Abstract
The genus Betula contains many important forest and ornamental species and a method of rapid clonal propagation of superior genotypes is needed. Thidiazuron (TDZ) is a potent synthetic plant growth regulator with cytokinin-like activity. TDZ was used to differentiate shoots after long term exposure to dichlorophenoxyacetic acid (2.4-D) as part of a larger study on clonal fidelity. Birch calli were cultured on Woody Plant Medium supplemented with 10-5 M 2,4-D for up to 30 weeks. The calli were transferred to media containing TDZ at concentrations of 10-6 to 10-9M. Most of the tissue which had not been exposed to 2.4-D differentiated shoots five weeks after being exposed to 10-6M TDZ. Increasing the of time exposure to 2.4-D or decreasing the concentration of TDZ delayed differentiation. Calli exposed to 2.4-D for more than 18 weeks rarely differentiated shoots regardless of the concentration of TDZ used.
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