Effects of Temperature Variations during Light Period on Growth and Tipburn Incidence of Hydroponic Leaf Lettuce Grown under Artificial Lighting

Closed facilities, such as plant factories and growth chambers, can precisely control environmental factors influencing plant growth (Kozai et al., 2019; Ahmed et al., 2020). The controllable environmental factors that influence plant growth in these facilities are light, temperature, humidity, air velocity, and CO2 concentration (Yabuki and Miyagawa, 1970; Mortensen, 1986; Kitaya et al., 1998; Shibuya and Kozai, 1998; Goto, 2003; Park et al., 2012; Becker and Kläring, 2016). Temperature is a major environmental factor influencing plant growth and development (Tollenaar et al., 1979; Cao and Moss, 1989; Karlsson and Werner, 2001). Particularly, temperature influences the growth rate of lettuce (Gent, 2016) and the occurrence of tipburned leaves in lettuce (Choi and Lee, 2003). Tipburn in lettuce, which usually occurs at inner and newly developed leaf margins, is a serious problem in vegetable production under controlled environments (Cox et al., 1976), such as closed plant production systems with artificial light (Son and Takakura, 1989). Therefore, optimizing the temperature control is critical for lettuce production in plant factories and in greenhouses. In greenhouses, there is a method that controls temperature by setting different temperatures depending on the time of day. This method of temperature control enhances photosynthesis, promotes the translocation of photosynthates, and reduces consumption due to dark respiration, resulting in increased productivity (Kawashima, 2008). In tomato and cucumber, optimum temperatures for the translocation and inhibition of respiratory consumption at night have been identified, and both growth and yield have been demonstrated to be enhanced by temperature management at night (Suzuki et al., 1983; Toki, 1995). The control of nighttime temperature in cut-roses (Mito et al., 1980) and short-term heating treatment at the end of day in spraytype chrysanthemums have also been reported (Douzono, 2012; Kawanishi et al., 2012). In addition to temperature management at night, Ehara et al. (2017) reported that fruit growth was accelerated by maintaining higher air temperatures in the afternoon than in the morning and quickly dropping them in the early evening in greenhouse cucumber. However, previous studies conducted in plant factories and growth chambers investigated the influence of temperature on the growth of lettuce by controlling air temperature at a constant temperature during the light and dark periods (Choi and Lee, 2003; Gent, 2016; Lee et al., 2019). There have been few reports that examine the influence of timedependent temperature control on the growth of leaf lettuce grown under controlled environment with artificial lighting. This study aims to identify the benefits of a temperature control that varies air temperature, depending on the time of day on the growth of leaf lettuce grown in a controlled environment with artificial lighting. In this study, a 16-h light period was divided into the first and second halves, and air temperature in a growth chamber was controlled by varying the temperatures with time. The effects of temperature variations during the light period on the growth and tipburn incidence of leaf lettuce grown in the growth chamber with artificial lighting were investigated.