Genetic modification of the flavonoid pathway alters growth and reveals flexible responses to enhanced UVB – Role of foliar condensed tannins

Accumulation of certain phenolics is a well‐known response of plants to enhanced UVB radiation (280–315 nm), but few experiments have compared the relative importance of different phenolic groups for UVB resilience. To study how an altered phenolic profile affects the responses and resilience of silver birch (Betula pendula) to enhanced UVB, we used RNA interference (RNAi) targeting dihydroflavonol reductase (DFR), anthocyanidin synthase (ANS), or anthocyanidin reductase (ANR) to change the accumulation of phenolics. The unmodified control line and RNAi‐modified plants were grown for 51 days under ambient or +32% enhanced UVB dose in a greenhouse. RNAi greatly affected phenolic profile and plant growth. There were no interactive effects of RNAi and UVB on growth or photosynthesis, which indicates that the RNAi and unmodified control plants were equally resilient. UVB enhancement led to an accumulation of foliar flavonoids and condensed tannins, and an increase in the density of stem glands and glandular trichomes on upper leaf surfaces in both the control and RNAi‐modified plants. Our results do not indicate a photoprotective role for condensed tannins. However, decreased growth of high‐flavonoid low‐tannin DFRi and ANRi plants implies that the balance of flavonoids and condensed tannins might be important for normal plant growth.