Cells, Genes & Molecules

Photosynthesis on the far side of the rainbow

Light you cannot see, in the infrared part of the spectrum, is surprisingly important for plant photosynthesis.
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Botanists know that plants can use infrared light, light with a longer wavelength than the human eye can see. This discovery has led to research on enhancing plants’ ability to use this light to improve photosynthetic efficiency. Now research by Shuyang Zhen and colleagues on plants in sunlight finds that plants already use a lot of this hidden light in broad daylight. Their study, published in a forthcoming issue of New Phytologist, suggests that more is happening in the leaves in the shade of a crop canopy than botanists thought.

Their results could have significant implications for the modelling of crops and how they use light. Zhen and colleagues point much research defines Photosynthetically Active Radiation as light in the spectrum between 400 and 700 nm. 

Low rolling hills of green grass with woods on the horizon. Above is a dark cloudy sky cut by the arc of a rainbow.
Image: Canva.

“The photosynthetic activity of far-red photons under full sun and under vegetation shade suggests that crop and ecosystem models may significantly underestimate canopy photosynthesis,” write Zhen and colleagues. “These findings also suggest that genetically engineering plants to produce chl d and f to harness far-red photons may not achieve the anticipated high degree of yield enhancement; the hypothesis behind this approach is that plants currently do not use photons above 700 nm. However, our findings show that plants already use photons up to about 750 nm efficiently, both under full sunlight and canopy shade.”

The botanists examined how effective the infrared photons were in daylight by using a shortpass filter. This is a screen that allows visible light through but blocks around 95% of the light in the infrared. The filter effectively starved the plants of infrared light. After putting the filter in place over Sunflower (Helianthus annuus) and Corn (Zea mays), the team then measured what happened to the photosynthesis in leaves.

The scientists found that leaves’ photosynthetic activity dropped by over six per cent within seconds of the filter blocking the infrared light. A side-effect of this filter was that it blocked ultraviolet light too, so the team tried another filter that only removed ultraviolet light. This cut photosynthesis by just one per cent, showing that the infrared barrier had the bigger effect.

Five sunflowers under a dark sky cut by another rainbow. Three of them have their head slightly bowed, almost like they are not happy about being the 'rain' part of the rainbow.
Image: Canva.

Zhen and colleagues argue that the fixation on the range of visible light means that there are problems both with modelling and experimentation. LED lights that emit photons only in the visible spectrum will lack the infrared light that plants use in natural conditions. Likewise, older halogen lamps may over-emit infrared light. For this reason, they say that scientists investigating photosynthesis should use special IR-LEDs that they can calibrate for experiments.

Infrared light is particularly important in the shade, where it can account for over half the light a leaf receives. This shade would include many leaves beneath the canopy of crops.

The authors conclude: “We recommend that the definition of photosynthetically active radiation be extended to include photons from 400 to 750 nm with the acronym ePAR (extended PAR), an improved metric that better predicts photosynthesis than PAR.”

READ THE ARTICLE

Zhen, S., van Iersel, M.W. and Bugbee, B. (2022) “Photosynthesis in sun and shade: the surprising importance of far-red photons,” New Phytologist. https://doi.org/10.1111/nph.18375

Translations by Google Translate.

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