Red light is the portion of the visible light spectrum with wavelengths of approximately 600 to 700 nanometres (nm). It is highly efficient at driving photosynthesis because chlorophyll absorbs red light strongly. Most horticultural lighting systems therefore provide a substantial amount of red light to support plant growth.
In addition to powering photosynthesis, red light regulates plant development through the phytochrome photoreceptor system. It influences seed germination, stem elongation, leaf expansion, flowering, dormancy, and shade avoidance. Plant responses depend on the balance between red and far-red light, as well as the overall PPFD and Daily Light Integral (DLI), rather than the amount of red light alone.
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