by winter (11.38) and summer (10.86) season. The trees species growing at various distances from the industrial hub of Baddi also exhibited significant variations in their APTI values. The tree growing at 0-100 m distance found to have maximum APTI value of 12.58. Whereas, those growing at a control site (> 800 m) registered minimum value of 10.91.The results are in line with the findings of Randhi and Reddy³⁸ who revealed that tree species in the polluted environment improve their capacity to survive and become tolerant to air pollution. Relatively less APTI value of plants growing away from industrial hub and at control site may be ascribed to less level of industrial pollution.The interaction between species x season and distance was found to be statistically significant, out of the three selected species A. indica growing within  0-100 m distance registered highest APTI value of 14.98 during rainy season. On the other hand minimum APTI value of 8.30 was noticed in case of D.sissoo growing at the control site (> 800 m) during summer season.

Table 5: Seasonal variation in leaf APTI of plant species growing at different distances (m) in industrial area  Click here to View table

Conclusion

The plants growing near the industrial region developed tolerance to air pollution by adjusting their biochemical parameters as compared to those growing at farther distances. Relatively A.indica emerged as tolerant species among the selected species and therefore should be recommended for developing green belt in the industrial region of Himachal Pradesh.

Acknowledgment

The authors acknowledge the Department of Environmental Sciences, Dr. Y.S Parmar UHF, Nauni, Solan for providing necessary facilities and sincere gratitude is expressed to Dean College of forestry for providing financial support to conduct the study.

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