Blue radiation interacts with green radiation to influence growth and predominantly controls quality attributes of lettuce

Authors: MENG, QUINGWU - Michigan State University; Boldt, Jennifer; RUNKLE, ERIK - Michigan State University

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2019
Publication Date: 1/10/2020
Citation: Meng, Q., Boldt, J.K., Runkle, E.S. 2020. Blue radiation interacts with green radiation to influence growth and predominantly controls quality attributes of lettuce. Journal of the American Society for Horticultural Science. 145(2):75-87. https://doi.org/10.21273/JASHS04759-19.
DOI: https://doi.org/10.21273/JASHS04759-19

Interpretive Summary: Indoor farming has recently been expanding rapidly in urban and peri-urban areas due to an emerging interest in producing local, fresh, and nutritious food throughout the year. Leafy greens and herbs are common crops suitable for commercial indoor vertical farming. Light-emitting diodes (LEDs) are the predominant light source because they can be placed close to the crop canopy, are energy efficient and long-lasting, and have customizable spectral distributions. Characterizing and understanding crop responses to different wavelengths of light is crucial to achieving desired yield and quality attributes, such as shape, texture, nutritional value, and sensory properties. We grew lettuce ‘Rouxai’ in a growth room under warm-white light or different combinations of red, blue and green light. All treatments received the same light intensity. An additional treatment was grown in the greenhouse. Biomass and leaf expansion decreased as the blue photon flux density increased. Adding green light did not affect shoot dry weight at low intensities of blue light (=20 µmol·m–2·s–1) but decreased it at higher intensities of blue light (=60 µmol·m–2·s–1). Otherwise, green light minimally influenced morphology, foliage coloration, essential nutrients, or sensory attributes. Increasing the blue photon flux density increased red foliage coloration and the concentrations of several macronutrients and micronutrients. Consumers preferred plants grown under LEDs to those grown in the greenhouse, which were more bitter and less acceptable, flavorful, and sweet. We conclude that lettuce phenotypes are primarily controlled by blue light, and green light maintains or suppresses growth depending on the blue photon flux density.

Technical Abstract: Adding green (G; 500–600 nm) light to blue (B; 400–500 nm) and red (R; 600–700 nm) light creates white light to improve crop inspection at indoor farms. Although G light can drive photosynthesis and elicit the shade-avoidance response, its effects on plant growth and morphology have been inconsistent. We postulated G light would counter suppression of crop growth and promotion of secondary metabolism by B light depending on the B photon flux density. Lettuce (Lactuca sativa) ‘Rouxai’ was grown in a growth room under nine sole-source light-emitting diode (LED) treatments with a 20-hour photoperiod or in a greenhouse. At the same photosynthetic photon flux density of 180 µmol·m–2·s–1, plants were grown under warm-white LEDs or increasing B photon flux densities at 0, 20, 60, and 100 µmol·m–2·s–1 with or without substituting the remaining R light with 60 µmol·m–2·s–1 of G light. Biomass and leaf expansion negatively correlated with the B photon flux density with or without G light. For example, increasing the B photon flux density decreased fresh and dry weights by up to 63% and 54%, respectively. The inclusion of G light did not affect shoot dry weight at 0 or 20 µmol·m–2·s–1 of B light, but decreased it at 60 or 100 µmol·m–2·s–1 of B light. Results suggest the shade-avoidance response is strongly elicited by low B light and repressed by high B light, rendering G light ineffective at controlling morphology. Moreover, substituting R light with G light likely reduced the quantum yield. Otherwise, G light barely influenced morphology, foliage coloration, essential nutrients, or sensory attributes regardless of the B photon flux density. Increasing the B photon flux density increased red foliage coloration and concentrations of several macronutrients (e.g., nitrogen and magnesium) and micronutrients (e.g., zinc and copper). Consumers preferred plants grown under sole-source lighting over those grown in the greenhouse, which were more bitter and less acceptable, flavorful, and sweet. We conclude that lettuce phenotypes are primarily controlled by B light, and G light maintains or suppresses lettuce growth depending on the B photon flux density.