Prospect and challenges in Rhododendron micropropagation

Rhododendron L. is a large genus of ornamental shrubs and trees native to Asian and American continents. There are currently 1,062 accepted species of Rhododendron; while, an estimated 25% of them are endangered. Due to the importance of micropropagation on conservation and production of Rhododendron, literature on Rhododendron micropropagation was reviewed. 48 research articles on micropropagation of Rhododendron were found, involving 95 different species, subspecies, varieties, and cultivars. The taxa reviewed belongs to subgenus Hymenanthes section Pontica (49.5%) and Pentanthera (2.1%), subgenus Rhododendron section Rhododendron (21.1%) and Schistanthe (2.1%), subgenus Azaleastrum section Tsutsusi (8.4%) and Sciadorhodion (1.0%), and subgenus Choniastrum section Choniastrum (1.0%). Micropropagation was conducted through in vitro collection of seed and adult parts, shoot induction, callogenesis, root induction, and somatic embryogenesis on tissue culture media, such as Anderson’s medium, Woody Plant medium, Economou and Read medium, Murashige and Skoog medium, mostly supplemented with sucrose and adjusted to pH 4.5-5.8 before autoclaving. Basal media composition, plant growth regulator, and explant choice were the most tested and influential factors in Rhododendron micropropagation. Types of cytokinins were a significant factor for shoot multiplication, with thidiazuron and isopentenyladenin producing the best results. However, stunted shoot induced on thidiazuron required an extra elongation step. In addition, there were reports of tumorous tissue proliferation from micropropagated plants and certain genotypes. Ex vitro rooting was shown to be a cost-effective alternative to in vitro rooting. Mycorrhizae studies found that inoculation improved growth of rooted cuttings. Micropropagation should be explored on ornamental and endangered Southeast Asian rhododendrons.