©2023 International Society of Arboriculture 338 Mousavi Javardi et al: Relationships Between Leaf Characteristics and Air Pollutants season in Glycine max (L.) Merr. (Soybean). No sig- nificant effect on photosynthesis was observed. The primary effect of heightened O3 on leaf area index (LAI) was found to be through modification of the senescence rate. Reduction in plant leaf area with increasing O3 concentration was noted in soybeans (Oikawa and Ainsworth 2016). A meta-analysis con- ducted by Emberson (2020), which involved 263 peer-reviewed articles, investigated the impact of heightened O3 (an average of 64 ppb) on northern temperate and boreal trees and found an 11% reduc- tion in tree biomass in comparison with those grown in ambient O3 (Wittig et al. 2009). Moreover, signifi- cant declines in root: shoot ratio, leaf area, Rubisco, chlorophyll content, transpiration rates, tree height, and stem diameter were also detected. CONCLUSION In general, the results of this research emphasize that air pollutants affect the morphology of tree leaves. The type and amount of its effect depends on various factors such as species, type, amount of pollutants, and environmental conditions. Both study species are widely used in the green spaces of Tehran metropolis, but the use of P. orientalis is more common. The mor- phology of the 2 study species and the morphology of their leaves are very different. In general, P. orientalis mature trees are seen as tall trees with wide crowns in urban forests, but C. caucasica is shorter and pro- duces a small crown. Leaves of P. orientalis are big and wide, while C. caucasica has small leaves. Based on the results of this research, the morphological dif- ferences of leaves greatly affect their quantitative variables. Also, the 2 study trees have shown a nega- tive reaction to O3, and the reaction of C. caucasica was more severe. It seems that the increase in the amount of O3 in the air of Tehran in recent years has caused this pollutant to have a greater effect on the tree leaf morphology. It is suggested that in future studies, the impact of air pollutant on other tree spe- cies in Tehran urban forests is also evaluated in order to reach a suitable summary about how trees react to air pollutants. The answer to this question can be of great help to city managers in choosing suitable tree species for planting in different green spaces of Tehran. LITERATURE CITED Abbasi S, Hosseini SM, Khorasani N, Karbasi A. 2021. The effect of air pollution on the morphological traits of the leaves of Platanus orientalis. Journal of Environmental Science result in negative impacts on ecosystems, such as alterations in the specific composition of plants exist- ing in a forest, modifications in habitat quality, and changes in water and nutrient cycles. The specific reaction of a particular specimen to O3 injury is reli- ant on its capacity to offset such injury. Dose-response relationships, therefore, differ based on the plant spe- cies, crop cultivar, developmental stage, and external environmental factors, such as water availability and temperature that influence the opening and closing of stomata (Mauzerall and Wang 2001). Given that the external environmental factors, including tempera- ture, soil, and irrigation conditions, as well as the age of the 2 study species, were identical in each region of our investigation, it appears that the discrepancy in the impact of O3 on the leaf area of the 2 species can be attributed to species-specific factors. In urban areas, ozone (O3) primarily arises from intricate photochemical reactions involving volatile organic compounds (VOCs) and nitrogen oxides (NOx)(Thielmann et al. 2001; Mazzuca et al. 2016). VOCs assist in the oxidation of primary NO emis- sions from various sources, leading to the formation of NO2, while also competing with ozone to react with NO and thus preserving the existing ozone lev- els (Mazzuca et al. 2016). In the past, carbon monox- ide (CO) represented the primary air pollutant in Tehran. However, due to recent advancements in fuel quality for automobiles, particulate matter, specifi- cally PM2.5, has become the dominant air pollutant on most days when air quality in Tehran is considered unhealthy. Nevertheless, over the past few years, O3 has emerged as a major air pollutant during spring and summer, particularly on hot and sunny days. As a result, in the current year, Tehran has experienced unhealthy air conditions for a total of 9 days since the beginning of spring, with 7 of those days attributed to O3 pollution. This number is expected to increase with rising temperatures in the summer. The surge in carbureted motorcycles emitting hydrocarbons con- stitutes the primary factor behind the recent increase in O3 levels in Tehran’s air. In our investigation, it was observed that O3 had a more deleterious impact on the leaf surface of C. cau- casica. Certain scholars have underscored the harm- ful effect of O3 on leaf morphological indicators. For instance, Dermody et al. (2006) demonstrated that the heightened O3 expedited senescence, thereby reduc- ing LAI by 40% towards the end of the growing
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