Lights under the spotlight

Building on the success of last autumn’s Horticultural Lighting Conference staged in the US, the inaugural event in Europe was staged at the High Tech Campus in Eindhoven, writes Steven Vale.

Bringing growers and manufacturers together, the targeted one-day Horticultural Lighting Europe conference was designed as a fast-paced and informative event to share the latest insights and ideas in the supplementary lighting market. Attended by over 220 delegates, the presentations touched on a wide range of subjects, many of which focused on LEDs or hybrid systems using both HPS and LEDs.

Anja Dieleman of Wageningen University and Research suggests light is the single most critical factor impacting the success of a controlled environment. Her session included an overview on quantifying and qualifying lighting differentials – from white light to purple light and top light to side light. Marja van Dessel of Den Boschkant shared her knowledge on why the tomato nursery choose to invest in LEDs and how they are using them. Marja also explored and discussed how the lights have changed their working climate and the effects.

Understanding LEDs was the title of a presentation by Carolin Horst of Osram, who spoke about the advantages of LEDs in horticultural crops. “LEDs are very different from traditional lighting sources,” she said. “They’re solid-state devices that can be designed to deliver narrow-spectrum photon beams of defined wavelength. Now unique light-distribution techniques are possible that have important implications for targeted lighting of high-value crops.”

In Belgium, around 100ha of tomatoes are grown under HPS lights, and another 6.6ha under HPS combined with LED interlights, and just a couple of projects using 100% LEDs. Isabel Vandevelde and Stefaan Fabri from the Belgian Research Station for Vegetable Production spoke of practical experience with LED and hybrid LED-HPS systems in greenhouse vegetable production.

They provided a few answers to the questions of how they compare in terms of photosynthetic active radiation and heat output, and whether a combination of the two technologies deliver the best of both worlds.

One example is a trial with De Ruiter tomato variety Merlice, which was grafted onto Maxifort (also De Ruiter), and planted on October 19, 2016.

The three lighting regimes included a hybrid combination of top lights (Gavita 1000w HPS and Lemnis LEDs), the same Gavita top lights with Hortilux LED interlights and a compartment with only Gavita HPS top lights.

Stem density was 4.34m² two hybrid systems and 4.17m² for the HPS only trial. Measured from October 19 to March 9 2017, the increased HPS resulted in 12.7% extra light, the LED top lights an extra 26.2% light and the LED interlights an extra 25.9% light.

The results show that the increased HPS results in no difference in fruit quality, but did lead to more burnt leaf edges. The increased LED resulted in more vegetative growth, no significant difference in growth speed, head thickness, or truss ripening speed, and no significant difference in brix or hardness.

The conclusions with higher HPS levels are that plants are more sensitive to burnt leaf edges, there is no extra production and no difference in fruit quality. Increased LED results in increased vegetative growth and higher production, but no difference in fruit quality. The work continues with different lighting regimes, and in different crops, including lettuce.

There then followed a presentation by Dale Needham of Fourthwall on how to assess horticultural lighting equipment. How to use tunable LEDs to manipulate crop production was the title of the following presentation by Dr Phillip Davis of Stockbridge Technology Centre.

This began with an interesting slide – ‘More light equals more growth, but also higher costs and not always a better result’.

For example, he showed a slide of four trays of petunia 35 days after sowing with four different lighting recipes of 100, 200, 280 and 360 µmol, which clearly highlighted the numbers and quality of flowers were best around the 200 mark. Pepper plants grow shorter as the light intensity increases.

Dr Davis reckons a combination of LEDs and lighting controls has the potential to change the way crops are grown in controlled environments. “LEDs offer unprecedented opportunities to manipulate wavelength, pulse duration, synchrony and spectral output,” he said, “and can modify the morphological and chemical characteristics of plants, enabling growers to extract greater value from crop production.”

Lunch provided an opportunity to tour the technical stands, after which the afternoon session began with a presentation (LEDs: What the research tells us in lettuce and bell peppers) by Joris Van Lommel and Liesbet Van Herck of the Belgian Research Station for Vegetable Production.

They illustrated the possibilities of LED lights in greenhouse vegetable production, with small variety trials in lettuce. The research confirms lettuce responds differently to different light and historical data already shows us that far red light promotes stem and leaf elongation, while red light leads to vegetative growth. In some studies, green and yellow light increases yields, while blue light has a beneficial effect on plant structure and leaf color, and promotes stomatal opening. With UV-A, more red pigments produced. Most studies happened in absence of natural light What happens in a greenhouse?

The two HPS/LED hybrid lighting regimes included HPS with 95% red/5% blue and HPS with 80% red/20% blue. Plus, there were two trials with LEDs only – 95% red/5% blue and 80% red/20% blue. The total umol/m² was 75 (50 µmol with HPS + 25 µmol LED) µmol with the two LED trials.

The results show there were no significant yield differences (only LED 20% blue somewhat lighter), more compact growth with just LED (most compact under 20% blue), lettuce coloured redder under only LED (no difference 5 – 20% blue), and there was 10% more root mass (under only 5% blue LED).

Also, more blue light increases the incidence of tip-burn in butterhead lettuce, but tip-burn sensitivity is very variety dependent. The tested varieties best able to handle the light were Saturdaï (red oak leaf) and Fairly (butterhead), up to 120 µmol. 80 µmol works for all tested varieties – more is very risky.

Their conclusions were that a broad versus narrow light spectrum has no notable added value in lettuce, and there was a more compact growth under a LED only regime. The message is to be careful with too much blue light, which promotes stomatal opening, results in increased evaporation and complicates calcium distribution in the crop. Also, there is a higher incidence of tip-burn under increased blue light, but no effect seen on stem length (no induction of flowering). A combination of 95% red and 5% blue is fine.

The next topic was the use of LEDs to control pathogens, and the results of findings by Jaimin Patel and Mark Rea of the Lighting Research Centre at the US Rensselaer Polytechnic Institute, and David Gadoury of the American Cornell University. Critical problems with current disease control strategies include fungicide resistance in pathogens and pesticide residues in crop produce. However, organic production is challenged by the need to suppress powdery mildews and downy mildews.

The three researchers are looking at the role light can play in the control of plant diseases and pathogens. The starting point is whether ultraviolet (UV) radiation can not only suppress pests but also induce disease resistance in plants. Also, is it possible to exploit light-dependent mechanisms for enhancing crop marketability while suppressing plant pathogens in controlled environments?

They have found that modifying the light environment could provide growers with a potential disease control alternative. Fungal pathogens have been attacking plants for at least 100 million years, and plants and pathogens have evolved in 24-hour cycles of light and darkness.

Consequently, all evolved systems regulate development via a 24-hour clock. Plants and pathogens also sense, interpret and respond to light. We know a lot about plants, and less about pathogens with respect to the above.

What the three researchers have found is that UV-B LED induces downy mildew resistance in basil, while blue light suppresses lettuce downy mildew sporulation. UV-B + red LEDs suppress powdery mildew of cucumber and red LEDs suppress downy mildew of basil (wavelength 625nm red LEDs, light intensity 12 µmol/m² when used for 12 hours at night).

The underlying conclusion is that optical radiation can suppress downy mildews and powdery mildews in tested crops. So, why is application of optical radiation not commonly used for disease management? That is an easy one to answer because there is less awareness among the major light manufacturers, and growers are not aware of the possibilities. The three are keen to collaborate with lighting manufacturers who want to develop products and recipes for plant health.

Finally, Christine Zimmermann-Loessl of the Vertical Farming Association and Jasper den Besten of HAS Hogeschool, spoke of the development of vertical farming systems and touched briefly on how to design and develop a LED lighting infrastructure for vertical farming, and of some of the investment costs involved.