How Higher Plants Respond to Excess Light: Energy dissipation in photosystem II

In this chapter we present current views concerning the adaptation and acclimation of the higher plant photosynthetic apparatus to excess light levels. The primary focus is at the level of the chloroplast thylakoid membrane which is where the primary events of photosynthetic energy transduction occur. We first summarize our current understanding the molecular composition and macromolecular organization of Photosystem II (PS II) because these factors pertain directly to photosynthetic function during environmental stress. We then discuss the biochemical and biophysical interpretations obtained from the most commonly used tool for probing the photosynthetic function of PS II, namely PS II chlorophyll (Chl) a fluorescence. We explain how PS II Chl a fluorescence yield measurements have provided insights into the dynamic relationships between the primary photosynthetic light-energy transduction processes and the important light adaptation and acclimation strategies utilized by PS II. The basic biochemical and biophysical aspects of the light-energy dissipation, avoidance and damage-repair mechanisms that influence PS II function are discussed in relation to a physiological gradient of increasing environmental stress. The future areas of research interest and importance regarding the optimization and preservation of PS II function during environmental stress are also briefly highlighted.