Effects of Nutrient Solution Temperature on Lettuce Grown in a Nutrient Film Technique System

Abstract

Increases in global population coupled with predicted depletion of natural resources has created a push for more intensive and efficient food production. Greenhouse production has been shown to significantly increase water, nutrients, and land use efficiencies over conventional field production. Despite the many advantages of greenhouse vegetable (GHV) production, a lack of wide spread adoption can be attributed to a significant increase in developmental costs and energy needs. Compared to traditional field cropping systems, GHV production requires higher energy inputs, mainly for heating and cooling systems. Although high production is possible with CEA, high energy requirements have limited expansion. This research examines the effects of heating nutrient solution temperature during cooler months or chilling nutrient solution during warmers months on Lactuca sativa L. ‘Rex’ grown in a nutrient film technique (NFT) system. Fifty lettuce seedlings were transplanted at a uniform size two weeks after germination to a NFT system located at Paterson Greenhouse Complex at Auburn University. Nutrient solution was heated with two Aquarium heaters (Odyssea Aquarium Appliance Co. Ltd) or chilled solution using a thermostat-controlled chiller (Penguin Chillers ½ HP Water Chiller). Size index was measured at 15 and 30 days after transplant and market fresh weight and head fresh weight were measured at termination. Head fresh weight, market fresh weight and size index were greater for the heated nutrient solution than the unheated, control solution. Head fresh weight increased 182%, MFW increased 136%, and SI increased 27% at termination. Chilling nutrient solution decreased MFW, HFW, and density but increased SI. Lettuce grown in nutrient solution chilled to the target temperature of 24°C had a 14% lower MFW, 17% lower HFW, 18% lower density and a 2% higher Size index (SI) than those grown in the non-chilled, control solution. Nutrient solution temperature also affected nutrient uptake. Chilling the nutrient solution led to a 24% increase in foliar calcium (Ca) percentage, and increased foliar manganese (Mn), sodium (Na), and zinc (Zn) by 69, 40, and 177%, respectively, but decreased foliar iron by 33%.