Updated: March 11, 2020 11:00:52 am
Photosynthesis is a life-sustaining process by which plants store solar energy as sugar molecules. However, if sunlight is in excess, it can lead to leaves being dehydrated and damaged. To prevent such damage, plants dissipate extra light as heat. While this was known, there has been a debate over the past several decades over how plants actually do so.
Now for the first time, researchers at MIT, the University of Pavia, and the University of Verona have directly observed one of the possible mechanisms through which plants dissipate extra sunlight.
Published on Tuesday in Nature Communications, a peer-reviewed journal, the new research has been able to determine–by using a highly sensitive type of spectroscopy–that excess energy is transferred from the pigment chlorophyll, which gives leaves their green colour, to other pigments called carotenoids. The carotenoids then release the energy as heat.
“During photosynthesis, light-harvesting complexes play two seemingly contradictory roles. They absorb energy to drive water-splitting and photosynthesis, but at the same time, when there’s too much energy, they have to also be able to get rid of it,” said Gabriela Schlau-Cohen, the Thomas D. and Virginia W. Cabot Career Development Assistant Professor of Chemistry at MIT.
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Previous research has shown how plants quickly adapt to changes in sunlight intensity. Even in very sunny conditions, only 30 per cent available sunlight is converted into sugar, and the rest is released as heat. The excess energy, if not released, leads to the creation of free radicals that can damage proteins and other important cellular molecules.
So far, it had been difficult to observe the heat dissipation phenomenon, given that it occurs on a very fast time scale, in femtoseconds or quadrillionths of a second. Also, the energy transfer takes place over a broad range of energy levels.
Then in 2017, MIT researchers developed a modification to a femtosecond spectroscopic technique, which allowed them to observe over a broader range of energy levels– spanning from red light to blue light. Using the new technique, researchers could observe that chlorophylls absorb red light and carotenoids absorb blue and green light, thus being able to monitor energy transfer.
Schlau-Cohen explained, “By broadening the spectral bandwidth, we could look at the connection between the blue and the red ranges, allowing us to map out the changes in energy level. You can see energy moving from one excited state to another.”
After the carotenoids accept excess energy, most of it is released as heat, thus preventing damage to the cells.
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