Quantification of the Effects of Alternating and Simultaneous Red and Blue Irradiations on Plant Morphology and Shoot Fresh Weight in Leaf Lettuce‘Greenwave’

Abstract

In the closed-type plant factories, artificial light sources, such as fluorescent lamps and light-emitting diodes (LEDs), are widely used as alternatives to sunlight. They, along with setting optimum environmental conditions (light, temperature, humidity, CO2, concentration of nutrient solution, and so on), can improve the productivity per unit area, and consequently increase the profit from small cultivation areas (Goto, 2012). However, only a few enterprises make a profit, due to the high initial cost for construction of closed-type plant factory units and the running costs of electric power such as lighting and air conditioning (Benke and Tomkins, 2017; Kozai, 2013). To reduce the running cost, LEDs are used as artificial light sources for plant growth, because of their lower power consumption and longer life span compared to fluorescent lamps. The morphogenesis of plants depends on the light spectra, and the illumination of LEDs is composed of a narrow range of wavelengths on the electromagnetic spectrum. Chlorophylls in plants mainly absorb red and blue lights and perform photosynthesis (Mohr and Schopfer, 1998); thus, several previous studies investigated the ability to increase yields by enhancing the efficiency of using these two colors. Shimokawa et al. (2014, 2020) investigated the growth of three cultivars of leaf lettuce, i.e., Summer Surge, Black Rose, Green Span (Lactuca sativa L. var. crispa) using Shigyo’s method, which involves alternating irradiation with red (LED; 660 nm, 100 mmol m 2 s ) and blue (LED; 450 nm, 60 mmol m 2 s ) lights. They found that this method enhanced the growth of two cultivars, i.e., Summer Surge and Black Rose, by increasing SFW compared to those under fluorescent light or simultaneous irradiation with red and blue lights. Using romaine lettuce (Lactuca sativa L. cv. Cos Lettuce), Jishi et al. (2016) examined the irradiation patterns of various combinations of red (LED, 90 mmol m 2 s ) and blue (LED, 90 mmol m 2 s ) lights, such as simultaneous red―blue irradiation as well as irradiation with shifted red-light period, based on the study of Hanyu and Shoji (2002), in which they found that using blue light irradiation just before entering the light period promoted the growth of spinach. When a photoperiod was set using a cycle of blue monochromatic light; red monochromatic light; and dark period, the total leaf area and SFW were maximized. Kuno et al. (2017) investigated the cultivation of leaf lettuce (Lactuca sativa L. var. crispa ‘Greenwave’) under simultaneous irradiation, alternating irradiation and shifted irradiation of red (LED, 120 mmol m 2 s ) or blue (LED, 120 mmol m 2 s ) light periods. The SFW obtained using alternating irradiation was significantly larger compared to that obtained using simultaneous irradiation. The SFW obtained using red monochromatic light was also large. Takasu et al. (2019) examined the optimum conditions for using alternating irradiations (ALTs) to cultivate Lactuca sativa L. var. crispa ‘Greenwave’ under nine different ratios of red light (LED, 100 mmol m 2 s ) and blue