LED efficacy, UV, and plant feedback highlight horticulture presentations (MAGAZINE)

The second US Horticultural Lighting Conference was packed with informative presentations, reports MAURY WRIGHT, with prevailing themes including SSL comparisons with legacy lighting, the use of ultraviolet spectrum to boost secondary metabolites, and the future of horticultural lighting.

In Denver, CO on Oct. 17, 2017, many of the foremost experts in the area of horticultural lighting gathered at LEDs Magazine's second annual US conference on the topic. The day was packed with informative talks and the networking in the tabletop exhibits and at the evening reception was fast and furious. Here we will highlight a few presentations that looked at LED lighting relative to alternative light sources for plants, ultraviolet (UV) lighting as a way to sculpt the flavor or potency of cultivars, and future efforts that might take feedback directly from plants to control the lighting applied.

FIG. 1. Petunias exhibit differences when grown in a greenhouse under no supplemental lighting (left), HPS supplemental lighting (middle), and LED lighting (right).

Interested in articles & announcements on horticultural lighting?

Before we get started, we will suggest that you peruse some past content if you aren't familiar with the concepts that underlie LED-based horticultural lighting. An article that we ran last year examined solid-state lighting (SSL) in the horticultural role, and contemplated key metrics that differ from those used in lighting for humans. And coverage of our 2016 Horticultural Lighting Conference provides insight into many issues surrounding the burgeoning application. Moreover, Philip Smallwood, director of research at our Strategies Unlimited market research business, opened the conference looking at the challenges of the application and at market data. For a limited time, a webcast that reprised that presentation is viewable on demand.

Steven Newman keynote

The keynote presentation in the opening session at the conference came from Steven Newman, the greenhouse crops extension specialist and professor of floriculture at Colorado State University (CSU). Newman had been given a unique opportunity to oversee construction of the new CSU Horticulture Center a few years back when the university built a new football stadium on the site of the old facility. The project included more than 21,000 ft² of greenhouse space, more than 6000 ft2 of classroom space and a 6-acre outdoor gardening area.

As the new facility was being planned, Newman described how a chance meeting with Philips Lighting led to the greenhouse facility being equipped with LED-based Philips GreenPower Toplight luminaires despite the fact that Newman had no prior SSL experience.

In spite of objections from the university engineering department that also had no experience with LED lighting, Newman went forward with the SSL installation based on the economics. Newman offered the data in the nearby table at the conference, where the projections indicated that LED lighting would cost a half-cent per day per square foot to operate and the high-pressure sodium (HPS) alternative would cost more than a cent and a half. Newman said, "That tells me the return on investment is faster than you think."

LEDs and floriculture

Newman said he welcomed the next phase of his career as a chance to learn about working with LEDs and set about to study "What could we do with floriculture plants to speed the production?" Newman said he has already studied a number of bedding plants, but described one set of tests in particular that directly compared 600W LED lighting with 1000W HPS lighting. He said the HPS lighting delivered PAR (photosynthetically active radiation)-band PPFD (photosynthetic photon flux density) of 65 μmol/m²/sec compared to 84 μmol/m²/sec for the LED lighting - with variables such as bench height, temperature, and on/off cycles for the lighting held constant. Newman took measurements at night to ensure that the PPFD levels were accurate with only a security light providing in the range of 0.47 μmol/m²/sec in stray light.

The cultivar studied was the Bada Bing Scarlet variety of begonia. Newman showed side-by-side photos of plants grown with no supplemental light, with the HPS supplemental light, and with LED supplemental light. The plant that was grown with HPS light was considerably smaller than the other two. Newman classified it as suffering from "stunting." He said he could not explain the impact for sure, but speculated that it may have been due to the spectral power distribution (SPD) of the HPS lighting.

The plants grown with no supplemental lighting and with LEDs appeared about equal in height. But the plants grown under the LED lighting appeared more dense.

FIG. 2. Peter Barber of SETi explained how UV lighting can impact the production of secondary metabolites in plants, affecting flavor and other qualities.

Next, Newman showed similar photos of TriTunia Pink Veined petunias (Fig. 1). In this case, the plant grown with no supplemental lighting was clearly taller than the one grown under LED lighting, but Newman cautioned that a closer look was required to discern the superior plant. Newman said in this case, the compactness of the plant grown under LED lighting compared to the stringiness of the plant grown with no supplemental lighting means that the more compact plant is more likely to survive the trip through big-box retail to successful transplant into a consumer's garden. Meanwhile, the plant grown under HPS had no evident flowering whereas the other two each had nice flowering.

Napa Valley of beer

Newman then described another surprising crop being grown in the facility. Early on in his talk, he had touted the Fort Collins area (home to CSU) as a great place to visit with one reason being the prevalence of 23 craft breweries. He called the region "the Napa Valley of beer." We will let others debate that characterization, but Newman's CSU colleague Bill Bauerle is growing hops hydroponically for the local brewing community.

Newman rhetorically asked, "Why grow hops in a controlled environment?" He immediately admitted that it could not be as economical as outdoor growing operations in Oregon and Washington that are using 25-ft-high trellis systems with machine harvesting and an automated drying process.

But Newman said every time hops are touched by a person or equipment, the quality goes down and the essential oils of the selected plant are compromised. He said brewers cherish the flavor profile of the minimally-processed fresh or wet hops and will pay a premium for that product to differentiate their beer. Moreover, Newman said the economics work out better than you might think with the LED lighting enabling continuous production and as many as five crop cycles per year.

UV LEDs in horticultural lighting

Among the most compelling of presentations at the conference was a talk focused on the use of UV spectrum in horticulture. Peter Barber (Fig. 2), director of product marketing and business development at SETi, presented "The myriad ways that UV LEDs will impact society through horticultural lighting." SETi is a UV technology specialist that was acquired by Seoul Viosys in early 2016. Seoul Viosys is focused on UV LEDs and is a sister business to visible-light LED manufacturer Seoul Semiconductor.

Barber briefly discussed lighting in the PAR region before jumping into the UV topic with the proclamation that UV energy has applicability over the complete cycle of vegetable growth and consumption, or what he termed "seed to belly" with the farmer of course in the middle of the cycle. Seedlings can benefit from UV energy in two ways, according to Barber - UV treatments can strengthen root systems and can prevent or suppress mold.

FIG. 3. LESA's Tessa Pocock projected a future in which sensors would enable a closed-loop system for horticultural lighting where plants would tell the system what they need.

For the farmer, mold suppression remains a UV benefit, but there are many additional benefits. As we have covered previously, UV energy can influence the appearance, smell, and taste of plants. Indeed, UV energy can increase nutritional value or perhaps potency in the case of a cultivar such as cannabis, according to Barber.

There are secondary uses for UV lighting in a farm as well. For example, it can be used to disinfect hydroponic lines that feed water and nutrients to plant roots. We covered such a usage in a shipping-container-based vertical farm. UV exposure can also increase the shelf life of products after harvest, benefitting the farmer and the consumer. Barber said UV light can even be used to treat mold spots on produce.

Secondary metabolites

Still, it's the impact on the look, flavor, and potency of a plant that may be the most interesting result of UV light in horticulture, and Barber explained some of the details of plant physiology relative to UV exposure. In response to UV-B spectral energy (UV-B is the middle UV band spanning 280-315 nm), a plant reacts through a stress mechanism to protect itself. The Mitogen-Activated Protein Kinase signaling - MKP1/MPK3/MPK6 - initiates the response.

A molecular signaling pathway called UVR8 then is responsible for increased secondary plant metabolites such as flavonoids in vegetables or THC in cannabis. But the plants are very selective in terms of the spectra to which they react. Barber used the analogy of an arcade skee ball game where the inner targets deliver more points to the player than the outer targets.

Barber said UV emission in the range of 280-300 nm only requires a fluence rate of 0.1 μmol/m²/sec to achieve the desired boost for secondary metabolite production. A plant would need ten times more energy from emission in the 301-310-nm band. Barber said, "That's why LEDs are so preferred. You can provide that targeted region." That statement could apply to LEDs in the PAR and other bands as well as to the UV energy that Barber was discussing.

The mechanism by which the plant reaction occurs is due to a plant's epigenetic memory, according to Barber. As an example, he said cannabis grown at high altitude in Colorado has higher concentrations of THC and terpenes than do plants grown at sea level. The plants grown at higher altitude get more UV. Barber likened the plant reaction to a natural sunscreen. And he pointed out that sporadic exposure can trigger the reaction. Conversely, too much exposure can lead to cell death. Barer said the closer you get to 280 nm, the greater the risk for permanent damage, noting that "UV-C is unbiased when it comes to DNA" and that it would destroy cells.

Closing Plenary

The final talk at the conference was the Closing Plenary and it included some data from studies on effective spectra, but more importantly it looked into the future of horticulture. The speaker, Tessa Pocock, is certainly equipped with the knowledge and experience to provide a forward look. Pocock is a senior research scientist at the Center for Lighting Enabled Systems and Applications (LESA) at Rensselaer Polytechnic Institute. LESA is a National Science Foundation (NSF) lab with US government funding, and as such is charged with looking at technologies that might come to fruition as much as a decade in the future.

Pocock opened by saying, "I'm a plant physiologist and my main concern is the plant." She said there have been around 400,000 species of plants characterized in nature, yet only around 30 species are cultivated for food. In contrast, she said around 21,000 taxa are used in pharmaceutical applications, so the horticultural lighting application may be far larger than the food supply.

Lighting is critical in more ways than you might think for plants. Pocock said, "Lighting is the primary information source for the plant. It tells the plant what to do and when." Moreover, she said light cues the plant as to what to expect and do next.

Pocock said there are more than 100 plant genes and 26 biochemical pathways that are regulated by light. She explained that photoreceptors in plants are quite often discussed but the chloroplasts are elements in plant physiology for which the details are often ignored.

Plant's sensor networks

Photoreceptor control has mainly a developmental impact on plants including things such as flowering, photoperiodism, leaf expansion, and stomatal opening. Chloroplasts are critical to photosynthetic control and the operation elements of plant growth including light capture, photosynthesis efficiency, CO₂ assimilation, and protection mechanisms and memory.

There are also elements of plant growth that are shared in terms of impact by the separate photoreceptor and chloroplast networks. Examples include circadian rhythm, height, pigments, immunity, and defense. And Pocock said the separate networks could in some cases combine favorably in terms of such impact or in other cases oppose one another in terms of impact on the plant.

Pocock quickly got back to a detailed look at some plants that would make the complexity of lighting impact clear. First, she showed the red lettuce called Rouxai grown under four different types of lighting - a commercial LED fixture with red, blue, and white LEDs (R/B/W), a phosphor-converted white LED (PC LED), the lab's basic reference cool-white fluorescent lamp (CWF), and a fixture with red and blue LEDs (R/B LED).

The lettuce grown under the PC LED spectrum was essentially green. Pocock said her students called it "green red lettuce." It had the lowest level of anthocyanin pigment concentration, which has antioxidant properties along with providing the attractive red color. The lettuce grown under CWF light had the highest concentration of anthocyanin, although the other LED lighting performed reasonably well also. We didn't know it at the time, but we would learn more about Pocock's Rouxai research later in 2017 and we published a news article on that work.

Unfortunately, there is no one spectral recipe that is beneficial for all plants, and Pocock made that clear on her next slide. A red butter lettuce Pocock tested did not respond well to CWF and did much better under PC LED, although the red coloration was missing with that cultivar and also with another called Salanova that is in the red oakleaf family to which Rouxai also belongs. Referring to the PC LED lighting, Pocock said, "Under this there is no photochemistry going on with respect to secondary metabolites."

Closing the loop on horticultural lighting

"The future is knowledge-based controlled environment agriculture, understanding these metabolic pathways," said Pocock. Rather than just experimenting with different light recipes, Pocock said we need to "get inside the plant cell and go from there with the lighting." She said for the past seven years, she has been working on remote detection of plant health through reflection of light from plant leaves along with chlorophyll fluorescence emitted from foliage.

In her LESA lab, Pocock has been working to develop a low-cost reliable sensor that can be used in a grow facility. She said that "the chlorophyll molecule intrinsically fluoresces." She added that this known property has been used since the 1950s by plant physiologists to determine if a plant is stressed. Pocock has what she described as a very-expensive Heinz Walz pulse amplitude modulation (PAM) fluorometer in her lab that allows her and her students to see how every photon is being used on a research basis. She showed images from a stress test done on basil where temperature was lowered. The results can be expressed as the effective quantum yield or the probability that a photon will be utilized by the plant.

Of course, such a PAM fluorometer isn't usable on a scalable basis in a production grow facility. So Pocock and students have been pursuing a sensor design, albeit in a project that is not presently funded. The second-generation prototype used an LED emitting at 470 nm and a nearby photodetector, both located 40 cm above a plant with a field of view diameter of about 5 in. The prototype proved accurate at detecting stress in basil.

The project has since moved to a third-generation prototype that is housed in a more reliable and robust growing chamber with a larger field of view, which feeds data continuously to a computer. The system can accurately detect day and night, when a plant is undergoing photosynthesis, and conditions such as drought, simply by monitoring the response from the plant.

"Our future is self-regulating light control," said Pocock. She acknowledged that there is much research to be done. But she asked, "Why should I impose a spectrum on a plant when it can tell me how it is doing?" She believes that we will ultimately "use the physiological state of the plant to control the light."

Of course, such a future would be good for the world of LEDs and SSL. LEDs with tunable spectrum would still be a perfect match for such a closed-loop system. The attendees at the conference may not have expected to learn such a lesson about the future, but their positive reaction to the Closing Plenary made it clear that they greatly valued the insight as well as the rest of the knowledge imparted by a stellar lineup of informed speakers. Stay tuned for more details and updates on our Europe and US conferences for 2018 (horticulturelightingconference.com).