Traditionally cultivated in China, primrose (Primula sp.) is popular for its early spring blooming in December through April. Although some species have medicinal uses, most of them are being bred and cultivated as ornamental plants, including common primrose (Primula vulgaris), chinese primrose (Primula sinensis), poisonous primrose (Primula obconica), and baby primrose (Primula forbesii) (Šarić-Kundalić et al., 2010). Baby primrose is a biennial temperate wildflower commonly found in Yunnan, Sichuan, and Tibet in China. Seeds of baby primrose were originally collected from the wild habitat of Kunming and were selectively bred and cultivated to obtain new cultivars with desirable flower traits at the end of 20th century (Jia et al., 2010b). With its long flowering season, proliferation of showy blooms, and pleasant fragrance, baby primrose is valuable as both a container plant and landscape plant (Jia et al., 2014). However, greenhouse-cultivated baby primrose often develop long flower peduncles that can result in lodging and decreased commercial marketability. Therefore, controlling the height of baby primrose in greenhouse environments is critical to its potential commercial success.
Plant growth regulators (PGRs) are synthetic chemical compounds or extractions of natural plant hormones that regulate plant growth and development. They are often used to influence plant cell division, rooting, germination, branching, flowering, and abscission at lower concentrations (Rademacher, 2015). Applications of PGRs are common in the production of bedding plants to inhibit stem elongation to meet the specific plant target heights. The effectiveness of PGR applications depends on several factors, including the active ingredient, application volume and concentration, time and method of application, crop species, and environmental conditions (Runkle, 2015, 2017). They can be applied by a substrate drench, a foliar spray, or a combination of both (Barrett, 1999; Blanchard and Runkle, 2007). Many PGRs are known to block the multistep biosynthesis pathways of gibberellic acid (GA). Depending on the compound, some act during earlier steps of the GA biosynthesis pathways, such as chlormequat chloride (CCC), whereas others, such as daminozide, act later (Runkle, 2017).
Plant growth regulators have been tested on a wide array of plants, including herbaceous perennials and agronomic crops (Hamaker et al., 1996; Shekoofa and Emam, 2008; Whipker, 2015). Chlormequat chloride and uniconazole (UNI) have been successfully used in floriculture as a drench or foliar spray (Rademacher, 1995). Chlormequat chloride was first found to inhibit stem elongation, increase stem thickness, and enhance leaf greenness of poinsettia (Euphorbia pulcherrima) and chrysanthemum (Chrysanthemum ×morifolium) by Lindstrom and Tolbert (1960), and it is now commonly used for height and flowering control in greenhouse or container production operations (Runkle, 2014). Chlormequat chloride is easily absorbed by plant leaves and roots, metabolized rapidly inside plants, and works efficiently in drench and foliar spray, with a common rate of 750 to 1500 ppm (Runkle, 2014). For example, poinsettia height was 8% to 23% shorter when 1500 ppm CCC was applied after pinching compared with poinsettia without CCC application (Karunananda and Peiris, 2010). Similarly, CCC suppressed the height of scarlet rosemallow (Hibiscus coccineus), monarch rosemallow (Hibiscus radiatus), and venice mallow (Hibiscus trionum) by 87%, 42%, and 52%, respectively, 28 d after the application of 2000 ppm compared with untreated plants (Warner and Erwin, 2003). However, phytotoxicity, including leaf yellow spotting, discoloration, and necrosis, may occur when CCC spray was more than 1500 ppm (Whipker et al., 2001). A combined application of CCC and daminozideis is often used to provide greater height control at a lower application rate and reduce the potential for phytotoxicity (Whipker, 2015). For example, Lewis et al. (2004) reported chlorotic spots on ‘Winter Rose Dark Red’ poinsettia leaves 2 d after 1500 ppm CCC spray application alone, but a mixture with daminozide at varying rate from 1500 to 4500 ppm with the same rate of CCC did not cause phytotoxicity but did reduce poinsettia sensitivity to CCC phytotoxicity.
Uniconazole is the most active, persistent, and effective PGR on a parts per million basis (Runkle, 2011). Unlike CCC, it is primarily absorbed by plant stems; therefore, spraying is the most common application method, but it can also be successfully applied as substrate drench and liner soak (Runkle, 2011; Whipker, 2015). Dependent on the species, single UNI sprays at 1 to 2, 4 to 6, and 10 to 15 ppm have been recommended, respectively, for bedding plants with moderate vigor such as celosia (Celosia argentea), more aggressive crops such as petunia (Petunia ×hybrida) and marigold (Tagetes erecta), and aggressive herbaceous perennials such as phlox (Phlox paniculata) and echinacea (Echinacea purpurea) (Runkle, 2011). The height of chrysanthemum was suppressed by 53%, 43%, and 17%, respectively, when 30 ppm of UNI was applied 0, 2, and 4 weeks after pinching (Gilbertz, 1992). In addition to CCC and UNI, several other PGRs, including daminozide, ancymidol, flurprimidol, paclobutrazol, ethephon, and benzyladenine, have also been registered and successfully used in the production of ornamental crops (Latimer and Whipker, 2019; Whipker, 2015).
At application rates of 500 ppm or more, CCC failed to achieve optimal height control of container-produced baby primrose (Jia et al., 2010a). Therefore, additional research examining baby primrose height in response to lower rates of CCC is warranted. Moreover, to find a more effective way to control baby primrose plant height, UNI application was also evaluated. In addition, little information is available regarding the efficacy of CCC and UNI application on baby primrose. To directly address these needs, the specific objectives were to 1) investigate the effects and efficacy of CCC and UNI as foliar spray on plant height control and flowering of baby primrose, and 2) investigate the cytological basis of PGRs for baby primrose height control. Results of this study will provide critical information and guidance regarding PGR application for height control of greenhouse-produced baby primrose.
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