When Plants Get Too Much Sun

PHOTOSYNTHESIS

The molecule that plants use to protect themselves from the dangers of getting too much sun during photosynthesis has now been revealed by a combination of ultrafast spectroscopy and plant genetics.

Plants rely on sunlight to generate chemical energy via photosynthesis. It's been known for more than two decades that plants protect themselves from excess light, which can lead to oxidative damage to chlorophyll and other key photosynthetic pigments. But the biophysical mechanism of this protective process--known as feedback de-excitation--has remained mysterious.

Chemist Graham R. Fleming, plant biologist Krishna K. Niyogi, and coworkers at Lawrence Berkeley National Laboratory and the University of California, Berkeley, have now shown that zeaxanthin, a carotenoid known to be produced by plants in response to bright sunlight, is responsible for the protective effect [Science, 307, 433 (2005)].

Using femtosecond spectroscopic techniques and plant genetics, the team shows that exposing spinach leaves to intense light triggers the formation of zeaxanthin cation radicals. These cation radicals form when zeaxanthin binds to potentially dangerous photoexcited chlorophyll molecules. The zeaxanthin gives up an electron to the excited chlorophyll, yielding a chlorophyll anion radical and a zeaxanthin cation radical. These products subsequently undergo charge recombination, allowing the excited-state energy of chlorophyll to be safely dissipated as heat.

The study is "a major advance in understanding how feedback de-excitation works at the molecular level," comments Robert E. Blankenship of Arizona State University. It "points the way to studies in which this mechanism might be manipulated for practical uses," he adds, such as protecting artificial photosynthetic systems used for solar energy generation from light-induced damage.