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.

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