Light-converting material enhances plant growth

Lettuce leaves basking in the red glow underneath a light-converting plastic film grew more than 20% larger than their counterparts in normal LED or sunlight (Nature Food 2021, DOI: 10.1038/s43016-021-00307-8).

Plants soak up sunlight to grow, but they don’t use the spectrum equally. Photosynthesis is maximally triggered by red light wavelengths. “One red photon generates a lot more biomass than one green photon,” says Xiaobo Yin of the University of Colorado Boulder. Horticulturalists have attempted to leverage this feature of plant biology by glazing greenhouse windows or dyeing greenhouse films red, hoping to boost plant growth, but without much success.

Many commercially available fluorescent dyes can absorb broad-spectrum light and re-emit red light very efficiently, but much of the emitted red light from these dyes remains trapped within the dye-containing material by internal reflections, explains Yin. An added challenge is to direct emissions such that they only shine on the plants.

To help the emitted red light break free and shine in one direction on plants, the researchers designed a surface texture comprising of many micrometer-sized bumps on one side of the film. The bumps’ curvature breaks the symmetry of internally reflected light and directs its escape through the opposite side of the film.

The researchers molded the bumpy texture onto a thin film of polymethyl methacrylate doped with a commercially available red-emitting dye. In controlled indoor experiments using broad-spectrum LED lighting, buttercrunch lettuce growing under the textured light-converting film weighed almost 20% more than lettuce grown under the textured, dye-free film. Lettuce grown in mini red greenhouses under natural sunlight grew as much as 30% heavier than controls.

For Michael Loik at the University of California, Santa Cruz, the surface modifications that allow light to pass through more efficiently represent a significant improvement. “They made a big advance in that regard.” But to be deployed in greenhouses, the material must be produced cost-effectively, he said. Loik also cautioned that there might be unintended consequences from exposing plants only to red light. For instance, some plants may need green or yellow light to produce flowers. To that end, Loik suggests testing the material with other common greenhouse crops such as tomatoes.

Yin and his team can prepare sheets that are about the size of letter paper. They are now working to adopt a continuous roll-to-roll technique for large-scale manufacture. Yin also hopes that the film could help eliminate the need for additional horticultural lighting in northern regions, where weak, natural sunlight is supplemented with LED lamps in colder months.