Assessment of air pollutants in an urban agglomeration in Poland made by the biomonitoring of trees

In 1970-2000, the state of the natural environment in many regions of Poland was unsatisfactory. This situation has been improving since the integration with the European Union, mainly owing to the reduction of emissions of sulphur and nitrogen oxides. The present study included three tree species: rowan (Sorbus aucuparia L. em. Hedl.), small-leaved lime (Tilia cordata Mill.), and aspen (Populus tremula L.). The trees grew in a pollution free environment (Huszlew) and in an urban agglomeration (Lublin). The aim of this research was to make comparative observations of the structure of leaves in relation to the content of lead, zinc, copper, nitrogen and sulphur determined in these organs. The N/S ratio in the leaves was accepted as an indicator of proper metabolic processes. Observations of leaves were made with using light microscopy and the scanning electron microscopy (SEM) technique. The content of heavy metals in leaves was determined by atomic absorption spectrometry (AAS) after dry mineralization. Total sulphur was determined by the turbidimetric method, while nitrogen by the Kjeldahl method after mineralization in sulphuric acid. When influenced by polluted air, the epidermis of the examined leaves was composed of smaller cells with higher stomatal density per unit area and had a lower number of open pores compared to the epidermis of the leaves developed in a clean environment. Among the species investigated, aspen was characterized by the highest ability to bioaccumulate heavy metals in its leaves. Among the elements determined only the Zn content exceeded the permissible limit, whereas Pb and Cu were found to occur in the range of values considered to be optimal. The dominant share of lead in suspended dust caused changes in the epidermis of leaves, which led to disorders in plant water relations.