Reviewing the Impact of Vehicular Pollution on Road-Side Plants—Future Perspectives

: With population explosion, automobiles have also exploded and so has pollution due to vehicular emissions. Road-side plants and highway vegetation are the ﬁrst targets of these vehicular emissions. This review brieﬂy presents a snapshot of how vehicular emission can affect plants. On the contrary, the positive perspective of how road-side plants may be able to affect and inﬂuence the harmful effects of vehicular emissions has also been discussed. Ways and means by which plants can become potential bio indicators of air pollution have also been speculated. The fact that the nanocarbon particulate aspect of automobile pollutants and their interactions with road-side plants and more so on road-side agricultural crops, has not been appropriately investigated has been raised as a key concern. The need to arrive at mitigation methods to identify permanent solutions to these rising concerns has been highlighted.


Introduction
The recent decades have seen a gradual increase in the global population from 7.4 billion in 2016 to 7.7 billion in 2019, 7.8 billion in 2020 to 7.9 billion in 2021. The world population is projected to reach 9.9 billion by 2050. With urbanization, industrialization, and various commercial activities rising to meet the demands of this exponentially increasing population, transportation needs have increased too. The use of private vehicles has also stealthily increased because the public transport systems are inadequate. Two-wheelers (motorcycles, scooters), being an easy and relatively cheaper means of commutation compared to four wheelers, have increased, mostly in developing countries [1]. Globally, 5 decades ago about 53 million automobiles were approximated, this has boomed to 500 million in 2000 and every year about 19 million vehicles are added [2]. It is an understandable increase in two wheelers, since they are an economic mode of transport. Four wheelers are also anticipated to double up in the near future.
Automobile-based air pollution amounts to 60 to 70%, with harmful emissions such as carbon monoxide, hydrocarbons, nitrogen oxide, volatile organic compounds, fine particles, carbon nanomaterial, and lead being released into the atmosphere (Figure 1). Old age, poor performance, and inadequate maintenance of vehicles enhance these emissions. Narrow roads, frequent traffic jams, poor geometrics, and congestion aggravate this further, affecting human health and the environment. Two and three wheelers emissions are about twice the amount emitted by all other sources. Researchers have reported 800,000 deaths annually as a result of urban air pollutants [3]. Treshow [4] defined air pollutants as "aerial substances that have adverse effects on plants, animals or materials". Air pollutants consist of particulate and gaseous pollutants. Particulate pollutants may be solid or liquid particles that may be large or small. Larger particles (sand grains, water droplets) quickly deposit, while smaller dust stays in the air for longer durations of time. Based upon their chemical nature, particulate pollutants could be either inorganic particulate matter (I.P.M) or organic particulate matter (O.P.M) [5].
This review presents the impact of automobile-based effects on highway/roadside plants. The existing knowledge on the physiological and morphological changes these highway plants are subjected to, has been briefly discussed. The positive influence highway plants have on reducing automobile-based effects has also been discussed. The practical implementation of this acquired knowledge has been speculated and discussed.

Impact of Vehicular Emissions on Highway Plants
Plants are susceptible to air pollutants, deposition of pollutant particulates on soil indirectly affects plant growth. Automobile-based pollutants deposit on leaves, blocking the stomata and eventually affecting transpiration. These depositions obstruct CO2 absorption and eventually decrease photosynthesis and affect the growth of plants and their productivity. Agarawal and Agrawal [6] have elaborately evaluated pollution in Dhaka city and discussed its associated health impacts. They have also confirmed that the high density of motor vehicles to be the reason for high pollution levels. This automobile linked pollution is expected to worsen with further increase in economic growth and industrialization. Rawat [7] studied the negative effects of automobile pollution on roadside plants on Mussoorie Road. Bakshi [8] investigated roadside air quality of Jammu city. They reported higher levels of suspended particulate matter (SPM) owing to heavy automobile traffic. Raina and Sharma [9] studied the impacts of automobile pollutants on leaf anatomy, morphology, and chlorophyll of Syzgium cumini L and reported a decrease in all parameters. Raina et al. [10] studied the influence of exhaust on trees growing along the road sides, they found that Albizzia lebbek was more adaptive to polluted environments. Nithamathi and Indira [11] reported damages on flowers and seeds of Ceasalpinia sepiaria L. in Tuticorin city. Many researchers around the world have explored the negative effects Treshow [4] defined air pollutants as "aerial substances that have adverse effects on plants, animals or materials". Air pollutants consist of particulate and gaseous pollutants. Particulate pollutants may be solid or liquid particles that may be large or small. Larger particles (sand grains, water droplets) quickly deposit, while smaller dust stays in the air for longer durations of time. Based upon their chemical nature, particulate pollutants could be either inorganic particulate matter (I.P.M) or organic particulate matter (O.P.M) [5].
This review presents the impact of automobile-based effects on highway/roadside plants. The existing knowledge on the physiological and morphological changes these highway plants are subjected to, has been briefly discussed. The positive influence highway plants have on reducing automobile-based effects has also been discussed. The practical implementation of this acquired knowledge has been speculated and discussed.

Impact of Vehicular Emissions on Highway Plants
Plants are susceptible to air pollutants, deposition of pollutant particulates on soil indirectly affects plant growth. Automobile-based pollutants deposit on leaves, blocking the stomata and eventually affecting transpiration. These depositions obstruct CO 2 absorption and eventually decrease photosynthesis and affect the growth of plants and their productivity. Agarawal and Agrawal [6] have elaborately evaluated pollution in Dhaka city and discussed its associated health impacts. They have also confirmed that the high density of motor vehicles to be the reason for high pollution levels. This automobile linked pollution is expected to worsen with further increase in economic growth and industrialization. Rawat [7] studied the negative effects of automobile pollution on roadside plants on Mussoorie Road. Bakshi [8] investigated roadside air quality of Jammu city. They reported higher levels of suspended particulate matter (SPM) owing to heavy automobile traffic. Raina and Sharma [9] studied the impacts of automobile pollutants on leaf anatomy, morphology, and chlorophyll of Syzgium cumini L. and reported a decrease in all parameters. Raina et al. [10] studied the influence of exhaust on trees growing along the road sides, they found that Albizzia lebbek was more adaptive to polluted environments. Nithamathi and Indira [11] reported damages on flowers and seeds of Ceasalpinia sepiaria L. in Tuticorin city. Many researchers around the world have explored the negative effects of air pollutants on plants [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Other workers have explored the impacts of air pollutants on the external morphology and anatomy of plants [30][31][32][33][34][35][36][37][38][39][40]. However, negative impacts of vehicular emission on plants are rather few and limited. Incomplete combustion of petrol emanates nitrous oxide, sulfur dioxide, lead, carbon monoxides, and particulate matter that damage the foliage of plants [41]. The growth and morphological characteristics of plants can be affected by exposure to heavy metals released from automobile exhausts. Pyatt [42] and Feder [43] observed foliar injury, chlorosis on leaves and other parts of plants, and morphological effects on younger leaves and stunted growth as a result of exposure to air pollutants. Figure 2 portrays the overall damages inflicted by vehicular emissions on road-side plants. of air pollutants on plants [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Other workers have explored the impacts of air pollutants on the external morphology and anatomy of plants [30][31][32][33][34][35][36][37][38][39][40]. However, negative impacts of vehicular emission on plants are rather few and limited. Incomplete combustion of petrol emanates nitrous oxide, sulfur dioxide, lead, carbon monoxides, and particulate matter that damage the foliage of plants [41]. The growth and morphological characteristics of plants can be affected by exposure to heavy metals released from automobile exhausts. Pyatt [42] and Feder [43] observed foliar injury, chlorosis on leaves and other parts of plants, and morphological effects on younger leaves and stunted growth as a result of exposure to air pollutants. Figure 2 portrays the overall damages inflicted by vehicular emissions on road-side plants. Other authors [44] reported that gaseous pollutants are absorbed by leaves, while the particulate ones are absorbed on the outer surfaces of the plants and are not that injurious. Furukawa et al. [45] correlated injury to the magnitude of SO2 absorbed. Plants sensitive to SO2, absorbed higher quantities of gas than those that are resistant. Treshow [4] found that the interference of pollutants affects the green coloration of leaves and brought about death of plant tissues. Bhatti and Iqbal [37] studied the impact of automobile exhausts on roadside trees in Karachi. In all the exposed plant species, foliage productivity was highly reduced in heavily polluted sites. Guaiacum officinale was most affected and Azardirachta indica was least affected and therefore A. indica has been proposed for road-side plantation.
Dust is yet another cause of concern. Dust from automobiles evolves from the exhausts as well as due to vehicular movement on roads. Dust is not just a concern for highway plants, as a matter of fact, every road-side plant is vulnerable to dust. Recently, there is a lot of concern over particle pollution. Automobile emission is one of the predominant sources of such particle pollution. These particles have been named as PM2.5 and PM10 which are very dangerous. These, not just settle on the surface, but move into the system (humans and plants). PM2.5 refers to the atmospheric particulate matter that has a diameter of less than 2.5 micrometers, while PM10 are the particles with a diameter of 10 micrometers. These particles have been studied elaborately with respect to their interactions with the human body, nothing much has been explored with respect to plants. Vora and Bhatnagar [46] observed high dust accumulation in areas exposed to dense automobile exhausts. Dust on road-side vegetation is usually due to automobile, vehicular Other authors [44] reported that gaseous pollutants are absorbed by leaves, while the particulate ones are absorbed on the outer surfaces of the plants and are not that injurious. Furukawa et al. [45] correlated injury to the magnitude of SO 2 absorbed. Plants sensitive to SO 2 , absorbed higher quantities of gas than those that are resistant. Treshow [4] found that the interference of pollutants affects the green coloration of leaves and brought about death of plant tissues. Bhatti and Iqbal [37] studied the impact of automobile exhausts on roadside trees in Karachi. In all the exposed plant species, foliage productivity was highly reduced in heavily polluted sites. Guaiacum officinale was most affected and Azardirachta indica was least affected and therefore A. indica has been proposed for road-side plantation.
Dust is yet another cause of concern. Dust from automobiles evolves from the exhausts as well as due to vehicular movement on roads. Dust is not just a concern for highway plants, as a matter of fact, every road-side plant is vulnerable to dust. Recently, there is a lot of concern over particle pollution. Automobile emission is one of the predominant sources of such particle pollution. These particles have been named as PM2.5 and PM10 which are very dangerous. These, not just settle on the surface, but move into the system (humans and plants). PM2.5 refers to the atmospheric particulate matter that has a diameter of less than 2.5 micrometers, while PM10 are the particles with a diameter of 10 micrometers. These particles have been studied elaborately with respect to their interactions with the human body, nothing much has been explored with respect to plants.
Hussain et al. [76] studied some road-side trees of Peshawar city. They confirmed that the leaf area and chlorophyll contents of all the polluted plants were reduced. Another study [77] on the influence of road dust on the absorption of radiation and energy balance of leaves, confirmed that dusty Hedera helix leaves near the pavement absorbed 30% more radiation and Rhododendrom catawbinse dust covered leaves absorbed 16% more radiation. Chlorophyll 'a', 'b', and carotenoids contents have also been reported to be affected by air pollutants [78][79][80][81][82][83][84]. Similarly, the leaves of vegetables such as cauliflower, okra, cabbage, spinach, radish, and brinjal were also affected [85][86][87]. According to Azmat et al. [84], automobile pollutants also affect the morphological glabrous hairs. Contamination is increasing in Quetta because of high automobile use, strongly affecting the Vitis vinifera (grapes) plants [88,89]. Leaf surfaces are also affected due to different trace elements settling on the leaf surfaces and because of gaseous discharges from road traffic. Therefore, road-side plants are main receivers and reservoirs of all vehicular discharges. Heavy metal automobiles pollutants are highly toxic and reduce growth and morphological characteristics. Ahmad et al. [90] proved that cadmium displayed 5 mg L −1 toxicity in case of root and shoot growth. Flowers of Nuttall's larkspur (Delphinium nuttallianum), Lewis flax (Linum lewisii), scarlet gilia (Ipomopsis aggregata), and sulphur paintbrush (Castilleja sulphurea) growing 1-2 m from a road were prone to more dust and less pollen than those 40-50 m away. This dust accumulation on flowers was significantly observed to affect pollination. Energy developmental projects have affected the Uinta Basin, the Colorado Plateau, and other areas of North America. These roads have enhanced the dust loads on plants and pollinators, which reduced plant growth and reproduction [91]. Lewis et al. studied the effects of an unlaid roads on the reproduction and growth of an endemic shrub [92].

Impact of Road-Side Plants on Minimizing Vehicular Emissions
While vehicular pollution as listed above has been adversely affecting roadside and highway vegetation. Plants do also contribute significantly and play a role in interfacing and reducing automobile-induced pollution. Vegetation directly/indirectly affects air quality. We briefly present reports of these plant-based impacts. Plants act as efficient filters of road dust [93,94]. Shetye and Chaperkar [95] employed plants for monitoring dust in Bombay. Their report confirms that leaves of Mirabilis were able to capture dust because of its epidermal hairs. Erythrina, Mangifera indica, Polyalthia, and Thespesia growing in Bombay were proven for their (leaves) reliability as dust indicators. Das et al. [96] conducted an elaborate study on the potential of Indian ornamentals and avenue trees to collect dust. His work confirmed that Ficus, Mangifera, Tectona, and Polyathia were better dust collectors. Das et al. [96] reported that the upper surface of leaves gathered more dust.
Trees are capable of removing gaseous air pollution via stomata, while plant surfaces also assist in removal of gaseous pollutants [97]. Trees intercept airborne particles and resuspend it in to the atmosphere, these particles are washed off by rain, or return to the ground owing to leaf and twig fall. In urban areas with 100% tree cover, short-term improvements in air quality (one hour) from pollution were observed. Removal of pollutants by trees was as high as 13% for particulate matter, 14% for sulfur dioxide, 15% for ozone, 8% for nitrogen dioxide, and 0.05% for carbon monoxide [98].
Vegetation are an important sink for the atmospheric pollutants. SO 2 , NO 2 and NH 3 , O 3 , and HF, metabolize readily, producing several damaging effects. Carbon dioxide, a combustion pollutant, acts as an essential ingredient of photosynthesis. Carbon monoxide is absorbed in gaseous phase. It is known that grass-like vegetation better absorbs several gaseous pollutants like nitrogen dioxide, sulfur dioxide, and ozone from the polluted air. Calculated on a 12 h exposure basis [99], maize was reported to absorb 0.2 g/m 2 of ozone; bean, 0.52 g/m 2 of CO; alfalfa, 0.17 g/m 2 of NO 2 ; and grasslands, 0.17 g/m 2 of SO 2 from a Sustainability 2021, 13, 5114 5 of 14 polluted area. Plants in turn can also be used as an air pollution indicator [100,101], as they can effectively adsorb air particles [102][103][104][105][106] and reduce air pollutants [107].
Dust can affect the aerial parts of plants. The size of the particles and their characteristics affect plants. Plants vary in their ability to collect particles from air [108,109], larger dust particles are easily filtered by plants than fine particles [110]. Unpaved roads produce more dust [111] and unpaved roads with vehicular traffic will obviously produce more dust than those without. Authors [112] have reported that there was about 10 gm −2 day −1 logarithmic decline in dust deposition, in an unpaved road in Alaska. Leaf is the most vulnerable part for attack of air pollutants compared to other plant parts, because all the crucial physiological processes are conducted by the leaf. Further, this is why the leaf can be marked as an excellent indicator of air pollution. The plant-based response to dust, varies species to species, because deposition of dust varies within plant species due to leaf orientation, phyllotaxy, leaf surface geometry, cuticular and epidermal features, height and canopy of roadside plants, leaf pubescence [44,93,113,114]. Dust accumulation of roadside plants may induce them to adopt to adaptive responses through changing their morphologies or physiologies.
Air pollution due to vehicular emission [37,115,116] affects all varieties of road-side plants. Cassia siamea plants growing in polluted localities in Agra city, exhibited significant differences in their flowering phonology and floral morphology [117]. Air pollution stress affects stomatal closure, reducing CO 2 availability in leaves and C fixation. This is why the net photosynthetic rates of roadside plants can be used as an indicator of the impacts of air pollutants on tree growth [118]. In accordance with their sensitivity levels, plants could alter the biochemical processes and lead to accumulation of certain metabolites [119]. SO 2 , NO x , and CO 2 and suspended particulate pollutants from automobile exhausts, absorbed by the leaves, affect chlorophyll and carotenoids, which in turn affect plant productivity [120][121][122][123][124][125]. The photosynthetic pigments can also be used as bioindicators of automobile pollution. Deleterious effects are caused by reactive oxygen species (ROS) in plants [124]. It has been reported that proline act as a free radical scavenger to protect plants from damage by oxidative stress and is said to accumulate under environmental stress [126]. Tankha and Gupta [127] confirmed an increase in proline with increasing SO 2 concentration and a similar increase in proline in Albizia lebbeck and Callistemon citrinus has also been reported [116]. All these above-mentioned automobile-induced responses of plants can be in turn used as pollution indicators.
Reports confirm that dust can influence plant community structures. Parish [128] reported a shift in plant community close to some cement factories [129]. Lotschert and Kohm [130] reported bark pH alterations and changes in Ca 2 content in the bark of trees on long-term exposure to dust. Some field-based studies have confirmed the damages on road-side plants [131] and agricultural crops grown beside highway traffic in London in the 1980s [132].
The above-mentioned information emerged as a consequence of a handful of researchers and their reports published till date. The negative effects are not merely limited to what has been reported. This review found that the research focus has been basically very limited and ideally lacking in this area. Not just highways, but even plants all along walkways and our neighborhood are vulnerable to the fury of the vehicular emissions. Table 1 summarizes the list of road-side plants that have been affected through vehicular emissions. There are definitely many more road-side plants that are affected and are yet to be observed and reported. This review envisions a scientific awareness to look into the magnitude of damage incurred so that remedial measures may be considered. Table 1. Effect of vehicular pollution on road-side plants.

The Nanoparticle Aspect of Automobile Exhausts
While the gaseous and dust fractions of automobile exhausts have been somewhat explored and presented, the nanoparticular aspects involved in vehicular emissions are relatively less documented. Diesel and automobile exhaust have been reported as the primary sources of atmospheric nano-and microparticles [133]. Vehicular exhausts are said to contain spherical nanoparticles in the size ranges of 20-130 nm in diesel engines and 20-60 nm in case of gasoline. Carbon nanotubes and fibers were recently found to be present in engine exhaust as a by-product of diesel combustion [134][135][136]. A high concentration of nanoparticles was localized on expressways, showing that vehicular pollution is a source of nanoparticles. The daily profile of nanoparticles could be directly correlated with local vehicle usages [137]. High pollution, proximity to high-traffic, increased the concentration of nanoparticles, several times [138]. These reports confirmed the presence of nanomaterial in automobile exhausts. Childhood cancers, myocardial infarction heart attack, lung cancer [139], and activation of one or more signaling pathways that cause proinflammatory, prothrombotic, and hemolytic breakdown of red blood cells responses are few of the noteworthy exhausts-based nanomaterial toxicity reports [140,141]. Carbon nanoparticles, nanotubes, and nanospheres from exhausts have been well documented for their human and animal health and welfare. Although the adverse effects of carbon nanotubes and particles on plants are well known [142][143][144][145][146][147], this review found that in-vestigations specifically on the effect of road transport-induced carbon nanomaterials on roadside plants were lacking.
Concerns on carbon nanomaterial-based toxicity on highway plants, like avenue trees, shrubs, weeds are already pressing. This concern gathers seriousness, when the emissions-induced carbon nanomaterial will affect edible and staple food crops (Figure 3). In most of the developing countries, farmland and agricultural fields lie in close proximity to highways and road, in this way, the affected plants are no longer merely roadside plants, but edible crops. This way, the automobile-induced carbon toxicity has chances of entering the food chain and affecting human health and welfare. The potential toxicity of carbon has been discussed in our earlier review [148]. A study conducted by Lin et al. [147] showed that C70 fullerenes and multi walled carbon nanotubes (MWNT) delayed flowering in rice by a month and reduced the yield of exposed rice plants. Exposure of wheat plants to carbon nanotubes made them susceptible to uptake of pollutants [149]. Moreover, it is also reported that carbon nanotubes can penetrate the cell wall of the roots of wheat plants, making a pipeline for entry of pollutants into the cells. The most disastrous projection is when seeds exposed to C70 fullerenes passed on the carbon nanomaterial to the next generation seeds. Rice seeds exposed to SWCNTs, MWCNTs, and C70 showed the presence of carbon nanomaterials in their second generation. The highest was in the seed, followed by the root and stem and leaf respectively. This confirms the transmittance of C70 to the next generation. Carbon NMs accumulate in roots and can delay germination of rice seeds by a month [150,151]. Given the toxicity fact file of carbon nanomaterials and that these nanomaterials are present in automobile exhausts, it is rather disappointing to see that not much attention has been paid to unravel the status quo of the carbon nanomaterial effects of road-side vegetation and agricultural crops.

Future Perspectives and Conclusions
Plants being the primary producers and the first level of the food chain need to be sustained with care and concern. The use of two wheelers is steadily on the increase, especially in underdeveloped and developing countries. Taking into account the economicity of two wheelers, one can only expect an escalation in the purchase and utility of two wheelers much more than four wheelers. As discussed above, two wheelers are sort of the major contributors to air pollution. A

Future Perspectives and Conclusions
Plants being the primary producers and the first level of the food chain need to be sustained with care and concern. The use of two wheelers is steadily on the increase, especially in underdeveloped and developing countries. Taking into account the economicity of two wheelers, one can only expect an escalation in the purchase and utility of two wheelers much more than four wheelers. As discussed above, two wheelers are sort of the major contributors to air pollution. Most importantly, this review calls to attention the need to assess the carbon nanomaterial load of automobile exhausts and further the impact of these automobile exhaust borne carbon nanomaterials on roadside vegetation. More so, since no reports exist on the effect of such vehicular exhaust induced carbon nanomaterials on road-side agricultural fields, this grey area needs research focus to gain appropriate future perspective directions. Moreover, there is lack of quantitative data and information in this direction. Such data, would be helpful in precisely determining the impact of plants on trapping vehicular emissions as well as understanding the impact of emissions on various plants. Figure 4 gives an overview of the current scientific awareness on (i) impact of automobiles on road-side vegetation, (ii) impact of road-side vegetation on automobile induced pollution, and (iii) impact of carbon nanomaterials on road-side plants and agricultural crops. When the gaseous pollutant, dust aspects of automobile exhausts have been studied extensively and reviewed elaborately, hardly any reports exist on assessing the carbon nanomaterial-based aspect of automobile induced pollution and its effect on plants. This review draws the attention of researchers towards this lacuna. The actual gravity of the situation should be disclosed, so that appropriate mitigation methods can be assorted to.  There is an urgent need for answers to queries such as: what is the perimeter of dispersal of the automobile emission? This information is needed in order to come up with guidelines to plan road infrastructure designing, so that vegetation can be allotted spots beyond the emission plume. This calls for research in this direction, which will spell out the minimum distance that vegetation need to avoid road and highway impacts and evade the emission plume. This emission plume cannot be standardized, it will vary with the automobile traffic of each region, and hence local research groups need to get to determine this, so that guidelines for road-side vegetation, specific for each locality, can be dictated. Especially with respect to agricultural crops, on whom the impact of vehicular emissions could be more detrimental, there needs to be clear cut guidelines issued, which will dictate There is an urgent need for answers to queries such as: what is the perimeter of dispersal of the automobile emission? This information is needed in order to come up with guidelines to plan road infrastructure designing, so that vegetation can be allotted spots beyond the emission plume. This calls for research in this direction, which will spell out the minimum distance that vegetation need to avoid road and highway impacts and evade the emission plume. This emission plume cannot be standardized, it will vary with the automobile traffic of each region, and hence local research groups need to get to determine Sustainability 2021, 13, 5114 9 of 14 this, so that guidelines for road-side vegetation, specific for each locality, can be dictated. Especially with respect to agricultural crops, on whom the impact of vehicular emissions could be more detrimental, there needs to be clear cut guidelines issued, which will dictate the minimum distance these agricultural plantations need to be from the road to escape the impact of the vehicular emissions. This will need research groups working out impacts and intensity of impacts specifically with respect to various food crops. Thus, there are a lot of unknowns that need to be addressed through focused research work in this direction and yet as this review points out, nothing much is happening and no real progress is in the air. This review hopes to draw the attention of researchers to concentrate on addressing the highlighted issues.