2014 ASHS Annual Conference

Greenhouses in northern latitudes must compensate for the attenuation in total light availability especially during the prolonged winter with short daylight hours, where supplemental artificial lighting is required in order to maintain a consistent crop yield throughout the year. Conventional greenhouse lighting systems utilize broad-spectrum light sources, such as high pressure sodium (HPS) or fluorescent lamps. These lamps were tailored for human vision and therefore are not ideally suited for plant growth. LEDs can be selected to target the wavelengths absorbed by plants, enabling the growers to customize the wavelengths of light required to maximize production and limit wavelengths that do not significantly impact plant growth. Plant response to light from the red and the blue spectra has been documented extensively. The purpose of this study was to determine which combination of red and blue LED light ratio was optimum for plant production.

In this study three red to blue ratio levels (5:1, 10:1 and 19:1) compared to 100% red and the plants of significant commercial importance to the greenhouse horticultural industry were used as subjects, namely Lettuce, Spinach, Basil, Tomato, Pepper and Strawberry. Seedlings were cultured in a growth chamber at 25+/-2.5°C, ambient CO₂, and a 16 hour light, 8 hour dark photoperiod under fluorescent light. Seeds were germinated and two weeks seedlings were planted in one inch rockwool cubes in trays for soilless hydroponic culture with half-strength Hoagland’s solution and transplanted in LED light chamber. Leafy vegetables were harvested after 4 weeks and fruiting crops were harvested after four months of transplant. Fruit and flower numbers were counted at two weeks 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 ºC for no less than 72 hours and subsequently weighed. One representative ripe fruit was collected from each plant during the final harvest, freeze dried, and stored at -80 ºC for future fruit quality measurements. Among the four treatments of light, the 5:1 ratio of red to blue LED was found superior. The lowest significant growth (number of flower, fruit, fresh mass, dry mass) was observed in 100% red LED. This research will facilitate the improved selection of LEDs for commercial control environment production.