Influence of Far-Red Intensity during the Seedling Stage on Biomass Production and Photomorphogenic Characteristics in Leafy Greens under Sole-Source Lighting

It has been well documented that far-red radiation (FR; 700–799 nm) elicits a shade-avoidance/shade-tolerance response across a wide range of plant species. Most sole-source lighting is relatively low in FR compared to sunlight (i.e., 2% vs. 20% of photons, respectively, integrated between 400 and 799 nm). The objective of this experiment was to determine if the photomorphogenic response to FR is a useful strategy during the seedling stage to promote leaf expansion in the hopes that subsequently transplanted seedlings would increase radiation capture resulting in higher harvestable biomass. Lettuce (cv. ‘Rex’, ‘Red Oak’, and ‘Green Grand Rapids’) seedlings were exposed to 5, 10, 20, or 30 µmol·m⁻²·s⁻¹ of supplemental FR for a duration of 10 d in a growth chamber for 20 h daily. During this stage, all seedlings received background light levels of 195 µmol·m⁻²·s⁻¹ PAR light from white LEDs for 20 h daily. Seedlings were transplanted into a nutrient film technique (NFT) hydroponic system in a separate growth chamber with LED fixtures that supplied white light at 295 µmol·m⁻²·s⁻¹ for 16 h daily (DLI = 17 mol·m⁻²·d⁻¹) until they were harvested at 35 d from seeding. At transplant, fresh weight, leaf area, and plant height were significantly greater for all cultivars exposed to 30 µmol·m⁻²·s⁻¹ of supplemental FR radiation compared to the 5 µmol·m⁻²·s⁻¹ control. Fresh weight increased by an average of 35% under 30 µmol·m⁻²·s⁻¹ FR. Mature plant dry biomass increased by 14% when seedlings were exposed to 30 µmol·m⁻²·s⁻¹ of supplemental FR radiation. Increasing far-red radiation consistently increased plant growth at the seedling stage, but these increases were generally overcome by maturation.