Web Extension 5.1: How Do Plants Cope with Too Much Light?

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).

Three microscopic images, A, B, and C, show chloroplasts within mesophyll cells of duckweed in darkness, weak light blue, and strong blue light. — **Figure 1 Chloroplast Movements in Response to Light** Chloroplasts within mesophyll cells of duckweed (*Lemna*) can shift their position within the cell to absorb more (A, B) or less (C) light. This mobility of chloroplasts helps to enhance photosynthesis at low light levels and avoid damage at high light levels. (Courtesy of M. Tlalka and M. D. Fricker)

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.

A graph plots concentration of xanthophylls in micro molars per molar of chlorophyll by time of day. Three curves are plotted on the graph. The approximate data from the graph are as follows.
- Light: A downward parabolic curve is at 0 at 6 in the morning, reaches a peak at 80 by noon, and falls to 0 by 6 in the evening.
- Zeaxanthin and Antheraxanthin: A concave down curve is at 10 at 6 in the morning, reaches a peak at 40 by noon, and falls to 10 by 6 in the evening.
- Violaxanthin: A concave up curve is at 90 at 6 in the morning, falls to a minimum at 45 by noon, and rises to 85 by 6 in the evening. — **Figure 2 The Xanthophyll Cycle and Dissipation of Light Energy** When leaves are irradiated with intense light at midday, the carotenoid pigment violaxanthin is converted into two other carotenoid pigments, antheraxanthin and zeaxanthin. Zeaxanthin and antheraxanthin absorb light energy and dissipate it as heat. This chemical conversion, known as the xanthophyll cycle, helps prevent the buildup of excess light energy and subsequent damage to the photosynthetic membranes. (After B. Demmig-Adams and W. W. Adams. 1996. *Trends Plant Sci* 1(1): 21–26.)

Literature Cited

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.