Growth and Bioactive Compounds of Lettuce as Affected by Light Intensity and Photoperiod in a Plant Factory Using External Electrode Fluorescent Lamps

External electrode fluorescent lamps (EEFLs) are a new, efficient light source that can be used in plant factories. We examined the effects of light intensity and photoperiod combinations on growth, total phenolic content, antioxidant capacity, and light use efficiency of lettuce (Lactuca sativa 'Cheongchima') in a plant factory employing EEFLs. Two-week-old seedlings were grown for 3 weeks at a photosynthetic photon flux density of 150 (150P) or 200 (200P) mu mol.m(-2).s(-1) under 12, 16, 20, or 24 h photoperiods. The air temperature was maintained at 20 +/- 2 degrees C and Yamazaki nutrient solution was supplied using a deep flow technique. Fresh shoot and root weights increased as photoperiod was extended, becoming greatest under the 150P/24 h condition. The shoot/root ratio was lowest at the 24 h photoperiod under 150P and 200P conditions. Leaf length decreased at longer photoperiods, but leaf width and number was increased; therefore, leaf shape became broader under longer photoperiods. Leaf area increased at the 150P/20 h condition but decreased at the 200P/24 h condition. Specific leaf weight (thickness) increased significantly as photoperiod was extended irrespective of light intensity and became greater under 200P than 150P. Total phenolic content and antioxidant capacity increased continuously with increasing photoperiods under 150P; however, in the 200P treatment, both increased up to 20 h, then decreased under the 24 h photoperiod. Light use efficiency was generally higher under 150P, but became similar at either light intensity under the 24 h period. Considering the growth rate, leaf size, antioxidant capacity, and cropping cycle, the 150P/20 h condition was deemed to be the most efficient and economical for growth of 'Cheongchima' lettuce in a plant factory system.