Weigela Thunb. is a genus of 12 recognized species of deciduous shrubs in the family Caprifoliaceae (Yokoyama et al., 2002). All species are native to eastern Asia and generally hardy and easily grown (Huxley, 1992). They have decorative flowers in spring and early summer varying from white through pink to red. Among them, W. florida (Bunge.) A. DC. is the most commonly produced species (Touchell et al., 2006). It was collected by Robert Fortune from North China in 1845 and commonly known as old fashioned weigela. This species has a dense and rounded canopy that typically grows to 1–3 m high and may spread over time to up to 4 m wide. It is pest resistant, tolerant to a wide range of environmental conditions, and cold hardy to USDA zone 4–5 (Touchell et al., 2006). It was from this species that most hybrids or cultivars have been developed (Duron and Decourtye, 1990).
Weigelas are no longer old fashioned plants; they have regained popularity in the ornamental plant industry during the last 20 years. More than 180 weigela hybrids or cultivars with different foliar and flower colors, growth forms, and reblooming characteristics are available (Wood, 2016). They are propagated mainly through stem cuttings (Weigle and Stephens, 1991). Cutting propagation of newly bred hybrids or cultivars; however, could be hampered by the availability of appropriate stems. Additionally, cutting propagation may carry and spread diseases, such as crown gall caused by Agrobacterium tumefaciens and gray mold caused by Botrytis cinerea (Jones and Benson, 2001). In vitro micropropagation could be a solution to provide disease-free propagules of the new hybrids or cultivars year-round to the ornamental plant industry (Chen and Henny, 2008).
The first in vitro culture of Weigela dates back to 1975, when meristems of five cultivars were cultured by Duron (1975), followed by stem internode culture (Duron, 1981), and bud culture by Calvert and Stephens (1986). All these cultures used Murashige and Skoog (MS) (Murashige and Skoog, 1962) medium; however, details of multiplication rates were not reported. Weigle and Stephens (1991) briefly mentioned that W. florida ‘Red Prince’ could be micropropagated using MS medium. Ochatt (1993) regenerated plantlets of W. florida ‘Bristol Ruby’ using protoplasts as explants. The MS medium was also used for shoot culture of W. florida ‘Red Prince’, but vitrification was a problem (Wang et al., 2000). Additionally, callus was induced from ‘Red Prince’ (Yuan and Zhang, 2006), and somatic embryos were produced from ‘Red Prince’ pollen (Wang et al., 2012); but plant regeneration from either calluses or embryos was not documented. As far as is known, reliable methods for in vitro propagation of Weigela have not been well established.
The objective of this study was to develop a method for in vitro propagation of W. florida using cultivar Tango as a model plant. Tango has a compact size (0.6 to 1 m tall), produces profuse spring flowers and purplish foliage, and is considered one of the most commonly produced cultivar. Different culture media, cytokinins, and auxins for axillary shoot induction were evaluated and microcuttings were rooted in vitro and ex vitro. A reliable protocol for in vitro shoot culture of W. florida ‘Tango’ was developed.
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