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Supplemental lighting can improve the growth of greenhouse crops, but the electricity required for supplemental lighting can be a significant expense for greenhouse growers. Lighting control strategies that use the dimmability of light-emitting diodes (LEDs) have the potential to decrease this cost. In our experiments, we tested the hypothesis that providing ‘Little Gem’ lettuce ( Lactuca sativa ) plants with the same daily amount of light, spread out over a longer photoperiod and at lower average photosynthetic photon flux densities ( PPFD s), would improve growth because light is used more efficiently to drive photosynthesis at lower  PPFD s. We conducted two greenhouse experiments wherein supplemental light was provided to reach a minimum daily light integral (DLI) of 17 mol·m −2 ·d −1  with a 12, 15, 18, or 21-hour photoperiod using adaptive lighting control of LED lights. As the photoperiod for supplemental lighting was increased and supplemental light was provided at lower average  PPFD s, plant dry weight increased. Conversion efficiency, the estimated increase in dry weight per Joule expended on supplemental lighting, increased as the photoperiod was extended from 12 to 21 hours. Leaf size and chlorophyll content index increased with longer photoperiods. The number of plants with symptoms of tipburn, including apical and marginal necrosis, also increased as the photoperiod was extended. These results demonstrate that adaptive lighting control can be used to increase the growth of ‘Little Gem’ lettuce and the energy use efficiency of supplemental lighting by providing supplemental light at relatively low  PPFD s.

Longer Photoperiods with Adaptive Lighting Control Can Improve Growth of Greenhouse-grown ‘Little Gem’ Lettuce (Lactuca sativa)