Photosynthesis and Morphology of Leaf Lettuce (<i>Lactuca sativa</i> L. cv. Greenwave) Grown under Alternating Irradiation of Red and Blue Light

Abstract

The mechanism for accelerating leaf lettuce growth by alternating irradiation of red (R) and blue (B) lights was investigated in this study. Leaf lettuce was cultivated under nine light conditions with different time ratios of R/B alternate irradiation; R0B24, R3B21, R6B18, R9B15, R12B12, R15B9, R18B6, R21B3 and R24B0. As a result, R21B3 treatment (21 hours of R irradiation and 3 hours of B irradiation, in an alternating pattern without a dark period) was determined to be the optimum condition for leaf lettuce growth, since shoot fresh weight under this treatment was significantly the highest. Moreover, photosynthetic ability and morphology were studied under alternating irradiation (R21B3 and R12B12) and simultaneous irradiation (RB24; simultaneous irradiation of R and B lights without a dark period). Photosynthetic ability of lettuce grown under R21B3 and R12B12 was significantly higher than that under RB24. A morphological index, PA/LA (Projected Area divided by Leaf Area), was greater in R21B3 and R12B12 than in RB24. This result suggested that alternating irradiation causes plants to have an effective posture for receiving light. Therefore, it seems reasonable to conclude that growth acceleration of plants under alternating radiation was caused by high photosynthetic ability and morphological superiority.

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

“…In closed-type plant factories with artificial light, improving production efficiency without increasing electricity costs is a challenging issue. Several recent studies have showed alternating R/B could be a promising approach to enhancing lettuce productivity and income in closed-type plant factories (Shimokawa et al, 2014;Chen et al, 2017Chen et al, , 2019Kuno et al, 2017;Takasu et al, 2019). However, the physiological reason why alternating R/B can enhance the productivity is largely unknown.…”

“…'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 growth relative to RB under similar day length and daily light integral (Chen et al, 2017;Kuno et al, 2017;Ohtake et al, 2018;Takasu et al, 2019). More recently, reported that an alternating interval of 1 hour promoted growth and sugar accumulation in lettuce more than that under an alternating interval of 4 hours.…”

Rerouting Artificial Light for Efficient Crops Production: A Review of Lighting Strategy in PFALs

“…From above, alternate lighting is characteristic of creating a light environment where crops are alternately irradiated by different spectral compositions. Molecular mechanisms underlying the regulation of alternate lighting on growth still remained to be discovered, whereas it is widely reported that the stimulating effects of alternate lighting are attributed to positive effects on morphology, metabolism, and photosynthesis efficiency [40,41]. Lettuce cultivated under optimal alternating blue and red light treatment tends to have a higher PA/LA (projected area/leaf area) index; meanwhile, photosynthetic efficiency was significantly enhanced [41].…”

“…Molecular mechanisms underlying the regulation of alternate lighting on growth still remained to be discovered, whereas it is widely reported that the stimulating effects of alternate lighting are attributed to positive effects on morphology, metabolism, and photosynthesis efficiency [40,41]. Lettuce cultivated under optimal alternating blue and red light treatment tends to have a higher PA/LA (projected area/leaf area) index; meanwhile, photosynthetic efficiency was significantly enhanced [41]. Besides, different monochromatic lights or spectral compositions possess varied physiological or phytochemical effects [42].…”