Phenolic compounds and cell structure in bilberry leaves affected by emissions from a Zn-Pb smelter

Vaccinium myrtillus L. is a common shrub in the herbaceous layer of pine forests. In connection with resistance to pollution it has been widely used in the environmental monitoring based on heavy metal accumulations in the foliage. The present study investigates leaves of bilberry growing under influence of emissions from a Zn-Pb smelter. Determination of heavy metal levels by AAS revealed enhanced accumulations of Zn, Pb, Cd, and Fe, and the level of Zn fell into the toxic range compiled for plants of average sensitivity. To verify the significance of metal accumulations occurring in the studied plant, the concentrations of phenolics were quantified by measuring UV absorption of foliage extracts. Phenolics can be used as a potential biomarker of pollution because they participate in plant's response to accumulation of heavy metals, acting as antioxidants able to scavenge free radicals produced by metal ions. Leaves collected near the smelter contained significantly higher amounts of phenolics. Since these compounds are also responsible for ecological interactions, changes in their concentrations resulting from heavy metal accumulations might also influence other organisms in the ecosystem. To study the role of phenolics in heavy metal accumulations at the cellular level, a special fixation was applied for TEM, involving glutaraldehyde with caffeine, to precipitate phenolics in the cell. A histochemical reaction showed significantly higher electron-opacity, most probably deposits of phenolics, in cells of the leaves collected near the smelter. The subsequent degenerative changes in mesophyll of these leaves were characterized by increased contents of heterogeneous multivesicular structures, degeneration of chloroplasts, and disorganisation of membranes leading to death of the cell and necrosis.