Light-emitting diodes (LEDs) have the potential to replace high-pressure sodium (HPS) lamps to provide supplemental lighting (SL) in greenhouses and consume less energy. Different wavebands of light can be delivered by LEDs to potentially regulate plant morphology by stimulating photoreceptors. However, we postulated that differences in the light quality of SL would be diminished given the broad spectral distribution of sunlight plants receive in greenhouses. Two series of experiments were performed with seedlings of tomato, pepper, geranium, petunia, and snapdragon. Plants were grown at 20 °C under five SL treatments (four from LEDs and one from HPS lamps) at a PPFD of 90 μmol∙m-2∙s-1 or at 10 μmol∙m-2∙s-1 from HPS lamps, all for 16 h·d-1. Regardless of light quality, growth was greater under the higher SL intensity. There was little or no effect of the percentage of blue (400-500 nm) light (from 10 to 45%) on leaf area, leaf number, plant height, and dry shoot and root weight when seedlings were grown under the same SL intensity. When SL was applied during the seedling phase alone, there was no effect of SL quality on subsequent flowering, but when SL treatments were continued after transplant, geranium under 45% blue + 55% red (600-700 nm) was shorter at flowering than those grown under HPS lamps. Including far-red (700-800 nm) radiation in SL showed some promotion of flowering but also extension growth. The LEDs used in this study were more energy efficient than HPS lamps while seedling and flowering plants were of similar quality. Thus, while LED SL did not elicit specific morphological effects, the LEDs used in these experiments are suitable and more energy-efficient replacements for HPS lamps.
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