Light Emitting Diodes for Greenhouse Crops

Light emitting diodes (LEDs) show promise to complement and replace current supplemental lighting equipment for greenhouse production. LEDs are characterized to use less electrical energy, offer opportunities to customize the spectral energy distribution, and have extended operational high output time frames with limited heat generation and release to the growing environment. Sizeable LED panel configurations are now available and suitable for crop production. Panels developed to support crop growth often provide light in the blue and red portions of the spectrum in support of photosynthesis. Most physiological processes in addition to photosynthesis are however, affected by the overall spectrum of the incoming light. Research is needed on proper LED wavelength compositions, configurations and photosynthetic photon flux (PPF) to most effectively support growth and proper crop development. Panels (300 W) with red LEDs (peak emission at 665 nm) supplemented with 10 percent blue LEDs (peak emission at 456 nm), tri-band LEDs with 40 percent red (660 nm), 40 percent orange-red (630 nm) and 20 percent blue (460 nm), and white LEDs, were tested in a controlled environment production area. Blackeyed Susan (Rudbeckia hirta 'Toto') was grown in 10 cm containers underneath the LED panels and as a comparison under high-pressure sodium-, metal halide- and 5T fluorescent lamps. The air temperature was maintained at constant 20 ± 2°C. PPF (400 to 700 nm) at plant height was approximately 150 μmol·m^-2s^-1 for all lamp types. An LI-6400 photosynthesis system (LI-COR, Lincoln, NE) with a clear top leaf chamber was used to measure net photosynthetic rate (Pn) of plants developed to the stage immediately prior to open flower. Measurements were made on exposed single leaves at 400 ppm CO₂. Pn was in the range of 3.5 to 4.0 μmol CO₂·m^-2s^-1 independent of lamp type suggesting PPF to be more important for the rate of photosynthesis than the spectral energy distribution.