Poinsettia, Euphorbia pulcherrima, is an important holiday symbol and is the number one flowering potted plant in the United States. The technique of chromosome doubling has been utilized to increase size of flowers, stems, and leaves of many species, and has been used in poinsettia breeding to obtain new cultivars. Application of colchicine or oryzalin to in vitro tissues may be used to enlarge the inflorescences and brackets and reduce the height of `Winter Rose'™ poinsettias, reduce the likelihood of chimeric tetraploids, and provide a rapid means for producing many tetraploid plants. The purpose of this research was to evaluate the effect of colchicine and oryzalin on callus and adventitious shoot formation of `Winter Rose'™ poinsettia with in vitro grown leaf tissues and its potential for tetraploid induction. In vitro grown leaf midvein sections were placed on various media supplemented with either colchicine or oryzalin at various concentrations for 1–4 days. Colchicine was least damaging to leaf tissues at concentrations of 0.25 or 250.4 μm. A large amount of callus, as well as adventitious shoots, were produced. Regenerated shoots were found to be diploid, determined by flow cytometry. On media with oryzalin (28.9–144 μm), leaf tissues produced callus, but not adventitious shoots. Calluses produced on oryzalin-containing media were tested using the flow cytometer and were found to be diploid.
The mitotic inhibitors, colchicine and oryzalin, were evaluated for their effects on callus, adventitious shoot formation, and tetraploid induction of Euphorbia pulchurrima `Winter Rose'. In vitro grown leaf sections were placed on various media supplemented with either colchicine or oryzalin at various concentrations for 1 to 4 days. Colchicine was less damaging to leaf tissues than oryzalin. On various colchicine-containing media, prolific calluses were produced and adventitious shoot formation was observed. Regenerated shoots were found to be diploid as determined by flow cytometry. On media supplemented with oryzalin (28.9 μm to 144 μm), leaf tissues produced callus but failed to form adventitious shoots. Samples of calluses produced on oryzalin-containing media were subject to analysis using flow cytometry and were found to be diploid. These results suggest that the colchicine is less toxic on poinsettia tissues and shoot induction than oryzalin. Additional experiments are needed to establish a protocol for in vitro induction of poinsettia tetraploid with colchicine and oryzalin.