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This loss of primary chemical defense is compensated for by the thickness and chemical constituents of the bark.
Unlike birch stem glands, birch leaf trichomes have so far received considerably less attention, although recent studies have suggested that they may play a role in birch resistance against UV-B radiation (Kostina ., 2003).
(4) Are changes in trichome density and concentrations of flavonoid aglycones due to changes in leaf size or to changes in trichome number per leaf and in synthesis of flavonoids?
The birch ( they have uneven distribution within leaf and high variation within and between leaves, trees and provenances.
In the present study, seasonal changes in leaf trichomes and epicuticular flavonoid aglycones in three Finnish birch taxa ( Trichome number and ultrastructure were studied by means of light, scanning and transmission electron microscopy, while flavonoid aglycones in ethanolic leaf surface extracts were analysed by high-pressure liquid chromatography.• Density of both glandular and non-glandular trichomes decreased drastically with leaf expansion while the total number of trichomes per leaf remained constant, indicating that the final number of trichomes is established early in leaf development.
Samples for light, transmission and scanning electron microscopy were fixed and studied as described by Valkama . The presence of small plastids, vacuoles, lipid droplets and osmiophilic material was recorded from each sample to estimate seasonal changes in trichome structure and the secretion of exudates.
Ten leaves from each clone, provenance or origin were collected for each stage of leaf development and then frozen and stored at −80 °C. Total concentrations of flavonoid aglycones were calculated as a sum of concentrations of individual flavonoids for each stage of leaf development.
It has been suggested that, in young leaves lacking epidermis, trichomes and their exudates may serve as a functional analogue of the epidermis in mature leaves (Karabourniotis and Fasseas, 1996) since they play a similar protective role against biotic and abiotic factors such as water deficit (Ehleringer, 1982; Mauseth, 1988), insect herbivores (Levin, 1973; Juniper and Jeffree, 1983; Wagner, 1991), phytopathogenic fungi (Allen ., 2000).
At later stages of leaf development, when the formation of the epidermis is completed, the functional role of the trichomes becomes less important, and they often senesce and shed.