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Maria Hellström
maria.hellstrom@slu.se, 040-41 51 52, 0729-620635
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Different wavelengths of light can improve plant protection
Published: 24 February 2025
Plant surfaces are not always a hospitable environment for beneficial bacteria that help protect plants from pests. However, in a new doctoral thesis, Maria Hellström has investigated how different wavelengths of light can support these bacteria, both in lab and greenhouse environments. Different bacteria survived best under different wavelengths, and in the future, lighting could become part of sustainable cultivation systems.
To grow healthy crops without harming the environment, we need to reduce the use of chemical pesticides. One alternative is to use beneficial bacteria as biological control agents. These bacteria can protect plants by producing substances that disrupt or kill pests or by simply occupying space so that pests cannot establish themselves on the plant.
Certain Wavelengths Affect Bacteria
But there is a challenge—plant surfaces are not always a welcoming environment for bacteria. To be effective, bacteria must be able to attach to leaves, survive and compete with other microorganisms, and reproduce.
– This is where an unexpected helper comes in: light! Although many bacteria do not use light as an energy source, certain light wavelengths can still influence their behaviour. Some bacteria, for example, can form protective biofilms or produce substances that help them survive longer, says Maria Hellström.
In her doctoral thesis, Maria tested how three different beneficial bacteria responded to visible light in both laboratory and greenhouse environments.
– In the laboratory, two of the bacteria—Bacillus amyloliquefaciens and Pseudomonas chlororaphis—survived best under blue light, while Streptomyces griseoviridis preferred red light. In the greenhouse, all bacteria survived longest in green and white light, and S. griseoviridis performed well regardless of light conditions. However, B. amyloliquefaciens disliked red light.
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A leaf imprint after the application of P. chlororaphis on tomato leaves in a nutrient plate in the lab. Photo: Maria Hellström.
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Tomato plants grown for light experiments in a greenhouse at Alnarp. Photo: Maria Hellström,.
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Maria Hellström presenting results from her dissertation at the International Horticultural Congress in Angers, 2022. Photo: Isabella Kleman.
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Lighting as a future component of sustainable greenhouse cultivation
There was no universal solution—the effect of light depended on the type of bacteria and the environment.
– But it is exciting that by combining biological control with light regulation, we can develop new plant protection strategies. By understanding how light affects beneficial bacteria, we may one day use lighting as part of a more sustainable cultivation system in greenhouses.
Advice for greenhouse growers
What can greenhouse growers think about in regard to lighting strategies?
– If you are already using white light in your cultivation, you could continue doing so. The three biological control organisms we studied survived best under full-spectrum light. Both the bacteria and the plants seemed to thrive best under white and green light.
– In our case, it was a relatively cool white light, meaning it had multiple peaks in the blue spectrum. It could be important to examine what lamps are currently being used in the greenhouse to determine the actual light composition. One thing to consider is that if you use long periods of monochromatic light treatments in your cultivation, it might be best not to apply biological control agents immediately before such a treatment, as we saw that different organisms survived to varying degrees depending on the wavelength.
Combining light treatment with beneficial bacteria and pest control
If Maria were to continue her research in this field, she would like to determine how specific wavelengths affect bacteria that do not utilize light in more detail such as which metabolic functions are influenced by the treatment, and whether light intensity plays a significant role.
– We have observed that light can induce biofilm formation, but it would be even more interesting to see if there are gene expression changes based on the wavelength. Another fascinating approach would be to study the combination of light treatment, biological control organisms, and pests. Could we find a setup that benefits the beneficial bacteria and the plant while hindering the pest? That would be quite exciting to explore.
Looking for the next adventure
Now, Maria is looking for her next adventure and is open to various opportunities.
– My dream job would be to continue doing research in some way, either at a university or in a company. Right now, I am particularly interested in how different ecological concepts could be applied in selecting new biological treatments and how we can use them to enhance their effectiveness. I am passionate about working with