Status and prospect of improved variety selection in Eucommia ulmoides.

Eucommia ulmoides is not only an unique Chinese traditional medicine, but also a potential temperate rubber resource tree species, which has great value in industrialization exploitation of pharmaceutical and rubber production. In this paper, status of collection and conservation of germplasm resources, evaluation of genetic variation in traits, selection breeding, cross breeding, polyploidy breeding and vegetative propagation of varieties in E. ulmoides were reviewed. Leaves with many advantages, such as large quantity, low cost, easy to collect, are the best raw material for pharmaceutical and rubber production when Eucommia spp. plantations were developed in some regions with poor site conditions and labor resource shortage. Because polyploidy breeding could result in an increasing content of secondary metabolites, triploid hybrid production by gametic chromosome doubling may be the most promising way to greatly improve the rubber content and medicinal components of leaves and leaf yield. In future polyploid breeding of E. ulmoides, we should focus on selecting crossing parents with higher combining ability in rubber content and medicinal composition content of leaves. Not only ploidy advantages resulting from gene dosage effects but also heterosis derived from allogametes fusing can be fully utilized. To increase the efficiency of clonal selection, developing a detection techniques with simple and low cost characteristics for secondary metabolites, such as E. ulmoides rubber, active ingredients, is necessary and required. A simple and rapid vegetative propagation technique characterized with low cost is also required to shorten the period of seedling cultivation and speed up high quality seedling production of improved varieties. With the successful development of triploid E. ulmoides, it is predictable that multiple trait improvement via one round polyploidy breeding will promote the development of green circular economy with the core of pharmaceutical and rubber production in China.