LED-interlighting
Researchers “grow” model of ideal plant in simulated greenhouse
3D modelling is set to play an ever greater role in research. It’s an important tool in the quest to design better LED modules, better greenhouses and better cultivation systems. With the calculations produced in this way, researchers can pre-test the designs and then try out the best options in greenhouse trials.
“Just as an engineer designs a new industrial product and tests it using CAD/CAM, we too can model the ideal plant and ‘grow’ it in a simulated greenhouse under various conditions,” says Pieter de Visser of Wageningen University & Research in the Netherlands. The simulated greenhouse can then be optimised, bringing the ideal crop ever closer.
New research questions
De Visser and his colleague Gert-Jan Swinkels receive requests from both colleagues in the Kas als Energiebron research project and suppliers of greenhouse roof materials to calculate which aspects of a greenhouse design can be improved in order to achieve a more productive crop. Issues they look at include how diffuse the light should be to achieve the best result in both summer and winter. De Visser can’t comment on the outcomes for businesses since this type of research is strictly secret.
3D modelling has been rapidly gaining ground at Wageningen over the past two or three years. The models have steadily improved over time, which is just as well: new greenhouse and agricultural technologies raise so many questions that it would take vast numbers of practical trials to answer them, at a time when budgets are being slashed. Understanding of plant processes has also improved greatly. This benefits the model calculations on the one hand, but it also raises a whole set of new research questions.
Temperature distribution in the crop
In recent years, for example, it has become increasingly clear that the shape of the plant is a very important factor, particularly in the early stages of the crop. So what is the ideal shape? Does a cucumber crop with smaller leaves perform better in winter? Are leaves in a horizontal position better? What root system is best? What aspects of plant shape can you steer using light colours? And is there any point in steering in this way if the LAI (leaf area index, or the total leaf area per ground surface area) is already at the optimum level?
With so many questions, the 3D calculations are an effective tool for separating the meaningful from the less relevant ones and sorting them by their potential outcomes.
“Utilisation of light is still very much at the top of the list,” de Visser says. “We are doing a lot of modelling in that area, both with natural and artificial light. But we have gradually started to focus more on temperature distribution in the crop. After all, changing the light often changes the way the temperature is distributed. And if you do a lot of tinkering with a greenhouse, you get other temperature gradients and the crop really does start growing differently. You invariably get places where the crop grows more slowly or more rapidly. We can identify those places.”
Another area being given increasing attention is the shape of the root system, both in open-field systems and in greenhouse crops grown in the ground.
Lambertian distribution
The researcher is involved in a lot of light-related research at WUR’s Bleiswijk site, but commercial companies also call on his services. One example is Philips, who have developed a new type of higher-performance LED based on his 3D calculations. The greenhouse trial with the new LEDs in Bleiswijk was screened off on all sides to prevent people from looking inside.
But de Visser is happy to share some general principles for improving LED lighting: “With LED interlighting, it’s easy to work out the optimum height of the modules. We now know that the light incidence on the upper and lower surfaces of the leaf impacts differently on photosynthesis and growth. That is something we never used to take into account. Another important factor is the LEDs’ emission patterns. With a Lambertian distribution (evenly decreasing light output to the side), the modules are positioned quite low down. With an optimised emission pattern, they are hung half a metre higher and the plant makes better use of the light. There is also less light loss.”
Better models lead to new insights
Light loss is an important criterion, particularly in the Winterlight greenhouse. In winter it may make sense to grow a more open crop. According to de Visser’s calculations, the aisles should then be as narrow as possible. It also makes sense to space the plants slightly further apart in the row so that they are evenly distributed over the surface.
“Another important issue is light penetration,” he says. “When plenty of light reaches the lower leaves, they photosynthesise better. But the more light there is at the bottom of the crop, the more falls on the aisles between the rows and is therefore wasted. So increasing light penetration is not always better.”
The improved model calculations are leading to new insights. Three years ago, the researcher was able to demonstrate that interlighting was more effective than top lighting because the light from top lighting reflects off the crop and therefore can’t be used for assimilation. But the picture has since become even more nuanced. “The model shows that lighting from above delivers slightly more photosynthesis than interlighting, provided you can limit the light loss,” de Visser says. That loss can consist of reflection but also of light that falls unused on the ground.
More photosynthesis
A particular criterion to consider is the colour of the light, such as the ratio between red and far-red light. In a trial with far-red light in tomatoes, the model calculations supported the conclusion that the increase in production under far-red light was to a very small extent due to slightly higher photosynthesis, to a slightly greater extent due to the changed plant shape, and largely due to dry matter being distributed differently, probably as a result of hormonal changes. Hormones can’t yet be modelled, by the way.
In addition, the trials in which red light was alternated with pure green or blue light for a few hours were also analysed with model calculations. “The colour changes the shape of the plant,” de Visser says. “This has a dramatic effect on light interception to begin with, but there is almost no difference at all above a LAI of 3. What’s more, green is absorbed less than blue but green delivers more photosynthesis, so there is virtually no difference between green and blue at the crop level. Red scores much better, both in terms of absorption and photosynthesis, so red light is the best choice for assimilation lighting.”
Ideal plant
The model constantly needs to be updated with new knowledge, so measurements have to be taken on plants on an ongoing basis. Not enough is known about green light as yet to enable everything to be predicted on the basis of a model, for example. De Visser again: “We calculate the results before the greenhouse trials, and this enables our colleagues to structure the trials better. An important question is what plant shape is best for intercepting light from LEDs. We are currently studying this in a project in collaboration with Bayer CropScience. You need to know what the ideal plant is and whether a new technique could help achieve it. So the model informs the trials and the trials inform the model.”
Summary
New greenhouse and agricultural technologies are producing so many options that it is impractical to investigate them all in greenhouse trials. Calculations with 3D computer models act as a filter so that only the most promising options are studied in the greenhouse setting. The main focal points are the shape of the plant and its roots, utilisation of light and temperature distribution in the greenhouse and the crop. One example being studied is the optimum position and radiation of LED modules.
Text: Tijs Kierkels.
Images: Wilma Slegers and WUR.
Related
Vertical interlighting for high-wire crops
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The Apollo can be combined with the Venus I top lighting strip. Since it radiates little heat, this strip light can be used near the plants so there is no loss of light into the aisles. In tomato and cucumber crops, LED is now not only more effective but also cheaper than conventional HPS lights.
Vertical interlighting
With this new LED interlighting, nurseries can reduce their electricity consumption by approximately 50% compared to HPS lighting. Since LED produces less heat, less CO2 is lost through ventilation to get rid of excessive heat. Water consumption is also slightly lower because the plants transpire less in a cooler environment.
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‘Switch to LED-interlighting requires new way of growing’
Biesbrouck Greet bvba is the first Belgian tomato nursery with LED-interlighting. After eight months, owner Luc Coghe sees a big rise in yield, better quality and taste and more vigorous plants. But actually the first season has just been a year of learning. Things could be even better and that will be necessary to achieve the desired profitability.
Tomato production under glass is rare in Wallonia, the French speaking part of Belgium. Therefore Luc Coghe’s 7 ha greenhouse in Pecq (Henegouwen province, on the border with West-Vlaanderen) certainly catches the eye. Actually it’s just coincidence that he’s ended up here about 45 kilometres from his old nursery in Ardooie (the Roeselare horticultural region). “We’d been looking for expansion possibilities around Ardooie for several years. But there due to the fragmented plots available it was difficult to build a nursery of about 10 ha. Here in Pecq we came across a landowner who wanted to discontinue his own business. The dimensions of the plot appealed to me and the price also played a role. It is somewhat further from the REO auction, but much is sold directly so that’s not a problem.”
That was ten years ago. The business, which is in the name of his wife Greet Biesbrouck, now has two locations. Mrs Biesbrouck runs the old 2.3 ha site in Ardooie.
Regional brand
They grow three types of tomato in Pecq: loose (variety Kanavaro), cluster (Plaisance) and cluster cocktail (Brioso). They are members of the growers association Tomabel, which has additional specifications; a set of requirements that go beyond those of the umbrella brand Flandria. “These specifications cover varieties, presentation, sorting standards, nutrition, pruning strategy and planting distance. They put additional emphasis on quality which costs a small fee in return,” explains the grower.
He has a strong innovative attitude with an eye for opportunity. In this way he is starting to promote himself as a local producer. “This is interesting for our buyers. We now have a customer in Wallonia for whom we do the packing ourselves. We use the label ‘Agriculture of Wallonia’ although this mark still has to become established. It is not yet recognised as a regional brand such as Flandria, but it is starting.”
Five years ago, with advice from Frank Florus, he equipped 2.6 ha of his greenhouse with SON-T lighting. Florus supervises many growers who use lighting, also in the Netherlands. “The reason for doing it was both financial and for labour planning. There is definitely a market for lit tomatoes and it provides a more continuous flow of income. Also you keep your personnel in a job. The profit from a traditional crop is not very good but there are still opportunities for winter production of tomatoes,” he says.
Hybrid system
Therefore last year he made an additional step in lighting in order to expand the winter production. Together with the advisor he had figured out two options: An extra 2 ha of tomatoes lit with SON-T, or hang additional LED-interlighting in the current lit section. “I had been interested in using interlighting in a hybrid system (together with SON-T) for quite some time. LEDs are an innovation and that appealed to me; this is the future. I visited the Dutch pioneers Jami, of Bergschenhoek, and Wim Peters, of Someren. They are convinced about the advantages of interlighting. We made calculations based on performance data supplied by Philips and a certain price level for the tomatoes. This indicated that the investment was justified.”
Now he has 6,600 Philips Greenpower LED interlighting modules (55 µmol/m2/s) hanging between the plants, at the height of the fourth cluster. The SON-T top lighting (170 µmol/m2/s) is used until 1 April; the interlighting is used for a month longer. But the lighting regime doesn’t stop there: the interlighting regularly comes on at sunset until the light level reaches 250 watt.
Grow differently
Interlighting requires a new way of growing. All pioneers say that the first season is mostly a year of learning and that’s just the same for Biesbrouck. In addition, each grower has his own strategy: do you first start the old way and gradually learn from experience or immediately try everything new.
“At first we didn’t make many adjustments,” says Coghe. “In principle the climate settings remained the same, just like the planting distance. But because the plants receive so much more light energy you have to respond to that. Otherwise the plant becomes too heavy. Therefore we have to use the pipe rail heating more. Next season we want to change things but in a structured way so we are writing down many of the lessons learned.”
Retain more crop
Decisions regarding the final adjustments to planting distance, pruning policy, type of plant, number of leaves and temperature strategy still have to be made. This will require some brainstorming with the consultant.
“You should aim for more crop in the greenhouse to optimally utilise the extra light energy. But whether you allow extra shoots to remain or start with more plants is still the question. But I’m sure that it makes sense to use the LEDs earlier. We started to use them at the third or fourth cluster but you could start earlier to give a few hours of extra lighting. Then you could plant closer together,” he says. “We could also allow the modules to move with the crop. Now they remain at the same height for too long.”
First season positive
Many decisions regarding cultivation still need to be made and the plant’s response brings all kinds of questions. But the fact remains that the impression after the first season is very positive. “The quality of the fruits is impressive,” he says. “The tomatoes are much more uniform and the green parts (crown and stem) are heavier. It’s striking that the taste is much better; that goes for all three varieties. Furthermore the harvest over the weeks is more even. The plant is much stronger, greener, resistant to diseases and leaf edges and it grows faster. Due to the more robust plant leaf plucking has become more difficult.”
He estimates that the extra production as a result of interlighting is around 20%. That is in line with the forecast for the first year. But such a high level is necessary to make the investment really interesting, he says. “The yield has to be better and there we still have work to do. The plant is often too strong and steering it generatively requires a different approach. I don’t expect that even after another year we’ll have learnt everything. But I’m confident it will work.”
Summary
After eight months experience Belgian tomato grower Luc Coghe is positive about the results of interlighting with LEDs. The harvest is more even and the tomatoes are more uniform. The plants are heavier and more robust. Production is 20% higher. The first season is mostly a learning process. Improvements in the cultivation should lead to higher profitability.
Text: Tijs Kierkels. Images: Wilma Slegers