Photoperiod extension in controlled environment agriculture - including the use of 24h continuous light - can be used to reduce light fixture and electricity costs in regions with lower night electricity rates. However, many plant species develop photoperiodic injury characterized by leaf chlorosis and yield reduction at a critical species-specific photoperiod threshold. Here we will discuss the response of different plant species to continuous lighting strategies. We will first challenge the conventional notion that dark adapted chlorophyll fluorescence (Fv/Fm) measurements are the most appropriate when assessing photoperiodic injury. We provide evidence that light adapted (φPSII) measurements allow for a more in-depth understanding of the light capture process at photosystem II. Through RNA-sequencing in tomato, we determined that the use of a dynamic 24h lighting treatment (i.e., red light during the day and blue light during the night) lead to normal gene expression of chlorophyll a/b binding (CAB) proteins. However when tomato plants were grown under a static continuous lighting strategy (i.e., red blue lighting for 24h) at the same daily light integral, gene expression of CAB proteins were drastically reduced, resulting in chlorosis and yield reduction. In comparison to tomatoes, cucumbers tend to be more tolerant to long photoperiods and therefore continuous lighting can have an immediate impact in commercial production. Initial results in cucumbers show that a continuous lighting strategy can decrease the lighting electricity costs by 26% and greenhouse gas emissions by 38.9% per unit of produce compared to a 16h control treatment. Using the knowledge gained throughout our studies, we propose a lighting strategy which gamifies the electricity market to further reduce costs and greenhouse gas emissions.
