2017 ASHS Annual Conference

Manipulating light quality is a potential alternative method of regulating plant height in the commercial production of ornamental crops. The red (R, 600-700 nm) to far red (FR, 700–800 nm) ratio (R:FR) of end-of-day lighting can regulate extension growth and development. Poinsettia, which is one of the most economically important floriculture crops, is sometimes shorter than market height specifications. We investigated the impacts of the R-FR and duration of day-extension lighting in controlling the extension growth and flowering of two poinsettia cultivars, ‘White Glitter’ and ‘Marblestar’. Plants were grown at 20 °C under 9-h short-days with or without end-of-day lighting- provided by two different types of light-emitting diode (LED) bulbs: R+white+FR (subsequently referred to as R+FR) and FR only. The R:FR ratios were 0.73 and 0.04 respectively, and the photon flux density between 400 to 800 nm was adjusted to 2-3 μmol·m^–2·s^–1 at plant canopy. The six end-of-day lighting treatments were 2 h of R+FR, 2 h of FR, 2 h of R+FR followed by 2 h of FR, 4 h of R+FR, 4 h of FR, and 4 h of R+FR followed by 2 h of FR. Compared with the 9-h short-day control, all lighting treatments significantly promoted plant height (from 47% to 195%), with the greatest promotion under the longest lighting periods. There were no differences in days to first bract color and days to anthesis when the 9-h day was extended by 2 h, but anthesis was delayed under all 4- or 6-h lighting treatments, especially in the R+FR treatments. Low-intensity FR significantly promoted extension growth for ‘White Glitter’ when delivered alone for 2 h and delayed anthesis for both cultivars when delivered alone for 4 h. We conclude that end-of-day lighting promotes extension growth of poinsettia and that low-intensity FR is not as effective as R+FR light in regulating flowering. Further research is needed to determine whether a higher-intensity of FR has greater impact on regulating flowering and extension growth.