Integrating the stages of photosynthesis

The overall process of photosynthesis requires the integrated activity of a network of subprocesses. Efficient and effective integration is important because carbon dioxide fixation, perhaps the most crucial activity of photosynthesis, arises from the combined activity of linked reactions in the thylakoid membranes and chloroplast stroma. Understanding how the various constituent processes of photosynthesis work together is important is not only an important question in plant physiology but is also at the heart of understanding the overall light-use efficiency of photosynthesis and the diversity of photosynthetic activities encountered in nature. There is no one single rate or activity of photosynthesis. Rather the process is very variable and highly responsive to the environment. This variability is part of ecological specialisation and optimisation, and the environmental dependence of photosynthesis (something which is still 'work in progress' seems to reflect compromises between maximising the light (and other resource) efficiencies of photosynthesis on the one hand, and protecting photosynthesis from damage largely due to the formation of reactive oxygen species. It is now widely recognised that how photosynthesis responds to environmental fluctuations is an essential part of operation. The physical environment of the plant and its leaves (e.g. temperature, humidity, carbon dioxide levels, and especially irradiance-this is not a complete list) changes over many time scales. These fluctuations are now seen to be the cause of increases in photosynthetic inefficiency beyond that encountered when photosynthesis is allowed to adjust and reach a steady-state optimum. Responses to rapid fluctuations in irradiance, which are physiological in nature, are because of their relative slowness in comparison to the speed at which irradiance can change now seen to be major source of increased short-term photosynthetic inefficiency in the field. Consequently, it is now widely considered that a complete understanding of the operation and regulation of photosynthesis must take into account the regulation of photosynthesis in response to environmental fluctuations. In this chapter, we will introduce something of the regulation of photosynthesis that occurs in response to changes in irradiance.The complexity of photosynthesis is best encapsulated by means of modes, and we will also introduce the use of models in photosynthesis