Management of LED Light Quality to Maximize Biomass and Chlorophyll Fluorescence in Sprouting Broccoli in Controlled Environments

Light quality plays a major role in plant growth and development in controlled environments.  Previous work by our group has shown blue/red wavelengths from light-emitting diodes (LED) influence biomass production in brassica microgreens differently than traditional fluorescent and incandescent lighting.  The objective of this study was to compare impacts of white LEDs with different ratios of blue/red LEDs on biomass and chlorophyll fluorescence parameters in Sprouting broccoli (Brassica oleracea var. italica).  Broccoli were seeded into soil-less media and grown for 7 days in greenhouse conditions before transfer to controlled environment chambers.  Chambers were set at an air temperature of 22 °C under a 14-hour photoperiod using a light intensity of 250 µmol·m^-2·sec^-1.  Experimental sole source LED light quality treatments were: 1) white; 2) 5% blue (447 ± 5 nm) / 95% red (627 ± 5 nm); 3) 10% blue / 90% red; 4) 20% blue / 80% red; 5) 40% blue / 60% red; and 6) 60% blue / 40% red.  The experiment was repeated three times.  All plants were harvested 30 days after seeding and shoot tissues were measured for height, fresh mass (FM), and chlorophyll fluorescence parameters.  Broccoli plant height (P≤0.001) and shoot fresh mass (P≤0.001) varied in response to light quality treatments.  Plants under the 5% blue/95% red treatment had the tallest height at an average of 16.1 cm, while plants under the 40% blue/60% red treatment had the lowest height (13.5 cm).  The highest shoot FM was found for plants under the 5% blue/95% red treatment, while the lowest shoot FM was under the 60% blue/40% red treatment.  Shoot tissue minimum fluorescence (F₀; P≤0.001), maximum fluorescence (F_M; P≤0.001), variable fluorescence (F_V; P≤0.001), and maximum quantum yield of PSII (F_V/F_M; P=0.02) were all influenced by light treatment.  The 5% blue/95% red treatment resulted in significantly higher F₀, F_M, F_V, and F_V/F_M values compared to all other light treatments.  The 40% blue/60% red treatment produced the lowest values for F₀, F_M, F_V, and F_V/F_M compared to all other light treatments.  Sole source 5% blue/ 95% red lighting resulted in the highest plant growth and chlorophyll fluorescence in baby Sprouting broccoli.  Higher ratios of blue light decreased broccoli height and shoot FM, which may be due to the influence of blue light stimuli on stem elongation and stomatal regulation.  Therefore, proper management of blue/red LED lighting ratios may be beneficial to baby leafy green production in controlled environments.