Contact: Department of Plant Biology and Biotechnology, Faculty of Life Sciences, University of Benin, Ugbowo, BEnin City
Background and Justification of the Study Air pollution has been defined as human introduction into the atmosphere of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organism, or damage the environment (Seyyednjad et al., 2013). While the earth is trying to maintain its biosphere consisting of land, water, and air connections, it cannot compete with “human activities” which are “disrupting the natural cycles of Earth” (Kobasa, 2009). Urban pollution has become a serious environmental problem to trees and crops (Chauhan and Joshi, 2008). Chlorophyll measurement is an important tool to evaluate the effects of air pollutants on plants as it plays an important role in plant metabolism and any reduction in chlorophyll content corresponds directly to plant growth (Joshi and Swami, 2009). The exposure of pollutants to leaves cause a reduction in the concentration of their photosynthetic pigments viz., chlorophyll and carotenoids, which affects the plants productivity, germination of seeds, length of pedicels, and number of flowers or inflorescence (Nithamathi and Indir, 2005). Chaurasia et al. (2013) from their investigation on the ‘effect of cement industry pollution on chlorophyll content of some crops at Kodinar, Gujarat, India’ stated that the concentration of chlorophyll in all the selected plant species; Arachis hypogaea, Sesamum indicum and Triticum species was more in all plants that were away from the cement industry in comparison to plants that were close to the industry. The cement kiln dust, containing oxides of calcium, potassium and sodium is a common air pollutant affecting plants in various ways i.e. cement dust and cement crust on leaves plug stomata and interrupt absorption of light and diffusion of gases, lowering starch formation, reducing fruit setting, inducing premature leaf fall and leading to stunted growth. The present knowledge of the effects of air pollution on plants is mostly based on experiments, where plants have been exposed to high concentrations of air pollutants for short periods under experimental conditions. The present study was, therefore, carried out to deter-mine the effect of air pollutant on the chlorophyll pigment and leaf area of Mangifera indica and Anacardium occidentale growing in Dangote Cement Factory and Fourth Republic Housing Estate, classified as polluted and non-polluted areas in Kogi State, Nigeria respectively in this study. The significant impacts of pollutants on the ecosystem call for urgent attention and hence the need to create the awareness to the general public of such impacts. Methodology A comparative study of the effects of air pollutants generated from the polluted site (Dangote’s Cement Factory, Obajana) and non-polluted site (Fourth Republic Housing Estate, Lokoja, Kogi State) on the chlorophyll content and area of leaves was carried out. The chlorophyll contents of some leaves sample of M. indica and A. occidentale in both areas of interest with potentially varying air pollution were determined. The plants selected for the study were those common to both locations. For each location, chlorophyll measurement was carried out on three (3) stands of each of the experimental plants. The chlorophyll contents of ten (10) leaves from each stand at same height from ground level were determined following the method of Hiscox and Israeltam (1979) using Dimethyl Sulphoxide (DMSO) and measuring the absorbance of each extract using 721G UV – VIS spectrophotometer at two wavelengths of 645 nm and 663 nm. Leaf area measurement was also carried out on selected leaves from three (3) stands of each of the experimental plants following the method of Eze (1965). Chlorophyll Extraction Using Dimethyl Sulphoxide (DMSO): One hundred milligrammes (100 mg) of leaf tissue was weighed using a sensitive weighing balance (Ohaus Adventurer). The weighed leaf tissue was shred into fractions and placed in a 20 ml test tube containing 7 ml dimethyl sulphoxide (DMSO) and the chlorophyll was extracted into the DMSO without maceration or by grinding at 65 °C by incubating for 30 – 35 minutes. Additional 3 ml of DMSO was added to make it up to a total volume of 10 ml. Three millilitres (3 ml) of the extract was pipetted into a cuvette using a micropipette and absorbance was read at 663 nm and 645 nm using a UV – VIS spectrophotometer. Determination of Chlorophyll Content: A 3.0 ml sample of chlorophyll extract was transferred to a cuvette, and the optical density (OD) values at 645 nm and 663 nm were read in a Beckman DBG spectrophotometer against a DMSO blank. Chlorophyll content was calculated following the equation of Arnon 1949 cited by Hiscox and Israelstam (1979). Extracts whose OD values were greater than 0.7 were diluted to 50 % with 90 % DMSO and the equation adjusted accordingly. The chlorophyll content was calculated using the following formulae: Chlorophyll a (mg/g tissue) = (12.70A663 – 2.690A645) x V 1000 x W Chlorophyll b (mg/g tissue) = (22.90A645 – 4.880A663) x V 1000 x W Chlorophyll content = Chlorophyll a (mg/g tissue) + Chlorophyll b (mg/g tissue) Where, A = absorbance at specific wavelength, V = final volume of the extract in 100 % DMSO W = fresh weight of tissue extract Determination of Leaf Area: The leaf area of M. indica and A. occidentale were determined using the method of Eze (1969). The leaf area of thirty leaves from each stand of the experimental plants were determined. The leaves were traced on graph sheets and the traced portions cut out and weighed using a sensitive weighing balance (OHAUS Adventurer). Having determined the weight of the traced portions, the areas of the leaves were determined following the same mathematical relations used by Eze (1969). Result and Discussion Results showed that the air pollutants from the cement factory had effects on the chlorophyll concentration of the leaves of Mangifera indica and Anacardium occidentale with no significant effects on the leaf areas. As seen in the Tables 1 and 2, there were reductions in the chlorophyll concentration of the leaf samples of M. indica and A. occidentale in the polluted site when compared to those in the non-polluted site. Site Chlorophyll a concentration (mg / g) Chlorophyll b concentration (mg / g) Total chlorophyll concentration (mg / g) Non - polluted 0.6937 ± 0.0281b 0.1812 ± 0.0234c 0.9693 ± 0.0373c Polluted 0.6357 ± 0.0264b 0.1171 ± 0.0115b 0.7528 ± 0.0339b Table 1: Chlorophyll Concentration Index of M. indica in the Study Areas ⃰ Mean value of thirty leaves ± standard error of mean (SEM) with different alphabets are significantly different from each other (P < 0.05). The one way ANOVA showed that there were significant differences (P < 0.05) in chlorophyll b and total chlorophyll concentration values of M. indica at the two study areas but no significant difference in chlorophyll a concentration (P > 0.05) was recorded. Reduction in chlorophyll a, b as well as total chlorophyll concentration of A. occidentale were significant at 0.05 % level as shown in Table 2 Site Chlorophyll a concentration (mg / g) Chlorophyll b concentration (mg / g) Total chlorophyll concentration (mg / g) Non - polluted 0.7590 ± 0.0246c 0.5150 ± 0.0691c 1.1795 ± 0.0285c Polluted 0.6356 ± 0.0345b 0.3682 ± 0.0367b 1.0038 ± 0.0625b Table 2: Chlorophyll Concentration Index of A. occidentale in the Study Areas ⃰ Mean value of thirty leaves ± standard error of mean (SEM) with different alphabets are significantly different from each other (P < 0.05). The percentage of chlorophyll reduction of M. indica and A. occidentale as presented in Table 3 is an indication that the plant species were under stress and also had reduced pigments due to pollution. Plant % Chlorophyll a Conc. Index Reduction % Chlorophyll b Conc. Index Reduction % Total Chlorophyll Conc. Index Reduction M. indica 8.36 % 35.38 % 22.34 % A. occidentale 16.26 % 28.50 % 14.90 % Table 3: The Percentage Chlorophyll Reduction of M. indica and A. occidentale in the Study Areas. ⃰ Mean value of thirty leaves ± standard error of mean (SEM) with different alphabets are significantly different from each other (P < 0.05). The photosynthetic pigments are the most likely to be damaged by air pollution. Chlorophyll pigments exist in highly organized state, and under stress they may undergo several photochemical reactions such as oxidation, reduction, pheophytinisation and reversible bleaching (Giri et al., 2013). According to Chauhan (2010), there was a decrease in the chlorophyll content of Ficus religiosa, M. indica, Polyalthia longifolia and Delonix regia at polluted site in comparisons to control sites. Pollutants when absorbed by the leaves may cause a reduction in the concentration of photosynthetic pigments viz., chlorophyll and carotenoids, which directly affect the plant productivity (Joshi and Swami, 2009). Satao et al. (1993) also reported that cement dust decreased the productivity and concentration of chlorophyll in a number of crops. The mean area of the leaves of M. indica in the polluted and non-polluted site were 81.66 ± 3.56 cm2 and 73.61 ± 3.91 cm2 respectively. While the mean area of the leaves of A. occidentale in the polluted and non-polluted sites were 66.54 ± 3.02 cm2 and 63.07 ± 3.01 cm2 respectively. There was no significant difference (P < 0.05) in the mean leaf area. In other words, the pollutants had no effect on the morphology but on the physiology of M. indica and A. occidentale. This is contrary to the observation made by Leghari and Zaidi (2013) who reported that there was a reduction in the leaf area of common plant species of Quetta City. CONCLUSION This study indicates that the total chlorophyll in non-polluted plants was higher than that of plants in the cement factory though there was no significant difference in the leaf area of the plants. Exposure to particulate deposition or pollutants may alter plant growth without physical damage to the plant. Moreover, accumulation of dust particulates on studied plant leaves could be a major problem in their production. The pigments content of the light harvesting complex is an important aspect related to the tolerance of plants to pollutants. Chlorophyll content is essential for the photosynthetic activity and reduction in chlorophyll content has been used as indicator of air pollution because it is fairly sensitive to air pollutants. The continuous cement industry pollution closes the stomata so interfering with gaseous exchange. This study can be used to determine how polluted an area is. Air pollution causes lots of damage to plants and the environment. Measures have to be taken to reduce the amount of pollutants released to the environment. REFERENCES Arnon, D. I. (1949). Copper enzymes in isolated chloroplasts, polyphenoxidase in Beta vulgaris. Plant Physiology 24: 1 – 15. Chauhan, A. (2010). Photosynthetic pigment changes in some selected trees induced by automobile exhaust in Dehradun, Uttarakhand. New York Science Journal 3(2): 45 – 51. Chauhan, A. and Joshi, P. C. (2008). Effect of ambient air pollution on photosynthetic pigments on some selected trees in urban area. 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Keywords: Chlorophyll concentration, bioindicator, air pollution, phyto-indicator, Mangifera indica, Anacardium occidentale, photosynthesis.