Light is a key ingredient in photosynthetic energy capture. However, the amount of light a plant captures can exceed the plant's ability to use it, particularly when other stresses limit another component of photosynthesis—for example, when water stress lowers the uptake of CO2 through the stomates or low temperatures limit enzyme activity. Excess light energy can generate toxic oxygen molecules (free radicals) that damage the photosynthetic membranes of the plant, a condition known as photoinhibition.
Plants have evolved several mechanisms to help dissipate the absorbed light energy that is not used in photosynthesis. Over periods of hours to days, some plants can move their leaf blades away from the sun or curl their leaves to lower their exposure to solar radiation. In addition, the chloroplasts of some plants may migrate within the cell to increase self-shading (Figure 1).
Another widespread mechanism for energy dissipation is the xanthophyll cycle (Demmig-Adams and Adams 1996, 2006), which involves the conversion of carotenoid pigments from one form to another and the release of absorbed light energy as heat (Figure 2). The xanthophyll cycle increases in importance as environmental constraints on photosynthesis develop due to stress, climate change, or as light energy increases.
Demmig-Adams, B. and W. W. Adams. 1996. The role of xanthophyll cycle carotenoids in the protection of photosynthesis. Trends in Plant Science 1(1): 21–26.
Demmig-Adams, B. and W. W. Adams. 2006. Photoprotection in an ecological context: the remarkable complexity of thermal energy dissipation. New Phytologist 172: 11–21.