2019 ASHS Annual Conference

The positive effects of supplemental red/blue light have been well documented for a variety of greenhouse crops. However, there is a lack of quantitative information to help growers predict the crop biomass gain in response to the amount of supplemental light provided. Therefore, we quantified the biomass increase per mole of photons of supplemental light for two greenhouse-grown herbs. We used red/blue LED lights to provide a wide range of supplemental light (up to 200 µmol m^-2 s^-1) with a red/blue ratio of 75:25. LED Lights were turned on for four hours before sunrise and again after sunset. A treatment with no supplemental lighting was used as the control. Each treatment was replicated ten times. The supplemental photosynthetic photon flux density (PPFD) at each plant was measured using a spectroradiometer. Basil ‘Spicy Bush’ (Ocimum basilicum var. minimum) and cilantro ‘Calypso’ (Coriandrum sativum) were directly seeded in 15 cm pots and placed under the lighting treatments. Plants were spaced 25 cm apart. Basil and cilantro plants were harvested 42 and 60 days after seeding, respectively. The number of photons (mol‧m^-2) received by each plant from supplemental lighting during the growing period was calculated. We observed significantly greater plant height, chlorophyll content index, and total plant dry weight for both basil and cilantro with supplemental PPFD, compared to the control treatment (P = 0.005). Even though we did not see a change in leaf area with the supplemental lighting for either of the herbs, the specific leaf area (leaf area per gram of dry weight) of both basil and cilantro decreased in response to supplemental light (P = 0.034). A regression of shoot biomass versus the amount of supplemental light received showed an increase of 1.06 mg of basil and 0.37 mg of cilantro per mole of supplemental light (P < 0.0001), far below the theoretical maximum of 1 g/mol. These values are important for herb growers to determine how much biomass and salable product increase can be expected from supplemental lighting. To maximize the financial return of supplemental lighting, research is needed to maximize the biomass produced in response to supplemental lighting.