Current World Environment

Introduction

Clean air is essential for the relationship between the environment and public health, but as pollution from many sources is released into the environment, the air quality is deteriorating, which has negative effects on both the climate and human health ^1-3. The main causes of air pollution emissions and poor air quality include growing urbanization, rising industrialization, and related anthropogenic activities.  Fine dust particles are a major problem today due to their undesirable effects on plants and animals.  By capturing particulates and absorbing gaseous pollutants, green belts are a natural way to reduce air pollution ^4.  

Air pollutants and metals like mercury and lead can be filtered by plants.. Conversely, certain air pollutants have the ability to negatively impact plant development and growth through their effects on the biochemical composition, photosynthetic rate, seed stomata, pedicle length, number of flowers on florescence, and germination. These alterations in biochemical traits are utilized to evaluate the plant's APTI. ^{5, 6.}

More tolerant plant species serve as sinks for airborne pollutants and can be employed to create green belts, whilst more sensitive plant species serve as bioindicators. ^7. The development of green belts surrounding urban and industrial areas will be aided by the addition of plants having capacity to withstand air pollution. The chlorophyll level, ascorbic acid, pH, and water content are helpful in calculating the tolerance level. This index has been used in studies on the construction of green belts, traffic noise reduction, and pollution mitigation near roadsides and next to businesses. It has been discovered that plants grown in conditions that appear to be contaminated have higher APTI than plants living in circumstances that appear to be less polluted. This is because various plants react to air pollution in different ways.

In the view of the above context, the leaves of Mangifera indica, Ficus religiosa, Psidium guajava, Annona squamosawere used for the present study  The objective is to find out the air pollutant tolerance capacity using Mangifera indica, Ficus religiosa, Psidium guajava, Annona squamosa already grown in polluted and unpolluted site.

Materials and Methods 

Leaf sample collection

Fresh leaves from the plants Mangifera indica, Ficus religiosa, Psidium guajava, and Annona squamosa were collected in sterile zip lock coversgathered from polluted sites locations near Adyar and the control site (College Campus-Unpolluted site). How many trees?? how many leaves per tree? 

Extract preparation

In accordance with standard protocols, fresh leaves were used. The entire experiment was carried out with aqueous extract.

Biochemical analysis

Evaluation of pH of leaves

1g of leaf sample was homogenized followed by centrifugation. Then supernatant was collected and using a pH meter, the pH was determined.⁸. 

Evaluation of relative water content  

Leaves weighing 5g were collected, washed, and weighed. After soaking the leaf samples in water for 24 hours, they were wiped dry and weighed to measure the turgid weight. After drying overnight at 70°C in a hot air oven, the leaves were reweighed. Singh's method for determining and computing RWC was employed. 

RWC= ((FW-DW)/(TWDW) x 100

 FW stands for fresh weight, DW for dry, and TW for turgid weight.

Estimation of ascorbic acid

Stock ascorbic acid solution

100mg of ascorbic acid is weighed & dissolved in 100ml of distilled water in a standard flask.

Working standard ascorbic acid

10ml of stock was pipetted out in to a measuring flask and made up to 100ml with distilled water in a standard flask.

Sample preparation:

5g of leaves crushed in mortar and pestle. Mix the sample with 50ml of 5% metaphosphoric acetic acid solution, transfer it into a beaker and add 50ml of phosphoric acid solution. Filter using whatmann filter paper 

 To the filtrate add few drops of bromine water. Then add few drops of thiourea solution to remove excess bromine water

85% H₂SO₄:

DNPH: 3g of 2,4 DNPH in 15ml of concentrated H2SO, then add 90ml of 74% ethanol.

Procedure

0.5, 1.0, 1.5, 2.0, 2.5 ml of working standard ascorbic acid was pipetted out into tubes labelled as standard from S1-S5. 0.1 ml of extract was pipetted out into tubes labelled as T. With 5% TCA, the volume in each tube reached up to 3 ml. 3 mL of 5% TCA acts as a blank. To each tube, 1.0 ml of DTC reagent was added and thoroughly mixed. The tubes were kept in a water bath for 1 hour at 60° C. The tubes were cooled followed by the addition of 5 ml of 9 N sulphuric acid. The tubes were maintained at room temperature for 20 minutes.  The intensity of the colour developed was recorded at 520nm. The amount of ascorbic acid was estimated from the standard graph ^[9]

Estimation of chlorophyll

Arnon's approach ^{10 9} was used to calculate the chlorophyll content. 1g of leaves were extracted in 20ml of  acetone for 15 minutes. The liquid portion was transferred  followed by centrifugation at 2,500rpm. In a normal flask, the contents were made up to 50 ml with 80% acetone. The optical density of chlorophyll a, b was recorded at 645nm and 663nm using a visible spectrophotometer, and the following formula was used to perform the calculations

Total chlorophyll: Chlorophyll a + Chlorophyll b 

The air pollution tolerance indices for the selected plants were calculated using the following formula

APTI= (A (T+P) + R)/ 10 is the formula. 

Ascorbic acid, T=Total chlorophyll, P=pH of the leaf extract, R=Relative water content of leaf percent).

Estimation of Total Soluble Sugar 

Anthrone procedure [1110] was used to calculate the total soluble sugar content of leaf tissue. 20 ml of distilled water along with 1 g of  leaf tissue was homogenised. The tubes were then hydrolyzed in a boiling conditon for three hours with 5 cc of 2.5 N hydrochloric acid. The tubes were allowed to cool to ambient temperature. After that, the sample was neutralisedneutralized with sodium carbonate solids. The sample was diluted to 100 mlL and centrifugation was performed. The supernatant was collected, and aliquots of one milliliter were utilized for estimate.

Working standards were prepared in separate tubes with various concentrations of glucose, and the volume of the standards and sample was reduced to 1ml using distilled water. The tubes were then filled with anthrone reagent ( 4 ml) and  kept in a warm condition for 8 minutes followed by cooling at normal temperature. The reading was recorded at 630 nm using spectrophotometer. Total soluble sugar concentration in the sample was calculated using the standard graph.

 Results and Discussion

Table 1: Chlorophyll content in leaves of Mangifera indica, Ficus religiosa, Psidium guajava, Annona squamosa   obtained from unpolluted and polluted sites.

Chlorophyll content in leaves of Mangifera indica, Ficus religiosa, Psidium guajava, Annona squamosa  collected from unpolluted and polluted sites is shown in table 1. 

From results obtained , it is well evident that chlorophyll level varied from 1.18 to 2.8 in unpolluted leaf samples and 0.12 to 0.42 in areas exposed to pollution. It is to be observed that if the pollutant level is high chlorophyll content will get decreased

From the study carried out, Mangifera indica, showed the highest total chlorophyll level of 2.82 mg/g, Psidium guajava 1.9. mg/g while Annona squamosa   and Ficus religosa   showed the lowest chlorophyll content of 1.18 and 1.17 mg/g in leaves collected from unpolluted sample.  The total chlorophyll content in polluted sample was found to be highest in Ficus religosa (0.76mg/g) followed by Psidium guajava (0.44 mg/g) and Mangifera indica   (0.42 mg/g), very lowest in  Annona squamosa (0.261 mg/g) respectively. 

The total chlorophyll content decreased in polluted samples than in unpolluted samples.

The levels of chlorophyll was decreased in samples obtained from polluted areas than in controlled sites.  The average chlorophyll content of leaves has been frequently used to assess how air pollution affect the photosynthesis ^12 1. It is believed that   chlorophyll serves as a measure of biomass output, photosynthetic activity, and growth. According to Kameswaran et al. (2019), Plants with a high chlorophyll content are often resistant to contaminants even after being exposed to traffic pollution. ^{13  2}. This shows that the plant's tolerance to SO2 from vehicular exhaust may be causing lower chlorophyll production or deterioration in the already-present chlorophyll concentration.

Table 2: pH of the leaf extracts of Mangifera indica, Ficus religiosa, Psidium guajava, Annona squamosa

The pH of the plant species collected from unpolluted and polluted site is given in the table 2. The pH of the leaves collected from unpolluted site ranged from 6.6- 5.5 and in polluted samples it was found to between 6.1 to 5.3. The highest pH was observed in A.squamosa (6.6) followed by M.indica (6.2), P.guajava (5.5), in unpolluted leaf samples.  There was a moderate decrease in the pH of samples from polluted sites. The pH of  A.squamosa from areas exposed to air pollutants was found to be 6.1 followed by followed by M.indica (5.9) , F.religiosa (5.6) and P.guajava (5.3),

According to Escobedo (2008), increased pH favors the production of ascorbic acid from hexose, whereas decreasing pH is assumed to be related to susceptibility to air pollution ^14 3-. The pH of leaves decreases when they are exposed to acidic pollutants like SOX and NOX, and this pH decrease is more prominent in sensitive plants compared to tolerant ones  ^15-16 14-15.

Figure 1: RWC (%) of plant species.   Click here to view Figure

The RWC of the of Mangifera indica, Ficus religiosa, Psidium guajava, Annona squamosa collected from unpolluted and polluted sites is shown in Figure 1

M. indica showed the greatest RWC (91%) followed by F.religiosa by 89% and P.gujava by 86%, A.squamosa by 81% in leaves collected from unpolluted site. In this study it was surprising to note that there was nearly 10% decrease in water content of leaves collected from polluted site. 

Under stressful circumstances, plants can  maintain their physiological equilibrium thanks to the high relative water content (RWC) in their leaves. Hence, a high RWC number indicates that plants are highly resistant to stress ^{17 16-}. Air pollution has been linked to reduced transpiration rates and damage to leaves that can pull water up from the roots, which prevents plants from bringing in minerals. The rate of transpiration in leaves is affected by the air pollutants with a decrease in relative water content that has been found in some plant species 

Figure 2: Ascorbic acid content in Mangifera indica, Ficus religiosa, Psidium guajava, Annona squamosa collected from unpolluted and polluted sites.   Click here to view Figure

Ascorbic acid of the leavescollected from unpolluted and polluted sites is shown in figure 2. From this study it was observed that there was slight change in the ascorbic levels of polluted and unpolluted leaves. The ascorbic acid level was found to be 7.9 mg/g in M indica and 7.8 mg/g in A.squamosa and the level was found to be 6.9mg/g in F.religiosa in unpolluted samples. A slight decrease was noted in polluted samples

Apart from synthesizing cell walls, ascorbic acid is essential for cell division, carbon fixation, and functions as a strong antioxidant to protect plants from the damaging effects of free radicals.. Because of this, trees are better able to withstand air pollution.  Studies reveal that ascorbic acid can lessen the quantity of ozone that passes through the cell wall and enters the plasma membrane ^18-19 17-18.  Plant tolerance limits increase as ascorbic acid level increases. As a result, ascorbic acid is required for plants to endure the damage caused by air pollution to their tissues..  ^{[20-21 19-20-]}. According to one study, when there is pollution, ascorbic acid levels in plants fall ^22 21.  However ascorbic acid levels in plants increases if plants withstand pollution ^23-24 22-23 

Soluble sugar content in leaves of   Mangifera indica, Ficus religiosa, Psidium guajava, Annona squamosa   from unpolluted and polluted sites is depicted in figure 3. The sugar levels were found to be elevated in all the unpolluted leaf samples when compared to the polluted ones. The amount of Sugar is more in M.indica (24 mg/ g) and least in F.religiosa (21 mg/g) in Unpolluted leaves and in polluted leaves M.indica  (20 mg/ g) and least in A.squamosa sp  (17 mg/g).

Figure 3: Soluble sugar content in plant leaves collected from Polluted and unpolluted site.   Click here to view Figure

Starch is the end result of photosynthesis and is composed of glucose. In higher plants, sugar serves as both the primary result of photosynthetic energy production and the primary metabolic pathway for carbohydrates. It is believed that the amount of soluble sugar in a plant indicates its physiological activity and its susceptibility to air pollution. Polluted leaves may have less sugar because of decreased photosynthesis brought on by lower pH, less chlorophyll, and less CO2 fixation. APTI showed variations among the various plants obtained from polluted and unpolluted areas. The APTI of plant species were represented in fig 4. 

Figure 4: APTI of plant species   Click here to view Figure

The APTI values were in the order M.indica (16.21) > A.squamosa (14.1) > F.religiosa (13.7) > P.guajava (13.3) in unpolluted leaves and in polluted leaves APTI values were 9.8 for A.squamosa, 10.7 for Psidium gujava, 11.7 and 11.9 for F. religiosa and M.indica  respectively. 

The APTI values provide a reliable way to assess the tolerance and vulnerability of any location's flora to automobile pollution. Plants with APTI values between 12 and 16 are categorised as intermediate, while those with APTI values between 11 and 17 are known to be tolerant.

Conclusion

Plants serve as living filters to reduce air pollution and are a major component in enhancing the green belt. From this study it can be concluded that planting plants like Mangifera indica, Psidium guajava, Annona squamosa, Ficus religosa   in urban areas can reduce air pollution. Less than 11 for the APTI value indicates sensitivity; values between 12 and 16 indicate moderate tolerance; values over 17 indicate high tolerance. It is shown that M. indica is extremely tolerant. Also, the other plants used in the study are found to be intermittently tolerant.Thus, attention is needed to promote and develop green belt zones in polluted areas using these type of plants.   

Acknowledgements 

The authors wish to thank the department and management of Dwaraka Doss Goverdhan Doss Vaishnav College for having provided the necessary chemicals to carry out this project work successfully.    .

Conflict of Interest

There is no conflict of interest

Funding Sources

There is no funding or financial support for this research work

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