PROBLEM TO BE SOLVED: To provide methods of constructing a tetraploid Populus stably with DNA doubling by a simple method without tissue culture.SOLUTION: A colchicine solution is dropped on an axillary bud of a shoot of a diploid Populus and cultured, and then an emerged side shoot is subcultured and a tetraploid Populus is constructed. Preferably, the concentration of a colchicine solution is 0.002% w/v or more to less than 0.1% w/v, more preferably, 0.005% w/v or more to less than 0.05% w/v. A gelling agent is added to the colchicine solution, and preferably a dropping is performed to an axillary bud at the root of a leaf in the condition that a main stem is cut off and the lower leaves of the shoot that is subcultured and grown is left. Preferably, the frequency of dropping is twice and a dropping interval is one day. Preferably, the amount of the colchicine solution is in the amount which a drop is made at an axillary bud and which the drop does not run down from the axillary bud.SELECTED DRAWING: Figure 1
