2017 ASHS Annual Conference

Real-time dynamic control over the light output from LEDs has the potential to improve the efficiency of supplemental lighting in greenhouses. Most current methods for supplemental lighting control are based on incident photosynthetic photon flux (PPF), daily light integral (DLI), and/or photoperiod. However, an approach to lighting control that also accounts for plant physiological responses could improve the overall efficiency of supplemental lighting. We developed a model for the photosynthetic response of lettuce (Lactuca sativa ‘Green Towers’) based on 35 days of diurnal chlorophyll fluorescence monitoring to measure the quantum yield of photosystem II (Φ_PSII, a measure of how efficiently leaves use absorbed light). Electron transport rate (ETR) was described as function of PPF with an asymptote at ETR = 126 µmol∙m^-2∙s^-1. Integrated over entire days, the daily electron transport rate reached an asymptote of ~3 mol∙m^-2∙d^-1.  The instantaneous light response curve was used to model different lighting approaches to reach a daily ETR of 3 mol∙m^-2∙d^-1. Daily light use efficiency (LUE_D) was defined as the ratio of total daily ETR to DLI. Supplemental lighting use efficiency (LUE_S) was calculated as the ratio of the increase in daily ETR to the increase in DLI resulting from supplemental light. We modeled supplemental lighting scenarios based on observed ambient PPFs for a day with a 12.2 h natural photoperiod, a DLI of 8.46 mol∙m^-2∙d^-1, and a daily ETR of 2.0 mol∙m^-2∙d^-1 (LUE_D = 0.236). Thus, supplemental lighting had to provide enough PPF for an additional ETR of 1.0 mol∙m^-2∙d^-1. The modeled scenarios included traditional lighting control (lights are on at a constant intensity), and a dynamic lighting approach, that controls supplemental light intensity based on ambient PPF (i.e. more supplemental light is provided when PPF is lower), for various supplemental photoperiods. Supplemental lighting was most efficiently provided using a dynamic strategy to maintain a minimum PPF of 102 µmol∙m^-2∙s^-1 over a 20 h photoperiod. Providing light in this way resulted in a LUE_S of 0.276 and increased the overall LUE_D (include value) compared to sunlight alone. In the least efficient scenario, light was provided at a constant PPF of 140 µmol∙m^-2∙s^-1 for 12 h during the natural photoperiod, resulting in a lower LUE_D (0.209) than sunlight and a much lower LUE_s than the dynamic control (0.171). Our model predicts that greater energy efficiency and lower lighting costs can be achieved by extending the photoperiod and providing supplemental light at low PPFs.