A bittersweet nightshade plant under experimental light conditions with extra red and far-red light. [Image: Utrecht University]
Some plants never see direct sunlight; they live their lives in light filtered by a forest canopy. Other plants grow under artificial light, like LED fixtures that can be tuned to provide optimum wavelengths for growth. But scientists still don’t know exactly how plants carry out photosynthesis under these conditions.
Researchers at two universities in the Netherlands have found that common plants in shady situations use more light than previously believed (Plant Cell Environ., doi:10.1111/pce.15340 ). While standard scientific models of photosynthesis assume that the glucose-making process requires light within the wavelengths of 400 to 700 nm, the team found that light just outside the low end of the visible spectrum—from 700 to 750 nm, dubbed “far-red”—contributes to photosynthesis just as well.
Models versus reality
For more than 40 years, scientists have used a particular mathematical model to predict the carbon assimilation rate of plants carrying out photosynthesis. The model takes into account ambient temperature, partial pressures of carbon dioxide and oxygen, and visible light intensity, plus other parameters. The original model didn’t consider the spectral differences of incoming light: daylight versus artificial illumination, cloud cover, or tall plant canopies that absorb blue and red wavelengths and pass more green and far-red light to the ground below.
Environmental scientist Hugo J. de Boer of Utrecht University and his colleagues used both theory and experiment to tweak the original model to account for chlorophyll excitation by photons of different wavelengths and the resulting electron transport chain.
The team chose three species of plants for the experiments: two agricultural crops, lettuce and beans, and a perennial vine called bittersweet nightshade (Solanum dulcamara), which usually grows in the understory of forests. The researchers placed leaves of the young plants in an enclosed chamber to measure gas exchange rates and illuminated them with various combinations of LED lights and filters to control radiation intensity and spectra. Finally, the group calculated how much the far-red light supplementation added to the plants’ rates of photosynthesis.
Researcher Hugo J. de Boer places a bittersweet nightshade plant under experimental light conditions with reduced red and far-red wavelengths. [Image: Utrecht University]
“In addition to showing morphological changes, our shade plants started growing much quicker when we tried to fool them by installing LED lamps to supplement far-red light,” said de Boer. “To our astonishment, our plants were perfectly capable of using the additional far-red light for photosynthesis.”
Findings and impact
The scientists confirmed that the original model does not accurately account for the amount of photosynthesis stimulated by the far-red region of the spectrum. “But it turned out to be much more difficult to quantify the color effect on photosynthesis, because the available mathematical models and measurement methods were based on the assumption that plants only use light from the visible spectrum,” explained de Boer. “So we adapted a commonly used photosynthesis model to quantify the color effect using combined measurements of photosynthesis and the full light spectrum that reaches the leaf.”
From the modeled data, the group derived a single parameter to express how the addition of far-red wavelengths affects plants’ biological processes. The study could yield new clues to the ecological balance of forests and could also improve indoor horticulture.