Abstract
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.