Prunus laurocerasus (common or English cherrylaurel) is an evergreen shrub or small tree in Rosaceae generally used as a hedge or screen. This species is a cytological anomaly due to its high ploidy (2n = 22x = 176; Meurman, 1929). Common cherrylaurel grows to more than 6 × 9 m and has a coarse texture with large leaves that can be up to 20 cm or more in length and generally are oblong with serrated margins. The species produces prolific numbers of white flowers in axillary racemes up to 12 cm or longer. There are a number of cultivars of common cherrylaurel in the trade, but ‘Otto Luyken’ and ‘Schipkaensis’ are most common. ‘Otto Luyken’ is a compact form that generally grows to 1.2 m high and 1.8 m wide with narrower leaves than the species and the leaves for the most part lack serration. ‘Otto Luyken’ maintains very dense growth and has prolific flower production even when shaded. ‘Schipkaensis’ is reportedly confused in the trade and often growers are producing what Dirr (2009) refers to as ‘West Coast Schipkaensis’. The original ‘Schipkaensis’ has leaves up to 11.4 cm long with entire margins or only a few teeth toward the apex. ‘West Coast Schipkaensis’, a name used by retailers to describe an alternate form of the original (M. Dirr, personal communication) is characterized by having more serrated leaves and a distinct upright habit.
Fruits of common cherrylaurel are large drupes of up to 1.25 cm and are produced in large numbers. In managed landscapes, they are a nuisance when they drop on walkways or are deposited by birds. Common cherrylaurel has escaped cultivation in parts of northwestern United States including western Oregon. It is regularly found in native forests and is ranked as a medium (M) to high (H) impact potential invasive species by the Emerald Chapter of the Native Plant Society of Oregon. Because of its characterization as an invasive species, a fruitless form of common cherrylaurel is desirable. Ploidy manipulation is a common technique used to reduce fertility by doubling chromosomes and then backcrossing to produce a plant with odd ploidy. Polyploidization often is performed in vitro due to the relatively small meristem size and ability to treat many plants (or meristems). In vitro polyploidization has been successfully performed on a wide array of plant species in many families using various antimitotic agents (Dhooghe et al., 2011). Although colchicine is the historical method of polyploidization and has proven effective for in vitro chromosome doubling, the dinitroaniline herbicide oryzalin has recently become more popular. Oryzalin has been used to double chromosomes of woody landscape plants in Rosaceae including Chaenomeles japonica (Thunb.) Lindl. ex Spach (Stanys et al., 2006), Malus Mill. (Bouvier et al., 1994), and Rosa L. (Kermani et al., 2003). Our goal was to induce whole genome duplication of ‘Otto Luyken’ and ‘Schipkaensis’ using in vitro application of oryzalin.
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