Air quality in selected sampling sites
A higher concentration of CO, SO2, NO2 and VOC values were recorded at the arterial road sites in comparison to the control. It has been reported by Saxena et al. (2012) that in the urban areas, traffic flow is among the foremost emission sources. Thus, the three arterial roads could had higher pollutant vehicular air pollutant levels than the control site.
In this study, Terminalia catappa, Mangifera indica, Ficus platyphylla and Polyalthia longifolia had higher relative water content at the experimental sites than at the control site. Similar result was obtained by Jyothi and Jaya (2010). Thus, the higher relative water content at the arterial road sites might be responsible for normal functioning of biological processes in these tree plants. Under the condition of stress, high relative water content inside a plant’s organs will keep up its physiological equilibrium.
pH signals the occurrence of detoxication process in plant necessary for tolerance (Thawale et al. 2011). Terminalia catappa, Mangifera indica, Ficus platyphylla and Polyalthia longifolia leaf extract pH in this study were found to be acidic in nature at the arterial road sites. Similar observation was reported in Gladiolus gandavensis (Swami et al. 2004). Low pH values is an indication of sensitivity of the plant species to air pollutants, while high pH could provide tolerance to pollutants (Govindaraju et al. 2012; Saxena et al. 2012). Plants exposed to air pollutants (specifically, SO2) generate substantial H+ to react with SO2, which enters through the stomata, resulting H2SO4 and lowering of leaf pH (Zhen 2000). It has been reported that higher leaf extract pH values leads to higher plants absorption of SO2 and NOx (Zou 2007). In study, lower pH values were recorded at the arterial roads, where SO2 values were higher; this characteristics suggest that leaf extract pH could be used as an indicator for vehicular air pollution.
Ascorbic acid is an antioxidant commonly found in growth plants parts that depicts its resistance to air pollution (Pathak et al. 2011). In this investigation, ascorbic acid in the leaves of the studied plants were higher at the arterial road sites with respect to the control site in Terminalia catappa, Mangifera indica, Ficus platyphylla and Polyalthia longifolia. This is in agreement with the reports of Nwadinigwe (2014) and Rai et al. (2013). However Rai and Panda (2014) reported higher ascorbic acid at the control site and reduced ascorbic acid at the experimental sites. The increased in ascorbic level reported in these tree species suggests their tolerance to the pollutants especially automobile exhausts and a defence mechanism of the respective plants. Previous studies have shown that ascorbic acid reduces reactive oxygen species (ROS) concentration in leaves, thus higher ascorbic acid content of a plant is a sign of its tolerance against SO2 pollution (Jyothi and Jaya 2010; Varshney and Varshney 1984).
It was observed in this study that photosynthetic pigments in the tree species leaves were lowered with higher concentration of vehicular air pollutants in the arterial roads lends credence to Tripathi and Gautam’s (2007) assertion that chloroplast is the first site of attack by vehicular air pollutants which consist of SPM, SO2 and NOX. This is in agreement with earlier studies (Wei et al. 2014). Air pollutants gain entry into the tissues across the stomata and partially denaturises the chlorophyll, thus a decrease pigment content in the polluted leaves cells (Pant and Tripathi 2012).
The reduction in chlorophyll has been credited to the interruption of the chloroplast layer because of SO2 phytotoxic nature (Winner et al. 1985) bringing about leaching of pigment (Rath et al. 1994). The promotion of secondary processes which breakdown chlorophyll and kills the cells is believed to be associated with SO2. Acidic polllutants like SO2 brings about phaeophytin formation by acidification of chlorophyll brings about reduction of leaf chlorophyll (Jyothi and Jaya 2010). Similar reduction in the photosynthetic pigment were observed in other studies (Mandal and Mukherji 2000; Wagh et al. 2006; Joshi and Swami 2009; Chauhan 2010). The study on cyto-architectonics’ destruction of Cucurbita moschata under SO2 and NO2 stress showed a damaged chloroplast and mesophyll cells caused by air pollution (Ding and Lei 1987).
In this study, carotenoids content of the tree leaves species sampled at the arterial road sites were lower in comparison to the control site. Chauhan (2010) reported that carotenoids are sensitive to SO2. Since SO2 is a by-product of vehicular air pollution, it is suggested that this pollutant could have caused the reduction of carotenoid content of the leaves of the studied species at the road sites. Several researchers had reported that carotenoid content reduced under air pollution (Sharma and Tripathi 2009; Tripathi and Gautam 2007; Verma and Singh 2006). The decrease in carotenoid contents of the tree species leaves at the arterial road sites agrees with Joshi and Swami (2009) that vehicular emission or vehicle-induced air pollution reduced photosynthetic pigments in tree leaves exposed to roadside pollution.
It was observed that the independent variable volatile organic compounds (VOCs) in all the tree species did not relate significantly to the dependent biochemical variables, which suggested that the dependent variable (VOC) may not be used to predict changes in the biochemical variables. It was not surprising from the result that not all the four biochemical variables (pH, relative water content, ascorbic acid and total chlorophyll) used in the computation of Air Pollution Tolerance Index (APTI) were significantly related to a single air pollutant. This reveals that APTI gives a general idea of pollution tolerance of plant rather than indicating its specific tolerance to air pollution and as such does not differentiate between various air pollutants.
In the present study, all the four tree species had mean APTI value of more than 17. Therefore, it is considered as tolerant to vehicular air pollution. Plants with higher APTI qualities were more tolerant to air pollution than those with low APTI values; those with low APTI qualities are sensitive plants and may go about as bio-pointers of air contamination (Shannigrahi et al. 2004; Chandawat et al. 2011). Hence, on the premise of their indices, different plants might be classified into tolerant, moderately tolerant, intermediate and sensitive plants (Chandawat et al. 2011).
Thakar and Mishra’s (2010) approach is effective in the identification of comparatively tolerant species by comparing the tolerance grades between the plant species under the same environment irrespective of how tolerant the investigated species is. Whilst, Padmavathi et al.’s (2013) approach is useful in the selection of the true tolerant plant species using three absolute APTI Index values in spite of the environmental conditions (Zhang et al. 2016). The combination of the tolerance results of the tree species based on the two approaches give a better tolerance evaluation. Tree species classified as tolerant based on Padmavathi approach could be used for urban greenery. But if classified as sensitive or intermediate based on Padmavathi approach, a reassessment as opposed to Thakar and Mishra method should be carried out. For instance, Polyalthia longifolia was classified as intermediately tolerant in Arterial road II and control site using Padmavathi’s approach; on re-categorisation, this tree species were sensitive to the vehicular air pollution. Hence, its consideration for usage in urban greening should be the least priority. However, other tree species in the arterial road sites were moderately tolerant and/or tolerant based on the two approaches, thus they are highly recommended for urban greening. The consideration of these tree species for urban greenery stems from the fact that they are tolerant and moderately tolerant at least two or three of the studied sites. Zhang et al. (2016) opined that plant species with tolerant and moderately tolerant grades may be applied in green belt planning for urban and suburb areas. It was also observed that tree species had different tolerant grades at different study sites with different classification. This could be as a result of differentials in air pollution and other environmental factors that may have influenced the four parameters in the APTI formula.
The association of the four biochemical parameters amongst them and with the dependent parameter APTI was illustrated in this study which suggests that total chlorophyll and ascorbic acid are the determinants on which the tolerance of the tree species depends on in Arterial road I. It also suggested that relative water content and total chlorophyll are the determinant of tolerance in the studied tree species in Arterial road II, whilst relative water content and ascorbic acid are the determinants of tolerance in the studied tree species in Arterial road III. At the control site, it was indicated that total chlorophyll and ascorbic acid are the most significant determining factors on which tolerance of tree species is dependent on.
qqqqqEnvironmentalists have consistently advocated for urban greenery in urban areas and roadsides as well. Green belts naturally cleanse the atmosphere by absorption, diffusion of gaseous and particulate pollutant through their leaves that function as efficient pollutant trapping device (Thambavani and Prathipa 2012). Anticipated Performance Index is an evaluating framework where a tree species is graded in view of air pollution tolerance index, morphological characteristics and alongside socio-economic parameters. In the evaluating framework, a tree gets a greatest score of 16 points, which are scaled to rates and in the light of the score got; the class is determined. From this study, Mangifera indica and Ficus platyphylla were considered under very good category are highly recommended for planting as urban tree for auto exhaust mitigation. These tree species possess dense canopy of evergreen leaves as well as economic values. It has been reported also that Mangifera indica is fast growth tree and stores high amount of carbon in its tissues, thus its high priority rating (Miria and Khan 2013). Terminalia catappa was judged to be a good performer. Polyalthia longifolia was found to be unsuitable as a pollution sink because of its low anticipated performance.