RESPONSES OF THE MORPHOLOGICAL TRAITS OF ELM (ULMUS MINOR 'UMBRACULIFERA') LEAVES TO AIR POLLUTION IN URBAN AREAS (A CASE STUDY OF TEHRAN METROPOLITAN CITY, IRAN)

In this study, changes in the morphological traits of the leaves of elm tree (Ulmus minor 'Umbraculifera') were investigated against the air pollution in Tehran City, capital of Iran. For this purpose, the healthy and fully developed leaves were sampled from even-aged trees of the same diameter class from 3 districts of Tehran City with different traffic volume and air pollution load. Then, the micromorphological traits of the stoma including length, width, density, size, stomatal shape, trichome density, and theoretical stomatal resistance were measured using Scanning electron microscopy (SEM) images. The macro-morphological traits, including length, width, shape, and area of the leaf, as well as the number of teeth on the blade, length of petiole, and asymmetry of the blade were measured. Determining the wet and dry weights and leaf turgor, the specific leaf area and relative water content were estimated. The results showed that the macro-morphological traits of the leaves were resistant and did not change significantly in the presence of air pollutants. In the contaminated areas, unlike the stomatal density that showed an increase, the stomatal size decreased. With increasing leaf thickness in the polluted areas, the stomatal density also showed an increase. Due to the inherent resistance of the leaves of elm tree to the reduction of leaf water potential, the failure of the photosynthesis rate, caused by the presence of pollutants, is compensated. In the species of elm, the traits of stomatal density and size are better descriptors for reflecting leaf changes in different environmental conditions.