2019 ASHS Annual Conference

Light emitting diodes (LEDs) have many advantages over older lighting technologies, including the ability to control the spectrum of light produced. The light spectrum is an important way plants receive information from their environment and different wavelengths can illicit certain morphological responses to help the plants adapt to their surroundings. Far-red (FR) light (710 – 750 nm) is transmitted through leaves more than 400 – 700 nm light and is a signal indicating that a plant is being shaded by other plants. We determined whether supplemental FR LED lighting can be used to induce a shade avoidance response, increase light interception, and accelerate plant production. Sixty ‘Green Salad Bowl’ lettuce plants were grown at a 15-cm spacing in 10-cm pots in a growth chamber with eight white LED panels providing a mean photosynthetic photon flux density of 207 ± 13 µmol m^-2 s^-1. Post germination, plants received supplemental light from two custom built FR LED light bars (spectral peak at 735 nm) on one side of the growth chamber, creating a gradient of FR in the growth chamber. The total amount of FR at plant height was the highest directly beneath the LEDs (~ 21 µmol m^-2 s^-1) and diminished the further away the plants were placed (~ 5 µmol m^-2 s^-1). Half of the plants were harvested after 18 days, and the remainder were harvested after 25 days. Canopies of all plants were digitally imaged using an Aris TopView Multispectral Digital Imaging System (Eindhoven, Netherlands) and subsequently phenotyped for projected canopy size using ImageJ (National Institutes of Health, Bethesda, Maryland, USA). Projected canopy size was increased by FR in both harvests (P = 0.0005; P < 0.0001), as was the total leaf area of the plants for both the first and second harvest (P = 0.0022; P = 0.0005). Projected canopy size was strongly correlated with total leaf area (r² = 0.91; r² = 0.89). The length and width of the longest leaf of each plant also increased, as did shoot dry weight and specific leaf area. An increase in specific leaf area is a typical shade avoidance response. The results from this study suggest that supplementing lettuce with FR wavelengths can increase light interception and expedite the growth of the plants