2.10 APTI analysis
Biochemical parameters of the leaves (e.g., pH, RWC, ascorbic acid, total chlorophyll) and their APTI data are shown in Table 4.
2.10.1 pH
The pH values of leaf extract ranged from 5 to 7 (Table 4). The highest amount (7.05) was related to Hylotelephium sp from Crassulaceae, and the lowest amount (5.4) was related to Malephora crocea from Aizoaceae. According to the literature, the pH extracted from plant leaves is lower in the presence of acidic contaminants (Achakzai et al., 2017; Pandey et al., 2016; Scholz & Reck, 1977). The presence of acidic pollutants such as SO2 and NO2, and PM from industrial emissions in the air may change the leaf pH to acidic in the plant (Chauhan, 2010; Swami et al., 2004). When plants are exposed to air pollution (especially SO2), they produce large amounts of H+ ions in their cell fluid to combine with the SO2, which enter through guard cells, stomata; therefore, H2SO4 is produced and decreases the plant pH (Zhen, 2000). This reduction rate is much higher in susceptible plants than tolerant plant species (Scholz & Reck, 1977). Besides, the high pH of plants, especially in polluted conditions, indicates an increase in their tolerance to acidic air pollutants (Govindaraju et al., 2012).
2.10.2 RWC
The RWC of the green wall plants was summarized in Table 4. Their average values vary significantly from 30 to 90%. Malephora crocea and Hylotelephium sp (85%) had the highest amounts (86 and 85%), and K. Prostrata had the lowest (38%). The high leaf water content helps plants retain physiological balance under stressful conditions such as air pollution (Dedio, 1975; Meerabai et al., 2012). When plant exposure to air contaminants, the rate of transpiration frequently becomes higher, which may lead to their drying; therefore, RWC can be considered an influential factor in resistance to pollution stress (Krishnaveni et al., 2013). Increasing the amount of RWC in plant species indicates better performance concerning drought tolerance, indicating the typical performance of biological processes (Rai et al., 2013). Besides, differences between RWC values are dependent on plant species(Jyothi & Jaya, 2010; Singh et al., 1991).
2.10.3 Ascorbic acid
Ascorbic acid contents of plants located in the green wall were summarized in Table 4, displaying a range of 0.5 - 9 (mg/g) (Table 4). The highest values (8.875 mg/g) were related to R. officinalis from Lamiaceae, and the lowest values were related to Malephora crocea (0.878 mg/g) and Hylotelephium sp (0.845 mg/g) from Aizoaceae and Crassulaceae. A statistically significant variation was perceived in the concentration of ascorbic acid among plant families. Ascorbic acid plays a crucial role in cell wall synthesis, defense, photosynthetic process, and cell division (Conklin, 2001). Ascorbic acid acts as an antioxidant in the plant, often found in the growing parts of plants, and increases the plant's tolerance to air pollution (Liu & Ding, 2008; Pathak et al., 2011). In other words, it reduces the accumulation of active oxygen in the leaves of plant species as a defense mechanism, thus raising the plant's tolerance to air pollution (Chaudhary & Rao, 1977; Pandey et al., 2016). Due to its importance in plant life, it is one of the factors examined in the APTI formula (Nwadinigwe, 2014). Ascorbic acid, as a stress-reducing agent, is generally higher in stress-tolerant plant species, while its low content in plant species makes them sensitive to air pollution stress(Rai, 2016; Zhang et al., 2016).
2.10.4 Chlorophyll content
Table 4 shows the total chlorophyll content of the studied plants, which ranges between 0.05 and 1.5 (mg/g). The highest amount (more than 1.4 mg/g) of total chlorophyll is related to R. officinalis from Lamiaceae and Hylotelephium sp from Crassulaceae. The lowest value (0.08 mg/g) was found in S. reflexum from Crassulaceae. Chlorophyll is one of the most significant plant metabolites in stressful situations, and its high levels cause tolerance to environmental contaminants (Joshi & Chauhan, 2008; Prajapati & Tripathi, 2008). Air pollution degrades photosynthetic pigments in plant leaves, and this degradation is widely used as an indicator of air pollution (Joshi et al., 2009; Joshi & Chauhan, 2008; Ninave et al., 2001; Rai, 2016). In air pollution stress, the alkaline and acidic contaminants (SOx and NOX) cause chlorophyll degradation in the plant by blocking the guard cells and forming pheophytin (Joshi et al., 2011; Rai, 2016). In general, high levels of chlorophyll in plants increase tolerance to air pollution (Prajapati & Tripathi, 2008). However, the chlorophyll content of plants varies based on the level of contamination in their environment and their tolerance or susceptibility (Rai & Panda, 2015).
2.10.5 Linear regression analysis
Figure 2 exhibits the linear regression analysis between biochemical variables and APTI values. As shown, there is no significant influence of pH (R2= 0.059) and Total chlorophyll content (R2= 0.001) on APTI. On the contrary, the leaf water content (R2 = 0.2959) and ascorbic acid (R2= 0.33) showed a positive effect on APTI. These results agree with Kaur and Nagpal (Kaur & Nagpal, 2017), which reported the significant strong positive impact of ascorbic acid on APTI.
2.10.6 APTI
Calculated APTI values of green wall plants are shown in Table 4. The APTI values of the plants varied between 5 and 12. The highest value (more than 12) of APTI was obtained for C. edulis and R. officinalis, while K. Prostrata presented the lowest quantity (5.7). APTI is calculated using those as mentioned above four biochemical parameters, which examine the level of sensitivity of any plant to air pollution (Singh et al., 1991). The importance of APTI in detecting tolerance or susceptibility of plant species was investigated by many researchers (Bamniya et al., 2012; Kaur & Nagpal, 2017; Prajapati & Tripathi, 2008; Rai, 2016). In general, plants with high levels of APTI are tolerant to air pollution and can be used as filters to absorb and reduce air pollution, while plants with low levels of APTI are sensitive and can be used as environmental bioindicators (Nayak et al., 2018). Different plants presented various APTI index values (shown in Table 4), which depend on the concentration of air pollution and the environment they are planted or grown (Gupta et al., 2016; Rai, 2016). For instance, suspended particles increase APTI in the plant after deposition on the leaf surface (Gupta et al., 2016). This study indicated that C. edulis s and R. officinalis are tolerant to air pollution, while K. Prostrata is sensitive species.
2.10.7 API analysis
The calculated API index of the plant species planted in green walls is shown in Table 5. Based on the evaluation of the tolerance index, biological, economic, and social characteristics, C. edulis is the best plant for planting in industrial, urban areas of the city. After that, lavender and R. officinalis were assessed as very good plants, while M. crocea and S. reflexum were considered as good plants. Hylotelephium sp and Frankenia thymifolia were classified as poor and very poor plants due to the lack of suitable characteristics, e.g., low APTI values. V. minor and K. Prostrata obtained the lowest API index value are not recommended for planting in polluted areas.
The API score, like the APTI value, can be used as a bioaccumulation indicator, while a low API value is considered a biomarker of vehicle pollution. Determining the performance of plants using APTI and API is a reliable method for selecting appropriate species for planting in green spaces of industrialized regions and traffic points (Kaur & Nagpal, 2017).
2.10.8 SEM analysis
The leaf surface characteristics of plants play a critical role in capturing atmospheric PM and their different effect on their capability to retain atmospheric PM (Mo et al., 2015). To investigate the effects of plant leaf structure on adsorbing particles, the leaves of plant species located in green walls were observed with SEM. From Table 6, many particles adsorbed on the adaxial and the abaxial leave surfaces and the vicinity of the stomata in Lavender, C. edulis, V. minor, and Hylotelephium sp. Some researchers, for example, Ram et al. (Ram et al., 2014), Ottel´e et al. (Ottelé et al., 2010), and Weerakkody et al. (Weerakkody et al., 2017; Weerakkody et al., 2018b), proved that the highest accumulation of particulate matters happened on adaxial surfaces of leaves. Although the PM accumulation on the adaxial and abaxial leaf surfaces was different, some plant structure parameters have affected this, which cannot be observed by SEM images. Plant hairs, trichomes, non-smooth surfaces have been identified as auxiliaries in the accumulation and storage of suspended particles (Barima et al., 2014; Räsänen et al., 2013; Weerakkody et al., 2018a; Weerakkody, 2017; Zhang, 2017). Besides, the grooves and their properties, such as deep or shallow, play a key role in PM adsorption. In other words, deep grooves capture more particles. Stomatal density in the plant leaf dreaminess the quantity of PM capturing. The plants, which have relatively low stomatal density, exhibit a high potential retenting of fine particles (Mo et al., 2015). This phenomenon can be seen from the SEM image of R. officinalis, S. reflexum, and K. Prostrata, in which large numbers of particles accumulated on its adaxial leaf surfaces. The S. reflexum image shows the accumulation of large numbers of particles, probably due to small, needle-shaped leaves, as well as the existence of grooves.
Although the PM accumulation on the adaxial and abaxial leaf surfaces was different, some plant structure parameters have affected this, which cannot be observed by SEM images. Plant hairs, trichomes, the non-smooth surface have been identified as auxiliaries in the accumulation and storage of suspended particles (Barima et al., 2014; Räsänen et al., 2013; Weerakkody et al., 2018a; Weerakkody, 2017; Zhang, 2017). In this study, lavender has unique morphological characteristics and many hairs, which demonstrating its ability to trap and adsorb suspended particles on the leaf surface.
2.10.9 Elemental composition analysis
Table 6 displays the elemental composition of particulate matter deposited on plant leaves using EDX analysis. From the figures inside this table, it can be seen that all the plants had a very similar elemental composition. Carbon had the highest amount of detected metals across all species, ranging from 19 to 47%. L. angustifolia, C. edulis, Hylotelephium sp, and K. Prostrata displayed more than 40% carbon. The second most abundant element observed in the DEX of all species was oxygen, with a value in the range of 9-36%. K. Prostrata, F. thymifolia, and L. angustifolia had the maximum quantity (more than 30%). Ca, K, Mg, Si, and Al were found in the EDX of all plant species with values less than 3%. It is good to mention that three plants have higher Ca contents of 3% (L. angustifolia; 7%, S. reflexum; 5%, and Hylotelephium sp; 3%). PM is composed of inorganic such as nitrates, sulfides, carbon black, and organic matter [1]. The elemental composition of trapped particulates is classified into three categories; (1) mineral particles, (2) metallic particles, and (3) biogenic particles. The minerals particles comprised Al, Si, O, Ca, Fe, K, Mg, which originated from various kinds of aluminum silicates of soil. Pollen is the major part of biogenic particles. The central elements of pollen are C, O, Si (Heredia Rivera & Gerardo Rodriguez, 2016). Thus, it can be concluded that this particle originated from dust. Moreover, as shown in Table 6, Fe, Mn, and Cr concentrations in plant leaves ranged from 0.07 to 0.5 percent. The metal elements such as Fe, Mn, Cr formed the metallic particles derived from industrial additives (Heredia Rivera & Gerardo Rodriguez, 2016). These metals on the leaf surface are the result of air pollution from motor vehicles.
2.10.10 Heavy metal analysis
Figure 3 depicts the accumulation of heavy metals in the leaves of plants grown on the green walls. As shown in Fig. 1, The highest chromium (Cr) accumulation was found in the S. reflexum (5.38 mg/kg) followed by F. thymifolia (4.21 mg/kg), while the lowest was found in the C. edulis (1.61 mg/kg). Cr content in plants ranges from 0.02–0.2 mg/kg with phytotoxicity at concentrations greater than 10 mg/kg (Pais, 1997). According to FAO, The maximum allowable limit for Cr concentration in plants is 5 mg/kg (WHO/FAO, 2007). In this study, the amount of Cr in all plant species except S. reflexum was almost below the standard. The Cr content of air ranges from 0.001 to 1 mg/m3, but in industrial areas, it can reach 30 to 50 mg/m3. Cr concentrations in green wall plants were higher than in those grown under controlled conditions (control sample). This increase was in the 1 to 60 (for M. crocea ) percent range.
From Fig. 3, S. reflexum had the highest iron (Fe) accumulation (307000 mg/kg) followed by F. thymifolia (160000 mg/kg), while R. officinalis had the lowest (2277 mg/kg). Plants have iron levels ranging from 10 to 1000 mg/kg dry matter. Besides, the maximum permissible limit for iron-based on FAO is 450 mg/kg (WHO/FAO, 2007). Iron is the critical constituent of plants, aiding in the stabilization of N2 and acting as a catalyst in forming chlorophylls (Caselles, 2002). Iron concentrations in green wall plants were much higher than in control sample plants in this study. This increase ranged from 8 to 95% (for the M. crocea ).
Zinc (Zn) accumulation was most outstanding in the M. crocea (425.37 mg/kg) and lowest in the R. officinalis plant (16.41 ppm). K. Prostrata (56.87 mg/kg) and Stone crop (41.62 mg/kg) ranked second and third in zinc accumulation, respectively. The maximum permissible limit for Zn concentration in plants is 60 mg/kg (WHO/FAO, 2007). Zinc concentrations in green wall plants were higher than in control plants. The amount of this increase ranged between 15 and 70 (for M. crocea) percent.
As evidenced in Fig. 3, The highest accumulation of Pb was found in the S. reflexum (2.21 mg/kg), while the lowest accumulation was found in the V. minor (0.3 mg/kg). The second and third ranks belonged to the F. thymifolia (1.69 mg/kg) and Hylotelephium sp (1.02 mg/kg). FAO has determined that the maximum acceptable concentration of Pb in all plant parts is 0.3 mg/kg (WHO/FAO, 2007). Pb concentration in green wall plants was much higher than controlled plants. The increment percentage change was between 4 and 76% (for M. crocea).
As proved in Fig. 3, the highest Cd accumulation was in the Hylotelephium sp (0.65 mg/kg), followed by S. reflexum (0.36 mg/kg), and the lowest accumulation was in the M. crocea (0.03 mg/kg). Cadmium levels in plants are permitted to range between 0.2 and 0.8 mg/kg, with toxic accumulation estimated to range between 5 and 30 mg/kg (Kabata-Pendias A, 1992). Cadmium is involved in the absorption, transport, and utilization of several elements, including potassium, calcium, magnesium, and phosphorus by plants. Besides, Cd concentrations were higher in green wall plants than in control plants. Accumulation increased by between 33 and 100 percent (for L. angustifolia, R. officinalis, S. reflexum ).
As seen in Fig. 3, accumulation was most remarkable in S. reflexum (1.39 mg/kg) and F. thymifolia (1.02 mg/kg). The R. officinalis had the most negligible accumulation (0.27 mg/kg). Arsenic is an unnecessary and generally toxic element that prevents root spread and mass production in plants. According to FAO, the maximum acceptable concentration of As in all plant parts is 0.1 mg/kg (WHO/FAO, 2007). All of the plants had higher arsenic levels than that. The concentration of as in all plants grown in the wall was higher than in control plants. The increase was between 5% and 100% (for R. officinalis).
From Fig. 3, the highest accumulation of Ni was found in S. reflexum (7.34 mg/kg), followed by F. thymifolia (4.52 mg/kg), and R. officinalis had the lowest accumulation (1.45 mg/kg). FAO has established a nickel permissible level of 67 mg/kg. The concentration of Ni in green wall plants was much higher than in plants grown under controlled conditions in this study but much lower than the standard. The increase ranged from 5–70% (for C. edulis ).