2019 ASHS Annual Conference

Adding green (G; 500–600 nm) radiation to blue (B; 400–500 nm) and red (R; 600–700 nm) radiation creates white light to improve crop inspection at indoor farms. Although G radiation can drive photosynthesis and elicit shade-avoidance responses, its effects on plant growth and morphology have been inconsistent. We postulated G radiation would counter suppression of crop growth and promotion of secondary metabolism by B radiation, depending on the B photon flux density. We grew lettuce (Lactuca sativa) ‘Rouxai’ in a temperature-controlled growth room under nine sole-source light-emitting diode (LED) treatments with a 20-hour photoperiod or in a greenhouse. At the same total photon flux density (400–800 nm) of 180 µmol∙m^–2∙s^–1, we grew plants 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 radiation with 60 µmol∙m^–2∙s^–1 of G radiation. Biomass and leaf expansion negatively correlated with the B photon flux density with or without G radiation. For example, increasing the B photon flux density decreased fresh and dry mass by up to 63% and 54%, respectively. The inclusion of G radiation did not affect shoot dry mass at 0 or 20 µmol∙m^–2∙s^–1 of B radiation, but decreased it at 60 or 100 µmol∙m^–2∙s^–1 of B radiation. Results suggest shade-avoidance responses are strongly elicited by low B radiation and repressed by high B radiation. Green radiation 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 to those grown in the greenhouse, which were more bitter and less acceptable, flavorful, and sweet. We conclude lettuce phenotypes are primarily controlled by B radiation, and G radiation maintains or suppresses lettuce growth, depending on the B photon flux density.