The operation of the lutein epoxide cycle correlates with energy dissipation

A new xanthophyll cycle involving de-epoxidation of lutein epoxide (Lx) into lutein in the light and epoxidation back in the dark has been recently described in parasitic plants and in trees from the genus Quercus. To explore the role of the Lx cycle in photoprotection, shade leaves of red oak (Q. rubra), with a relatively high Lx pool, were exposed to different light intensities. Both violaxanthin and Lx were de-epoxidised to the same extent, although the initial kinetics differed, with a rate proportional to the light intensity. De-epoxidation of violaxanthin and Lx was inhibited by dithiothreitol, suggesting that the same enzyme, violaxanthin de-epoxidase (VDE), catalyses both reactions. Dark recovery lagged in the case of Lx, and after 5 h in darkness, the Lx cycle was much more de-epoxidised than the violaxanthin cycle. The different rates of epoxidation of the violaxanthin and Lx cycles were used to study the role of the Lx cycle in photoprotection. Statistical approaches (partial correlation and multiple regression) indicate that in these leaves, maximal photochemical efficiency of PSII (F_v/F_m) and non-photochemical quenching are correlated with the level of Lx de-epoxidation. The potential implications of this finding for the understanding of the photosynthetic process in shaded and understorey leaves are discussed.