AGING-RELATED ANATOMICAL AND ULTRASTRUCTURAL-CHANGES IN LEAVES OF BIRCH (BETULA-PENDULA ROTH) CLONES AS AFFECTED BY LOW OZONE EXPOSURE

Effects of ozone on the leaf anatomy and ultrastructure of five birch (Betula pendula Roth.) clones were studied during one growing season in open-field conditions. Cumulative ozone exposure was 1.5 times higher than ambient. Ozone exposure decreased total leaf thickness in one, ozone sensitive, clone. The effect on palisade and spongy mesophyll thickness was clone-specific, while the amount of palisade intercellular space was reduced in all clones. A second effect was a change in the relative amounts of adaxial and abaxial epidermis. In palisade and spongy parenchyma cells of all clones, ozone increased the number of irregular and spherical shaped chloroplasts, the electron density of chloroplast stroma, swelling and curling of thylakoids, translucency of the mitochondrial matrix and also the amount of cytoplasmic lipids. In the sensitive clone shorter chloroplasts and reduced amount of starch were observed in ozone-exposed plants, whilst, in the tolerant clone, the size of chloroplasts and the amount of starch were unaffected. Ozone effects on number, size and electron density of plastoglobuli and vacuolar tannin were clone-dependent. At the ultrastructural level, the normal leaf ageing process progressed at different rates in the birch clones. Ozone accelerated senescence-related structural changes, in accordance with earlier observations of deciduous species.