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Mengzi Zhang and Erik S. Runkle

Manipulating light quality is a potential alternative method of regulating plant height in the commercial production of ornamental crops. In particular, end-of-day (EOD) lighting with a high red (R; 600–700 nm) to far-red (FR; 700–800 nm) ratio (R:FR) can suppress extension growth, whereas a low R:FR can promote it. We investigated the effects of the R:FR and duration of EOD lighting in regulating extension growth and flowering of two poinsettia cultivars, White Glitter and Marble Star. Plants were grown at 20 °C under 9-hour days with or without EOD lighting provided by two types of light-emitting diode 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 and 800 nm was adjusted to 2 to 3 μmol·m−2·s–1 at plant canopy. The six EOD lighting treatments were R+FR or FR for 2 or 4 hours, 2 hours of R+FR followed by 2 hours of FR, and 4 hours of R+FR followed by 2 hours of FR. We also investigated the impact of a 4-hour moderate-intensity (13 μmol·m−2·s–1) EOD FR treatment in the second replication. EOD lighting generally increased poinsettia extension growth, 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-hour day was extended by 2 hours, but flowering was delayed under 4- or 6-hour EOD treatments, except for the 2-hour R+FR + 2-hour FR and 4-hour FR treatments. Four hours of moderate-intensity EOD FR greatly promoted extension growth and delayed or prevented bract coloration in both cultivars. We conclude that EOD lighting promotes extension growth of poinsettia, and specifically, EOD FR at a low intensity (2–3 μmol·m−2·s–1) is not perceived as long-day signal, whereas a higher intensity (13 μmol·m−2·s–1) of FR delays flowering.

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Mengzi Zhang, Jie Yang, Huitang Pan, and Brian J. Pearson

Baby primrose (Primula forbesii) is a newly cultivated and valuable ornamental plant with great market potential for both indoor and landscape use. As a container plant, baby primrose has long, weak flower stalks that can easily lodge, resulting in poor-quality plants, especially during transportation. To control plant height and subsequently prevent flower peduncle lodging, we investigated the effects of two plant growth regulators (PGRs), chlormequat chloride (CCC) at 0, 250, 500, or 750 ppm and uniconazole (UNI) at 25, 50, or 75 ppm on growth, development, and flowering of two cultivars of baby primrose, Fragrant Luolan and Red Star. Plant growth regulators at the proposed concentrations were applied twice throughout the experiment. Both PGRs significantly suppressed plant height in both cultivars, with a 16% to 27% reduction by CCC and 50% to 59% by UNI compared with untreated plants. Among CCC-treated groups, plants were shortest when CCC was applied at 500 ppm; plant height was suppressed more when treated with UNI. In both cultivars, UNI significantly suppressed the first, second, and third peduncle lengths. Furthermore, CCC affected peduncle length, but to a lesser extent than UNI. Plant growth regulator applications generally had little effect on flower characteristics of baby primrose. Neither PGRs influenced the inflorescence number and flower size; however, PGRs did increase the number of floral whorls and suppressed pedicel length of ‘Red Star’. New leaf growth was suppressed by both PGRs. In addition, peduncle cell length and width were both significantly suppressed by PGR applications. We concluded that two foliar applications of UNI at 25 ppm comprised the most effective method of controlling baby primrose plant height while maintaining desirable flower traits at a relatively low production cost. Results of this study provide guidance for techniques that can be used to effectively control the plant height of potted baby primrose for commercial greenhouse production.