Light-induced formation of 13-cis violaxanthin in leaves of Hordeum vulgare

The xanthophyll cycle is recognized as one of several mechanisms involved in the protection of plant membranes against damage resulting from excess excitation in high light. However, in most plants, only a proportion (typically similar to 60%) of violaxanthin undergoes de-epoxidation into zeaxanthin. When leaves of H. vulgare are illuminated at 650 mu mol m(-2) s(-1) of photosynthetically active radiation for 120 min, deepoxidation of violaxanthin is accompanied by trans --> cis isomerization of this xanthophyll. The most significant change upon illumination is the formation of the 13-cis isomer from the all-trans form of this carotenoid. The level of the 13-cis isomer increases from similar to 1% of total violaxanthin in dark-adapted leaves to similar to 9% in leaves that have been illuminated. An identical pattern of isomerization for violaxanthin is also observed in leaves which art: illuminated following pre-treatment with dithiothreitol (DTT). Following illumination, plants are returned to darkness and recovery of the original isomeric content and composition is observed. The role of such light-mediated isomerization of violaxanthin in terms of limiting de-epoxidation and its potential role in photoprotection of the light-harvesting complex is discussed. (C) 1999 Elsevier Science S.A. All rights reserved.