Somatic embryogenesis and cryopreservation of Stewartia species

Abstract

Stewartia is a genus of flowering shrubs and trees in the family Theaceae. Stewartia species are native to eastern Asia, with the exception of S. ovata and S. malacodendron, which are indigenous to southeastern North America. Despite having outstanding ornamental value and unique horticultural features, Stewartias are not readily available for landscaping in the horticultural trade due to difficulty with mass propagation. In addition to commercial propagation, there is a need to develop propagation techniques for the conservation of the North American species, which are rare (S. ovata) or endangered (S. malacodendron). This research examined somatic embryogenesis as a means to mass propagate Asian and North American Stewartia species and to provide material for cryopreservation of valuable Stewartia germplasm. By testing a variety of plant growth regulators (PGRs), somatic embryogenesis was induced from immature zygotic embryo explants of seven Stewartia species—five Asian and two North American. Additionally, embryogenic cultures were obtained from three North American cultivars, S. malacodendron ‘Delmarva’, S. ovata ‘Red Rose’, and ‘Royal Purple’. For two Stewartia species, S. ovata and S. sinensis, somatic embryogenesis was induced from immature endosperm. Picloram (0.05 or 0.1 mg/L) and 2,4-dichlorophenoxyacetic (2 or 4 mg/L) were the most effective PGRs for inducing somatic embryogenesis of North American and Asian Stewartia species, respectively. All seven Stewartia species examined produced somatic embryos that converted into plants that could be transferred to the greenhouse. Embryogenic tissue from five Stewartia species were successfully put into cryo-storage and recovered, providing a means of long-term germplasm storage. This is the first report of somatic embryogenesis in the genus Stewartia. This is the first report of somatic embryogenesis from immature zygotic embryos in the genus Stewartia and it was accomplished with seven different species.