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

The xanthophyll cycle is recognized as one of several mechanisms involved i n the protection of plant membranes against damage resulting from excess ex citation in high light. However, in most plants, only a proportion (typical ly 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 ph otosynthetically active radiation for 120 min, deepoxidation of violaxanthi n is accompanied by trans --> cis isomerization of this xanthophyll. The mo st significant change upon illumination is the formation of the 13-cis isom er from the all-trans form of this carotenoid. The level of the 13-cis isom er increases from similar to 1% of total violaxanthin in dark-adapted leave s to similar to 9% in leaves that have been illuminated. An identical patte rn of isomerization for violaxanthin is also observed in leaves which art: illuminated following pre-treatment with dithiothreitol (DTT). Following il lumination, plants are returned to darkness and recovery of the original is omeric 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 discus sed. (C) 1999 Elsevier Science S.A. All rights reserved.