Evaluating Light-emitting Diodes for Use in Supplemental Greenhouse Lighting for Seedling Plug Production

In temperate climates, seedlings are often grown in commercial greenhouses under supplemental lighting (SL) during the winter to increase plant quality. To increase the photosynthetic daily light integral, SL is typically delivered by high-pressure sodium (HPS) lamps. Light-emitting diodes (LEDs) have potential to be used in SL applications because of their increasing energy efficiency and ability to emit specific wavebands of light, which can increase photosynthesis and regulate plant morphology. We evaluated seedling growth under four fixture types that included red (R, peak=660 nm), blue (B, peak=453 nm), and/or white (W, peak=560 nm) LEDs or HPS lamps, all of which delivered a PPF of 90 μmol·m-2·s-1 (high SL); a sixth treatment delivered 10 μmol·m-2·s-1 from HPS lamps (low SL). Seedlings of seven ornamental and vegetable varieties were grown at 20 °C under ambient solar light and SL treatments under a 16-h photoperiod and for 21 to 40 days. At transplant, seedling height, leaf area, leaf number, and fresh root and shoot weight were measured and then plants were transplanted and grown in a common environment under a 16-h photoperiod until flowering. Preliminary responses were variable among varieties studied and characteristics measured, and no consistent treatment effects were apparent. However, plants under low SL were usually shorter, and smaller leaves, and had lower fresh shoot weight than the five high SL treatments. Seedlings under the LED treatments were usually at least as tall as those under HPS lamps. In four varieties, shoot weight was greater under some of the LED treatments than the high HPS treatment while the opposite was true for two varieties. Our initial conclusion is that the spectrum of SL has inconsistent effects on plant growth attributes and that both SL types are effective at producing high-quality seedlings.