Sticky stuff in the spring foliage – if not a product of aphid carbohydrate metabolism, then it is due to hairy or glandular trichomes that may be able to produce lipophilic flavonoid aglycones onto the plant surface as well
Flavonoid glycosides occur in foliage of all ages
Figure 1. Seasonal variation of flavonoid aglycones in different birch species shows similar patterns and the only differences come from the absolute concentrations and from the fact that they foliate at different times of the season. Figures on the righ show how both light and SEM micrographs reveal that the glandular trichomes of birch leaf surface are active during the spring and flatten or die as the season progresses during the summer.
The variation of flavonoid levels in plant tissues depends much on the types of flavonoids produced. Foliar flavonoid glycosides may show relatively stable levels during the season, perhaps with a small peak in content during the early summer. In general, flavonoid glycosides can be considered to be the most common group of plant polyphenols: they are present in almost all kinds of plant tissue and are found throughout the growing season.
Flavonoid aglycones co-occur with maximum trichome activity
The seasonal patterns with flavonoid aglycones at least in birch leaves seem much more straightforward. This is much due to the fact that these lipophilic compounds are synthesized and excreted onto the leaf surface by glandular trichomes. These trichomes are most active during the spring and their activity decline steadily during the summer so that autumn leaves already contain dead and flat trichomes (Fig. 1). The seasonal variation of flavonoid aglycones in birches is shown in Figure 1.
“What a moment – to see huge levels of the aglycones in birch for the first time!”