Alternating Red/Blue Light Increases Leaf Thickness and Mesophyll Cell Density in the Early Growth Stage, Improving Photosynthesis and Plant Growth in Lettuce

Cultivation in a closed-type plant factory with artificial lighting enables year-round production of crops with a stable yield and uniform quality. This is in contrast to cultivation in fields and in sunlight-type plant factories where environmental fluctuations can reduce crop yield and quality. Therefore, closed-type plant factories are expected to be applicable to harsh environments inadequate for crop production and to lead to improved global food security (Kozai, 2013; Anpo et al., 2019). However, the high electricity cost due to the artificial lighting diminishes the benefit of improved sales, which hinders new entry into the business (Kozai and Niu, 2020). Identifying optimal irradiation methods to maximize crop yield without increasing electricity costs could enhance the benefits of closed-type plant factories and lead to an expansion in operations. In most cases, artificial red (R) and blue (B) light from light-emitting diodes (LEDs) is used for cultivation in plant factories since these wavelengths are specifically absorbed by chlorophyll to drive photosynthetic processes (Pfündel and Baake, 1990; Massa et al., 2008). In addition, monochromatic R or B light alone is unsuitable for crop production, because compared to simultaneous R+B (RB) light or white (W) light, R light alone decreases photosynthetic rate and biomass, and leads to abnormal shape (Goins et al., 1998; Wang et al., 2015), and B light alone decreases stem length, leaf area, and photosynthetic rate due to chloroplast avoidance response (Wada et al., 2003; Kim et al., 2004). It is widely recognized that simultaneous RB light is a promising irradiation procedure for vegetable plants including pepper (Piper nigrum), lettuce (Lactuca sativa L.), spinach (Spinacia oleracea), radish (Raphanus sativus var. sativus), tomato (Solanum lycopersicum), rapeseed (Brassica napus), and cucumber (Cucumis sativus L.) (Brown et al., 1995; Yorio et al., 2001; Nanya et al., 2012; Li et al., 2013; Miao et al., 2019). Previous studies have attempted to find optimal controls of R and B light, including intensity of photosynthetic photon flux density (PPFD) (Yanagi et al., 1996; Zha and Liu, 2018), length of photoperiod (Jao and Fang, 2004; Jishi et al., 2016), and ratio of R light to B light (Okamoto et al., 1997; Hogewoning et al., 2010; Borowski et al., 2015; Wang et al., 2016). Recent studies also report that the patterns of R and B light irradiation affect plant growth. For instance, Shimokawa et al. (2014) found that alternating irradiation with R and B LEDs (12 hours R : 12 hours B) enhanced growth in leafy lettuce (Lactuca sativa L. cv. ‘Summer Surge’), compared with lettuce grown under W light or simultaneous RB light (12 hours light : 12 hours dark) with the same daily light integrals. This phenomenon cannot be explained by a difference in day length, because alternating irradiation of red and blue (R/B) light also promoted lettuce