Fluorescence as a Tool to Understand Changes in Photosynthetic Electron Flow Regulation

The physiological state of a chloroplast is strongly influenced by both biotic and abiotic conditions. Unfavourable growth conditions lead to photosynthetic stress. Chlorophyll a fluorescence is a widely used probe of photosynthetic activity (specifically PSII), and therefore stress which specifically targets the electron transport pathway and associated alternative electron cycling pathways. By manipulating the processes that control photosynthesis, affecting the chlorophyll a fluorescence, yields detailed insight into the biochemical pathways. Light that is captured by a chlorophyll molecule can be utilised in three competing processes; electron transport, energy dissipation (via heat) and chlorophyll a fluorescence emission. Electrons produced by water-splitting are not always used in carbon fixation; if the incident irradiance generates more electrons than the dark reactions can use in carbon fixation, damage will occur to the photosynthetic apparatus. If carbon fixation is inhibited by temperature or reduced inorganic carbon (Ci), ATP or NADPH availability, then the photosystem dynamically adjusts and uses alternate sinks for electrons, such as molecular oxygen (water-water cycle or Mehler ascorbate peroxidase reaction). The process of stress acclimation leads to a number of photoprotective pathways and we describe how inhibitors can be used to identify these particular processes. In this chapter, we describe the processes controlling electron transport as influenced by light-induced stress.