Background <p>Flowering time (referred to as heading date in rice) is a characteristic controlled by quantitative trait loci (QTLs). Heading date is an important agronomic trait that determines yield in rice and is precisely regulated by various exogenous and endogenous factors.</p> Objective <p>To investigate the role of the <i>ascorbate peroxidase 9</i> (<i>APX9</i>) gene in the flowering mechanism, we examined the expression levels of flowering time regulators under long-day conditions (14&#xa0;h light/10 hours dark) and natural long-day conditions.</p> Methods <p>A BC₄F₇ near-isogenic line (NIL), derived from an interspecific cross between the Korean elite <i>japonica</i> cultivar (<i>cv</i>. Hwaseong, <i>Oryza sativa</i> L.) and <i>Oryza rufipogon</i> (IRGC 105491), was used in this study to identify and characterize candidate genes associated with heading traits. The NIL contains a small <i>O. rufipogon</i> chromosome segment harboring the <i>APX9</i> gene. In addition, overexpression and T-DNA insertion knockout transgenic lines were used to examine the function of <i>APX9</i> gene.</p> Results <p>The NIL exhibited delayed flowering under both controlled and natural long-day conditions compared to Hwaseong. Real-time PCR analysis indicated that <i>APX9</i> influences the upstream pathway of flowering time regulation. Over-expression lines showed delayed flowering relative to Hwaseong, whereas T-DNA mutants flowered earlier than the control cultivar, Dongjin. These findings support a role for <i>APX9</i> in modulating flowering time in rice.</p> Conclusion <p>The <i>APX9</i> is known for its antioxidant activity and its response to various abiotic stresses. The <i>APX9</i> may play a role in the upstream regulation of the flowering pathway. These findings will be valuable in understanding the effect of <i>APX9</i> during flowering stages. It would also be interesting to explore the relationship between hydrogen peroxide (H₂O₂₎ levels and the <i>APX9</i> gene in signaling events related to heading.</p>