Advances in the Breeding of Tree Peonies and a Cultivar System for the Cultivar Group

Tree peonies native to China have become an internationalized ornamental plant and are widely cultivated in many countries in Asia, America, Europe and Australia. Breeding starting from domestication of wild plants through selection of bud mutations and seedlings to hybridization has been done for about 1500 years. At present a few thousand cultivars are cultivated globally in gardens or propagated in nurseries, all of which genetically originated from infraand inter-specific, inter-subsectional and -sectional hybridization and are included in different groups with distinct traits from each other. A cultivar system of tree peonies that considers the origin and diversity of cultivars should theoretically be set up at the base of a cultivar group, and would depend on recent advances from botanical studies and breeding achievements in the last few decades. Such a system composed of 17 cultivar-groups first identified by our work will explain the relationship between species and cultivar groups and among cultivar groups and would benefit tree peony breeding as an important reference for ascertaining breeding strategy and objectives, selecting and matching parents in crossing, and exploring and utilizing valuable genes controlling desirable ornamental traits and resistances. Attempts made in in vitro tissue culture, ovule and embryo culture, somatic embryogenesis and plantlet regeneration are uncovering a promising use of bio-techniques to overcome problematic issues of tree peony breeding such as cross incompatibility, hybrid abortion and sterility, slow growth to flower and difficulties in propagation. Combined with molecular marker-assisted and transgenic breeding, inter-subsectional and -sectional distant hybridization will effectively promote the breeding of tree peonies and undoubtedly bring about more opportunity to release better cultivars. A scientific strategy and an international cooperation on tree peony breeding are very necessary for effective work and objectives to be achieved in the future. _____________________________________________________________________________________________________________