OCCURRENCE OF TANNINS IN LEAVES OF BEECH TREES (FAGUS-SYLVATICA) ALONG AN ECOLOGICAL GRADIENT, DETECTED BY HISTOCHEMICAL AND ULTRASTRUCTURAL ANALYSES

Sclerophylly and synthesis of phenolic compounds are active responses of plants subjected to environmental stress (drought, low nutrient sup ply, u.v.-B radiation, ozone). Here we describe the morphological and histochemical alterations occurring in field-grown leaves of Fagus syl vatica L. from three sites located along an ecological gradient: from a site in cool and protected conditions to one located on a mountain r idge, where the trees grow on a thin layer of soil and are exposed to the wind and to intense solar radiation in summer. The morphological d ata show that, as the ecological conditions of the stand worsen, indiv idual leaf surface decreases, while the thickness of the leaves and th eir specific d. wt (i.e. d. wt per unit leaf area) increases. Histoche mical and ultrastructural tests show a marked increase of phenolics du ring the course of the year. These substances, present primarily in th e leaves of trees growing in stress conditions, have been identified m ainly as tannins. They accumulate in the vacuoles, especially those of the upper epidermal layer and the palisade mesophyll; at a later stag e they appear to be solubilized in the cytoplasm and retranslocated, e ventually impregnating the outer wall of the epidermal cells amidst th e cellulose fibrils, where they cluster together and form an electron- opaque layer between the wall and the cuticle. Observation of the epid ermal cells also reveals that the outer cell wall is thicker. The pape r discusses the roles of secondary metabolites in protection and detox ification processes; the possible ecological significance of these alt erations in the ecophysiology of beech trees.