Damage and protection of the photosynthetic apparatus from UV-B radiation. II. Effect of quercetin at different pH.

The effect of the exogenously added quercetin against the UV-B inhibition of the photosystem II (PSII) functions in isolated pea thylakoid membranes suspended at different pH of the medium (6.5, 7.6 and 8.4) was investigated. The data revealed that the interaction of this flavonoid with the membranes depends on the pH and influences the initial S0-S1 state distribution of PSII in the dark, the energy transfer between pigment-protein complexes of the photosynthetic apparatus and the membrane fluidity. Quercetin also displays a different UV-protective effect depending on its location in the membranes, as the effect is more pronounced at pH 8.4 when it is located at the membrane surface. The results suggest that quercetin induces structural changes in thylakoid membranes, one of the possible reasons for its protection of the photosynthetic apparatus.