2017 ASHS Annual Conference

Food safety, environmental impact, and efficient energy usage are growing concerns in horticultural production systems. Producing lettuce under artificial lighting can be a solution addressing these concerns. Light-emitting diodes (LED) offer the advantages of a narrow light spectrum, low power consumption, and little heat production. The objective of this study was to compare the effects of LED and fluorescent light and the sequence of the lighting on the growth of compact 'Tom Thumb' lettuce in a noncirculating hydroponic system. 'Tom Thumb' lettuce seeds were started in Oasis cubes under T5 high output fluorescent lighting in the lab. Seedlings were then transferred to 5.1-cm net pots, which were placed in 1.9-liter containers containing a hydroponic nutrient solution of Hydro-Gardens' Chem-Gro lettuce formula 8-15-36 hydroponic fertilizer with added calcium nitrate (19% Ca and 15.5% N) and magnesium sulfate (9.8% Mg and 12.9% SO₄). Half of the seedlings were grown under red+blue+white LEDs (110 µmol/m²/s, 12-hr photoperiod) and half under T5 high output fluorescent lighting (111 µmol/m²/s, 12-h photoperiod). After 12 days, half of the plants under LEDs were moved under fluorescent lighting, and half of the plants under fluorescent lighting were moved under LEDs for 16 more days. The four treatments were LED lighting (LL), fluorescent lighting (FF), initial LEDs followed by fluorescent (LF), and initial fluorescent followed by LEDs (FL). At the end of the study, differences among treatments were significant for plant height, leaf chlorophyll content, root dry weight, total plant dry weight, shoot dry weight produced per amount of nutrient solution used, and the pH and electrical conductivity of the nutrient solution. There were no significant differences among treatments for shoot dry weight, shoot-root ratio, percent dry weight partitioned to the shoots, percent dry weight partitioned to the roots, and the amount of nutrient solution that was used by each lettuce plant. In conclusion, the sequence of LED and fluorescent lighting could be an alternative to only LED or fluorescent lighting for lettuce plants.