Colchicine induces polyploidization in sweet marjoram (Origanum majorana)

Sweet marjoram (Origanum majorana L.) is an aromatic plant widely used in traditional medicine, food, cosmetics, and pharmaceutical industries. However, unfavorable climatic conditions resulting from global warming negatively affect its productivity. This challenge may be addressed by developing more tolerant cultivars. To support this, the present study evaluated the efficacy of colchicine for inducing polyploidy in sweet marjoram, with the aim of facilitating cultivar improvement. Seeds were treated with colchicine at concentrations of 0.000 %, 0.025 %, 0.050 %, 0.100 %, 0.200 %, 0.400 %, and 0.800 % for 24 h. Treated seeds were germinated in vitro, and surviving seedlings were established under greenhouse conditions. The ploidy status of all greenhouse plants was initially screened by stomatal assay and subsequently confirmed by flow cytometry. Results showed that seed germination percentages (30 %-46 %) did not differ significantly among treatments; however, seedling survival declined as colchicine concentration increased, with no seedlings surviving at 0.400 % and 0.800 %. Seventeen putative polyploids were identified, of which fourteen were confirmed as true polyploids and three as mixoploids. The fourteen true polyploids included two triploids, ten tetraploids, and two pentaploids. The efficiencies of polyploid induction were 11.1 % at 0.025 %, 40 % at 0.050 %, 75 % at 0.100 %, and 80 % at 0.200 % colchicine. Compared with diploids, polyploid plants exhibited wider leaves, larger stomata, and reduced stomatal density. The true polyploid plants represent valuable genetic resources for breeding new sweet marjoram cultivars.