There’s a lot of work going on into LED lighting at the moment. We already know from past research that light colour impacts on plant processes. The next step is to develop dynamic light recipes with the light colour adjusted to meet the needs of the plant at different times of the day or the growth phase. Last year saw researchers conduct the first trials with a dynamic light recipe in a semi-practical setting. Tomato plants were given blue or green light for part of the day and red light for the rest of the time.

This research was conducted in the context of a wide-ranging, innovative EU research project, the HI-LED project, which focused on three areas of application for LEDs: the use of light and light colour in the workplace, in museums and in greenhouse horticulture. The project saw the development of new lighting systems which can be controlled to provide the required light colour at any time. The four-year research project was launched on 1 December 2012 and has since ended.
Anja Dieleman was the project manager for greenhouse horticulture. Wageningen University & Research in the Netherlands worked on the project alongside Hortilux Schréder and the Spanish research institute IRTA. “IRTA looked at the effects of light colour on fruit quality, we studied the effect on the whole crop, and Hortilux supplied the specially produced lamps. When you use LEDs in museums or in the workplace, you only need one or two lamps. Cost and effectiveness also play an important role in greenhouse horticulture.”

Exploratory phase

The process started with the question: what does light colour do to plants? Dieleman explains: “Most of the research took place in climate chambers. The effect of red light there is also the effect of the absence of blue light, for example. In the greenhouse, the background is just sunlight.”
The exploratory trials with young sweet pepper and tomato plants, which have been reported on previously, looked at the effect of red, blue, amber, green and a combination of red and blue on the crop compared with white light as a reference. All six were against a background of natural daylight.
“The plants with red and amber light produced the same picture as the plants under the white reference light. The plants under blue light had shorter, smaller leaves, were darker green in colour and had higher chlorophyll levels. The measurements showed that photosynthesis increased in these plants once they were no longer under blue light. The upshot of this could be that plants that have been lit with blue light for a certain length of time process light more efficiently for the rest of the day. The plants that were under green light were more elongated and had a more open leaf structure. They looked similar to the plants that were under far-red light. You could make use of this in some way to improve light interception, at the start of the crop, for example,” Dieleman sums up.

Blue and green steering light

The basic trial with young plants and other trials with LEDs only give us a glimpse of the future. There are dozens of potential permutations for trials with different combinations of light colour, light intensity and times, and it was up to the scientists to make the right choices for the rest of the project.
In a preliminary trial, therefore, they first ran small-scale tests looking at the effects of blue light, which impacts on photosynthesis, and green light, which impacts on the shape and light interception of the crop. “In one series of plants we looked at the effects of green light at different times of the day. In terms of elongation it made no difference when we gave them a period of green light. Another series of plants was given different intensities of blue light (20, 100 and 200 μmol) at the same time of day. Even the lowest intensity seemed to have an effect. This means you can use blue light as a steering light to increase photosynthesis.”

Choice of lamp

There was no more time for preliminary research, as the start of the semi-practical greenhouse trial coincided with the normal time for artificially lit tomato plants and the researchers had to get their requirements to the lighting supplier beforehand. The supplier made the light fittings specially for the trial. It was decided to use LED fittings that gave green, blue or red light and were dimmable so that they could be fine-tuned for the trial. “We had the 0 series of these fittings; the first commercial series is available on the market in the meantime,” the project manager reflects.
The greenhouse trial with Komeett tomatoes ran from November 2015 to May 2016. “We had four 70 m2 greenhouse compartments at our disposal.” In the first compartment, the plants received 85 μmol/m2/s blue light for the first three hours in the morning, followed by 220 μmol/m2/s red light. The same was done in the second compartment but starting with green light instead of blue. The plants in the two reference compartments only received red light. The total light in the four compartments was the same. This meant that the reference plants were lit for a slightly shorter period of time in total. During the trial, large numbers of measurements were taken to monitor plant development, flowering, fruit development and quality and the effect on photosynthesis. “The expectation was that green light would mainly affect plant shape and blue light would impact on photosynthesis,” says crop researcher Kees Weerheim.

Different plant responses

Weerheim summarises the results. The plants that received blue light for the first three hours of the day increased production by 8% – a combination of the greater number of fruits produced and the heavier weight of the fruits. In addition, the plants were 10% shorter, at about 600 cm compared with 660 cm. The leaves contained slightly more chlorophyll but photosynthesis was not measurably higher.
The results of the plants that received green light for the first three hours were a little more difficult to explain. These were found to have lower photosynthesis, even though the leaves also contained slightly more chlorophyll which should allow them to absorb more light. The plants under green light were no different in length from the reference plants, although the crop was more open, allowing the light to penetrate through to the second leaf layer more easily. This could be beneficial for light interception, similarly to diffuse light through the greenhouse roof.

Sum total of little things

“The plants with blue light showed increased production, whereas the plants under green light did not. But the differences are small. We can’t pinpoint one specific factor as being responsible for the improvement in production. It’s the sum total of lots of little things: differences in photosynthesis, leaf position, chlorophyll content. What we do know is that pursuing this offers potential for the future. We don’t know or understand everything yet, but it is definitely the way to go,” project manager Dieleman confirms.
LED lighting in general is becoming more and more popular. Its potential lies in the use of light colours. “We need to generate knowledge and make growers and propagators aware of the opportunities.” Dieleman sees this as a big jigsaw puzzle to which more and more pieces can be added.


The EU’s four-year HI-LED project has ended. In the latest greenhouse trial with Komeett tomatoes, the plants received 85 μmol/m(sup>2/s blue or green light, supplemented with 220 μmol/m2/s red light. This treatment was compared with reference compartments in which the plants were only given red light. The plants under blue light increased production by 8% and were 10% shorter. Although the plants under green light had a more open structure which made for better light penetration, their production was more or less comparable to the plants in the reference section.

Text and images: Marleen Arkesteijn.