Micropropagation of the Relict Genus Cercidiphyllum (Cercidiphyllaceae)

The genus Cercidiphyllum (Cercidiphyllaceae) is endemic to Japan and China, consisting of two dioecious tree species, common katsura (C. japonicum Sieb. & Zucc.) and the broad-leaved hiro-ha-katsura (C. magnificum Nakai). Cercidiphyllum grows 5 to 10 meters in height and similar width in riparian forests, forest margins, and streams. Prized as specimen trees, all genotypes of the genus have potential for commercial nursery development. The micropropagation of woody ornamentals provide a stepping-stone to biotechnological approaches in plant improvement programs, aid in nursery production, and conservation efforts. To date, there exists no literature on the micropropagation of C. magnificum, C. japonicum (Weeping Group), and limited information on C. japonicum. This study focuses on many aspects of micropropagation, in respect to well represented genotypes within Cercidiphyllum. Factorials of nutrient salt and plant growth regulator concentrations were used for in vitro establishment, proliferation, and root initiation of Cercidiphyllum. Four nutrient salt formulations (MS, DKW, LP, or WPM) ranging from high to low salt formulations were studied to determine a suitable nutrient salt formulations for the establishment and proliferation of axillary explants. Factorial combinations of thidiazuron (TDZ) concentrations (0, 0.05, 0.10, 1.0 μM), 6-benzylaminopurine (BA) concentrations (0, 1.1, 2.2, 4.4 μM), indole-3-butyric acid (IBA) concentrations (0, 0.05 μM), and 1-naphthaleneacetic acid (NAA) concentrations (0, 0.05, 0.10, 1.0 μM) were used for micropropagation stages. Reduction of phenolic exudation of explants during the initiation phase may improve vigor and can be prevented by carbon source treatments. Nodal explants 2 cm in length were used to initiate cultures and maintained on various media with 0%, 0.1%, 0.3%, or 0.5% sucrose; correlating sucrose concentration with phenolic exudates. All micropropagation experiments included 0.07% agar and a pH 5.8. Explants were incubated approximately 30 cm beneath cool-white fluorescent lamps that provide a photon flux of approximately 40 μmol·m^-2·s^-1 for a 16-h photoperiod at 25 ± 3 °C. Preliminary results indicate lower nutrient salt formulations (WPM, LP, and MS, respectively) combined with low concentrations of auxin (0.5 μM IBA) and moderate cytokinin levels (2.2 μM BA) performed better at axillary shoot initiation and higher concentrations of cytokinin (4.4 μM BA) for proliferation. Further results show a reduced concentration of sucrose (0.1%) shortens initial bud-break time and increased shoot induction. However, explants lose vigor after 4 weeks compared to 0.3% sucrose and should thus be transferred immediately after initiation.