2018 ASHS Annual Conference

In the study, we quantified the effect of photoperiod while maintaining equal DLI in two experiments. First in growth chambers we grew red-leaf lettuce, Lactuca sativa ‘Red Sails’ under five photoperiods. We found biomass increased linearly with longer photoperiods, with 24-h plants averaging 82% increase in dry biomass in comparison to the 12-h treatment plants. Second, in a greenhouse we grew red-leaf lettuce, Lactuca sativa ‘Skyphos’, and found biomass increased in the 21-h treatment by an average of 49% in comparison to the 12-h treatment. This report demonstrates that greater production of lettuce can be achieved through precise control of DLI and photoperiod in the greenhouse.

The benefits of extending photoperiod while maintaining daily light integral (DLI) have been demonstrated in the past to increase profitable production. Many significant horticultural crops, like lettuce, require a specific DLI that should be obtained, but not surpassed. To achieve lighting goals, a control algorithm using supplemental lighting must adjust light intensity to achieve the optimum photoperiod, without exceeding DLI, based on the variable daily sunlight conditions. Previously, due to the lighting technology at the time, lighting control algorithms could not utilize dimmability to achieve photoperiod and DLI simultaneously, therefore allowing for the most electrically efficient use of lighting. However, with the introduction of LEDs into horticultural environments, significant gains in the ability to control artificial light through rapid and instantaneous dimmability is now possible. Findings from this project have directed current research using a control algorithm to integrate the information from an ambient sunlight sensor with supplemental LED lighting in the greenhouse to maintain photoperiod while achieving the consistent DLI to optimize crop production utilizing supplemental LED lighting in greenhouses.