Photosynthetic Responses of Plants to Excess Light: Mechanisms and Conditions for Photoinhibition, Excess Energy Dissipation and Repair

Abstract

Plants as sessile organisms must be capable of rapidly coping with changes in environmental conditions. In nature light is the most variable environmental parameter. During the day, plants must deal with changes of several orders of magnitude in the light quantity but also changes in light quality take place. Light is an absolute prerequisite for photosynthesis as an energy source; however, excess light can also be harmful and lead to a destruction of the photosynthetic apparatus. Photoinhibition of photosynthesis has been defined as a light-dependent decline in photosynthetic efficiency as a result of absorption of light. However, a strong consensus is still missing concerning the term photoinhibition and whether it describes a decrease in photosynthetic efficiency due to photodamage and thereby a reduction in the population of functional photosystems or regulatory adjustments, like reduced energy transfer from the antenna to reaction centers or both of these processes. Diurnal photoinhibition is a common phenomenon in most plants exposed to direct sunlight. Depending on the season and also on the diurnal cycle, plants have developed various adaptation systems to cope with highly, as well as frequently, changing light intensity and quality. Although a number of mechanisms have evolved to dissipate excess absorbed light energy by harmless pathways, the photosynthetic apparatus still remains a fragile system and vulnerable to damage by light. This chapter describes briefly the mechanisms of photoinhibition and plant response to light stress. In this chapter, we have used the term photoinhibition to describe the process that finally leads to a photodamage and repair of the reaction centers, while the dissipative regulatory processes are regarded as sole photoprotective processes.