Green may be the dominant colour in most plants, but this same colour in the light spectrum is rejected by plants. Green is also the most intense solar light. But why do plants reject it? Physicist Nathaniel M Gabor of University of California had been in wonder about this fascinating character of plants since his days as a PhD student. Now, a team led by him has come out with a theoretical model that can explain not only why plants reject green light, but also its remarkable ability to select light of specific wave length to maximise photosynthesis, the process through plants make food for their survival. Gabor and his team’s theoretical paradigm has been published in Science recently.
It is known that sunlight, which appears to our naked eye as white light, actually consists of several different colors with each having its own wavelength and energy. In fact, more the wavelength, lesser the energy associated with a light in the spectrum. But, solar energy is dynamic and rapid changes of light happens throughout the day. Plants have the remarkable capacity of using the solar energy in producing food from carbon dioxide and water, despite sudden changes in the solar energy. In other words, noisy input of solar energy is processed to bring output of near unity. Does there exist any design principles that render plants with the efficiency?
Gabor’s team investigated this question. They resorted to the approach of network modeling. They borrowed the idea from the theory of complex network used widely in cellphone networks, brain networks and also the power grid. Their model adopted the form of noise canceling network where the noisy inputs are filtered and a steady output is obtained. Their model could reproduce the photosynthetic light harvesting not only in plants, but also in green sulphur bacteria and purple bacteria.
“Our model shows that by absorbing only very specific colours of light, photosynthetic organisms may automatically protect themselves against sudden changes -- or 'noise' -- in solar energy, resulting in remarkably efficient power conversion. Green plants appear green and purple bacteria appear purple because only specific regions of the spectrum from which they absorb are suited for protection against rapidly changing solar energy,” said Gabor.
In photosynthesis, when the flow of the solar power falling upon the light harvesting portion of the photosynthetic organism is higher than the out flow, then the photosynthesis process must adapt itself to handle the change in energy. Failing to do that would lead the organism to expel the extra energy. This is a process where the photosynthetic organism is likely to undergo oxidative stress, thereby causing damage to itself.
The study says that plants take energy from different portions of the colour spectrum that depends upon the light condition. In a condition where light or brightness is in abundance it may absorb energy from light such as violet, whereas in a gloomy condition it may prefer to absorb energy from colours closer to green, for example yellow. This remarkable dynamic capability offers the photosynthetic organisms—plants and bacteria the ability to produce a unitary output from noisy inputs.
The model is significant from the aspect of connecting tools of physical science to understand the intricacies of biological systems.
Regarding the importance of their model, Gabor said—“Biologists know well that biological systems are not generally finely tuned given the fact that organisms have little control over their external conditions. This contradiction has so far been unaddressed because no model exists that connects microscopic processes with macroscopic properties. Our work represents the first quantitative physical model that tackles this contradiction."
So, plants reject green lights and reflect back in order to avoid the stress of the most intense solar energy. This reflecting back of green light also tells us why green overwhelms plants. Similarly, purple bacteria reject the purple light and it appears purple to us.