Photosynthetic Photon Flux, Photoperiod, and CO2 Concentration Affect Growth and Morphology of Lettuce Plug Transplants

Lettuce ( Lactuca sativa L. cv. Summer-green) plug transplants were grown for 3 weeks under 16 combinations of four levels (100, 150, 200, and 300 μmol·m -2 ·s -1 ) of photosynthetic photon flux ( PPF ), two photoperiods (16 and 24 h), and two levels of CO 2 (400 and 800 μmol·mol -1 ) in growth chambers maintained at an air temperature of 20 ±2 °C. As PPF increased, dry mass (DM), percent DM, and leaf number increased, while ratio of shoot to root dry mass (S/R), ratio of leaf length to leaf width (LL/LW), specific leaf area, and hypocotyl length decreased. At the same PPF, DM was increased by 25% to 100% and 10% to 100% with extended photoperiod and elevated CO 2 concentration, respectively. Dry mass, percent DM, and leaf number increased linearly with daily light integral (DLI, the product of PPF and photoperiod), while S/R, specific leaf area, LL/LW and hypocotyl length decreased as DLI increased under each CO 2 concentration. Hypocotyl length was influenced by PPF and photoperiod, but not by CO 2 concentration. Leaf morphology, which can be reflected by LL/LW, was substantially influenced by PPF at 100 to 200 μmol·m -2 ·s -1 , but not at 200 to 300 μmol·m -2 ·s -1 . At the same DLI, the longer photoperiod promoted growth under the low CO 2 concentration, but not under the high CO 2 concentration. Longer photoperiod and/or higher CO 2 concentration compensated for a low PPF.