Plant light use efficiency decreases as light intensity is increased, and a better understanding of crop-specific light responses can contribute to the development of more energy-efficient supplemental lighting control strategies for greenhouses. In this study, diurnal chlorophyll fluorescence monitoring was used to characterize the photochemical responses of ‘Green Towers’ lettuce ( Lactuca sativa L.) to photosynthetic photon flux density ( PPFD ) and daily light integral (DLI) in a greenhouse during a production cycle. Plants were monitored continuously for 35 days, with chlorophyll fluorescence measurements collected once every 15 minutes. Quantum yield of photosystem II (ΦPSII) decreased exponentially with PPFD , whereas electron transport rate (ETR) increased asymptotically to 121 µmol·m –2 ·s –1 . Daily photochemical integral (DPI) is defined as the integral of ETR over a 24-hour period; DPI increased asymptotically to 3.29 mol·m –2 ·d –1 with increasing DLI. No effects of plant age or prior day’s DLI and a negligible effect of PPFD s 15 or 30 minutes before measurements within days were observed. Simulations were conducted using the regression equation of ETR as a function of PPFD {ETR = 121[1 – exp(–0.00277 PPFD )]} to illustrate methods of increasing photochemical light use efficiency for improved supplemental lighting control strategies. For a given DLI, DPI can be increased by providing light at lower PPFD s for a longer period of time, and can be maximized by providing light with a uniform PPFD throughout the entire photoperiod. Similarly, the DLI required to achieve a given DPI is reduced using these same methods.
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Weaver et al. (2019) studied this question.