Induction of polyploidy in Dahlia pinnata using various concentrations of Oryzalin under in vitro culture

Polyploidy induction is widely used in ornamental breeding to generate novel phenotypes such as larger flowers, thicker stems, and increased vegetative biomass. Therefore, this study aimed to evaluate oryzalin-mediated chromosome doubling in in vitro Culture of Dahlia pinnata using a completely randomized 2×6 factorial design with two explant types (apical shoots and middle nodes) and six oryzalin concentrations (0–75 µM) applied by 24 h immersion. Plant height, node number, and leaf number were recorded weekly for 8 weeks after culture, and ploidy was determined by flow cytometry (BD Accuri C6+, USA). The results showed that oryzalin caused a clear dose-dependent growth suppression. Low doses reduced growth without changing ploidy, while 30 µM produced tetraploid sectors in apical shoot-derived plantlets (16.67% putative tetraploids) but with reduced vigor. Higher concentrations (60–75 µM) nearly arrested growth without further increases in polyploid recovery. Middle-node explants showed greater tolerance to oryzalin than apical shoots. These results identify a narrow concentration window for initial polyploid induction in Dahlia pinnata while indicating the need to balance induction efficiency against phytotoxicity.