To meet the challenges of producing plants for sale, growers frequently use plant growth regulators (PGRs) to control plant height, branching, and flowering. BA is a PGR that increases branching in floriculture crops when sprayed on containerized plants (Carey et al., 2009; Farris et al., 2009; Martin and Singletary, 1999). The addition of BA, a synthetic cytokinin, increases the ratio of cytokinin to auxin in the plant, disrupting apical dominance (Cline, 1991). This results in more basal and/or lateral branching and fuller plants. The effects of BA vary with cultivar: of 10 Hosta Tratt (Funkia K. Spreng; Niobe Salisb.) cultivars treated with 1250 to 3750 mg·L−1 foliar sprays of BA, five cultivars had an increased number of offsets (Garner et al., 1997).
Chemical PGRs are often used during plug and rooted cutting (liner) production but must be used with care for efficacy and to prevent unwanted effects such as phytotoxicity or delayed flowering. These PGR applications are typically made ≈2 to 4 weeks after liners have been transplanted, when plants are established and in active growth (Albrecht and Tayama, 1992). However, we are interested in testing the use of BA at an earlier stage of plant production on liners before transplant. Because liners are still in smaller cells, they are easier to treat with PGRs than plants that have already been transplanted into larger containers and less chemical is required per plant, so growers can save money by spraying earlier in the plant production cycle. Some commercial producers of liners have reported reduced root mass in plants treated with BA (personal communication). BA has been shown to reduce root growth in in vitro Arabidopsis seedlings (Auer, 1996). Sedum leaf cuttings treated with BA at concentrations of 8 to 500 mg·L−1 exhibited reduced root growth and increased shoot growth at all concentrations (Boe et al., 1972). Other research has shown BA has no effect on the root growth of treated plants (Khosh-Khui et al., 1978; Leclerc et al., 2006; Richards and Wilkinson, 1984). However, application of BA to unrooted cuttings of Verbena ×hybrida Voss, which had already initiated adventitious root formation, had no effect on root growth, whereas low rates of BA (30 mg·L−1) applied to cuttings before initiation of adventitious root formation increased root dry weight of liners at 12 d after treatment (Svenson, 1991).
The process of liner production has been divided into five stages: Stage 0, before arrival or harvest of cuttings; Stage 1, cutting arrival or harvest and sticking; Stage 2, callusing; Stage 3, root development; and Stage 4, toning the rooted cutting. During Stage 4 the roots are grown enough so that the root ball will hold together when pulled from the plug tray (Dole and Hamrick, 2006). The objective of this study is to evaluate the effects of the application of BA to herbaceous perennial plants during liner production, specifically during Stage 3, on root growth and shoot branching of the finished liners (Stage 4).
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