Many years ago, Hibberd (1862) noted “the time is fast coming when gardens of any pretensions to beauty will be judged by their collections of Berberis, for there is not any other class of evergreen shrub which affords so many points for interesting observation.” Now 150 years later, the genus Mahonia Nutt. (syn. Berberis L.; for review, see Rounsaville and Ranney, 2010) is finally making a transition from the gardens of collectors into mainstream horticulture. Mahonia have tremendous ornamental potential, owing to their durability, showy displays of brilliant-colored flowers, and evergreen foliage. Nevertheless, few selections of the genus have been introduced commercially. Mahonia ×media cultivars dominate retail sales, although these selections are less than desirable to many consumers as a result of their spindly growth and spine-covered leaflets.
Mahonia ‘Soft Caress’ (PP20183) is a new cultivar that has shown great promise as a landscape plant. This cultivar arose as a seedling from an open-pollinated M. eurybracteata Fedde (Ozzie Johnson, personal communication). Although M. eurybracteata is notable for its relative compactness and narrow evergreen foliage, ‘Soft Caress’ grows considerably denser and was aptly named for its delicate linear leaflets. Hardy to USDA Zone 7, ‘Soft Caress’ is adorned with terminal yellow racemes during fall and early winter.
Although Mahonia sp. can be successfully propagated by stem cuttings, plants tend to have few lateral branches and thus a limited number of shoots available for propagation. Development of in vitro propagation methods would provide a desirable option for more rapid multiplication. In addition to propagation, establishing tissue culture protocols provides an ideal platform for manipulating ploidy level, harvesting chemical compounds, and initiating embryogenesis (Alvarez et al., 2009; Herbert et al., 2010; Pierik, 1997).
Studies on micropropagation of ornamental varieties of Berberis and Mahonia have been limited to Berberis thunbergii DC. ‘Crimson Pygmy’ (Uno and Preece, 1987); M. aquifolium ‘Apollo’ and ‘Undulata’ and M. ×media ‘Winter Sun’ (Daguin et al., 1992b); and M. trifoliata (Mackay et al., 1996). These studies have demonstrated that protocols necessary for shoot growth and development can vary considerably among genotypes and different basal salts and plant growth regulator combinations may be necessary. Woody Plant Medium (WPM) (Lloyd and McCown, 1980) and MS media (Murashige and Skoog, 1962) were used for Berberis and Mahonia cultures by Uno and Preece (1987) and Daguinet al. (1992b), respectively. Additionally, Mackay et al. (1996) used a combination of WPM salts and MS vitamins for M. trifoliata.
In most micropropagation studies, 6-benzylaminopurine (BAP) has been an effective cytokinin for shoot growth for Mahonia and Berberis species when provided at 5 to 10 μM (Daguin et al., 1992b; Mackay et al., 1996; Uno and Preece, 1987). Mackay et al. (1996) found BAP alone was sufficient for shoot proliferation of M. trifoliata; however, for several other Mahonia sp., low concentrations of kinetin (Kin) and the auxin, indole-3-acetic acid (IAA), were necessary to stimulate shoot initiation and multiplication (Daguin et al., 1992b). Daguin et al. (1992b) also found it necessary to use higher concentrations of sucrose (45 versus 30 g·L−1), which are sometimes beneficial for younger tissues (Pierik, 1997). Furthermore, Daguin et al. (1992b) incorporated activated charcoal, which is often used to bind organic compounds secreted by plants in vitro, despite the undesirable binding of crucial growth regulators, particularly BAP (Thomas, 2008).
The ability of gibberellic acid (GA3) to break dormancy, induce bud growth, and promote internodal elongation (Pierik, 1997) makes it a valuable compound for in vitro culture of small bud explants. Daguin et al. (1992b) and Uno and Preece (1987) used GA3 at 0.6 or 10 μM for shoot multiplication of Mahonia and Berberis cultures, respectively. GA3 is also known to promote ethylene-induced necrosis and leaf abscission (Morgan, 1976), which can be a particular problem for cultures in sealed vessels. However, use of ethylene inhibitors such as cobalt(II) chloride hexahydrate (CoCl2), or silver nitrate (AgNO3) can be effective for mediating the negative impacts of GA3-promoted ethylene (Ma et al., 1998; Misra and Chakrabarty, 2009).
Rooting microcuttings of Berberis and Mahonia have been successfully achieved both in vitro and ex vitro using a variety of auxins. Mackay et al. (1996) achieved nearly 100% rooting of M. trifoliata in vitro using 1.0 μM naphthaleneacetic acid on cultures younger than 6 months. However, as culture age increased, microshoots required a cytokinin-free subculture followed by increased levels of auxin, which ultimately yielded lower (68%) rooting percentages. Conversely, Daguin et al. (1992b) transferred microcuttings of Mahonia directly to glasshouse flats after treatment of the microcuttings with a low concentration of IBA and achieved 80% rooting within 6 weeks. Karhu and Hakala (1990) treated microcuttings of Berberis thunbergii for 7 d in the dark using liquid media with low levels (2 to 6 μM) of either IAA or IBA (Karhu and Hakala, 1990). Interestingly, a subsequent transfer of the microcuttings to an auxin-free media gave significantly better rooting (85%) than those transferred to media with low auxin levels (35%) (Karhu and Hakala, 1990).
With development and introduction of new selections of Mahonia, it would be advantageous to have micropropagation protocols to facilitate rapid production and commercialization of these plants. Therefore, the objective of this research was to develop a rapid micropropagation protocol for Mahonia ‘Soft Caress’.
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