It’s not easy being green: Investigation of states involved with green light absorption in green plants and algae

Researchers studying light-harvesting complex II (LHCII) found that green light is absorbed by electronic (Qx) and vibronic Qy states, primarily those of Chlorophyll b (Chl b), using polarization-associated two-dimensional electronic-vibrational (PA-2DEV) spectroscopy.

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What is color? The color of an object arises from which wavelengths of light are absorbed and which are reflected. For example, we would say a leaf is green because when sunlight (composed of different wavelengths of light like a rainbow) hits the leaf, the leaf absorbs red, orange, yellow, blue, and purple light, and reflects green light back at us. Based on this description, we might conclude that green light has no effect on the plant as the very color tells us that the plant is rejecting that wavelength of light.

However, recent studies have shown that scientists may have been too hasty to overlook green light and its affect in photosynthetic organisms. The leaf is composed of many different parts, creating a high light scattering environment, which scatters green light throughout the leaf, increasing its effective path length and therefore the chance of absorption by the plant. A closer look at the amount of light of each wavelength used by the plant is nearly uniform across the photosynthetically active region, leading to renewed interest in the green region of the visible spectrum of light, especially as little is known about the states involved with green light absorption. 

Due to the close and overlapping energy levels of interest, Eric Arsenault and Prof. Graham Fleming developed the technique of polarization-associated two-dimensional electronic-vibrational (PA-2DEV) spectroscopy to study the mixed electronic and vibrational states of the light harvesting complexes in plants. By using a pump and probe of the same and different polarizations, researchers can distinguish states that would overwise be too close or convoluted to gain significant information, without a change in the target energy state. Eric Arsenault and Dr. Yusuke Yoneda used this technique on samples provided by Dr. Masakazu Iwai and Prof. Kris Niyogi to assign the enhanced ability to use green light to a specific type of chlorophyll, Chl b (rather than Chl a), and thus concluded that Chl b is responsible for harnessing the power of green light and distributing photosynthetic activity throughout a leaf. 

Ultimately, understanding light harvesting across all possible wavelengths of light, including green light, is necessary to have a complete understanding of the spatial and energetic landscapes of photosynthesis in green plants and algae. This study, in particular, may explain why green plants have evolved with different light harvesting pigments and how these distinct pigments may have led to the very high quantum efficiency of photosynthesis.

For more information, see the recent publication in Nature Communications: The role of mixed vibronic Qy-Qx states in green light absorption of light-harvesting complex II.

Kaydren Orcutt

Graduate Student, University of California, Berkeley

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