Regulation of extension growth and flowering of seedlings by blue radiation and the red to far-red ratio of sole-source lighting

Abstract

Manipulating the radiation spectrum of sole-source lighting (SSL) with light-emitting diodes (LEDs) enables the regulation of growth, development, and quality attributes of ornamental transplants. We quantified growth responses and subsequent flowering of annual bedding plant seedlings grown at 20 °C under eight SSL combinations of blue (B; 400−500 nm), red (R; 600−700 nm), and far-red (FR; 700−800 nm) radiation (subscript values indicate the photon flux density of each waveband, in μmol m–2 s–1): B20R160, B20R160FR10, B20R160FR20, B20R160FR40, B60R120, B60R120FR10, B60R120FR20, and B60R120FR40. Seedlings were also grown indoors under warm-white (WW180) LEDs or in a greenhouse for comparison. Among all nine species tested, the addition of FR at 40 μmol m–2 s–1 increased the seedling height of only snapdragon (Antirrhinum majus, by 64–134%) and zinnia (Zinnia elegans, by 52–96%), regardless of the proportion of B, compared with SSL treatments without FR or in the greenhouse. Similarly, WW180 promoted seedling stem elongation in snapdragon (by 75–139%) compared with SSL without FR or the greenhouse control, but not in the other species. Relative chlorophyll content of zinnia and petunia (Petunia × hybrida) generally decreased as the photon flux density of FR increased, and in both species was 13–14% greater under B at 60 μmol m–2 s–1 than at 20 μmol m–2 s–1. Radiation treatments did not influence leaf area and dry shoot weight in any species. The subsequent flowering of snapdragon was accelerated by 7–11 d with the additional 20 or 40 μmol m–2 s–1 of FR, or under WW180, compared with SSL without FR or the greenhouse control. We conclude that when ≥20 μmol m–2 s–1 of B is delivered to crops, the addition of FR at ≥20 μmol m–2 s–1 can accelerate flowering of at least some long-day plants, with little to no effect on extension growth, but has no effect on flowering of day-neutral or short-day plants.