The Energy Requirement for Supplemental Greenhouse Lighting Can Be Reduced by Considering 'Excess' Light from the Previous Day

The sunlight greenhouse crops receive varies and is often insufficient for consistent year-round growth in greenhouses. Supplemental lighting is commonly applied in winter, but this practice has a significant energy cost, accounting for 10-30% of operating expenses and impacting greenhouse profitability. Greenhouse lights are traditionally adjusted based on sunlight intensity to meet crops' daily light requirements. However, if plants can withstand lower daily light integrals (DLI) after a sunny day without reducing the growth, there is potential to reduce the energy required for supplemental lighting and increase the profit. To determine whether excess light received one day can be 'carried over' to the next, we grew oakleaf lettuce (Lactuca sativa 'Green Salad Bowl' and 'Red Salad Bowl') under six lighting regimes inside a vertical farm. Plants in all treatments received an average DLI of 15 mol.m(-2).d(-1), but DLIs alternated from day-to-day (15/15, 17.5/12.5, 20/10, 22.5/7.5, 25/5, and 27.5/2.5 mol.m(-2).d(-1)), resulting in DLI fluctuations from 0 to 25 mol.m(-2).d(-1). Plants had similar leaf area (similar to 800 cm(2)/plant) and dry weight (similar to 1.8 g/plant) when grown with DLI fluctuations from 0 to 15 mol.m(-2).d(-1), while higher DLI fluctuation reduced growth. To confirm this DLI "carrying-over" effect on plants grown under sunlight with supplemental light, we conducted a second study in a greenhouse with 'Green Salad Bowl' lettuce. In this study, plants were grown with five different DLI fluctuations (15/15, 16.75/13.25, 18.5/11.5, 20.25/9.75, and 22/8 mol.m(-2).d(-1)), ranging from 0 to 14 mol.m(-2).d(-1), while maintaining an average DLI of 15 mol.m(-2).d(-1) in all the treatments. We observed similar leaf area (similar to 750 cm(2)/plant) and dry weight (similar to 1.8 g/plant) in lettuce plants grown with DLI fluctuations from 0 to 10.5 mol.m(-2).d(-1). Higher DLI fluctuations reduced growth. Hence, carrying excess light from a sunny to an overcast day is possible within limits. Our study concluded that the DLI requirement can be reduced by approximately 5.25 mol.m(-2).d(-1) on the day following a sunny day. By analyzing historical weather data from five US locations, we quantified the potential annual energy savings from incorporating this 'carrying-over DLI' concept. This approach resulted in annual energy savings of approximately 75-190 MWh/ha in greenhouse lettuce production. Such reductions in supplemental lighting energy will enhance the profitability and sustainability of the greenhouse industry.