2019 ASHS Annual Conference

Including far-red (FR, 700-800 nm) radiation in indoor lighting can promote seedling growth and, in at least some long-day plants, can accelerate subsequent flowering. Here we investigated how the duration and timing of FR treatments during the seedling stage influence growth and subsequent flowering of common floriculture transplants. Seedlings of dianthus (Dianthus chinensis), geranium (Pelargonium ×hortorum), petunia (Petunia ×hybrida), and snapdragon (Antirrhinum majus) were grown at 20 °C under an 18-h photoperiod with a photon flux density of 32 µmol∙m^–2∙s^–1 of blue and 128 µmol∙m^–2∙s^–1 of red radiation. The 27 d seedling stage was divided into three equal phases, and 32 µmol∙m^–2∙s^–1 of FR radiation was provided for one-third, two-thirds, or the entire seedling stage. Seedlings were then transplanted and grown in a common greenhouse environment until flowering at 20 °C with a 16-h photoperiod. Generally, plant height increased (by 20 to 86%) as the duration of exposure to FR increased, and as the timing of FR radiation was delayed. However, there was little to no effect of FR treatment on leaf number and total leaf area in any species. In snapdragon and petunia, relative leaf chlorophyll concentration decreased when FR was delivered during the first and third phases, the last two phases, or the entire seedling stage. In petunia, 9 d of FR during the second or third seedling phase also reduced relative leaf chlorophyll concentration. Compared to without FR, shoot dry weight increased when FR was delivered during the entire seedling stage for petunia by 60%, or during the last two phases for geranium by 32%. The effects of the FR treatments on subsequent flowering will be also presented.