Urban pollution poses severe threats to both environmental integrity and public health, particularly in densely populated cities. As per the “Revision of World Urbanization Prospects,” the global urban population currently stands at 55 percent and is projected to reach 68 per cent by 2050 (UN, 2019). Particulate matter (PM), adversely affects plant growth by inducing oxidative stress and disrupting physiological processes (Yadav et al., 2019). Rapid urbanization has intensified vehicular emissions and declined sustainable energy use, escalating particulate matter and harmful gas concentrations across residential, institutional and recreational spaces (Shrestha et al., 2021). Vehicular emissions release particulate matter, heavy metals and toxic gases that adversely affect air quality, soil health and plant physiological processes (Dizaji et al., 2016). Increasing population growth, industrialization and infrastructure development exert significant pressure on natural resources, leading to the loss of productive agricultural land and environmental degradation (Radhika et al., 2026). Among natural mitigation strategies, vegetation stands out as the most effective biological solution. Plants uniquely eliminate suspended particulate matter (SPM), making green spaces indispensable in urban planning (Singh et al., 2021; Chaurasia et al., 2022; Patel et al., 2023; Sapkota and Shrestha 2024). Prolonged pollution exposure triggers measurable physiological changes in plants, including chlorophyll content, ascorbic acid, relative water content and carotenoid levels, serving as reliable bioindicators (Sapkota and Shrestha, 2024). The Air Pollution Tolerance Index (APTI) effectively quantifies species-level pollution tolerance (Leghari et al., 2019). This study addresses the estimation of APTI and the Anticipated Performance Index (API) to evaluate tree species under varying traffic conditions, distinguishing sensitive bio-indicator species from tolerant pollutant sinks (Correa-Ochoa et al., 2022). Hence, the present study is conducted to identify suitable ornamental tree species for urban green belt development.

The present study was conducted at SRM Institute of Science and Technology, Kattankulathur, Chengalpattu, Tamil Nadu, during the year 2025-2026. The investigation focused on assessing air pollution levels, particularly PM₁₀, in high-traffic zones such as the premises of SRM Lake, Valliammai Engineering College entrance and Golden Arch gate. PM₁₀ concentrations were monitored using a high-volume air sampler in two 12-hour intervals. A comprehensive tree survey recorded 22,928 trees belonging to 70 species, of which twenty predominant species (Table 1) were selected for detailed morphological, socioeconomic and biochemical analysis of leaf samples (Rajakaruna and Masakorala, 2019).


       
The Dust capturing capacity (DCC) on leaf samples was estimated using the gravimetric method suggested by Manisha and Pal (2014). The leaf extract pH was determined by the ratio of adsorbed hydrogen to metallic ions, indicating its acidity or alkalinity (Sadasivam and Manickam, 2005). The method described by Henson et al. (1981) was utilized to determine the relative water content. The total chlorophyll concentration was estimated using Arnon (1949) approach. Ascorbic acid was measured using the Ranganna (1986) method and reported as mg g^-1. The ability of the tree species to withstand air pollution was estimated by the method outlined by Singh and Rao (1983). The APTI is calculated using the formula.

 
Where,
A = Ascorbic acid content (mg/g).
T = Total chlorophyll content (mg/g).
R = Relative water content and
P = pH of the leaf extract.
       
The APTI of each selected species was combined with morphological plant traits, such as plant habit, canopy structure and plant type, to determine the API. The chosen plant species is given an API score as prescribed by Prajapati and Tripathi (2008) (Table 2). The formula used to determine the percentage score is (Table 3):

 
Particulate matter was assessed by measuring the concentration of PM₁₀ (Particulate Matter ≤10). The concentration of (PM₁₀) in ambient air was calculated using the equation, as suggested by CPCB (2013) at the elevated traffic zone:

Whereas,
W_f = Final weight of filter paper after sampling in g.
W_i = Initial weight of the filter paper before sampling in g.
(W_f=W_i) = Total mass of collected particles in g.
10⁶ = Factor for conversion of g to µg.
V_air = Volume of air sampled (m³).
       
V_air is calculated by the average flow rate (Q_avg in m³/hour) and during the time (t-total sampling time in hours) i.e.
 

V_air = Q_avg × t

 
Statistical analysis
 
Data were analysed using CRAN R (Version 4.5.3) and MS Excel 2013. A completely randomized design with three replications was used for statistical analysis. One-way ANOVA with LSD post-hoc test (p<0.05) compared treatment means. Pearson’s correlation and Principal Component Analysis (PCA) were evaluated between biochemical parameters.

Pollution concentrations
 
PM₁₀ concentrations in the high-traffic zone exhibited significant temporal and seasonal variations throughout the study period (Table 4). During March 2026, daytime PM₁₀ concentrations reached 169.35 µg/m³, whereas nighttime concentrations decreased to 28.97 µg/m³, likely due to rainfall, increased humidity and reduced anthropogenic activity. In April 2026, comparatively lower PM₁₀ concentrations were observed during both daytime and nighttime (12.70-10.32 µg/m³). Elevated daytime concentrations were primarily associated with vehicular emissions, construction activities and open burning practices.


 
Dust capturing capacity (DCC)
 
DCC showed variations among selected ornamental tree species under elevated traffic environments (Table 5).  DCC of the studied species ranged from 0.102 mg/cm² in Terminalia arjuna (Lowest) to 0.513 mg/cm² in Ficus religiosa (Highest). The following trend in the dust-capturing capacity was noted: Ficus religiosa > Delonix regia > Azadirachta indica > Cassia fistula > Pongamia pinnata > Saraca asoca > Pisonia grandis > Ficus microcarpa > Bombax ceiba > Ficus benjamina > Ficus natalensis > Samanea saman > Plumeria pudica > Plumeria rubra > Callistemon citrinus > Terminalia arjuna. Rough leaf surfaces and shorter petioles significantly enhanced particle retention in superior species. These findings are confirmed by the previous study (Bharti et al., 2017).


 
Tolerance index of selected tree species towards air pollution
 
APTI was estimated and presented in Table 5. Ficus religiosa (20.10), Delonix regia (19.76) and Azadirachta indica (19.39) exhibited the highest pollution tolerance, which may be due to robust antioxidant defenses and effective leaf tissue buffering, making them ideal for urban greening initiatives (Akilan and Nandhakumar, 2016; Correa-Ochoa et al., 2022). Cassia fistula demonstrated moderate tolerance, whereas Termialia catappa, Pongamia pinnata and Saraca asoca displayed intermediate tolerance. Twelve species were categorized as sensitive bio-indicators. On the whole, the metabolic characteristics significantly influenced the APTI values (Anake et al., 2022).
 
Performance of tree species in relation to air pollution
 
The API evaluates a plant’s green belt suitability by incorporating ecological, socioeconomic and biological characteristics (Pathak et al., 2011). Among the species evaluated, API grades ranging from 1 to 6. Ficus religiosa (87.50%), Azhadirachta indica and Delonix regia (81.25%) attained the highest grade (6), reflecting superior pollution tolerance (Table 6). Cassia fistula, Ficus microcarpa, Ficus natalensis, Pongamia pinnata and Terminalia catappa (62.50% each) were rated “Good” with Grade 4, whereas Peltophorum pterocarpum (56.25%) demonstrated moderate tolerance with Grade 3. Species including Bauhinia purpurea, Bombax ceiba, Ficus benjamina and Samanea saman ranked as “Poor,” with certain others classified as “Very Poor”. These outcomes corroborate earlier findings (Pandey 2015; Patel et al., 2023), reinforcing API’s effectiveness as a tool for species selection in urban greening and pollution mitigation efforts.


 
Principal component analysis (PCA)
 
The PCA biplot revealed substantial biochemical variation among ornamental tree species in high-traffic areas (Fig 1). PC1 and PC2 together accounted for 75.1% of the total variance (49.7% and 25.4%, respectively). PC1 showed strong positive correlations among chlorophyll content, RWC and ascorbic acid. Delonix regia, Ficus religiosa and Azadirachta indica demonstrated superior pollution tolerance, while species like Pongamia pinnata excelled in water retention. PC2 highlighted leaf pH differences, with Bombax ceiba and others showing strong acid-buffering ability. Several species exhibited poor stress resilience. These findings corroborate established research on urban plant functional responses to pollution gradients (Zhu and Xu, 2021).


 
Correlation matrix between APTI and biochemical parameters
 
Significant correlations between APTI and key tree biochemical characteristics were identified through correlation matrix analysis (Table 7). Chlorophyll content showed strong positive associations with both RWC (r = 0.737, p<0.001) and APTI (r = 0.731, p≤0.001), highlighting its role in pollution tolerance. RWC also exhibited a strong positive correlation with APTI (r = 0.626, p≤0.01). Ascorbic acid demonstrated the strongest positive correlation with APTI (r = 0.871, p<0.001), confirming its critical role in oxidative stress reduction, while its associations with RWC (r = 0.279) and chlorophyll (r = 0.415) were moderate and non-significant. pH showed weak, non-significant relationships with all parameters, including APTI (r = 0.247). These findings underscore the importance of Ascorbic acid, RWC and Chlorophyll in determining APTI and consistent with previous studies (Enitan et al., 2022; Kaur and Nagpal 2017; Ashick Rajah et al., 2025) and findings of Sabitha and Thambavani (2011), who reported significant positive correlations among chlorophyll, ascorbic acid and APTI.

This study demonstrated significant interspecific variation in physiological, morphological and biochemical responses of twenty ornamental tree species to traffic-induced dust pollution. Ficus religiosa, Delonix regia and Azadirachta indica recorded the highest APTI values, with Azadirachta indica showing superior API performance. These species, along with others, are recommended for greenbelt development in high-traffic areas to enhance air quality and ecological sustainability.

The authors acknowledge the Dean, SRMCAS and the Associate Director (Campus Life), SRM IST, for granting permission to collect samples from the experimental area.

Funding
 
The authors express their sincere gratitude to the SRM Institute of Science and Technology (SRM IST), Chengalpattu, for financial support provided under the Selective Excellence Research Initiative (SERI) project (No. SRMIST/R/AR(A)/SERI2024/174/78/342) during 2024-25.
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.
 
Informed consent statement
 
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The authors declare no conflicts of interest.