The Effect of Different Ratios of Red and Blue LED Light on Greenhouse Tomato Production

Tomato is one of the most consumed crops in the world. In northern climates, tomatoes have a short time period for growing in the field; thus, to maintain supply and reduce shipping costs, tomatoes are grown locally in greenhouses. Conventional lighting systems for greenhouses utilize broad-spectrum light sources, such as high-pressure sodium (HPS) or fluorescent lamps. These lamps are excellent luminous sources for the human eye, but are not the most efficient light sources for plant production due to their low levels of blue light and other photosynthesis-sensitive wavelengths. Light-emitting diodes (LEDs) are a 50-year-old-technology that is showing potential in the greenhouse industry. With LEDs, specific wavelengths can be produced, creating a custom light spectrum targeted for maximum plant production. The purpose of this study was to determine which combination of red and blue LED light ratio was optimum for greenhouse tomato production. In this study three red to blue ratio levels (5:1, 10:1 and 19:1) were compared to HPS light for tomato. Seedlings were cultured in a growth chamber at 25 °C (+/–2.5 °C), ambient CO₂, and a 16-hour light, 8-hour dark photoperiod under fluorescent light. Seeds were germinated and in two weeks seedlings were planted in 1-inch rockwool cubes in trays for soilless hydroponic culture with half-strength Hoagland’s solution and transplanted in LED light chamber in greenhouse. The greenhouse set points were 21 °C during the day and 16 °C during the night. The supplemental lighting was left on for 16 hours a day, from 06:00 HR to 22:00 HR. A central computer in the greenhouse was used as the control system. It controlled the lighting (on at 06:00 HR and off at 22:00 HR) and the ventilation, coupled with a mister, in order to keep the temperature close to the set points. The internal greenhouse temperature was monitored and controlled; relative humidity was monitored, but not controlled. Tomato fruits were harvested after four months of transplant. Fruit and flower numbers were counted at two-week intervals and at final harvest. The fresh biomass harvested (aerial and fruit) was dried according to the ASABE standard (2007), with a temperature of 65 °C for no less than 72 hours and subsequently weighed. The lowest significant growth (number of flower, fruit, fresh mass, dry mass) was observed in control. This research will facilitate the improved selection of LEDs for commercial control environment production.