The latest on lighting

The 8th International Symposium on Light in Horticulture was held at Michigan State University in East Lansing, Michigan from May 23 to 26, 2016, and was hosted by Dr. Erik Runkle and Dr. Roberto Lopez. This symposium, which takes place only once every four years, serves as a platform for scientists around the world to share current research, exchange ideas and to establish collaborations. The symposium was packed with quality information about the use and manipulation of light in horticulture. The total number of attendees was approximately 250.

Two of the most prominent topics were the use of light-emitting diodes (LEDs) and the production of plants indoors. The symposium featured 52 oral presentations and 78 poster presentations. Also, four internationally recognized scientists participated as invited keynote speakers:

• Bruce Bugbee, Professor of Crop Physiology, Utah State University

  — Toward an optimal spectral quality for plant growth and development

• Kevin Folta, Professor of Horticultural Sciences, University of Florida

  — Controlling plant growth, development and metabolism with commands from the electronic canopy

• Wim van Ieperen, Assistant Professor, Wageningen University (The Netherlands)

  — Plant growth control by light spectrum: Fact or fiction?

• Jason Wargent, Associate Professor in Horticulture, Massey University (New Zealand)

  — UV LEDs in horticulture: from biology to application

A publication that summarizes the research presented is available from the International Society of Horticultural Science (ISHS). You can find more information at actahort.org/books/1134

I’d like to share with you a short summary of some of the presentations that caught my eye.

Dr. Marc van Iersel from the University of Georgia is working on a chlorophyll fluorescent-based biofeedback system. The objective is to maintain high photosynthetic efficiency and reduce electrical consumption. In other words, by measuring real-time chlorophyll fluorescence, the plant serves as the sensor to control the light intensity of the LED array. The idea is to match the plant’s capacity to use light with the provided light, which is done by electronically adjusting the light levels in real-time by measuring the plant response to light (chlorophyll fluorescence).

Dr. Jason Wargent from Massey University in New Zealand is working on the use of UV-B LEDs to improve young plant quality. For example, he has demonstrated that applying UV-B using irrigation booms increases the yield, plant hardiness, leaf thickness and transplant success of young seedlings.

Dr. Kevin Folta from the University of Florida introduced the idea of pairing plant genetics with LED technology in order to improve the overall production outcome (a better and unique crop). He also showed how one cultivar of edible microgreens is able to produce many different harvestable products (long seedling, purple seedling, short, green, etc.) just by manipulating the light spectrum. Folta emphasized that plant breeding and light recipes should go hand-in-hand to improve indoor production systems.

Another prominent topic in the conference was the use of LED lights for the increase of nutritional content of common horticultural crops such as lettuce, leafy greens and tomatoes among others. For example, Ph.D. student Nikolaos Ntagkas from Wageningen University in the Netherlands exposed harvested tomato fruits to different LED light treatments, and he was able to increase Vitamin C in tomatoes by 32 percent compared to tomatoes without the light treatment.

An emerging topic in this conference was the environmental impact of horticultural production in terms of carbon footprint (CO2 emissions) and energy consumption. For example, Dr. Anja Dieleman from Wageningen University quantified the carbon-footprint of tomato greenhouse production when grown under HPS supplemental lighting and LED supplemental lighting.

Dr. Bugbee presented the following quote by Dr. Frits Went, author of the book “The experimental control of plant growth”: “Photosynthesis does not control plant growth, plant growth controls photosynthesis.” Other speakers agreed with this quote, and it became one of the discussion points throughout the conference. We often think of photosynthesis as the main factor driving plant growth. However, photosynthesis by itself it is not a good indicator of overall plant growth. Environmental factors such as light intensity and quality alter plant development and morphology, which then change the plant’s capacity to carry out photosynthesis.

For example, in the last article I wrote (“Luminous possibilities,” accessible here: bit.ly/1UlfPhS), I showed how cucumber seedlings’ net photosynthetic rate increased with the increase of blue photon flux. However, the plant’s dry mass decreased with the increase of blue photon flux. In this particular example, plant leaf area was affected by the light quality, which then affected the plant’s capacity to intercept light. Here, the growth rate was better predicted by the relationship of leaf area and photosynthesis, and not by photosynthesis alone.

Another example is the increase of CO2 concentration in greenhouse production. Generally, when the CO2 concentration is increased, the photosynthetic rate also increases. However, if the higher concentration of CO2 is maintained for a long time (days) the plant can start changing its leaf anatomy and potentially develop fewer stomata cells, thereby reducing CO2 diffusion into the plant.

With the current capacity to create customized spectrums using LEDs, scientists are now working to find the spectrums that will positively affect plant development and morphology to directly increase plant growth or other desirable characteristics. Growers can later use that information to increase yield and profitability.

On the last day of the conference, we enjoyed a technical tour of east Michigan. We visited Mast Young Plants, the Frederik Meijer Gardens and Henry Mast Greenhouses. Overall, the conference was a unique and rich experience.

The 9^th International Symposium on Light in Horticulture will be held in 2020 in Sweden.