Induced polypoidy under In Vitro conditions and highly efficacious screening of hexaploids in purple coneflower (Echinacea purpurea (L.)Moench)

Leaf, petiole, and root explants from in vitro maintained triploid purple coneflower plantlets were treated with120 mg·L-1colchicine for the induction of hexaploid plants. Best result for induction of hexaploid was obtained by treating the triploid root explants with 120 mg·L-1colchicine for 25 days. Regeneration of adventitious buds from the triploid root explants on Murashige and Skoog (MS) medium with 6-Benzyladenine (BA) and Naphthalene acetic acid (NAA) after the colchicine treatment persisted for a time longer than from those untreated. Only slow-growing plantlets in a same genotype-and-treated population were selected and detected to increase the screening efficiency as well as save time and efforts. Chromosome counting confirmed that most early-regenerating and fast-growing buds were triploid, while most late-regenerating and slow-growing buds were hexaploid, screening only those slow-growing regenerated plantlets could increase effectively the hexaploid. In the present study, screening efficiency increased from 21% (detected all plantlets) to 53% (detected only the slow-growing plantlets). On the other hand, hexaploid plants had much larger stomata and more stomatal guard cell chloroplasts. The stomatal guard cell chloroplast number in hexaploid plants has a good linear relationship with those in the diploid, triploid, and the tetraploid progenitors. Results indicate that the colchicine-induced hexaploid could be induced and screened out with high efficiency, making this process worth further exploring in other species as well.