Abstract
The morphology of plants growing under combined blue- and red-light irradiation is affected by the presence or absence of time slots of blue- and red-light mono-irradiation. The purposes of this study were to investigate the morphology and growth of cos lettuce grown under light irradiation combining several durations of blue and red light simultaneously and independent mono-irradiations of blue and red light during the day, and to clarify the effects of the durations of blue-light mono-irradiation and blue-light irradiation. Young cos lettuce seedlings were grown under 24-h blue-light irradiation with a photosynthetic photon flux density (PPFD) of 110μmol m−2 s−1 (B+0R) or under 24-h blue-light irradiation with a PPFD of 100μmol m−2 s−1 supplemented with 8 (B+8R), 16 (B+16R), and 24-h (B+24R) red-light irradiation with PPFDs of 30, 15, and 10μmol m−2 s−1, respectively (Experiment 1). The daily light integral was 9.50mol m−2 in all treatments. In Experiment 1, leaf elongation was promoted as the duration of red-light irradiation decreased and the duration of blue-light mono-irradiation increased. The maximum shoot dry weight was observed under the B+8R treatment. Growth was likely promoted by the expansion of the light-receptive area caused by moderate leaf elongation without tilting. In Experiment 2, young cos lettuce seedlings were grown as for Experiment 1, but blue- and red-light irradiation intensities were reversed (R+0B, R+8B, R+16B, and R+24B). Leaf elongation was promoted by the absence of blue-light irradiation (R+0B). The leaf surface was increasingly flattened, and the shoot dry weight was enhanced, as the duration of blue-light irradiation increased. Thus, cos lettuce leaf morphology may be manipulated by adjusting each duration of blue-light mono-irradiation, red-light mono-irradiation, and blue- and red-light simultaneous irradiation, which can, in turn, promote cos lettuce growth.
Highlights
The effects of lighting patterns on plant growth have been studied to improve plant cultivation with artificial light
The photosynthetic photon flux density (PPFD) values described here were measured at 2 cm above the surface of the urethane cube, in which the cos lettuce seedlings were planted, using a light quantum sensor (LI-190SA; LI-COR, Lincoln, NE, United States)
Leaf widths were similar among the treatments, but leaf lengths tended to increase along with blue-light mono-irradiation; the leaf length/width ratio tended to be greater as the bluelight mono-irradiation duration increased (Figures 5D–F)
Summary
The effects of lighting patterns on plant growth have been studied to improve plant cultivation with artificial light. When plants are irradiated with blue and red light simultaneously, growth rates are highest at a blue/red photosynthetic photon flux density (PPFD) ratio of 80–90/20–10 (e.g., Hernández and Kubota, 2016) at an identical total PPFD. In these studies, the spectral photon flux density distribution (SPFD) was constant during the light period. There have been attempts to promote plant growth using lighting patterns by applying different SPFDs during different hours of the day (Jishi et al, 2016; Ohtake et al, 2018)
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