Dynamic Application of High and Low Red:Blue Ratios During Lettuce Development Shifts Growth and Metabolite Allocation.

Plants respond to light spectrum in different ways. When using red:blue (R:B) lighting during cultivation, high R:B can increase plant growth and carbohydrates. Conversely, low R:B can increase phytochemicals and nutritional compounds. The aim of this experiment was to utilize dynamic R:B spectra to define this interplay between growth and phytochemical production by determining if spectral effects on growth are due to preferential carbon allocation to different phenylpropanoids. Here, lettuce (Lactuca sativa L.) was grown under dynamic applications of high R:B (R:B) or low R:B (R:B) for three 9-day phases. Plant growth decreased based on cumulative exposure to low R:B. Two groups of phenylpropanoids, flavonoids and anthocyanins, were induced under low R:B, with similar concentrations regardless of whether plants were exposed to low R:B for the final 9 days or all 27 days. Red lettuce preferentially produced anthocyanins over quercetin, whereas green lettuce maintained high quercetin. However, lignin, structural phenylpropanoids, and cell wall-associated compounds were unaffected by R:B. Growth effects from R:B were instead linked to available energy from carbohydrates, which increased under high R:B. In conclusion, phytochemical content depends on the spectral conditions that plants receive during the final growth phase, while growth depends on the cumulative spectra. Using dynamic light application can therefore balance growth and phenylpropanoids.