Evaluation of growth and quality of hydroponic lettuce at harvest as affected by the light intensity, photoperiod and light quality at seedling stage

In order to obtain high-quality lettuce seedlings, effects of photosynthetic photon flux density (PPFD), photo-period and light quality at seedling stage on growth and quality of hydroponic lettuce were investigated at different growth stages. Hydroponic lettuce (Lactuca sativa L. cv. Frill ice) seedlings were grown under 12 combinations of two levels of PPFD (200 and 250 mu mol m(-2) s(-1)) and two photoperiods (14 and 16 h d(-1)) under three light quality: light-emitting diodes (LEDs) with red to blue ratio (R:B ratio) of 1.2 and 2.2, and fluorescent lamps with R:B ratio of 1.8 as control. After the 20-day treatment during seedling stage, lettuce seedlings were transplanted to a homogeneous lighting environment for another 20 days before harvest. Results showed that leaf length and ratio of leaf length to leaf width at 20 days after sowing decreased in a logarithmic way with increasing daily light integral and fluorescent lamps resulted in larger seedlings with bigger leaves and higher leaf and root dry weights compared to LEDs. Leaf and root fresh weights of harvested lettuces were higher in the seedlings grown under PPFD at 200 mu mol m(-2) s(-1) with a 16 h d(-1) photoperiod, regardless of light quality at seedling stage. Generally, vitamin C content of mature lettuce was lower in seedlings grown under LEDs with R:B ratio of 1.2 and no significant differences were observed in nitrate content among the three lighting sources. In consideration of mature yield and quality, lighting efficiency and electrical consumption of lighting sources, PPFD of 200 mu mol m(-2) s(-1) with a photoperiod of 16 h d(-1) provided by LEDs with R:B ratio of 2.2 was suitable for hydroponic lettuce (cv. Frill ice) seedling production.