Light Intensity during Green-Leaf Butterhead Lettuce Propagation Influences Yield and Carotenoids at Harvest

Abstract

Controlled environment agriculture (CEA) operations must increase resource-use efficiency, yield, and phytonutrient concentrations to remain competitive. Carotenoids are phytonutrients of interest due to their purported health promoting effects. Their content is impacted by environmental controls, including lighting. Light-use efficiency increases with greater planting density, which is highest during seedling production. This creates the opportunity to raise light intensity during seedling production to improve growth characteristics and phytonutrient concentrations at harvest. Therefore, the objective of this research was to quantify the extent to which light intensity influences carotenoid accumulation in green butterhead lettuce seedlings, and if differences remain at harvest. Lettuce ‘Rex’ (Lactuca sativa L.) seedlings were grown under fluorescent lighting with intensities of 60, 100, 200, 400, or 600 µmol·m−2·s−1 with a 24-h photoperiod. After 14 days, seedlings were transplanted into raft hydroponic systems in a common greenhouse environment and grown for 21 days. At transplant and final harvest, tissue samples were collected and stored at −80 °C for phytonutrient analysis. Carotenoids, β-carotene, lutein, neoxanthin, zeaxanthin, and violaxanthin, and chlorophylls a and b were quantified using high-performance liquid chromatography (HPLC). We observed a 475% fresh mass enhancement in seedlings grown under 400 versus 60 µmol·m−2·s−1, with a 174% improvement persisting to final harvest. Higher seedling light intensities also generally increased leaf numbers in seedlings and at final harvest, as well as seedling carotenoid concentrations. Final harvest carotenoid concentrations generally decreased with increasing light intensity. Thus, producers should be cognizant that seedling light intensity strongly influences seedling and finished production yield, morphology, and carotenoid content.