Blue Radiation Interacts with Green Radiation to Influence Growth and Predominantly Controls Quality Attributes of Lettuce

Adding green [G (500-600 nm)] radiation to blue [B (400-500 nm)] and red [R (600-700 nm)] radiation creates white radiation and improves crop inspection at indoor farms. Although G radiation can drive photosynthesis and elicit the shade-avoidance response, its effects on plant growth and morphology have been inconsistent. We postulated G radiation would counter the suppression of crop growth and promotion of secondary metabolism by B radiation depending on the B photon flux density (PFD). Lettuce (Lactuca sativa 'Rouxar) 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 [PPFD (400-700 nm)] of 180 mu mol.m(-2).s(-1), plants were grown under warm-white LEDs or increasing B PFDs at 0, 20, 60, and 100 mu mol.m(-2).s(-1) with or without substituting the remaining R radiation with 60 mu mol.m(-2).s(-1) of G radiation. Biomass and leaf expansion were negatively correlated with the BPFD with or without G radiation. For example, increasing the B PFD 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 mu mol.m(-2).s(-1) of B radiation, but it decreased it at 60 or 100 mu mol.m(-2).s(-1) of B radiation. Results suggest that the shade-avoidance response is strongly elicited by low B radiation and repressed by high B radiation, rendering G radiation ineffective at controlling morphology. Moreover, substituting R radiation with G radiation likely reduced the quantum yield. Otherwise, G radiation barely influenced morphology, foliage coloration, essential nutrients, or sensory attributes regardless of the B PFD. Increasing the B PFD increased red foliage coloration and the 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 concluded that lettuce phenotypes are primarily controlled by B radiation and that G radiation maintains or suppresses lettuce growth depending on the B PFD.