Allele‐informed QTL‐seq identifies <i>Hd18</i> as a minor modifier of heading date in elite rice cultivars

plants homozygous for the A allele headed 4.1 days earlier in the field and 6.2 days earlier in the greenhouse than those homozygous for the G allele. Analysis of 290 Korean rice cultivars further demonstrated that the Hd18 A allele was associated with approximately 2 days earlier heading under a nonfunctional hd1 background. Evaluation of two recombinant inbred line populations showed that Hd18 had minimal effects on major agronomic traits, while exhibiting background-dependent effects on specific traits. These results establish Hd18 as a minor effect modifier of heading date with utility for fine-scale phenological adjustment in rice breeding.