Sclerophylly and synthesis of phenolic compounds are active
responses of plants subjected to environmental stress
(drought, low nutrient supply, u.v.-B radiation, ozone). Here we
describe the morphological and histochemical
alterations occurring in field-grown leaves of Fagus sylvatica
L.
from three sites located along an ecological
gradient: from a site in cool and protected conditions to one located
on a mountain ridge, 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
data show that, as the ecological conditions of the stand worsen,
individual leaf surface decreases, while the
thickness of the leaves and their specific d. wt (i.e. d. wt per
unit leaf area) increases. Histochemical and
ultrastructural tests show a marked increase of phenolics during the
course of the year. These substances, present
primarily in the leaves of trees growing in stress conditions, have
been identified mainly as tannins. They
accumulate in the vacuoles, especially those of the upper epidermal
layer and the palisade mesophyll; at a later
stage they appear to be solubilized in the cytoplasm and retranslocated,
eventually impregnating the outer wall
of the epidermal cells amidst the cellulose fibrils, where they cluster
together and form an electron-opaque layer
between the wall and the cuticle. Observation of the epidermal cells
also reveals that the outer cell wall is thicker.
The paper discusses the roles of secondary metabolites in protection
and detoxification processes; the possible
ecological significance of these alterations in the ecophysiology of beech
trees.