2018 ASHS Annual Conference

The annual energy cost for horticultural lighting in the US is approximately $600 million. To lower these costs, it is essential to provide light in a way that allows for efficient photochemistry. Because the quantum yield of photosystem II (ΦPSII), the fraction of absorbed light used for photochemistry, decreases with increasing photosynthetic photon flux density (PPFD), we hypothesized that electron transport through photosystem II integrated over 24 hr, the daily photochemical integral (DPI), increases if the same amount of light (daily light integral, DLI) is spread out over longer photoperiods. To test this, we measured chlorophyll fluorescence to determine ФPSII and the electron transport rate (ETR) of lettuce (Lactuca sativa 'Green Towers'). Plants were grown at a PPFD of ~ 250 μmol·m^–2·s^–1. Chlorophyll fluorescence measurements were taken in a growth chamber equipped with LED lights. A datalogger controlled PPFD and photoperiod and collected ΦPSII and ETR data. DLIs of 15 and 20 mol·m^–2·d^–1, provided over photoperiods of 7, 10, 13, 16, 19, and 22 hours, were tested. PPFD during these measurements ranged from 189 to 796 μmol·m^–2·s^–1. ΦPSII decreased from ~ 0.69 at a PPFD of 189 μmol·m^–2·s^–1 to ~ 0.29 at a PPFD of 796 μmol·m^–2·s^–1, while ETR increased from ~54 to 100 μmol·m^–2·s^–1. DPI increased as a function of photoperiod and this increase was more pronounced at high DLI. At a photoperiod of 7 hours DPI was ~ 2.5 mol·m^–2·d^–1, regardless of DLI. However, with a photoperiod of 22 hr and a DLI of 15 mol·m^–2·d^–1, the DPI was ~ 4.2 mol·m^–2·d^–1 (68% higher than with a photoperiod of 7 hr), and with a DLI of 20 mol·m^–2·d^–1 the DPI was ~ 5.5 mol·m^–2·d^–1 (120% higher). Our results show that DPI is significantly higher with lower PPFD over a longer photoperiod than with higher PPFD over a shorter photoperiod, because the light is used more efficiently at low PPFD. Subsequent longer-term growth trials have shown that longer photoperiods with the same DLI do increase crop growth. These short-term physiological trials, combined with results from longer-term growth trials, indicate that applying supplemental light out over longer photoperiods results in more energy-efficient stimulation of crop growth. This research should encourage growers who use photosynthetic lighting to re-evaluate their current lighting protocols and consider using longer photoperiods.