A researcher from the University of Eastern Finland conducted a study that investigated the roles of flavonoids and condensed tannins in the silver branch tree's defense system against autumnal moth and ultraviolet B radiation (UVB). The results showed that foliar flavonoids have toxic and deterring effects against insect herbivores.
Two experiments were conducted using control and genetically modiefied (GM) silver birch trees. The GM plants have specific enzymes of the flavonoid-tannin pathway partially silenced using RNAi. These are the dihydroflavonol reductase (DFR), and anthocyanin synthase (ANS), or anthocyanin reductase (ANR). Silencing them led to the block in production of one or both types of condensed tannin monomers thereby reducing the accumulation or altering the structure of condensed tannins. Data from the experiment showed that DFRi resulted in strongly reduced photosynthesis in plant growth, while ANRi decreased growth compared to the unmodified control plants.
Another experiment was conducted to determine the potential roles of glandular trichomes in relation to heating and altered solid moisture using native silver birch. This exhibited that the accumulation of certain flavonoids or increased in glandular trichosome density deterred the feeding of autumnal moth larvae, while the condensed tannins reduced the growth efficiency of the larvae. For the UVB experiment, results showed that UVB had accumulating effects in the flavonoid-tannin pathway including condensed tannins altered phenolic composition did not affect the resilience of the silver birch to UVB enhancement. In general, silver birch flavonoid glycosides are responsive to UVB.
According to the author, the results of all the experiments suggest that foliar flavonoids in silver birch trees play roles in herbivore insect and UVB resistance. There was evidence that condensed tannins may have toxic effects against insect herbivores, while glandular trichomes on the upper leaf surface of the birch tree could contribute to the insect herbivore deterrence and/or UVB light resilience of the silver birch.
Research on genetically modified silver birches generates new information about chemical defences of plants | University of Eastern Finland (uef.fi)