INTRODUCTION:

The origin of urban air pollution is mainly in anthropogenic emission sources, which include automobiles, industries and domestic fuel combustion. The air pollutants so generated are detrimental to human health (Thambavani and Kumar, 2012). In urban areas, the mobile or vehicular population is predominant and significantly contributes to air quality problems. Automobiles produces volatile organic compounds (VOC), suspended particulate matter (SPM), oxides of sulfur (SOx), oxides of nitrogen (NOx) and carbon monoxide (CO), which have adverse effects on surrounding ecosystem (Wagh et al., 2006). Monitoring of air pollutants is a prerequisite to air quality control. Rapid industrialization and addition of the toxic substances to the environment are responsible for altering the ecosystem (Sarala Thambavani and Saravanakumar, 2011; 2012). Their impact on the chemical composition of plants is often used as an indicator of and a tool for monitoring environmental pollution (Rao, 1977; Posthumus, 1984, 1985; Agrawal and Agrawal, 1989; Kulump et al., 1994; Dmuchowski and Bytnerowicz, 1995).

Indian cities are facing serious problems of airborne particulate matter (Agarwal et al., 1999). Agricultural activities and vehicular traffic may generate local dust concentrations close to the source that exceed environmental guideline values (Leys et al., 1998; Manins et al., 2001). The deposition of gaseous pollutants and particulate matter and their interception are greater in woodlands than in shorter vegetation (Fowler et al., 1989; Bunzl et al., 1989). Trees act as a sink for air pollutants and thus reduce their concentration in the air. When plants are exposed to the environmental pollution above the normal physiologically acceptable range, photosynthesis gets inactivated (Miszalski and Mydiarz, 1990). Dust interception capacity of plants depends on their surface geometry, phyllotaxy, and leaf external characteristics such as hairs, cuticle etc., height, and canopy of trees. Leaf petioles are more efficient particulate impactors than either twigs (stems) or leaf lamina (Ingold, 1971).

The complex photosynthetic process also in greenhouse conditions are open field conditions are influenced by the degradation of chlorophylls, thus the quantification of the exact chlorophyll content of the two types of chlorophyll a and b can be used in the characterization of the photosynthetic capacity of the plants (Jakab and Pamfil, 2011). The photo-inhibition in additional way is associated but not necessary accompanied by the degradation of the chlorophylls (Critchley, 1998). During the last few decades’ increased human interference, urbanization and heavy vehicular activity in Madurai city has resulted the changes air quality (Sarala Thambavani and Saravana kumar, 2011).

The present investigation has been undertaken to study the effect of atmospheric pollutant on photosynthetic pigments of selected plant species. In the present study, the pollution effects on the performance of selected plant species was observed and the total chlorophyll content decreased significantly in response to automobile exhaust pollutants in polluted plant leaves compared with control of Azadirachta indica(L), Pongamia pinnata(L), Delonix regia(L), Polyalthia longifolia(L) and Ficus religiosa(L).

MATERIAL AND METHODS:

Study Area Description:

Madurai city has grown on both sides of river Vaigai and its terrain is mostly flat. The ground rises from the city, towards outward, on all sides except the south, which is a gradually sloping terrain. It is surrounded on the outskirts by small and prominent hills. The city is about 100 meters above mean sea level and it is situated on 9⁰55’ north latitude and 78⁰7’ east longitude and the city is covering 51.96 sq.kms that comprises of a total population of 25,78,201 persons (Census 2011). Whereas the Madurai Urban agglomeration comprising the city and surrounding settlements accommodates a population of 12,03,095 persons. The climate of Madurai town is hot and dry and the temperature range between a maximum and minimum of 42⁰C and 21⁰C respectively. April and May are the hottest months and rainfall is irregular and intermittent, with an average of approximately 85 cm per annum. The wind blows from northeast direction during January– February and from southwest direction during May to July. The phenomenal growth in population coupled with the growth of vehicles and increasing transport demand have created numerous transportation problems in the city, particularly deterioration of environmental quality, resulting in an increased air pollution and traffic noise (Sivacoumar, 2000).

Photosynthetic pigments Analysis:

Chlorophyll a and b impart the green color that one associate with plant leaves. Carotenoids, which are yellow pigments, are also present in leaves but are usually masked by the chlorophylls. It is only in the fall when the chlorophylls are degraded faster than the carotenoids that the yellow color becomes visible to us. The chlorophyll and carotenoid contents of plants can vary markedly with its age or depend on environmental factors such as light intensity or quality during growth. Carotenoids and chlorophylls are found in the chloroplasts and are associated with the thylakoids, the internal membrane network of these organelles. It is now established that all chlorophylls are organized as discrete chlorophyll-protein complexes within the lipid matrix of the photosynthetic membrane.

The majority of chlorophyll ‘a’ molecules (and all chlorophyll ‘b’ and carotenoid molecules) functions as antenna pigments. In combination with proteins, they form the light-harvesting complexes, which absorb and funnel light energy to the reaction center chlorophylls, thereby allowing the plant to utilize a broad spectrum of wavelengths for photosynthesis. Some of the chlorophyll ’a’ molecules serve specialized functions in the reaction centers of photo systems I and II, where the light energy is used to drive the reduction of components of the electron transport chain.

Experimental Procedure:

The studies were conducted on Azadirachta indica(L), Pongamia pinnata(L), Delonix regia(L) Polyalthia longifolia(L) and Ficus religiosa(L) plants growing under natural conditions. The plant samples were collected from 50-200 cm height from the ground in the roadside of the study area and control site of the selected plant species and the leaves were carefully removed from the bark, using a snapper blade and washed with water to remove the dust on the surface of the leaf samples. About 1g of leaves, torn into small pieces in a mortar ground with a pinch of quartz sand and a total of 10 mL of absolute acetone. Initially, add only a small amount of acetone to begin the grinding process. It is much easier to grind the leaves if the extract is a pasty consistency. Add more solvent in small increments while continuing to grind the leaves. For some species may need to add more than the suggested 10mL of acetone. Pour the extract into a 15mL centrifuge tube and centrifuge in the bench top centrifuge at 5000 rpm for 3 to 5 min. Remove the extract to a 10mL graduated cylinder using a Pasteur pipette. Transfer an aliquot of the clear leaf extract (supernatant) with a pipette to a 1-cm-pathlength cuvette and take absorbance readings against a solvent blank in a UV-VIS spectrophotometer at 662, 645, 470, 435 and 415 nm wavelength to determine the concentrations of photosynthetic pigments like chlorophyll-a, chlorophyll-b and carotenoids using the formula given by Lichanthaler (1987).The ratio of Absorbance 435nm to 415nm are the parameter for chlorophyll degradation in the experiment (Ronen and Galun, 1984; Sarala Thambavani and Saravanakumar, 2011).

Quantification of pigments (For 100% Acetone):

Chl-a (µg/ml) = 11.24 A_661.6 – 2.04 A_644.8

Chl-b (µg/ml) = 20.13 A_644.8 – 4.19 A_661.6

Carotenoids = (1000 A₄₇₀ – 1.90 C_a - 63.14 C_b)/214

Fig. 1 - Preparing of vegetal samples

Fig. 2. Weighing of the leaf samples

Fig.3- Extraction of chlorophylls in 100% acetone

Fig-4 Comparative analysis of the chlorophyll ‘a’ content in the analyzed samples

Fig-5 Comparative analysis of the chlorophyll ‘b’ content in the analyzed samples

Fig-6 Spectral absorbance of chlorophylls in the visible light

TABLE I. Determination of the optical density by spectroscopy of the chlorophyll extracts

Plant Name	Status	Optical Density A662	Optical Density A645	Optical Density A470	Optical Density A435	Optical Density A415
A.indica	Control	0.8408	0.4372	1.0145	1.5933	1.4065
P.pinnata	Control	0.851	0.5198	1.0978	1.6931	1.6385
D.regia	Control	0.618	0.2708	0.7915	1.2359	1.1178
P.longifolia	Control	1.2588	0.6681	1.3939	2.0569	1.9146
F.religiosa	Control	0.9177	0.3827	0.7974	1.5259	1.2846
A.indica	Polluted	0.5555	0.2382	0.6237	1.0855	0.9031
P.pinnata	Polluted	0.5979	0.2399	0.4303	0.9523	0.8164
D.regia	Polluted	0.5636	0.2366	0.5275	1.0493	0.8393
P.longifolia	Polluted	0.8088	0.3346	0.7153	1.3568	1.1202

Fig-7 Absorbance of the chlorophyll extract of selected healthy plant species

Fig-8 Absorbance of the chlorophyll extract of selected Polluted plant species

Analysis of the Absorbance Spectra

The chlorophyll molecules are components in different photo-systems integrated in the interior of the thylacoid membranes of the chloroplasts. The chlorophylls are absorbing the most intense light from the blue region of the electromagnetic spectra of light, followed by the red region, but they absorb very weak in the green region, from this derives the fact that the vegetal tissues containing the chlorophylls are becoming the green color after this pigment. The absorption of the chlorophylls is between the 400-700 nm, and this region is named the photo synthetically active region (PAR). The comparative analysis of the spectra of the two plant categories reveals the fact that there are notable differences between the absorbance of the chlorophyll content of healthy and polluted plants. The spectral curve of the healthy plants and polluted plants were noted in the figure. The symptoms of healthy leaves without any dust accumulation on the leaves have revealed higher levels of chlorophyll content

RESULTS AND DISCUSSION:

Overall growth and development of plants are functions of various environmental factors such as air, water, and soil (Katiyar and Dubey, 2000). Dust deposition affects the light available for photosynthesis and blocks the stomata pore for diffusion of air and thus put stress on plant metabolism (Eller, 1977; Hope et al., 1991; Keller and Lamprecht, 1995; Anthony, 2001). Decrease in total chlorophyll content in the leaves may be due to the alkaline condition created by dissolution of chemicals present in the dust particulates in cell sap which is responsible for chlorophyll degradation. The major importance in the light assimilation process is due to chlorophyll “a” due to the fact that these are the main responsible for the satisfactory functioning of the PS II photo-system in natural light conditions. The “b” type chlorophylls the main components of the proteins which collect LHCP light and the content of this pigment in the leaves is important for the capacity of the leave to accommodate in shadow conditions (conditions typical in greenhouse). Total chlorophyll content of polluted leaves is lower than that of control leaves and is reported by several researchers (Somashekar et al., 1999; Mandal and Mukherji, 2000).

Results observed that the concentration of photosynthetic pigments is affected by the traffic load and chlorophyll content may vary in all the selected plant species during the sampling period depending on their habitat, climatic condition, pollution level etc. it may vary depending on the air quality of that area. The vehicular emissions have a profound impact on the concentration of different photosynthetic pigments. The shading effects due to deposition of suspended particulate matter on the leaf surface might be responsible for this decrease in the concentration of chlorophyll in polluted area. It might clog the stomata thus interfering with the gaseous exchange, which leads to increase in leaf temperature which may consequently retard chlorophyll synthesis (Mark, 1963; Singh and Rao, 1981). The reason for degradation of chlorophyll pigments can also be attributed to action of SO₂ and NO₂ on the metabolism of chlorophyll (Lauenorth and Dodd, 1981), both of these gases are the constituents of vehicular emissions.

Analyzing the fig. 7 it is revealed the fact that the chlorophyll content is higher in the case of healthy plants , this varying from 7.60 mg/g to the quantity 12.78 mg/gin comparison with the polluted plants at which the chlorophyll “a” content was at a lower level. The content in chlorophyll “a” of the polluted leaves varied between 5.76 mg/g respectively 8.41 mg/g concluding the fact that the chlorophyll content was significantly reduced regarding the analyzed in selected plant leaves. The analyzed leaves presented chlorothic bands and the reduction of the chlorophyll content cannot due because of the leaves senescence, the polluted plantlets low chlorophyll content was probable due to alterations in the biosynthesis of chlorophylls. Similar phenomena were observed in the case of the chlorophyll “b” content analysis at the healthy and polluted plant leaves (Fig.8). According the analysis the chlorophyll “b” content varied 3.86mg/g - 8.17mg/g at the healthy plants. In the case of the polluted plants the chlorophyll “b” content varied between 2.32 - 3.35 mg/g . The reduction in total carotenoids varied from 1.27-2.28 mg/g at polluted leaves compared to healthy leaves in all the plant species. The reductions in chlorophyll ‘a’, chlorophyll ‘b’ and total chlorophyll due to air pollution have been noted (Joshi and Chauhan, 2008). Rao and Leblanc (1966) have also reported reduction in chlorophyll content brought by acidic pollutants like SO₂ which causes phaeophytin formation by acidification of chlorophyll.

The present study revealed a decrease in photosynthetic pigments in all the selected plant species, growing at sites with heavy vehicular traffic area as compared to control area where vehicular traffic is low. This shows clearly the effect of vehicular exhaust on the roadside vegetation in the city. Significant reduction in total chlorophyll content at traffic area was recorded in all plant species. Pongamia pinnata(L) showed much reduction in total chlorophyll content as compared to control. Chlorophyll measurement is an important tool to evaluate the effect of air pollutants on plants. Chlorophyll plays an important role in plant metabolism. The reduction in chlorophyll concentration corresponds directly to the reduction in plant growth and chlorophyll degradation measurements were intended as a parameter of air pollution experiment. The present study suggests that plants have the potential to serve as excellent quantitative and qualitative indices of pollution level. Present study with all the selected plant species showed reduction in the concentration of chlorophyll at heavy traffic area. The rapid urbanization imparts more stress on the vehicular use, which release toxic air pollutants in the urban atmosphere in the developing countries. Monitoring of air pollution, biomonitoring of plants is an important tool to evaluate the impact air pollution on plants. The concentration of different photosynthetic pigments recorded for the selected plant species collected from polluted and control sites have been presented in the table. The following observations were made.

Chlorophyll degradation:

From the result it was found that chlorophyll degradation is highest in polluted area than control which received clean air, no traffic and other smoke producing activities are also absent. It means that if there is less chlorophyll degradation, the amount of chlorophyll content will be more. However the lowest degree of chlorophyll degradation was observed at control sites while the highest were observed at polluted sites.

The amount of chlorophyll degradation was recorded in A.indica at polluted sites varied from 1.20 mg g-1 in comparison with control site 1.13 mg g-1 in the study period. This is due to A.indica(L) has greater pollution retaining capacity the lower amount of chlorophyll degradation was monitored at polluted sites during the study period. In pongamia pinnata(L) the average amount of chlorophyll degradation was observed in control site and polluted site are 1.03 mg g-1 and 1.17mg g-1 respectively. The amount of chlorophyll degradation increases drastically at polluted sites in all the periods. There was maximum chlorophyll degradation in pongamia pinnata(L) leaf samples at polluted site due to heavy traffic load compare to control. It indicates pongamia pinnata (L) suffered by automobile exhaust pollution.

The average concentration of chlorophyll degradation in control site and polluted site in Delonix regia(L) are 1.11 mg g-1 and 1.25mg g-1 respectively at various days. This is due to less concentration of chlorophyll ‘a’ in D.regia at polluted site on that days. It means that larger number of chlorophyll ‘a’ concentration the pigment degradation will be less, if it is low the degradation will be high. There was chlorophyll degradation is minimum in control site of Polyalthia longifolia(L) are 1.07 mg g-1 compared to polluted site 1.21 mg g-1 The average amount of Chlorophyll degradation in Ficus religiosa(L) was recorded 1.19mg g-1 in control compared to polluted site (1.18mgg^-1).

CONCLUSION:

The dust interception capacity of different leaves depends on leaf structure, phyllotaxy, presence or absence of hairs, presence of wax on leaf surface, size of petioles, and canopy structure. Plants with a waxy coating, rough leaf surfaces, and short petioles tend to accumulate more dust than plants with long petioles and smoother leaf surfaces. Dust particles affect leaf biochemical parameters, bringing about some morphological symptoms. The extent of such effects depends on plant tolerance toward dust particles and on the chemical nature of the dust. Decline in pigments may be because of a drop in pigment synthesis due to the shading effect of dust, the alkaline condition caused by dissolution of dust particles in cell sap that may lead to pigment degradation (due to photo bleaching), and/or the inhibition of enzymes essential for biosynthesis of pigments. All these changes exert stress on plant physiology.

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Received on 25.09.2012 Modified on 03.10.2012

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