Opportunities Using Light Emitting Diodes to Produce High Quality Crops

Light emitting diodes (LEDs) are considered an alternative to light sources currently used for year round high quality greenhouse crop production. The opportunity to customize the spectrum may be an advantage although many questions remain in respect to the most appropriate configuration and wavelength composition to best support crops for desired growth and development. LED grow lights are often designed with two monochromatic peaks centered on blue (~ 450 nm) and red (630-660 nm) wavelengths. The higher rate of photosynthesis observed in the blue and red spectral regions, is the primary reason for developing these types of LED grow lights. Although the carbon assimilation rate may be high, morphology and flowering responses are also affected resulting in potential undesired plant qualities compared to natural or continuous spectrum light sources. The addition of orange (600 nm), orange-red (630 nm) and white LEDs has been suggested to improve and balance the overall spectrum for crop growth. Studies are needed to identify crop production applications where LEDs of limited as well as more balanced spectral energy distribution offer advantages. For instance, poinsettias exposed to panels of red LEDs (peak emission at 665 nm) supplemented with 20 percent blue LEDs (peak emission at 456 nm) for a limited time period prior to short days resulted in overall shorter plants. Rooted poinsettia cuttings were transplanted to 12.5 cm diameter containers, the plants were pinched 28 days later and grown under LEDs for two weeks. Photosynthetic photon flux (400 to 700 nm) at plant height was approximately 200 μmol·m^-2s^-1 from the LEDs. Plants were returned to the greenhouse and short days (8:00 AM to 5:00 PM) initiated 10 days later. The conditions in the greenhouse were natural light supplemented with high-pressure sodium irradiance. For the three cultivars ‘Freedom Red’, ‘Freedom White’ and ‘Freedom Jingle Bell’, overall plant height was reduced with 3 to 5 cm at the marketing stage. Freedom White and Jingle Bell produced one additional flowering shoot following initial exposure to LEDs. Additional studies are underway to evaluate if pre-conditioning of propagation material with a monochromatic LED spectral distribution may improve final branching, flower formation and yield of greenhouse and field grown crops.