Biological weighting function for xanthophyll de‐epoxidation induced by ultraviolet radiation

Abstract

The light‐induced de‐epoxidation of xanthophylls is an important photoprotective mechanism in plants and algae. Exposure to ultraviolet radiation (UVR, 280–400 nm) can change the extent of xanthophyll de‐epoxidation, but different types of responses have been reported. The de‐epoxidation of violaxanthin (V) to zeaxanthin (Z), via the intermediate antheraxanthin, during exposure to UVR and photosynthetically active radiation (PAR, 400–700 nm) was studied in the marine picoplankter Nannochloropsis gaditana (Eustigmatophyceae) Lubián. Exposures used a filtered xenon lamp, which gives PAR and UVR similar to natural proportions. Exposure to UVR plus PAR increased de‐epoxidation compared with under PAR alone. In addition, de‐epoxidation increased with the irradiance and with the inclusion of shorter wavelengths in the spectrum. The spectral dependence of light‐induced de‐epoxidation under UVR and PAR exposure was well described by a model of epoxidation state (EPS) employing a biological weighting function (BWF). This model fit measured EPS in eight spectral treatments using Schott long pass filters, with six intensities for each filter, with a R2 = 0.90. The model predicts that 56% of violaxanthin is de‐epoxidated, of which UVR can induce as much as 24%. The BWF for EPS was similar in shape to the BWF for UVR inhibition of photosynthetic carbon assimilation in N. gaditana but with about 22‐fold lower effectiveness. These results demonstrate a connection between the presence of de‐epoxidated Z and the inhibition under UVR exposures in N. gaditana. Nevertheless, they also indicate that de‐epoxidation is insufficient to prevent UVR inhibition in this species.