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.”


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