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As plants grow, various environmental stressors such as temperature fluctuations, strong light, desiccation, osmotic pressure, and lack of oxygen or minerals around the rhizosphere affect the plants. The time taken for an environmental stressor to affect a plant depends on the type of environmental stress and the extent of influence depends on plant species, plant organ, growth stage, and growth conditions of the affected plant. It has been estimated that the average yield of crops produced by farms is only 21.6% of the record yield of the crops (Boyer, 1982). This difference in crop yield is mainly attributed to abiotic factors such as water deficit, soil nutrient availability and toxicity, and heat stress or chilling/freezing, each of which can combine with other factors to lower crop productivity (Mittler, 2006). Crops cultivated in a plant factory are less affected by changes in the abiotic environment than crops cultivated on a farm (Gruda, 2009). Therefore, one can always anticipate a stable supply of crops grown in a factory. In addition, these plants are increasingly seen as safe and reliable because they are unaffected by environmental pollution such as by leakage of chemical substances or radioactive materials. Plant factories are classified into those that use sunlight and those that use artificial light. Of these, the type that has a completely controlled light source can cultivate chemical-free vegetables regardless of weather or location because the plant factory is a closed system where all nonbiological environmental factors are controlled. Accordingly, the completely controlled type of plant factory may produce the most volume of green vegetables such as spinach or lettuce in the future. However, cost problems have been identified (Takatsuji, 2013). In the completely controlled type of plant factory, vegetables grow optimally due to the regulation of light and temperature. For example, it has been reported that the content of anthocyanin, a flavonoid in red leaf lettuce, increased on exposure to blue light during growth (Shoji et al., 2010). In addition, previous studies have shown that the quality and value of vegetables are improved when salinity (seawater or table salt) is provided as an abiotic environmental nutrient, as seen in tomatoes (Kitano et al., 2008), onions (Akamatsu, 2007), and broccoli (LopezBerenguer et al., 2009). However, there have been few reports documenting the results of cultivation conditions in which salt is added to the growing cultures of green vegetables, which are extremely vulnerable to salt stress, with regard to their impact on the quality of the plants produced in a closed-type plant factory. In this study, we cultivated lettuce in a hydroponic culture containing either seawater or sodium chloride (NaCl). The quantity of nutrients and pigments in lettuce was measured and compared to attempt to improve the quality and value of lettuce.

Effects of Salinity and Nutrients in Seawater on Hydroponic Culture of Red Leaf Lettuce