Use of far-red radiation in sole-source lighting for floriculture crops

Most indoor sole-source lighting from light-emitting diodes (LEDs) is aimed to deliver photosynthetically active radiation (400-700 nm) for efficient plant growth. Commercial fixtures usually contain a combination of blue (B, 400-500 nm) and red (600-700 nm) LEDs, or white LEDs alone, which emit a low photon flux density of far-red (FR, 700-800 nm) radiation. However, FR radiation controls a range of plant growth and developmental processes, including stem elongation, leaf expansion, branching, and flowering and thus, can induce desired plant responses. We have investigated how FR radiation, with different spectral and photosynthetic photon flux density (PPFD) combinations, in sole-source lighting regulates plant growth and quality attributes in a wide range of floriculture crops. During the seedling stage, increasing FR radiation from 6 to 50% of the total photon flux density (or decreasing R:FR from 8:1 to 1:1) promoted extension growth of leaves and stems and subsequent dry mass accumulation, regardless of PPFD. In some long-day species, providing at least 16 µmol m^‑2 s^‑1 of FR (or R:FR=8:1) to seedlings accelerated flowering after transplant, especially under a low PPFD. In addition, 80 µmol m^‑2 s^‑1 of B radiation diminished the effects of FR radiation on stem elongation but had little effect on FR-mediated leaf expansion and subsequent flowering promotion. The results of our research indicate that inclusion of FR in sole-source lighting, especially with a moderately high photon flux density of B radiation, can efficiently increase seedling growth, elicit a desirable growth habit, and promote flowering of at least some long-day species.