Longer Photoperiods with the Same Daily Light Integral Increase Daily Electron Transport through Photosystem II in Lettuce

Controlled environment crop production recommendations often use the daily light integral (DLI) to quantify the light requirements of specific crops. Sole-source electric lighting, used in plant factories, and supplemental electric lighting, used in greenhouses, may be required to attain a specific DLI. Electric lighting is wasteful if not provided in a way that promotes efficient photochemistry. The quantum yield of photosystem II (Φ_PSII), the fraction of absorbed light used for photochemistry, decreases with increasing photosynthetic photon flux density (PPFD). Thus, we hypothesized that the daily photochemical integral (DPI), the total electron transport through photosystem II (PSII) integrated over 24 h, would increase if the same DLI was provided at a lower PPFD over a longer photoperiod. To test this, Φ_PSII and the electron transport rate (ETR) of lettuce (Lactuca sativa 'Green Towers') were measured in a growth chamber at DLIs of 15 and 20 mol m^-2 d^-1 over photoperiods ranging from 7 to 22 h. This resulted in PPFDs of 189 to 794 μmol m^-2 s^-1. The Φ_PSII decreased from 0.67 to 0.28 and ETR increased from 55 to 99 μmol m^-2 s^-1 as PPFD increased from 189 to 794 μmol m^-2 s^-1. The DPI increased linearly as the photoperiod increased, but the magnitude of this response depended on DLI. With a 7-h photoperiod, the DPI was ≈2.7 mol m^-2 d^-1, regardless of DLI. However, with a 22-h photoperiod, the DPI was 4.54 mol m^-2 d^-1 with a DLI of 15 mol m^-2 d^-1 and 5.78 mol m^-2 d^-1 with a DLI of 20 mol m^-2 d^-1. Our hypothesis that DPI can be increased by providing the same DLI over longer photoperiods was confirmed.