By Brian Poel

New technologies in controlled-environment agriculture allow growers to manage crop production in ways like never before. Dr. Xiuming Hao and his team at the Harrow Research and Development Centre (HRDC) are guiding Ontario greenhouse vegetable growers to implement these technologies in their production systems. Their mission is to “develop and transfer new technologies for the production and protection of greenhouse vegetables and ornamentals.”  With federal, provincial, and industry support, mainly the Ontario Greenhouse Vegetable Growers, their research prioritizes growers’ problems and needs.

Dr. Xiuming Hao and his staff at the Harrow Research and Development Centre are exploring the application of supplemental lighting on greenhouse vegetables. Photo courtesy of Dr. Xiuming Hao.
Dr. Xiuming Hao and his staff at the Harrow Research and Development Centre (Ontario, Canada) are exploring supplemental lighting applications in greenhouse vegetable production. Photo courtesy of Dr. Xiuming Hao.

Reliable results of new practices are important for growers considering their adoption. “We have a really good research facility… 7,000 m2 of greenhouses with researchers, provincial greenhouse extension specialists, technicians, and eight dedicated crew just to maintain the crop—what we’d call semi-commercial conditions” explains Dr. Hao. “We grow our crop just like they would in a commercial greenhouse—the same watering, greenhouse climate controls, crop cultivars—but we also have multiple greenhouse compartments allowing for strict experimental design and proper replication so the data is scientifically sound.” During the late stages of research projects, HRDC also collaborates with growers to conduct trials in their own commercial greenhouses. “That way when growers see (new practices) performing really well, they are quick to implement them,” says Dr. Hao.

One successful implementation is dynamic temperature control. For crops such as tomatoes and sweet peppers, temperature of leaves and fruit changes at different rates. Using a pre-night temperature dip, growers have seen increased fruit production while capitalizing on energy savings from this dynamic temperature control.

Growing crops is difficult when light levels are low in winter, so growers need supplemental electric lighting for fast growth. Greenhouse vegetable growers traditionally do not use supplemental lighting. Instead, they reserve winter time for greenhouse cleanout and re-planting. However, they are gradually adopting supplemental lighting to enable year-round production, which can demand higher prices. As an alternative light source for horticulture, cheaper and more efficient light-emitting diodes (LEDs) are making their way into greenhouse vegetable production.

The daily light integral (DLI) is the total photosynthetic light accumulated each day and as DLI increases, especially when low, there is a direct increase in crop production. Dr. Hao says that the DLI can be increased using either a lower intensity for a longer photoperiod (e.g., 100 µmol for 24 hours) or a greater intensity for a shorter photoperiod (e.g., 200 µmol for 12 hours). Each method provides the same DLI but has their caveats. A higher intensity requires greater investment in light fixtures, but a longer photoperiod can have negative effects on some crops. Dr. Hao explains that crops such as tomatoes are susceptible to damages induced by long photoperiods. Leaf chlorosis occurs when the photoperiod exceeds 17 hours. Thus, an extended photoperiod does not necessarily increase the yield.

Fortunately, dynamic temperature control can help mitigate the negative effects caused by an extended photoperiod. Dr. Hao says that long photoperiods induce excess photo-assimilate in the leaf. They can damage the chloroplast membrane if not exported to the fruit efficiently. A quick temperature drop renders the fruit warmer than the leaf because the fruit has less surface area relative to its volume. The resulting change in the plant growth balance draws photo-assimilate to the fruit, preventing accumulation in the leaves. This method reduces leaf chlorosis under long photoperiods, especially in tomatoes, and promotes early-season fruit production. Dr. Hao says that they can now use 20 hours of light in tomato production. Since a moderate intensity is sufficient over a long photoperiod, capital costs for supplemental lighting can be reduced. Dynamic temperature control also reduces heating costs. For these reasons, this method is already being adopted by the industry.

Creating different cooling rates between leaves and fruit can mitigate issues caused by long photoperiods under supplemental lighting. Photo courtesy of Dr. Xiuming Hao.
Creating different cooling rates between leaves and fruit can mitigate issues caused by long photoperiods under supplemental lighting. Photo courtesy of Dr. Xiuming Hao.

Dr. Hao is now turning his attention to the vertical light distribution within greenhouse vegetable canopies. Vine-type crops like tomatoes, peppers, and cucumbers can exceed 10 feet in height from the fruit up to the youngest leaves. Intracanopy lighting in tomatoes is not a new application, but Dr. Hao is rethinking the light spectrum used, “A lot of current light recipes (spectral compositions) are formulated based on researches on leaf vegetables. They are good for growing leaves, but that’s not our product. We need more than that.” Using carbon-14 radioisotopes, Ph.D. candidate Jason Lanoue is comparing photo-assimilate translocation under different light spectra among other environmental conditions. They are seeking light recipes that can improve crop yield, quality, and nutritional value. While a universal light recipe for all crops seems unlikely, they hope this approach can point them in the right direction, specifically for tomatoes, cucumbers, and sweet peppers, the three most important greenhouse crops in Ontario.

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Vine crops require a unique approach to the appropriate light spectra of supplemental lighting. Photo courtesy of Dr. Xiuming Hao.

Research results from the HRDC are communicated to the industry each October at the Canadian Greenhouse Conference. Dr. Hao and his team also present at various academic conferences. For example, his group presented seven papers and six posters at last year’s 8th International Symposium on Light in Horticulture.


About the Author: Brian Poel obtained his M.S. in Horticulture at Michigan State University in 2016. Based in Southwestern Ontario, Canada, he is now the Plant Lighting Specialist at LumiGrow Inc. He can be reached via email: brian.poel@gmail.com.

Edited by Qingwu (William) Meng

Canadian researchers advance supplemental lighting for the greenhouse industry