Comparing Flowering Responses of Long-day Plants under Incandescent and Two Commercial Light-emitting Diode Lamps

Abstract

<jats:p>When the natural daylength is short, commercial growers of ornamental long-day plants (LDP) often provide low-intensity lighting to more rapidly and uniformly induce flowering. Incandescent (INC) lamps have been traditionally used for photoperiodic lighting because their spectrum, rich in red [R (600 to 700 nm)] and far-red [FR (700 to 800 nm)] light, is effective and they are inexpensive to purchase and install. Light-emitting diodes (LEDs) are much more energy efficient, can emit wavelengths of light that specifically regulate flowering, and last at least 20 times longer. We investigated the efficacy of two new commercial LED products developed for flowering applications on the LDP ageratum (<jats:italic>Ageratum houstonianum</jats:italic>), calibrachoa (<jats:italic>Calibrachoa</jats:italic> ×<jats:italic>hybrida</jats:italic>), two cultivars of dianthus (<jats:italic>Dianthus chinensis</jats:italic>), and two cultivars of petunia (<jats:italic>Petunia</jats:italic> ×<jats:italic>hybrida</jats:italic>). Plants were grown under a 9-hour short day without or with a 4-hour night interruption (NI) delivered by one of three lamp types: INC lamps (R:FR = 0.59), LED lamps with R and white (W) diodes [R + W (R:FR = 53.35)], and LED lamps with R, W, and FR diodes [R + W + FR (R:FR = 0.67)]. The experiment was performed twice, both at a constant 20 °C, but the photosynthetic daily light integral (DLI) during the second replicate (Rep. II) was twice that in the first (Rep. I). In all crops and in both experimental replicates, time to flower, flower or inflorescence and node number, and plant height were similar under the R + W + FR LEDs and the INC lamps. However, in Rep. I, both petunia cultivars developed more nodes and flowering was delayed under the R + W LEDs compared with the INC or R + W + FR LEDs. In Rep. II, petunia flowering time and node number were similar under the three NI treatments. Plant height of both dianthus cultivars was generally shorter under the NI treatment without FR light (R + W LEDs). These results indicate that when the DLI is low (e.g., ≤6 mol·m<jats:sup>−2</jats:sup>·d<jats:sup>−1</jats:sup>), FR light is required in NI lighting for the most rapid flowering of some but not all LDP; under a higher DLI, the flowering response to FR light in NI lighting is apparently diminished.</jats:p>

Academic Research Perspective of LEDs for the Horticulture Industry

“…Throw in a little green light along with red and blue, and plant species with some mutual leaf shading grow even better (Kim et al, 2004;Lu et al, 2012). Then add some far red light and plants grow taller (Brown et al, 1995;Chia and Kubota, 2010); and some photoperiodic classes of plants flower better when far red light is present during night-break lighting (Deitzer et al, 1979;Downs, 1956;Kohyama et al, 2014). Some species develop physiological disorders (e.g., gall-like tumors on their leaves and shoot tips) under narrow spectrum and/or dim light (Massa et al, 2006;Morrow and Wheeler, 1997).…”

“…If it is just to increase DLI, then it may not matter whether the red : blue ratio or electrical efficiency is optimized in order for an LED product to be effective. But, if the goal is to make sure that hypocotyls elongate sufficiently for seedlings to be used as rootstocks for grafted transplant production (Chia and Kubota, 2010), for certain cultivars to flower on a specific schedule (Craig and Runkle, 2013;Kohyama et al, 2014), or to not inhibit leaf expansion of a leafy-green crop (Cope and Bugbee, 2013;Dougher and Bugbee, 2001;Hoenecke et al, 1992), then precise spectral mixes may be very important. Research with flexible LED technology ultimately will determine what LED hardware requirements must be.…”

Low red: Far-red light ratio causes faster in vitro flowering in lentil

“…Newer options include LED light sources, which have the highest energy efficiency and life span, but require costly infrastructure (Kohyama et al 2014). Here, using LEDs with an R:FR = 3.09 resulted in floral induction in the two lentil cultivars considered in 33 d. The other option considered was T5 bulbs, which are highly efficient with respect to PPF and have minimal heat release with a slight variation in their spectrums.…”

Regulation of flowering by green light depends on its photon flux density and involves cryptochromes

“…Plants were subject to 0 (short days without day‐extension lighting), 2, 13 or 25 μmol m −2 s −1 of green light (peak = 521 nm; 15 W, green LED light bulb, model 03419; Westinghouse, Cranberry Township, PA) or 2 μmol m −2 s −1 of R + white (W) + FR light (14 W, GreenPower LED flowering DR/W/FR; Philips, Eindhoven, the Netherlands). The R + W + FR LEDs were used as a long‐day control because they were as effective as incandescent lamps at regulating flowering of a wide range of ornamental crops (Kohyama et al , Meng and Runkle ). We achieved desired photon flux densities and light distributions by applying layers of aluminum mesh and adjusting the LED layout and the distance between the LEDs and the plant canopy.…”