How do plants protect themselves against sunburn?

Ultra-fast switching prevents "burn through" the photosystem

A chlorophyll (top) and a carotenoid molecule (bottom) temporarily unite quantum mechanically and collectively dissipate excess energy so that not too much of it reaches the reaction center. © TU Braunschweig
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Not only most people - even plants have to protect themselves from too much solar radiation. Your photosynthesis reaction center works so efficiently that any "too much" sun would make the system burn out. How plants protect themselves has now been discovered by a research team. As they report in the journal "Proceedings of the National Academy of Sciences" (PNAS), the sophisticated mechanisms could be harnessed, among other things for photovoltaic systems.

In the so-called reaction center of the plant, nature creates something astonishing: every light quantum or photon captured by the green plant pigment chlorophyll is almost 100 percent used for electrical charge separation. This electrical energy is then used to drive all other biological processes. Because of the high efficiency, the system needs only little light. In nature, however, the sun's radiation can become a thousand times stronger within seconds, for example when clouds break up or a shadow donor disappears. Accordingly, a thousand times more energy would have to be converted in the reaction center.

Safety switch for the green energy factory

"That's as if a car engine suddenly had to make three million three-turns instead of three thousand. No apparatus can handle this unscathed, "explains Professor Peter Jomo Walla from the Institute for Theoretical and Physical Chemistry of the Technical University of Braunschweig. He has studied with his colleagues how plants manage to switch quickly and effectively in such a situation. You need to quickly get rid of the excess energy to save your photosystem from burning through. How this happens in detail has been unknown for decades and has been the subject of intense research.

Laser look inside the living plant

"Only since the invention of ultra-short-time lasers has it been possible to observe these extremely fast and elementary steps at all, " explains Walla. His research group has several ultra-short-time lasers that produce flashes of only a few femtoseconds in duration to scan the ultrafast photosynthetic processes. Through a special modification of the ultra-short-time lasers, the researchers were able to directly demonstrate in living plants, during this regulation, exactly how the switching works.

Ultrafast reaction dissipates energy as heat

As soon as the sun irradiates a leaf too much, the green chlorophyll combines in a matter of seconds with orange carotenoid leaf pigments, albeit not chemically but electronically. By quantum mechanical "tricks" the two molecules then behave together like a single display

Molecule. Each molecule now has some of the properties of the other molecule.

While the green dye usually stores the light energy electronically in the short term, carotenoid molecules quickly generate heat from it. By "mixing" this carotenoid property with the chlorophyll, the surplus energy is therefore immediately safely converted into heat. With decreasing light intensity, the two molecules immediately release their liaison, and the chlorophyll molecules supply the reaction center with energy as usual.

These processes take place in an incredibly short time, namely within a few femtoseconds - trillionths of a millionth of a second. By comparison, nothing is faster than light, and yet it takes more than 100 femtoseconds to cover only the distance, the diameter of a

corresponds to human hair.

Benefits for photovoltaics and plant breeding

"It gives us insights that may, for example, be important for the breeding of crops that are productive even under extreme climatic conditions, such as those in Third World countries, " says Walla. "A medium-term goal of our research is the development of artificial photosynthesis and synthesis

novel photovoltaic systems, for example with special nanoparticles. These behave in a similar way to chlorophylls or carotenoids, but they are much more stable. "

First model systems, whose efficiency depends on the speed of the energy conversion processes, similar to those in nature, are already being developed at the TU Braunschweig and investigated with the ultra-short-time lasers.

(Technical University Braunschweig, 20.08.2009 - NPO)