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Substituting green or far-red radiation for blue radiation induces shade avoidance and promotes growth in lettuce and kale

Meng, Qingwu, Kelly, Nathan, Runkle, Erik S.
Environmental and experimental botany 2019 v.162 pp. 383-391
Brassica oleracea var. sabellica, Lactuca sativa, air temperature, biomass production, chlorophyll, color, crops, hydroponics, kale, leaf development, leaf morphology, leaves, lettuce, light emitting diodes, photons, photosynthesis, pigmentation, plant growth, seedlings
Although red (R; 600–700 nm) and blue (B; 400–500 nm) radiation can be sufficient for plants grown indoors, other wavebands such as green (G; 500–600 nm) and far red (FR; 700–800 nm) can also regulate photosynthesis, plant morphology, and secondary metabolism. The objective of this study was to determine how substitutions of B radiation with G and/or FR radiation influence growth of leafy greens grown indoors under light-emitting diodes (LEDs). We postulated G and/or FR radiation (and low B radiation) would trigger shade-avoidance responses and thus promote biomass accumulation through increased radiation interception. We grew lettuce (Lactuca sativa ‘Rex’ and ‘Rouxai’) and kale (Brassica oleracea var. sabellica ‘Siberian’) under warm-white (WW) LEDs at 180 μmol·m–2·s–1 (400–800 nm) for 9–11 days and then transplanted seedlings into a hydroponic system with ten different lighting treatments. The air temperature (20 °C), photoperiod (20 h), total photon flux density (180 μmol·m–2·s–1; 400–800 nm), and fertility were maintained the same across treatments. In addition to WW and equalized-white (EQW) controls, combinations of B (peak =449 nm), G (peak =526 nm), and FR (peak =733 nm) LEDs, each at 0, 20, 40, or 60 μmol·m–2·s–1, were delivered in a R background (peak =664 nm) of 120 μmol·m–2·s–1. One month after seed sow, we collected data on shoot mass, leaf morphology, and pigmentation. Substituting G or FR radiation for B radiation promoted leaf expansion and increased shoot mass but decreased chlorophyll concentrations in all crops. For example, lettuce ‘Rex’ grown under 60 μmol·m–2·s–1 of G +120 μmol·m–2·s–1 of R radiation was 38% greater in plant diameter and 54% greater in shoot dry mass compared to those under 60 μmol·m–2·s–1 of B +120 μmol·m–2·s–1 of R radiation. Substituting B radiation with G radiation at 60 μmol·m–2·s–1 also reduced red coloration of lettuce ‘Rouxai’. At the same photon flux density, FR radiation increased leaf expansion and decreased red foliage coloration more than G radiation. We conclude that substituting G and/or FR radiation for B radiation triggers shade-avoidance responses, accelerating plant growth while decreasing pigment concentrations.