Lighting for year-round vegetable production in Alberta greenhouses

Photo courtesy of Saeid Mobini, Ph.D

This is the second article in this series. You can read the first article, from the February 2019 issue of Produce Grower, "Confront heat stress or save energy"

Testing new lighting systems requires sizable investments such as large greenhouses outfitted with a high-wire hydroponic system — similar to modern commercial greenhouses, various lighting fixtures and light-measuring equipment. Further, growers and researchers should determine the characteristics of unique lighting systems, and their strengths and weaknesses for each plant production system, considering factors such as local climate and existing cultivation style. At the Brooks Crop Diversification Centre, a government research facility in Alberta, facilities were all available for four crop cycle trials, using both poly and glass greenhouses. This gave me the opportunity to test this goal during the past three winters.

This research project had a replicated block design. Data was collected over three years for both cucumber and tomato production. We evaluated the effect of light spectrum and distribution on several quantitative and qualitative morphophysiological traits, as well as yield quality and quantity for these crops. The main objective of this study was to evaluate the application of intra-canopy lighting LEDs for cucumber and tomato growth to illustrate the effect of lighting systems on energy use efficiency and crop yield improvement.

Experimental protocol

Long English cucumbers (vars. Bonbon and Verdon) each at two crop cycles and Beefsteak tomatoes (var. Torero grafted on Maxiford) were cultivated in a high-wire hydroponics system located at Crop Diversification Centre South, Brooks, Alberta during 2016-18.

Lighting sources were:

• High-pressure sodium (HPS): 600 watt HPS lamp with a light intensity of 120 µmol·m-2·s-1 in the photosynthetically active radiation (PAR)
• Inter-canopy LED lighting: Philips Green Power LED Interlighting with a light intensity of 60 µmol·m-2·s-1 in the PAR

Light was measured using line quantum sensors (apogee Logan, Utah) at a distance of two feet from the light source within the four replications at midnight in order to eliminate any probable light pollution. Data was collected from several locations of plant canopy to assess the light distribution pattern across the plant canopy.

To evaluate plant phenological response to the lighting treatments, plant morphophysiological parameters including photosynthetic rate (measured by EARS-PPM-300, the Netherlands), chlorophyll content (measured by CCM-200 plus-Opti-Sciences, U.S.), stem growth, stem diameter, leaf number, leaf area and fruit load were measured for five plants in each experimental unit every week. To determine the effect of light source on yield improvement, fresh fruit weight and quantity (for each fruit harvest) were recorded and analyzed. Statistical analyses were performed using randomized complete block design (RCBD) with four replications and 16 experimental units. During statistical analysis, the computation of a correlation coefficient for critical difference was carried out between different experimental runs to ensure there was homogeneity of variance within the data. The PROC-MIXED procedure in PC-SAS (SAS Institute, Cary, North Carolina) was used to determine the difference in means using LED’s range test at P<0.05.

Result — Part 2. Yield component improvement

Plant morphology response

The plant morphophysiological responses to light treatments have been demonstrated in Fig. 1. In general, vegetative growth has depicted the same pattern across all lighting treatments with a similar oscillation (quickening and slowing down of growth) in each crop cycle related to growing conditions. Stem thickness was higher in all supplemental lighting treatments as compared to the control for both Long English cucumber varieties. As expected, weekly stem growth, which indicates plant stretching at lower light levels, was higher in the control group rather than lighting treatments for the BonBon variety of cucumber. However, this phenomenon was not observed for the Verdon variety, and weekly stem growth remained at the lower level in the control group compared to lighting treatment groups. This result may explain why Verdon is recommended for winter season conditions, which have lower light intensity, when plants are more prone to stretching, and BonBon isn’t. In both cucumber varieties, leaf size was the smallest in the control group except during the last two weeks of data collection. Nevertheless, the leaf number was almost the same across all treatment groups and varieties, likely due to leaf-trimming as part of routine operation; so, a similar leaf number indicates the same canopy size across treatments and varieties. Overall, plants grown under the HPS, LED and LED+HPS systems had thicker stems and larger leaves than the control group as illustrated in Fig. 1. There was no significant difference across light treatments.

Plant chlorophyll content and photosynthesis rate

Monitoring chlorophyll content (chl) during eight critical weeks of plant production clearly showed that chl content responded to the light intensity within the PAR range (Fig. 2). Chlorophyll content was almost the same across treatment groups at the top of the canopy, where there is no light competition. Significant differences in chlorophyll content were observed in the middle and bottom of the canopy where leaves are competing for photon absorption. As hypothesized, the control group showed the lowest chl content whereas the HPS + LED treatment displayed the highest chl content, likely due to greater light intensity. We expected to observe a remarkably higher chl content for the intra-canopy LED treatment rather than overhead HPS treatment at the middle and bottom of the plant canopy, but this difference was found to be not statistically significant. This discrepancy may be explained by the lack of adequate light intensity provided by the current LED fixtures (60 µmol·m^-2·s^-1) and higher crop demand in regard to light intensity at the middle and bottom of the canopy in order to show an assessable response of chl content. Looking at the photosynthetic rate difference between HPS and LED treatments at the bottom of canopy also supports the fact of low light intensity produced by LED, as the lowest rate of photosynthesis occurred in the LED light treatment group, where we expected to see a higher photosynthetic rate for the intra-canopy LED light treatment.

Overall, the photosynthesis rate was higher using hybrid light rather than intra-canopy LED or overhead HPS alone, with the control group producing the lowest photosynthetic rate across the plant canopy (Fig. 2).

Light quality of supplemental lighting fixtures

Light quality was evaluated using a spectroradiometers (measured by apogee PS-200). The HPS spectrum had the highest peak at 820 nm. This peak generated a heat shock. This puts other peaks at 465 and 499 nm (Blue light), and the majority of radiation was distributed between 550 to 700 nm, with some peaks at 565, 576 and 594 nm (Fig. 3A).

The current intra-canopy LED lighting had a concentrated spectrum at two peaks of 460 nm (Blue light) and 660 nm (Red light), which are expected to be the most effective spectrums for a photosynthetic reaction. However, we had anticipated to have a better balance between Blue and Red lights as recommended by the lighting manufacturers for cucumbers and tomatoes which is usually ranged between 1:4 up to 1:6. Our data showed the Blue:Red ratio was about 1:19 due to existing 5% Blue and 95% Red in this LED spectrum. New LEDs in the market offer 10% more Blue for vegetable cultivation. It seems that 5% Blue is far from optimal Blue:Red ratio for tested crops (Fig. 3B). The hybrid light spectrum, created by the combination of HPS lamps and intra-canopy LED at the middle of the plant canopy, showed the mixture of both spectrum with a peak at 460 nm as Blue light, and the highest peak at 660 nm as Red light, with another peak at 820 nm, representing Far-red light (Fig. 3C). Overall, the tested LED light showed a better spectrum for plant growth compared to HPS or hybrid light. However, it could be improved by adjusting the Blue:Red ratio and increasing the light intensity to double or more.

Effect of supplemental light treatments on yield improvement

As expected, crop productivity was influenced by supplemental lighting. The number of marketable fruit, as main yield component, was significantly increased in the lighting treatments rather than control group. The hybrid light followed by intra-canopy LED lighting provided greater marketable fruit numbers than overhead HPS light alone, which was more remarkable in the BonBon variety than Verdon.

The fresh yield for the BonBon increased by 78.7% under HPS lighting, 97.8% under LED lighting, and 174.6% under the hybrid lighting treatment, as compared to the sunlight. The fresh yield also increased for the Verdon under various lighting treatments, with an 11.7%, 57.6%, and 88.4% increase under HPS, LED, and hybrid lighting systems, respectively (Fig. 4).

Overall, to achieve all profits of horticultural LED lighting technology and its practical features, several technical aspects need to be considered. These include light intensity, quality and its distribution in order to select the right LED fixture, as well as implementing the right light recipe based on the plant light requirement, cultivation practices, available daily light integral over the plant canopy and local climate conditions. Local climate conditions will be discussed in part three of this series. The author is ready to share information and experiences, communicate with other researchers and growers, and respond to any question raised for selecting the right lighting recipe for their facilities.

Saeid Mobini, Ph.D in Horticulture, was a greenhouse specialist, Crop Research and Extension Division in the Alberta Ministry of Agriculture and Forestry in Canada. He is currently the master cultivator at SugarBud Craft Growers. mobinisaeid@gmail.com