The Tolerance Index for Different Growing Tree Plant Species in Jubail Industrial City, a Polluted Area, KSA

Air pollution is a major risk factor for urban and industrial areas in developing countries [1]. Uncontrolled industrialization and rapid urbanization are known to cause serious problems, due to inadequate emission control and lack of stringent environmental regulations [1]. Air pollution is global problem and is responsible for one in every nine deaths annually [2]. The environment and human well-being are adversely affected due to the industrial progress and expansion of cities. The plantations, local residents and atmosphere are enormously affected by the release of oxides of sulphur, carbon, nitrogen [3]. There are no known physical or chemical methods to upgrade air quality of polluted zone [4]. Urban vegetation plays a significant role in combating air pollution. Trees act as an important and cost-effective solution to combat air pollution [2]. Plant leaves are major air pollutant receptors [5]. In addition, plants may absorb particulate air pollutants through their roots. Studies have shown that plants can reduce the number of particles in the air by onethird as compared to areas with no vegetation [6]. The response of plants to pollutant absorption can be used as sustainable tool to estimate air quality [7]. Several studies illustrated the effectiveness of a green belt strategy in addressing air pollution through planting tolerant trees in areas that are polluted [8,9]. Therefore, they represent a sustainable approach for removing air pollutants through mechanisms such as leaf sorption, vegetation fallout, and deposition over the leaf surface [10-12].


Introduction
Air pollution is a major risk factor for urban and industrial areas in developing countries [1]. Uncontrolled industrialization and rapid urbanization are known to cause serious problems, due to inadequate emission control and lack of stringent environmental regulations [1]. Air pollution is global problem and is responsible for one in every nine deaths annually [2]. The environment and human well-being are adversely affected due to the industrial progress and expansion of cities. The plantations, local residents and atmosphere are enormously affected by the release of oxides of sulphur, carbon, nitrogen [3]. There are no known physical or chemical methods to upgrade air quality of polluted zone [4]. Urban vegetation plays a significant role in combating air pollution. Trees act as an important and cost-effective solution to combat air pollution [2]. Plant leaves are major air pollutant receptors [5]. In addition, plants may absorb particulate air pollutants through their roots. Studies have shown that plants can reduce the number of particles in the air by onethird as compared to areas with no vegetation [6]. The response of plants to pollutant absorption can be used as sustainable tool to estimate air quality [7]. Several studies illustrated the effectiveness of a green belt strategy in addressing air pollution through planting tolerant trees in areas that are polluted [8,9]. Therefore, they represent a sustainable approach for removing air pollutants through mechanisms such as leaf sorption, vegetation fallout, and deposition over the leaf surface [10][11][12].
The resistance of different plants to air pollution depends on the plant's ability to detoxify each pollutant, the concentrations of atmospheric pollutants, and the interactions between pollutants and plants [13]. The Air Pollution Tolerance Index (APTI) provides a reliable method for estimating the susceptibility level of tree species as the biofilter performance for managing ambient air quality. APTI value uses distinct biochemical parameters such as ascorbic acid, total chlorophyll, relative water content, and leaf extract pH that collectively reflect stress indicators [14][15][16]. Based on these four parameters, APTI values for the plant species can be

ARTICLE INFO ABSTACT
Tree species growing in Jubail Industrial City were screened for their relative resistance to air pollution. The leaf samples collected from study species were used to determine their Air Pollution Tolerance Index (APTI) by calculating the ascorbic acid, chlorophyll, pH and relative water content. The results demonstrated a substantial impact of pollutants on APTI and a strong positive correlation between APTI and pH as well as relative water content. APTI did not exceed value of (9) indicating that all study species are within the sensitivity range with varied degree of sensitivity. Ficus altissima had highest value of APTI (8.803) so it's considered as the most tolerant species (or the least sensitive species) among the study species in the investigated site so it's most desirable for planting in the industrial zone of Jubail City and all plants studied in the area are good bioindicators for pollution. Urban green planning in Jubail Industrial City should include the growing of more tolerant tree species that can be used as bio monitors of environmental pollutants in the city. measured. It indicates the sensitivity and tolerance of the different plant species growing in the same industrial area [17]. The plant species with lower APTI value can be used as bio-indicators, whereas the tolerant ones with higher values can serve as buffers [18,19].
Jubail Industrial City is the largest petrochemical industrial complex of its kind in the world, which includes many industries and high-end residential areas built according to the highest specifications. Since its inception, the Royal Commission for Jubail and Yanbu has worked to create a complete harmony between industrial development and environmental preservation, and one of the most important pillars for preserving the environment is the environmental monitoring program that includes air quality monitoring, meteorology, water industrial waste management, protection from oil and noise pollution and wildlife control. Jubail Industrial City is interested in afforestation on roadsides and within residential neighborhoods. The present work was carried out to study air pollution in Jubail Industrial City through measuring Air Pollution Tolerance Index (APTI) of three plant species grown in the study area so that we can identify the tolerant species to air pollution that may help in proper selection of plant species in urban plantation program at Jubail City.

Materials and Methods
Jubail is a city in the Eastern province on the Arabian Gulf coast of Saudi Arabia (longitude 40-49 degrees east and latitude 27,000 degrees north). The climate is hot, humid in summer and cold in winter. The study was carried out on eight tree plants species. The species were identified, and mature leaf samples were collected from each species in triplicate. The Air Pollution Tolerance Index (APTI) was determined by measuring four biochemical parameters that are ascorbic acid, chlorophyll, leaf extract pH and relative water contents in leaf samples according to the following methods.

pH of Leaf Extract
This was done following the method adapted by Agbaire and Esiefarienrhe (2009) [21]. 5g of the fresh leaves was homogenized in 10 ml ionized water. This was filtered and the pH of the leaf extract determined after calibrating pH water with buffer solution of pH 4 and 9.

Ascorbic Acid (AA) Content
Ascorbic acid content (expressed as mg/g) was measured using spectrophotometric method [22]. 1 g of the sample was placed into a test-tube, 4ml oxalic acid -EDTA extracting solution was added then 1 ml of orthophosphoric acid followed by 1 ml 5% tetraoxosulphate acid. To the mixture 2 ml ammonium molybdate was added and then 3 ml of water. The solution was then allowed to stand for 15 min, after which the absorbance at 760 nm was measured with a spectrophotometer. The concentration of ascorbic acid in the samples was then extrapolated from a standard ascorbic acid curve.

Relative Water Content (RWC)
RWC was determined according to Barrs and Weatherly (1962) [23]. RWC is a ratio of the amount of water in the leaf tissue at sampling to that present when fully turgid. A composite sample of leaf discs was taken and the fresh weight was taken and then leaf was floated on water for up to 24 h. The turgid weight was then recorded, and the leaf tissue was subsequently oven-dried to a constant weight at about 85°C for 24 h. RWC is calculated by using the following formula: , .

FW
Fresh weight DW dry weight and TW turgid weight = = =

Air Pollution Tolerance Index (APTI)
APTI of leaf was calculated by using the equation, which was developed by Singh and Rao (1983) [24]. The equation has been given as follows: Where, A is the ascorbic acid content in the leaf in mg/g dry weight, T is total chlorophyll content of leaf in mg/g dry weight, P is the pH of the leaf extract, and R is the percentage (%) RWC of the leaf.

Statistical Analysis
All data were expressed as mean ± SD by one-way ANOVA with plant sample as the main factor using statistical software of SPSS Ver. 24 (IBM SPSS, 2016). Comparisons of means when the factor had a significant effect were obtained using Duncan test (1955) [25]. A probability of P<0.05 was required for statements of significance.   As shown in Table 1 and Figure 1

Leaf Extract pH
Values of pH for leaves extract found in acidic rang as most species had pH values lower than six (

Ascorbic Acid (AA) Content
As indicated from the results in Table 1

Relative Water Content (RWC)
The relative water content in the study species was found to be ranged from a minimum value of 53.28% in leaves of P. aculeate to a maximum value of 87.603 and 85.70% in leaves of F. altissima and C. sebestena respectively (Table 1 & Figure 4).   Figure   5).

The Correlation of Biochemical Parameters with APTI
The correlation matrices were worked out among various analyzed parameters, and they are presented in Table 2  Note: Where: (*) Correlation is significant at the 0.05 level, and (**) Correlation is significant at the 0.01 level.

Discussion
Air quality affects human respiration and quality of life on earth and changes with the number of toxic pollutants in the atmosphere [26]. In dry climates, air quality becomes even worse due to various gaseous and particulate materials suspended in air. Air pollution not only affects humans, but plants as well, causing foliar chlorosis or necrosis [26]. The tolerance behavior of trees to air pollutants can be best described to assess their potential in remediating the air pollution problem and purifying the local air of an area [27]. Four biochemical parameters were analyzed for APTI and the results revealed that these parameters played a significant role to determine resistivity and susceptibility of plant species.
Among these four biochemical parameters pH and RWC had the greatest impact on APTI values so pH and RWC played important role in nullifying the detrimental effects of air pollution in plants as is evident from the results obtained. The role of pH is that it participates in the regulation of the sensitivity of a plant to pollution.
When gaseous air pollutants in the cell sap, such as SO 2 , NO 2 , and CO 2 diffuse, they convert into acidic radicals [33]. Leaves, with lower pH values are more influenced by atmospheric pollutants, while those with higher pH are more tolerant to pollutants [34].
Similarly, we observed law pH in species of high sensitivity, and high pH in species that showed law sensitivity in the study site.
The influence of pH on tolerance to pollution is loosely tied to the metabolism of ascorbic acid. pH apparently has a direct positive relationship in the breakdown of hexose sugar to ascorbic acid as high pH favors this process and promotes more ascorbic acid accumulation [35]. Ascorbic acid in plant leaves has multiple functions to perform through cell wall synthesis, cell division, photosynthetic carbon fixation and acts as a strong reducer protecting the plants against reactive oxygen species (ROS), thereby improving the tolerance ability of the trees against air pollution [27].
Water is a critical element that facilitates the maintenance of plant physiological balance when conditions are strenuous (difficult or stressful) due to air pollution [33]. Water in plants maintains leaf temperature, enables nutrient uptake, and sustains metabolic activities. Pollutants may increase membrane permeability, resulting in loss of water and nutrients as well as premature senescence of leaves [26]. High water content in plants enables the plant to dilute acidity that may occur in the cytoplasm of leaf cells [36]. The status of leaf water in plants is directly linked to numerous physiological conditions such as respiration, transpiration, and growth [37].
When the relative water content is high, the plant becomes more tolerant of air pollution under tough conditions [33]. In the present study, the more tolerant species exhibited high RWC up to 87.603 while low RWC was recorded in the species of high sensitivity.
Vegetation behaves as a sponge or sink of air pollutants including gases and particulate matter detrimental to the fauna [35].
The entry of air pollutants via the stomata causes the clogging up of the stomata leading to a drastic decrease in the photosynthetic rate which intern reduces the sugar, protein and chlorophyll quantities of the leaf [38]. Total chlorophyll content in the leaves of species under investigation didn't show high effective role in decreasing sensitivity to pollution in the study area. It was found that F.
altissima recorded as much more tolerant species in the study area contained lowest TCh while the most sensitive species in the study area (P. aculeate) had highest TCh.
Therefore, it can be inferred that the two most important parameters that maintained the defense mechanism in study tree species are pH and RWC, which was ensured by study of correlation coefficient between the four biochemical parameters and APTI.
A strong positive correlation was indicated between RWC and APTI as well as pH and APTI. The study revealed a strong positive correlation between AAC and TCh, suggesting that the chlorophyll content is related to the metabolism of AAC [39]. Based on the APTI values the plants were conveniently grouped as follows [40]

Availability of Data and Materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Ethical Approval
Not Applicable.

Consent to Participate
Not Applicable.