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

A mechanism of increasing the shoot fresh weight (SFW) of leaf lettuce under alternating red (R) and blue (B) irradiation by light-emitting diodes (LEDs) was investigated based on plant morphogenesis. Seedlings of leaf lettuce (Lactuca sativa L. var. crispa‘Greenwave’) were cultivated under seven alternating irradiations, i.e., R24B0, R20B4, R16B8, R12B12, R8B16, R4B20 and R0B24, in which RxBy implies x and y hours of irradiation for red and blue LEDs, respectively. The results of these treatments were compared to those of two simultaneous irradiations: one containing no dark periods and the other containing a 12-hour period, i.e., sR24B24 at a photosynthetic photon flux density (PPFD) of 100 μmol m-2 s-1, and sR12B12 at a PPFD of 200 μmol m-2 s-1. Multiple regression analysis was conducted to investigate the causal relationship between the plant morphological characteristics (PMs) and SFW. The increase in the number of leaves with the red-light period in alternating irradiation treatments increased the total leaf area and consequently the SFW. On the other hand, the difference between PMs in alternating and simultaneous irradiations depended on the individual leaf area (ILA), where ILA under alternating irradiation treatments was significantly larger compared to that under simultaneous irradiation treatments.