Optimization of temperature and light, and cultivar selection for the production of high-quality head lettuce in a closed-type plant factory

Abstract

Two separate experiments were conducted to identify suitable cultivars, to optimize light and temperature, and to understand the heading process in order to produce high-quality crisphead lettuce (Lactuca sativa L.) in a closed production environment. Three commercial crisphead lettuce cultivars (‘Adam’, ‘Manchu’, and ‘Sensation’) were grown on deep flow technique beds in a closed-type plant factory under three different fluorescent light intensities [150, 200, and 250 µmol m−2 s−1 photosynthetic photon flux (PPF)] and two temperature conditions. The plants were grown until 60 days after transplantation (DAT) under the high [22/18 °C (0–30 DAT) → 18/16 °C (30–60 DAT)] and low [18/16 °C (0–30 DAT) → 18/14 °C (30–60 DAT)] temperature conditions, with 50 ± 10% relative humidity and a 12-h day length. Growth parameters, including head formation rate and tip burn incidence, were measured in all experiments. All cultivars exhibited significantly higher growth rates in the 250 µmol m−2 s−1 light intensity compared to lower light intensities under both temperature conditions, and showed a cultivar-specific difference in the growth pattern at different light intensities. Our cultivation trials suggest that ‘Sensation’ is ideal for production in closed-type plant factory systems at 250 µmol m−2 s−1 PPF due to its stable head formation and superior head quality. Under the high temperature condition, head formation was only observed in two cultivars, ‘Sensation’ and ‘Adam’, while all cultivars showed head formation in the low temperature condition. The initiation of head formation began at the 8–10 true-leaf stage around 20 DAT, and then apparent head formation was observed around 30 DAT, followed by an increase of head density until harvest (60 DAT). Tip burn symptoms were observed throughout the experiment, with higher values in the high temperature condition. The results of our study suggest that good head formation can be achieved under high light intensities ( ≥ 200 µmol m−2 s−1 PPF) and low temperature [18/16 °C (0–30 DAT) → 18/14 °C (30–60 DAT)] in crisphead lettuce grown in a closed-type plant factory.