S06 - Session P7 - The Energy Requirement for Supplemental Lighting in Greenhouse Leafy Greens Production can be Reduced by Considering `Excess´ Sunlight from the Previous Day (SUBMIT FULL-TEXT)

Authors: Theekshana Jayalath *, Marc van Iersel

The amount of sunlight greenhouse crops receive varies throughout the year and often is not enough for consistent year-round production. Therefore, supplemental lighting is used to provide adequate light. However, the electricity cost associated with greenhouse lighting can account for > 20% of the operating costs. If plants can tolerate lower daily light integrals (DLI) following a sunny day with excess light, the electricity requirement for supplemental lighting can be reduced. To determine whether excess light received one day can be 'carried-over' to the next day, we grew oakleaf lettuce ( Lactuca sativa 'Green Salad Bowl' and 'Red Salad Bowl') under six lighting regimes inside a growth chamber. Plants received an average DLI of 15 mol‧m -2 ·d -1 , but high and low DLIs alternated day-to-day in each treatment with fluctuations of 0, 5, 10, 15, 20, and 25 mol‧m -2 ·d -1 , respectively. Lettuce grown with DLI fluctuations from 0 to 15 mol‧m -2 ·d -1 had similar leaf area and dry weight, while further increases in DLI fluctuation reduced growth. To confirm this DLI "carrying-over" effect on plants grown under sunlight with supplemental light, we conducted a greenhouse study with 'Green Salad Bowl' lettuce. Plants were grown with DLI fluctuations from 0 to 14 mol‧m -2 ·d -1 , while maintaining an average DLI of 15 mol‧m -2 ·d -1 . Lettuce plants grown with DLI fluctuations from 0 to 10.5 mol‧m -2 ·d -1 had similar leaf area and dry weight, while further increases in DLI fluctuations reduced growth. Thus, carry-over of excess light from a sunny to an overcast day is possible, within limits. To quantify the energy savings resulting from 'carrying-over' excess DLI to the following day, we used historical weather data of five US locations. Annual energy savings for greenhouse lighting were about 110 MWh·ha -1 . Such reductions in greenhouse electricity requirements for supplemental lighting will improve the profitability and sustainability of the greenhouse industry.