PARENT SESSION

Symposium S3B Photosynthetic acclimation: Ecophysiology, diverse environments
Tuesday August 31st, 2004 10:20 AM-12:20 PM Room 210A
Chair: Jun Minagawa
Co-Chair: Norm Huner

Light harvesting in plants: Molecular patterns of adaptability. Alexander Ruban^*,, Peter Horton,

ABSTRACT- The structure and function of the light harvesting system of photosystem II (LHCII) shows dynamic properties that are adapted to the large fluctuations in the external environment and developmental state of the plant. Structural organization of LHCII in vivo could determine the efficiency of light harvesting by means of controlling the cross-section and chlorophyll excitation density, developed as a result of a long-term adaptation. Therefore the study of the integrated antenna provides a fundamental background for understanding the flexible nature of light-harvesting. Adaptive tuning of light harvesting can also occur on the fast-time scale as a result of pH formation and xanthophyll cycle activity in the antenna. The process, called NPQ (qE) displays the kinetic features of an allosterically regulated co-operative process, involving a switch from energy dissipative to a non-dissipative state. This process is second order and is associated with absorption changes showing alterations in chlorophyll and carotenoid environment. In vivo spectroscopy has shown features of zeaxanthin activation by altering its physico-chemical properties. Similar alterations have been found in in vitro experiments showing zeaxanthin binding to isolated PsbS protein. Such binding could make zeaxanthin an effective excitation quencher in antenna and PsbS a vector delivering zeaxanthin into the NPQ site. Alternatively PsbS can be involved in sensing the proton gradient and modulating or triggering fast LHC antenna dynamics leading to NPQ. Studies in vitro reveal the extent of the dynamic capacity of each of the LHCII components. Regulation of the organization of the light harvesting system in different ways may be important not only in determination of NPQ capacity and dynamics, but also in the more sustained qI type of energy dissipation, which can play a major photoprotective role in some higher plants and diatom algae.

KEY WORDS: light harvesting, LHCII, NPQ, zeaxanthin