Comparison of LED Supplemental Lighting and High Pressure Sodium Energy Consumption and Plant Responses of Cucumber and Tomato Transplants

Supplemental lighting is a key technology in vegetable nursery greenhouses to improve transplant growth and quality. Light emitting diodes (LEDs) lighting technology is relatively new and must be evaluated in terms of economic feasibility and plant responses. We performed a side-by-side comparison of supplemental red-LED (632 nm), blue-LED (443 nm) and a conventional high pressure sodium lamp (HPS). The supplemental light provided 54 ± 1.2 μmol·m^-2·s^-1 PPF for 18 hours (2:00–20:00) or 3.5 mol·m^-2·d^-1 DLI over the plant canopy. Growth and morphological parameters were recorded for cucumber (Cucumis sativus cv. Cumlaude) and tomato (Solanum lycopersicum cv. Komeett). After 38 days of seeding, cucumber shoot dry mass, leaf number and leaf area were 29%, 16%, and 27%, respectively, lower under the blue-LED and 27%, 13%, and 22 % respectively, lower under the red-LED compared to the HPS. Cucumber hypocotyl length was between 38% to 47 % lower in the red-LED compared to the blue-LED and HPS treatments, respectively. After 40 days of seeding, tomato shoot dry mass was higher under HPS followed by red-LED (18% lower) and blue-LED (29% lower). Hypocotyl length was 18%–17% lower in the red and blue LEDs respectively, compared to HPS. The number of leaves under the first flower for red-LED was significantly less than those under blue-LED and was not different from HPS. Greater shoot dry mass in the HPS treatment can be attributed to the higher canopy temperature throughout the experiment due to the greater infrared radiation emitted by the HPS fixture. When considered a commercial scale application, we found that the blue-LED treatment consumed 13% less electric energy, and the red-LED treatment consumed 10% more energy than the HPS (the electrical energy consumption excluded that for fans and controllers). However, after calculating the dry mass per supplemental lighting power input (g/kWh) the HPS treatment was more efficient (tomato: 3.2; cucumber: 3.4 mg/kWh) than both LEDs, and the blue-LED (tomato: 2.5; cucumber: 2.7 mg/kWh) was more efficient than the red-LED (tomato: 2.3; cucumber: 2.2 mg/kWh). From this study it is evident that the selection of supplemental lighting should be species specific. Also grower’s expectations and priorities have to be taken into consideration to decide appropriate lamp type and its light quality. Efficiencies of the LEDs, controllers, and cooling systems have to be improved to match HPS efficiency.