2017 ASHS Annual Conference

Determining optimal light levels for efficient crop production is vital for controlled environment crop production. Operating lighting at sub- or supra-optimal levels can slow production or unnecessarily increase production cost. Our goal was to develop a system to automatically determine the optimal light level for different crops, based on how the electron transport rate (ETR) through photosystem II responds to increasing photosynthetic photon flux density (PPFD). To determine the optimal light level for four different species, we exposed plants to gradually increasing PPFD levels. After each increase in PPFD, ETR was estimated using chlorophyll fluorescence measurements. Optimal light levels were determined based on the increase in ETR with a 25 µmol·m^-2·s^-1 in PPFD. When the increase in ETR associated with an increase in PPFD no longer exceeded a specific threshold, the increase in PPFD was considered to be used inefficiently and the PPFD was lowered to the previous level, which was then maintained for the rest of the photoperiod. Helleborus x ballardiae 'HGC Pink Frost' settled at an average ETR of 69 µmol e^- m^-2 s^-1 at a PPFD of 265 µmol·m^-2·s^-1, Echinacea ‘Sombrero Salsa Red’ settled at an average ETR of 74 µmol e^- m^-2 s^-1 at a PPFD of 275 µmol·m^-2·s^-1 and Lactuca ‘Green Towers’ settled at an average ETR of 78 µmol e^- m^-2 s^-1 at a PPFD of 287 µmol·m^-2·s^-1. In both plants PPFD was stable during the remainder of the photoperiod, while ETR slightly decreased, because of a gradual decrease in the quantum yield of photosystem II (φPSII). Non-photochemical quenching of chlorophyll fluorescence, related to the conversion of absorbed light energy into heat, increased during the photoperiod. By utilizing this form of biofeedback control, it is possible to determine optimal light levels for crops, based on their physiological ability to use the provided light.