2018 ASHS Annual Conference

Although red and blue light can be sufficient for plants grown indoors, other wavebands have potential to increase crop yield and quality by regulating plant morphology, photosynthesis, and secondary metabolism. The objective of this study was to determine how blue, green, red, and far-red light interacted to regulate growth of leafy greens grown indoors under light-emitting diodes (LEDs). We grew kale (Brassica oleracea var. sabellica ‘Siberian’) and lettuce (Lactuca sativa ‘Rex’ and ‘Rouxai’) under warm-white light at 180 µmol∙m^–2∙s^–1 for 9–11 days and then transplanted them in a hydroponic system with ten different lighting treatments. The air temperature (20 °C), photoperiod (20 hours), total photon flux density (180 µmol∙m^–2∙s^–1, 400–800 nm), and fertility were maintained the same across treatments. In addition to warm-white and equalized-white controls, combinations of blue (peak = 449 nm), green (peak = 526 nm), and far-red (peak = 733 nm) light, each at 0, 20, 40, or 60 µmol∙m^–2∙s^–1, were delivered in a red-light (peak = 664 nm) background at 120 µmol∙m^–2∙s^–1. One month after seed sow, we collected data on shoot biomass, leaf morphology, and pigmentation. Substituting green or far-red light for blue light promoted leaf expansion and increased biomass but decreased chlorophyll concentration in all crops. For example, lettuce ‘Rex’ grown under green + red light was 35% greater in plant diameter and 40% greater in shoot dry weight compared to those under blue + red light. Green light at 60 µmol∙m^–2∙s^–1 also reduced red coloration of lettuce ‘Rouxai’. At the same intensity, far-red light increased leaf expansion and decreased pigmentation more than green light. We conclude that green light can counter blue-light-induced growth suppression and trigger the shade-avoidance response, accelerating growth while decreasing pigment concentration.