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

Abstract

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.