Abstract
The presence of an acidic lumen and the xanthophylls, zeaxanthin and antheraxanthin, are minimal requirements for induction of non-radiative dissipation of energy in the pigment bed of Photosystem II. We recently reported that ascorbate, which is required for formation for these xanthophylls, also can mediate the needed lumen acidity through the Mehler-peroxidase reaction [Neubauer and Yamamoto (1992) Plant Physiol 99: 1354-1361]. It is demonstrated that in non-CO2-fixing intact chloroplasts and thylakoids of Lactuca sativa, L. c.v. Romaine, the ascorbate available to support de-epoxidase activity is influenced by membrane barriers and the ascorbate-consuming Mehler-peroxidase reaction. In intact chloroplasts, this results in biphasic kinetic behavior for light-induced de-epoxidation. The initial relatively high activity is due to ascorbate preloaded into the thylakoid before light-induction and the terminal low activity due to limiting ascorbate from the effects of chloroplast membranes barriers and a light-dependent process. A five-fold difference between the initial and final activities was observed for light-induced de-epoxidation in chloroplasts pre-incubated with 120 mM ascorbate for 40 min. The light-dependent activity is ascribed to the competitive use of ascorbic acid by ascorbate peroxidase in the Mehler-peroxidase reaction. Thus, stimulating ascorbic peroxidase with H2O2 transiently inhibited de-epoxidase activity and concomitantly increased photochemical quenching. Also, the effects inhibiting ascorbate peroxidase with KCN, and the KM values for ascorbate peroxidase and violaxanthin de-epoxidase of 0.36 and 3.1 mM, respectively, support this conclusion. These results indicate that regulation of xanthophyll-dependent non-radiative energy dissipation in the pigment bed of Photosystem II is modulated not only by lumen acidification but also by ascorbate availability.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
