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.”
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.
Interest in artificial lighting with LEDs is on the rise. Recently tomato growers Verkade in ‘s-Gravenzande (Westland, the Netherlands) gained their first experience with a hybrid system consisting of SON-T HPS lights and LEDs in a greenhouse that is too low for intensive lighting using only SON-T lights. Growing manager Fred Schäpe is happy with the winter yields. Based on his first impressions, he believes this system could deliver even more. He is pleased with how things are going.
It’s the middle of summer. At Verkade the LED interlighting modules are often still on at 11 am, depending on the climatic conditions. They can be switched off after midday. In his first year with this lighting system the growing manager is already exploring how far he can take things. By taking a good look at how other growers are using artificial lighting, he is learning fast and making his own plan.
In the autumn of 2015, an artificial lighting system was installed at this ‘s-Gravenzande-based tomato grower’s 4.4 ha site with the support of the growers’ cooperative Van Nature, in order to meet rising customer demand for Dutch products of consistently high quality all year round.
The Venlo greenhouse has a bay width of 8 m and a post height of 4.85 m, which is too low for intensive lighting with SON-T lights because of the amount of heat they emit. “We could also have raised the height of the greenhouse, but a combination of SON-T lights and the cooler LEDs proved a better option in the end,” Fred Schäpe explains.
After much discussion, the two organisations decided to install a hybrid system. At the top they installed Hortilux 1,000 watt SON-T downlights delivering 135 µmol/m2/s. These lights are distributed in threes along the width of the trellis girder. The distance between the light and the tops of the plants is 1.2 m, reducing to around 1 m as spring arrives.
One strip of 55 µmol/m2/s LEDs was suspended between the plants in each row and can be moved as the plant grows taller. A total of 6,610 Philips GreenPower LED interlighting modules have been installed, bringing the system’s total output to 190 µmol/m2/s.
Start of growing
Verkade specialises in large cluster tomatoes. Fred Schäpe has always grown Merlice. In midsummer the plant load is spectacular – an image that is only enhanced by intensive leaf pruning which makes the trusses stand out even more.
The 64-day-old grafted and topped plants were planted at the end of October. At first the plant density was 2.3 per m2. A lateral stem was allowed to develop in week 46, increasing the density to 3.05 per m2. The last lateral stem was added in week 48, bringing the final density to 3.8 stems per m2. Production started in week 52.
Immediately after planting, the plants remained unlit for ten days, after which the light level was gradually increased. In the lighting season, which lasted until around 1 April, the SON-T lights were left on between midnight and 6 pm. They were switched off when the radiation reached 2,000 J/m2, so the number of lighting hours was lower in April.
Unlike the SON-T system, the LEDs are left on constantly from 4 am to 5 pm from 1 April onwards, except on very hot days when the system is switched off earlier.
Effect on truss development
At the start of cultivation the interlighting strips were hung as low as possible in the crop. Later on they were repositioned once, close to the developing trusses. The growing manager is already coming to the conclusion that he will have to move the modules several times over the course of the next crop to keep up with the trusses and to enable every set truss to get maximum benefit from the light. Schäpe: “We do this manually. We want the trusses to develop as well as possible with maximum plant load.”
By about week 50 the modules were in their highest position and were left there for the rest of the growing period. He soon noticed the positive impact of the LEDs on truss development and flowering speed. “Trusses seem to be developing a little better with LEDs than with only SON-T.”
In January last year, Schäpe was happy with the crop status. “It’s all going well,” he said at the time. “The crop has been steered generatively right from the start, and I can see that the plants are having no trouble putting their energy into the fruits.” He is happy with the winter production, and although it is only their first growing year, he looks ahead with optimism: “With the knowledge we gained last winter, I’m sure we will be able to do even better in the future.”
The crop manager has his own strategy for truss pruning and fruit load. Sometimes he prunes the Merlice to as many as six fruits if the truss is strong enough. By doing so he is looking for the ideal fruit load.
Looking back at his first season, Schäpe is positive about the impact of the LEDs on production and quality. Although it’s a heavy investment for large cluster tomatoes, Schäpe sees even more potential in optimising the crop even further. Only time will tell. But for the time being he can look back over his first season of artificial lighting with a smile.
With support from the growers’ cooperative Van Nature, a hybrid lighting system was installed at one of tomato grower Verkade’s sites in Westland, the Netherlands, because the greenhouse is too low for intensive lighting using only SON-T lights. Crop manager Fred Schäpe is happy with truss development and flowering speed. Although this is only their first year of artificial lighting, the company is pushing the boundaries in order to optimise growing.
Text and image: Pieternel van Velden.
Van Uffelen Flowers held an open-doors day to show off its newly delivered four-hectare chrysanthemum greenhouse at Herenwerf in Maasland on Saturday 16 April, together with its builders and installers.
The newly designed Greenhouse was built by Technokas. The greenhouse cover was executed in diffuse glass, with a haze factor of 70. Another interesting detail is the double screening system by Svensson (Harmony 2515) and Bonar (energy-saving blackout cloth), executed by Huisman Scherming. The lighting system was provided by Hortilux Schréder (1,000 Watt SON-T narrow-angle lighting fixtures, 10,000 lux), and the other water and electrical systems were by Stolze.
Next Generation Cultivation
The greenhouse is prepared for the installation of air handling units for mixing the air in the greenhouse with air blown in from outside, in accordance with the basic principles of Next Generation Cultivation. ‘This system allows the entire air content of a greenhouse to be complete renewed approximately once every hour,’ explains Hans van Tilborgh of Technokas. ‘It will not be replacing the air vents, but will comprise a useful addition to them.’ Before the outside air is blown into the greenhouse through a large hose, it is heated to the greenhouse temperature. This is to prevent climate differences in the greenhouse.
However, the system does not provide in heat recovery, like tomato grower Ted Duijvesteijn’s ID Greenhouse. ‘That would mean installing a much more complex system. It also saves energy. In combination with a double screen this greenhouse will allow us to save 30 to 40 per cent more energy than in a conventional chrysanthemum greenhouse,’ continues Van Tilborgh.
In addition to the greenhouse, Technokas also supplied Van Uffelen Flowers with hoistable heating frames and production halls, designed by the Poortinga & Zwinkels architecture firm. According to architect Hester Poortinga, Van Uffelen aims to have its new building reflect the brand identity and values of Zentoo: transparent, unifying and innovative. Zentoo is the trademark under which Van Uffelen chrysanthemums are marketed. The chrysanthemum varieties are supplied by Fides and Deliflor.
Other technical tours de force at Van Uffelen Flowers are the Robur fully automated spray boom, the ISO Group peat block planting machine and the Bercomex harvester. Once harvested, the flowers are transported to the shed on underground conveyor belts. The cooling facilities with pre-cooling units were supplied by Hamelink Koeling BV.
Text/photos: Mario Bentvelsen.
The application of LED lighting in greenhouse horticulture is still in its infancy. A few nurseries are already using LEDs, mostly red and blue. But still little is known about how the different light colours work precisely on the physiology of the plant. Research by Wageningen UR Greenhouse Horticulture in the Netherlands shows that blue and green LEDs can be used to steer a crop. A large greenhouse trial should give insight into how this can be applied in practise.
The European Union financed the research that was carried out by Anja Dieleman and Esther Meinen between September 2014 and July 2015. Dieleman puts it into context: “It is part of wider, innovative, EU-research into how LED-lamps can be applied, for example, in the work environment and in museums. Together with a Spanish Institute, we examined the possibilities within greenhouse horticulture. The question was, ‘How can light colours steer a crop in its growth and development?’ The ultimate goal is to develop a dynamic light formula for plants."
While almost only red and blue LEDs are used in horticulture, the first part of the research, carried out at the end of 2014, considered six colours: White, amber, red, blue, green and red/blue. Each colour has its own light spectrum, ranging from 400 to 700 nm. Hortilux supplied the specially manufactured lamps. Young tomato and pepper plants were chosen for the trial as these are very common crops.
Meinen: “We began very straightforwardly, purely to understand how the colours work.” In six compartments of 4.5 m2 the plants received 200 µmol/m2/s (micromol photons per square meter per second) of LED light of a single colour in addition to a small amount of daylight, averaging 4 per cent over the whole day, for period of 15 hours. “In this way we could best see what the light colour did to the plant.”
Steer with blue and green
Measurements were taken after three weeks which included plant length, leaf stretch, light absorption, photosynthesis and growth. The photo of six tomato plants shows very well where the largest effects occurred. Blue light (second from left) led to the plants becoming shorter and lighter in weight, the stomata were the most open and the leaves were the darkest in colour. “Although the growth lagged behind, blue LEDs do have potential as a means of steering growth. We measured a relatively high level of chlorophyll which is favourable for photosynthesis. And sometimes growers actually want compact growth,” explains Meinen.
Green light led to the tallest plants, the largest leaf surface area and leaves with an open structure. Dieleman: “It’s very interesting, because in general it’s thought that green light does little for the plant because green light is mostly reflected. It turns out that, just like far red light, it has a considerable effect on stretching of the crop. The effects of the other colours were similar to that of white light.”
More pigments with blue light
Because green and blue light had the most distinct steering effect, the second part of the trial, that took place between March and July 2015, concentrated on these two colours. Bearing in mind the light formula should ultimately be applicable in practise, it was decided to use a limited period of blue or green steering light at lower intensities.
The researchers started with blue LEDs. In addition to a control with white light they tested three options namely two hours blue light in the morning with an intensity of 20, 100 or 200 µmol/m2/s. For the rest of the day the plants received 'standard' red/amber light. The idea was that the plants would create more pigments which could be used later in the day for photosynthesis. Was this true?
They saw a clear line. At 20 µmol/m2/s there was no visible positive effect, at 100 µmol/m2/s there was a small increase in the production of pigments and at 200 µmol/m2/s there was a distinct effect. The photosynthetic capacity was the highest in plants that received 200 µmol/m2/s blue light in the morning. “The dry weight and leaf surface area of the plants that received two hours of blue light was similar to those in the control treatment,” says Meinen.
More open structure with green light
To test the effect of green light, the researchers also set up three trials in addition to a control treatment with white light. They exposed the plants to two hours of green LED light at various times of the day: in the morning, late afternoon and at the end of the day. In all cases the light intensity was 50 µmol/m2/s. For the rest of the day the plants were exposed to ‘standard’ red/amber light.
Here too, the results were positive. With just two hours of steering light, in all three situations the crop was more open and taller while the dry weight and leaf area of the plant were comparable with those in the control treatment. “The most notable feature was the different, more open leaf structure. With the same leaf surface area the leaves were larger in size. That can mean that light is better able to penetrate the crop. Did the time of day have an effect? From this trial it doesn’t seem to make any difference,” says Meinen.
Flowering and fruit development in greenhouse trial
In short, just two hours of blue and green ‘steering light’ per day is enough to change the morphology of the young plant. But what is the effect on flower and fruit development? Dieleman: “We couldn’t examine this in these experiments. Therefore in spring 2015 we started a larger greenhouse trial in Bleiswijk, that covers a complete winter crop of tomatoes. One area is the control in which we only use red LED lights. The other treatments use 50 µmol/m2/s of blue light and green light for a few hours during the morning. The main lighting for the rest of the day is red LED light. For all treatments we look at the effect of plant development, flowering, fruit development and quality.”
In the area with blue light the researchers are looking specifically at photosynthesis and in the area with green light they are looking at the openness of the crop and how that affects light interception. “Originally we wanted to test a single light formula comprising different colours. We’ve deviated from this in this trial because the effects of blue and green light are opposite. Therefore we’ve kept them apart.”
During the trial growers were interested in the results, even though the cost of LED lighting is still quite high. “Some tomato growers are already using LEDs, mostly red and blue. Plant propagators, ornamental and pot plants growers are also interested in using LEDs to steer growth, for example as an alternative to using growth retardants. However, we still have to gather much more knowledge about the exact effect of light colours,” says Dieleman. Meinen adds: “Therefore in the following trial we will take lots of measurements to understand the underlying plant processes.”
A trial has shown that it is possible to steer the growth and development of young tomato and pepper plants by using blue and green LEDs. A comprehensive follow-up trial with a winter crop of tomatoes should show what effect these colours have on fruit development. The ultimate goal is a dynamic light formula for optimal steering of the plant.
Previously, the intensity of lighting was expressed in lux. But this is a unit that expresses the sensitivity of the human eye to light, not the sensitivity of plants. Plants use light for photosynthesis. The more photos of light, the more photosynthesis occurs hence the measurement is based on micromol photons, which is expressed as a number in µmol/m2/s.
Text: Karin van Hoogstraten. Photos: Gert Janssen (Vidiphoto) and Wageningen UR