Somatic Embryogenesis of the Rare Cultivars, Stewartia malacodendron "Delmarva" and Stewartia ovata "Red Rose"

Propagation of rare and endangered plant species via seed is often challenging due to difficulties with seed collection, low seed yields and viability, and sometimes complicated requirements for germination. Also, sites for collection are few and the amounts of seed and plant material are limited. Other barriers to propagation of unique plant species include difficulty in rooting cuttings and successful acclimation. As an alternative to propagation by seed and/or cuttings, this research investigated the use of plant tissue culture methods, specifically somatic embryogenesis, to enhance the populations of two rare cultivars of woody plant species, Stewartia malacodendron Linnaeus Delmarva and Stewartia ovata Red Rose. The cultivars are difficult to propagate via cuttings because of each tree’s recalcitrance to rooting. Not only are the mother trees unique in ornamental characteristics and few in number, the trees are very old, 40 + years, and declining in health. Seeds from immature fruit capsules of each cultivar were surface-disinfested and zygotic embryos were aseptically excised and cultured on sterile semisolid culture media supplemented with various types and concentrations of plant growth regulators. Somatic embryogenesis was induced from embryo explants of S. malacodendron Delmarva and S. ovata Red Rose at a very low frequency in the presence of 0.1 mg/L picloram. A few somatic embryos of S. ovata Red Rose have converted to plantlets and will be transferred to soil shortly. Propagation of elite Stewartia cultivars demonstrates the ability to increase the populations of rare woody plant species with unique ornamental characteristics to support availability for the horticulture industry.