Specialty food production in controlled environments is gaining momentum with an increasing interest in supplying fresh, local, and nutritious produce throughout the year. Plant phenotypic plasticity enables trait manipulation through adjusting environmental variables such as light (quantity, quality, and duration). Changing the spectral composition can alter photosynthetic energy and photomorphogenic signals, thereby influencing yield, morphology, and secondary metabolism. Blue and red light are typically used in sole-source lighting because of their high photosynthetic photon efficacy. In contrast, potential benefits of other wavebands such as green and far-red light have been less explored. To elucidate how various combinations of blue, green, red, and far-red light regulate growth and quality attributes of leafy greens, we conducted experiments in controlled-environment growth rooms with sole-source lighting from adjustable light-emitting diodes and/or in a greenhouse. Plants were grown in a deep-flow-technique hydroponic system and/or in a soilless substrate. Here, adding far-red light to blue and red light elicited the shade-avoidance response of lettuce (Lactuca sativa) and basil ( Ocimum basilicum) seedlings, increasing leaf expansion, light capture, and biomass. These responses were more pronounced under a high ratio of blue to red light or a low photosynthetic photon flux density. Spectral interactions were further investigated among blue, green, and far-red light in mature hydroponic lettuce and kale (Brassica oleracea var. sabellica ). In a red-light background, substituting green or far-red light for blue light antagonized blue light-induced growth suppression and pigment accumulation. However, responses under increasing green light were confounded by decreasing blue light, which can also trigger the shade-avoidance response. This was addressed by a following experiment, in which red light was substituted with green light at various blue photon flux densities. With or without green light, increasing blue light decreased biomass and leaf size of red-leaf lettuce but increased red foliage coloration and concentrations of several essential nutrients. Green light marginally influenced biomass under low blue light but decreased it under high blue light. Thus, green light effects depend on interactions among blue, green, and red light in specific spectral contexts. In addition, consumers preferred lettuce grown under sole-source lighting compared with those grown in a greenhouse. Finally, sequential alternations of spectra revealed lasting effects of initial lighting treatments and dynamic lettuce growth responses over time. Collectively, these studies reveal how crop traits can be improved by wavebands beyond static red and blue light and help uncover complex spectral interactions in whole-plant physiology of herbaceous plants.
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