In vitro induction of chromosome-doubling in cultured shoots of three cultivars of mint (Mentha canadensis L.) treated with colchicine

To improve the yield and quality of essential oils, chromosome-doubling in mint cultivars was induced by treating shoots with colchicine in vitro. Shoot tips of three mint cultivars ('68-7', '73-8', and 'HU 39') were cultured in vitro and treated at 2 months using either of two methods to induce chromosome-doubling. Explants were immersed separately in each of three concentrations of colchicine [0.1, 0.2, or 0.3% (w/v)] for 24 h or 48 h. Alternatively, shoots were cultured on solid 1.0 x MS (Murashige and Skoog) medium supplemented with one of five concentrations of colchicine (10, 20, 30, 40 or 50 mg l(-1)) for 30 d. After each treatment, ploidy levels were measured by flow cytometry. The results showed that high yields of 4n plants were induced by the immersion of shoots in 0.2% (w/v) colchicine for 24 h (13.3%), or by culturing shoots on MS medium containing 20 mg l(-1) colchicine for 30 d (17.3%). The immersion method, which gave a survival rate of 93.3%, was more convenient and less phytotoxic to produce 4n mint plants. Compared with untreated plants, we observed fewer but larger stomata in chromosome-doubled plants. We also observed significant differences in the size, internode length, leaf length, leaf width, and stem width in chromosome-doubled mint plants.