far-red lighting
Trials show far-red light leads to large rise in tomato production
Far-red light enables a larger proportion of assimilates to reach the tomato. This is the conclusion of researchers at Wageningen University & Research who carried out trials in Bleiswijk, the Netherlands. “This is the first time that it has so clearly been shown that you can use light colour to steer tomatoes,” says Anja Dieleman.
That using far-red light could be favourable for tomato production was shown some time ago in a trial using high intensity far-red (140 µmol/m2/s). But the initial enthusiasm of the growers involved in the trial waned when later in the season the crop deteriorated. The early production was significantly higher but declined steeply in the spring as the crop developed short and small leaves.
Intensity important
It was time to take a more fundamental approach. “Small scale trials with different light colours were carried out by a PhD student within the research program Bio Solar Cells. These looked very promising,” explains Esther de Beer, of Philips Lighting.
Afterwards the treatments were adapted for practical trials carried out at Wageningen University & Research facilities in Bleiswijk. The goal was to determine if the addition of far red-light is attractive for commercial users. The research carried out previously had already shown that the intensity of the far-red light is important.
Three treatments were compared in Bleiswijk. The control was top lighting with red/blue LEDs (185 µmol/m2/s). This was supplemented with two treatments of far-red light of two intensities: 30 and 55 µmol/m2/s. The crop received less daylight than normal because the wall screens in the trial greenhouse were always closed to prevent radiation being emitted to neighbouring sections. Planting took place at the beginning of October and the variety used in the trial was Komeett.
Effect on photosynthesis
“Far-red light has a number of known effects,” says researcher Anja Dieleman. “It produces more elongation in the stems and leaf stalks so the crop structure is more open. This allows better light interception and thus greater crop photosynthesis. In addition, flowering is sooner, in extreme cases even leading to stress-induced flowering. Far-red also suppresses the apical dominance.”
The far-red light was on at the same time as the red/blue LEDs and remained on for 30 minutes longer. “Nothing at all happened at the beginning,” she says. “You would have expected more elongation but it was only in November that we started to notice differences arising. The difference in stem length between the treatments with or without far-red reached a maximum of 25 cm, so it was very little. Furthermore it was noticeable that the leaves under the far-red light were less green.”
The far-red light also has an effect on photosynthesis: Measurements show that photosynthesis was 12% higher when the far-red lamps were on. This is striking because far-red is not PAR-light (Photosynthetically Active Radiation). “Therefore the effect is not directly related to the plant using the extra light for photosynthesis but it is due to the alignment between photosystem I and II, which are protein complexes in the chlorophyll,” explains Dieleman.
Crop reacts quickly
The crop started to yield fruit in January and it was immediate bingo: The average fruit weight and total yield were significantly higher in the far-red treatments. The difference in fresh weight compared with the control (red/blue only) was 7% at 30 µmol and 17% at 55 µmol.
The researchers were, considering their experiences in previous research, very alert to the developments in leaf length. If the leaves should become too short the crop would intercept too little light and that would be at the expense of production. This happened at the beginning of March: The leaves in the far-red treatments were shorter than those without far-red.
“Therefore we stopped the far-red treatments on 25 March. After that the crop recovered again. Within three weeks the leaves were longer and the crop was darker again. This means that far- red light can be used to steer the crop and it responds quickly to it. Also the light intensities were well chosen: they produced relatively little vegetative effect but had a large generative effect. According to the experts we could easily have gone through the summer with this crop,” says Dieleman. However, the trial was stopped before the summer for financial reasons.
Distribution of assimilates
The question is: What caused the yield to increase? From weighing the leaves, stems and fruits it appears that more assimilates go towards the fruits and less towards the green parts. “That is remarkable,” says the researcher. “It is actually very difficult to change the distribution of assimilates in tomato. This is driven by hormones that set off a chain of reactions and much about this is still unknown.” Perhaps it has something to do with the PSS-value (phytochrome stationary state), which indicates the activity of the pigment phytochrome. “That begs the question as to whether you should keep the far-red lamps on all day or just at the end of the day. Perhaps that last half hour – when the other lights were already out – clinched it.”
But the far-red light costs extra electricity. “Therefore, could we increase the yield to the same magnitude if instead of using 30 or 55 µmol/m2/s far-red light we increased the intensity of the red/blue LED-light by the same values (so from 185 to 215 or 240 µmol/m2/s)? This appears to be so from our calculations,” says De Beer. “But we are not concerned about that here. The interesting part is the ability to steer the distribution of assimilates. You can achieve something different than when you just give more light.”
Summary
Far-red light, in addition to assimilation lighting, resulted in a large rise in tomato yield: 7% and 17% in two different research trial treatments. The green part of the crop remained in good condition for half a year but after that its ability to intercept light decreased. The crop recovered again after the far-red light was switched off.
Text: Tijs Kierkels. Photos: Wilma Slegers and Wageningen University & Research
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.