Influence of Heating Nutrient Solution on Greenhouse-grown Hydroponic Lettuce

Hydroponic lettuce production is increasingly popular in the southeastern United States. In order to compete with field-grown lettuce, production costs of greenhouse-grown lettuce should be decreased. A potential way to decrease production costs is to reduce energy usage by heating nutrient solutions instead of air. An experiment was conducted from 24 February 2016 to 4 April 2016 in a heated greenhouse to determine effects of heating nutrient solutions lettuce (Lactuca sativa L. ‘Rex’) grown in deep water culture. Lettuce seeds were germinated and grown for two weeks in OASIS® cubes (2.54 cm X 3.18 cm X 3.81 cm). Seedlings were fertilized with 50 mgŸL^-1 N from Gramp’s Original 8-15-36, 100 mgŸL^-1 N from calcium nitrate (15.5-0-0), and 40 mgŸL^-1 Mg from magnesium sulfate (10% Mg) before being transplanted to deep water culture containing the same nutrient solution mixture. Seedlings were randomly assigned to one of twelve Styrofoam boards (2.54 cm thick), which were floated on nutrient solutions contained in one of twelve, 42.5-L plastic boxes (AKRO-MILS® Multi-load Tote). Nutrient solutions were either unheated (control), continuously heated to a target temperature of 16 °C, or continuously heated to a target temperature of 22 °C using aquarium heaters (Hailea® Aquarium Heater 200w). Outdoor and greenhouse air temperatures, along with nutrient solution temperatures, were recorded hourly. The experiment was a completely randomized design, with four replications per nutrient solution temperature treatment and was repeated three times. Data were pooled across each repeated experiment and were subjected to analysis of variance in JMP Pro 12 and means were separated using Tukey’s HSD (α = 0.05). Greenhouse daytime and nighttime air temperatures averaged 25.9 and 17.6 °C, respectively. Nutrient solution temperatures for the unheated control, target 16 °C and target 22 °C averaged 20.4, 21.8, and 23.2 °C, respectively. Shoot fresh weight was highest when nutrient solution was heated to 22 °C (68.8 g), but was not significantly different between 16 °C and unheated control treatments (53.4 and 58.3, respectively). Root fresh weight was also highest when nutrient solution was heated to 22 °C (14.8 g), but was not different between 16 °C and unheated control treatments (12.1 and 11.9 g, respectively). Heating greenhouse lettuce nutrient solutions during production may decrease energy costs and production times, thereby improving potential profits.