LED-lighting
Trial aims to show how LEDs can be used to steer commercial crop
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."
First understand
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.”
In practise
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.”
Summary
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
‘Urban farming will not solve the problem of world hunger’
Urban farming is currently a hot item, capturing the interest of people all over the world. The roof of a former Philips building in The Hague is currently the scene of a large-scale 1,500 m2 urban farming project developed by UrbanFarmers and Priva for the cultivation of exclusive vegetables in combination with fish farming. Will urban farming, with cultivation on rooftops or indoors assisted by LED lighting, help shape the future of horticulture?
‘Urban farming is a hype. I get questions about this every week, and sometimes even every day! Many people believe that this is the solution we have been waiting for to alleviate world hunger. I do not share this conviction; simply because it is too expensive for bulk production.
I do believe that marketing concepts in big cities, with rooftop cultivation or cellar cultivation by restaurants, for example, are feasible. Consumers who find this appealing would be prepared to pay twice as much. You could also produce crops with a higher vitamin and nutrient content, which could be achieved through better crop control by using LED lighting, for instance. Consumers might very well be prepared to pay a higher price for this.
‘Rooftop cultivation at standard prices? I doubt that this is possible. Greenhouse cultivation on 10 hectares of land with only one crop type for efficient production is no longer unusual, but you can’t compare this to rooftop cultivation on a very limited surface area. Even if it is easier to control all the cultivation factors, it still has to be economically feasible. This is only possible if added value is offered that the consumer is willing to pay for.
‘I once saw a television commercial in Japan for vegetables, grown using LED lighting by employees wearing full laboratory clothing. The concept of growing healthy vegetables is entirely different in Japan. I wonder how the average European consumer would respond to this? Would he consider it to be rather artificial? And I’m not even taking the economic feasibility of this option into consideration.
‘There are many urban myths about the possibilities offered by urban farming. There will, without a doubt, be a market for this - one where products are sold that contribute some form of added value. This would certainly be possible in metropolises such as New York and Shanghai, also in terms of consumer experience. However, you can grow food in greenhouses located on the outskirts of such cities very effectively, all over the world.
‘We are investigating the cultivation of plants using only artificial light. We have access to all the knowledge we need to do this, and this approach to cultivation approach may indeed be feasible. However, we are still faced with the social aspect and the economic feasibility. Aside from that, I believe it could be possible to carry out specific phases in the cultivation process in climate chambers; phases that have a higher probability of something going wrong in the greenhouse and where the volume of plants is not so big. This would be particularly useful in the early stages of cultivation.’
There are consumers who cherish romantic ideals with regard to open-field farming.
‘Those people forget that greenhouse horticulture offers distinct advantages with regard to the consumption of water. In the Netherlands, the availability of water is generally not an issue. There have, however, been instances where hosepipe bans were imposed on growers of open-field crops on sandy soil in Limburg and Brabant in dry summers. The collection of rain water in greenhouse horticulture is not always sufficient, and in those cases the alternatives available to growers are not always adequate.
‘The availability of water on a global scale is a much bigger problem. The cultivation of one kilo of tomatoes in Israel or Spain requires 60 litres of water. When grown in a greenhouse, 30 litres will be sufficient. In the Netherlands, this is only 15 litres. Cultivation in a closed greenhouse requires even less water, because you can cool down the air temperature and collect the condensed water.’
Leo F.M. Marcelis (Elst Gld, 1963) studied horticulture at Wageningen University, where he obtained his PhD in 1994. He was a professor by special appointment of Crop Production in Low-Energy Greenhouses at Wageningen University until 2013 and team leader at Wageningen UR Greenhouse Horticulture. On 1 December 2013 Prof. Dr Leo Marcelis was appointed Professor of Horticulture and Product Physiology at Wageningen University.
Download the complete interview with prof. dr. ir. Leo Marcelis about diffuse glass, LED-lighting, urban farming, de-leafing and the effects on plants, energy consumption and cultivation strategy (login required).
Source/photo: Tuinbouwteksten.nl/Theo Brakeboer.
LEDs for energy savings of 50%
LED lighting allows energy consumption to be reduced in the cultivation of tomatoes with assimilation lighting. Even better: energy consumption can be cut in half. Eight PhD candidates and three post-graduate researchers are conducting research as part of the ‘Led it be 50%’ project to achieve this.
‘Switching from high-pressure sodium (SON-T) lamps to LED lighting will result in energy savings of 25% with regard to the conversion of electricity into light. In a few years that will even be 30%, since LEDs are becoming increasingly effective. A more even distribution of light across foliage by suspending LEDs at the right position dispersed throughout the crop will enhance light absorption by 15%.
‘Another variable is the application of different colours of light, which will enable you to control the intensity of the light throughout the day. This should lead to a photosynthesis intensification of 10%. We also want to investigate the possibility of sending relatively more assimilates to the fruit to enable 5% more fruit to be formed with the same photosynthesis level. A total amount of 60% in electricity can be saved on lighting.’
As LEDs produce little heat, will greenhouses require more heating?
‘Net energy savings of 50% are realistic. I don’t think that growers will need to raise their heating quotas, because my theory is based on the idea of crops being cultivated under higher humidity conditions. The humidity can be higher particularly during the night-time, so that less moisture will evaporate from the plants. Vaporisation costs energy, which is why we are seeking ways to cut back on vaporisation and to achieve cultivation under slightly more humid conditions.
‘A low evaporation rate and high humidity conditions allow you to save on heat. That has to compensate for the lack of heat otherwise produced by SON-T lamps.
‘Cultivation under higher humidity conditions, however, increases plants’ susceptibility to mildew and fungi. We hope to enhance the resistance in plants being grown under LED lighting through such measures as the controlled application of red light during the night.
‘We aim to achieve a production increase of 30% with the same amount of light - or the same production levels with 30% less light. But will professional growers opt for these possibilities?’ Marcelis has to smile. This question is reminding him of the introduction of a tomato variety 35 years ago. This variety could be grown at a lower temperature, but when exposed to normal temperature conditions the crop yielded decidedly more fruit. Growers unanimously preferred the latter option. ‘We examine the relationship between the amount of light used and the plant’s response to this. An entrepreneur will decide for himself where his priorities are.’
Is interlighting the answer to a more efficient use of light?
‘Light needs to be absorbed by a plant in order for it to contribute to its growth. Of all the light that shines on a plant from above, 5 to 7% is refracted. This is what you see when you fly over a greenhouse at night with the lighting on. It is not true that a portion of the light is refracted upwards on its own accord. The lamps direct their beams downwards. Another portion of the light is lost because it hits the ground. This is around 5 to 10%. In conclusion, another small portion of light is lost through transmission. This is the light that shines straight through a leaf.
‘The challenge lies in being able to reduce light loss, and to distribute light as evenly as possible. When placed directly beneath the lamps, a plant may receive an excess of light, and placed lower down, it may receive too little. In this case it’s better to consider not only vertical but also horizontal distribution. Interlighting, however, doesn’t solve this problem entirely, but it can cut back this loss considerably. You lose less light to the open sky and the ground.’
Interlighting doesn’t enable light to be projected at a big distance.
‘There is not a lot of light behind a leaf. There must be a way to improve that. Perhaps distribution could be improved with a different shape of leaf. Or you could reduce the size of the lead and experiment with adding colours to the light. I’m certain that much more can be achieved, but this will require a great deal more research.
‘SON-t lamps do not emit their light in a uniformly distributed manner across the crop; most of the light is absorbed by the topmost leaves. With a diffuse distribution improvements of 5% could be achieved.
‘Seventy per cent of all assimilates are absorbed into the fruit. This means that 30% remain inside the plant, but does the plant need this much? Suppose that you can get 75% to the plant through more efficient light control. This is an interesting aspect to take consideration.
‘Placing a diffuse sheet of glass under an SON-T lamp will take away too much light. And even if you make that light diffuse, the reflection remains and you still have less light at the bottom. The question for the industry is: this is what we can do with the sun, now what can you do with the lamp? There are still numerous possibilities with LEDS by placing lenses in front of the light source.
‘Five years from now growers will probably be using a combination of SON-T on top and interlighting in between the crop. But in the end, they will be using LEDs exclusively. I’m not clairvoyant. Perhaps SON-T lighting will make giant strides forward, but there are more development possibilities for LEDs.’
Do plants derive other substances when exposed to LED lighting in comparison to SON-T light?
‘LED lighting directed at the bunch in tomato plants will double the Vitamin C content. This immediately raises new questions for further research: how does that work? What colour light would you need to achieve this? Research on this is currently in full sway. Perhaps this will show us that we can increase other beneficial substances as well. It is doubtful that professional growers will soon be positioning their lighting directly around every bunch of tomatoes, but we do want to discover the principle behind this. Perhaps this will offer growers new possibilities. Everyone can grow tomatoes under diffuse glazing, but if you can grow tomatoes that have a beneficial effect on health, you can distinguish yourself on the market. Specific types of LED lighting could also increase these substances in other crops, such as herbs.’
Plant growth can be influenced by the colour of LED lighting. Marcelis refers to a test conducted on tomatoes in the Wageningen UR test greenhouses incorporating varicoloured LED lighting. Conventional lighting with red and blue light resulted in plants at chest height, while the plants in the test area that were exposed to far-red lighting grew above Marcelis’ head.
‘The research we are conducting should teach us which light combinations will result in optimum production. We are, for instance, also examining the results of applying far-red lighting for short periods during the night. Of all the spectral colours, red is the most efficient. Our knowledge of plant response to LED lighting is, however, still in its infancy.’
The research is funded by the STW technology foundation, LED lamp manufacturer Philips, three seed producing firms (Rijk Zwaan, Nunhems and Bejo), two automation firms (HortiMax and B-Mex), two plant nurseries (Van der Lugt and Westlandse Plantenkwekerij) and Wageningen UR University and Research Centre.
Leo F.M. Marcelis (Elst Gld, 1963) studied horticulture at Wageningen University, where he obtained his PhD in 1994. He was a professor by special appointment of Crop Production in Low-Energy Greenhouses at Wageningen University until 2013 and team leader at Wageningen UR Greenhouse Horticulture. On 1 December 2013 Prof. Dr Leo Marcelis was appointed Professor of Horticulture and Product Physiology at Wageningen University.
Download the complete interview with prof. dr. ir. Leo Marcelis about diffuse glass, LED-lighting, urban farming, de-leafing and the effects on plants, energy consumption and cultivation strategy (login required).
Source/photo: Tuinbouwteksten.nl/Theo Brakeboer.
‘No greenhouse should be without diffuse glazing, no matter where in the world’
Diffuse light is a hot item in greenhouse horticulture, not in the least thanks to Leo Marcelis. Professor Marcelis conducted research into the benefits of diffuse glazing as early as 1987. ‘Diffuse glazing may be more expensive, but with an increase in yield of 5% you can earn back your investment in no time at all. In many cases the increase in production will even exceed this 5%.’
‘When I conducted a study on diffuse light in 1987, this could only be performed through simulation in calculation models. Theoretically, we were able to conclude that diffuse glazing definitely had positive effects on the plant growth and crop production. Twenty-five years later we learned something, after all. The results achieved with diffuse glass were even better than those calculated at the time.
‘Around five years ago the production started of various types of diffuse glazing with good light transmission properties. Several tests were conducted on this, showing, above all, that diffuse glass works: it has a positive effect on production, resulting in production increases from 5 to 10% a year.’
The application of diffuse glass: Marcelis is convinced of its advantages. Whether the horticulture industry backs his conviction is not entirely clear, particularly because only few new greenhouses have been built in the past years.
‘Nevertheless, many of the greenhouses being built today are equipped with diffuse glazing.’ He continues, diplomatically: ‘Any grower not opting to use diffuse glass will probably have a good reason. Diffuse glass may be more expensive, but with an increase in yield of 5% you can earn back your investment in no time at all. In many cases the increase in production will even exceed this 5%.
‘The diffuse glass on the market today generally has poor light transmission properties, so I would not recommend it. What you gain on the one hand (light distribution), you lose on the other (light). Light is, of course, a basic requirement for plant growth. You have to use as much of it as you can, and as efficiently.’
Can diffuse glazing be applied in all countries of the world?
‘The positive effects produced by diffuse glazing are not restricted to the Netherlands; diffuse glazing will produce beneficial results in every other country in the world, and particularly in countries where there is a lot of light.
‘The more light, the greater the impact produced by diffuse glass. It is particularly beneficial when the direct light is strongest; the strong beams no longer focus as directly on the top of the plant. The intensity of the light is uniformly distributed throughout the greenhouse.’
Which crops benefit most from diffuse light?
‘With conventional glass, you will alternately have shady spots and spots with a high light intensity. This can cause damage to leaves, which is particularly undesirable if you are growing potted plants. This is why growers of potted plants frequently reduce the amount of light through screening or the application of chalk. This does, however, have a negative impact on the growth rate and the colour of the flowers.
‘Many growers of potted plants employ a low light intensity programme to prevent any chances of leaf scorch. Diffuse glazing will, however, allow for a higher light intensity. This could be of considerable advantage to growers.
‘When there is a lot of direct light, the temperature will rise while the humidity drops. Achieving a better balance in this will allow more light to be absorbed by the plant. Anthurium, for instance, is generally grown at a light intensity of 5 mol without humidifiers. Depending on the pot size and the season, a crop will take 22 weeks to grow. With diffuse glazing and a humidity that does not drop below 70% you could easily increase the light intensity to 10 mol. As a result, your crop will be ready for sale in 16 weeks and will be heavier, too. That is an astounding result. In the 25 years of my research, I have never conducted a study that produced such a big leap in production in comparison to what is being done in practice.
‘That you can achieve such leaps in production in 2014 is hardly believable. You would only expect this in developing countries, where all crops are heavily screened.’
Does this mean that you need to be able to control humidity to gain more light?
‘To fully enjoy the benefits of diffuse light you must be able to fully control the greenhouse climate. In those instances where screening is used during the day you would achieve better results with a diffuse screen.
‘Growers who already have their greenhouse set up won’t be eager to switch to diffuse glazing, because that would create a lot of havoc. In those cases, you could consider installing a diffuse screen; that will also produce positive results. Typically, new glass is only installed in newly built greenhouses; in that case diffuse glass should be used. All of this applies even more so to the cultivation of potted plants.’
What would happen if an outstanding replacement for glass were to be invented?
‘I assume that greenhouse covers will continue to be made of glass, but you cannot exclude the possibility of a good synthetic alternative being brought to the market in a few years. An alternative that is available today is an outstanding foil with excellent light transmission properties: F-clean. Apparently, there is also a diffuse variant. However, this foil is very expensive. If it were to drop in price, it may very well replace glass.’
Leo F.M. Marcelis (Elst Gld, 1963) studied horticulture at Wageningen University, where he obtained his PhD in 1994. He was a professor by special appointment of Crop Production in Low-Energy Greenhouses at Wageningen University until 2013 and team leader at Wageningen UR Greenhouse Horticulture. On 1 December 2013 Prof. Dr Leo Marcelis was appointed Professor of Horticulture and Product Physiology at Wageningen University.
Download the complete interview with prof. dr. ir. Leo Marcelis about diffuse glass, LED-lighting, urban farming, de-leafing and the effects on plants, energy consumption and cultivation strategy (login required).
Source/photo: Tuinbouwteksten.nl/Theo Brakeboer.