Oryzalin-induced autopolyploidy in sweet marjoram (Origanum majorana L.)

• Oryzalin had no effect on seed germination but reduced seedling development and survival. • Fifteen true polyploid plants were obtained, including nine triploids and six tetraploids. • The average polyploid induction efficiency among the surviving plants was 22.7 %. • Oryzalin-induced polyploids exhibited enlarged leaves, stomata, and trichomes. Sweet marjoram ( Origanum majorana L.) is a medicinal and aromatic herb valued for its applications in traditional medicine and culinary use. The growing demand for this crop has created the need to increase productivity per unit area. Mutation breeding through polyploidization can be employed to develop improved cultivars with enhanced productivity traits. This study investigated the effects of different oryzalin seed treatments (0, 7.5, 15, 30, 60, and 120 µM for 24 hours) on polyploid induction in sweet marjoram. Oryzalin-treated seeds were germinated and established in vitro for polyploid screening using stomatal assays. Ploidy levels were subsequently confirmed by flow cytometry of greenhouse-established plants. Results indicated that oryzalin neither promoted nor inhibited seed germination. However, seedling development and survival were reduced in plants from oryzalin-treated seeds. Oryzalin induced triploid, tetraploid, and mixoploid plants. Among the surviving plants, the average polyploid induction efficiency was 22.7 %. The highest induction efficiency (50.0 %) was observed at 120 µM, whereas the lowest (8.3 %) occurred at 30 µM. Intermediate concentrations of 7.5, 15, and 60 µM produced efficiencies of 16.7 %, 24.0 %, and 30.8 %, respectively. Tetraploid plants exhibited an average induction efficiency of 9.1 %, with the highest value (50.0 %) observed at 120 µM. Induced polyploid plants exhibited an increased leaf width-to-length ratio, changing from ovate or oblong-ovate to orbicular leaf shape. Stomatal size increased while density decreased compared with diploid controls. Similarly, trichome size increased and density decreased, indicating characteristic epidermal modifications typically associated with chromosome doubling and improved leaf surface traits.