©2023 International Society of Arboriculture 330 Mousavi Javardi et al: Relationships Between Leaf Characteristics and Air Pollutants permeability impairment, and loss of growth and yield (Tiwari et al. 2006). The effects of air pollutants on plants are multifaceted and depend on various fac- tors, such as exposure duration, pollutant concentration, species, age, and interactions with other environmen- tal factors including other pollutants (Abeyratne and Ileperuma 2006). Most plants experience physiologi- cal changes before showing visible damage to leaves when exposed to airborne pollutants (Liu and Ding 2008). Plants that are frequently exposed to environ- mental pollutants absorb, accumulate, and integrate these pollutants into their systems, and depending on their sensitivity level, they may show visible changes, including alterations in biochemical processes or accu- mulation of certain metabolites (Agbaire and Esiefar- ienrhe 2009). Visible injury to plants can take the form of collapse of leaf tissue with necrotic patterns, yellow- ing or other color changes, or alterations in growth or premature loss of foliage, depending on the type of pol- lutant, its concentration, the species, physiological con- ditions, and age of the plant (Bell and Treshow 2002). There are various indicators used to demonstrate pollution stress in plants. Research indicates that leaf characteristics reflect a plant’s mechanisms to its sur- roundings through changes in anatomical, morpholog- ical, and physiological characteristics in response to environmental changes (Ling et al. 2011). Air pollution stress leads to stomatal closure, which reduces the avail- ability of CO2 in leaves. Exposure to air pollutants, such as O3 and NOx, affects the metabolic function of leaves and interferes with the net carbon fixation by the plant canopy, which is measured by the net photo- synthetic rate, a commonly used indicator of increased air pollutant impacts on tree growth (Joshi and Swami 2007; Woo et al. 2007). Air pollution stress triggers the formation of active oxygen species in plant cells, which results in oxidative stress, affecting the biochem- ical processes of plants and decreasing their tolerance to other stress factors (Rai et al. 2011). Air pollutants first deposited on soil, such as heavy metals, affect the functioning of roots and interfere with soil resource capture by the plant, leading to reductions in resource capture (production of carbohydrates through photo- synthesis, mineral nutrient uptake, and water uptake from the soil), affecting plant growth through changes in resource allocation to various plant structures (Weber et al. 2002). In polluted urban areas, plant species’ branches and leaves accumulate significant amounts of chemical elements, acting as accumulation reagents. the most, accounting for about 38% of gaseous pollut- ants and approximately 14% of air-suspended particles (TAQCC 2019). Over the years, population growth, road transportation, vehicular traffic, and industrial activities have led to an increase in gaseous and par- ticulate pollutants, resulting in only 2 days of good air quality in Tehran during 2022. Tehran’s air quality monitoring network comprises 35 stations, of which 21 are monitored by the Tehran Regional Municipalities and 14 are monitored by the Department of Environ- ment. Most of these stations are equipped with ana- lyzers to measure the concentration of CO, SO2, NOx, O3, PM2.5, and PM10, which are the main air pollutants in Tehran. The number of parks in Tehran has increased from 75 in 2017 to 2,355 in 2023, and afforestation around Tehran currently covers an area of more than 45,000 hectares. The average per capita green space in Teh- ran is 16.7 m2, varying from 2.64 to 62 m2 in different regions. There are 113 deciduous and 83 evergreen tree and shrub species in Tehran. Iran is located in a dry region of the earth, and Tehran faces crises such as population growth, climate change, and various air pollutants (Taheri et al. 2019), making green spaces essential for the city. The purification of the city’s air is an important role played by plants (Jeanjean 2015; Slamet et al. 2018). Urban trees, although helpful in mitigating air pollution, are themselves vulnerable to high levels of ambient air pollution (Larcher 2003). The primary route through which air pollution affects plants is the uptake of pollutants through stomata in the leaves (Abeyratne and Ileperuma 2006). When stomata are open, pollutants can easily diffuse into the leaf tissue and cause damage. However, if pollutant levels are excessively high, injury can still occur even when stomata are closed. For instance, plants growing along busy roads in Khartoum, Sudan, demonstrated an increase in stomata and epidermal cells due to vehicle emissions (Achille et al. 2015). Therefore, stomatal aperture regulation by various factors is crucial in determining the extent of damage that plants will incur. The modification of stomatal frequency and size is an important mechanism for plants to control the absorp- tion of pollutants in response to environmental stress (Gostin 2009). Air pollutants can cause a range of damage to sensi- tive plant species, including leaf damage, stomatal clo- sure, premature aging, photosynthetic effects, membrane
November 2023
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