| Summary: | Optimizing flowering time in crop plants is critical for maximizing yield and quality under target environments. While there is a wide range of heading date variation in Korean rice cultivars, the underlying gene mechanisms are unclear. Here, we sequenced the protein coding regions of <i>Hd1</i>, the major rice heading date gene, from 293 Korean rice cultivars and investigated the associations between <i>Hd1</i> allele types and major agronomic traits under four different environments. There were four functional <i>Hd1</i> and five nonfunctional <i>hd1</i> alleles distributed among the 293 Korean rice cultivars. The effects of the <i>Hd1</i> allele types were highly significant for days to heading in all four environments, explaining 51.4–65.8% of the phenotypic variation. On average, cultivars carrying nonfunctional <i>hd1</i> headed 13.7 days earlier than those carrying functional <i>Hd1</i>. While the <i>Hd1</i> allele types exhibited highly significant effects on culm length and protein content under all four environments, the differences between cultivars carrying <i>Hd1</i> and <i>hd1</i> were minimal. The effects of the <i>Hd1</i> allele types on amylose content were significant in only one of the four environments. Our results provide useful information for fine-tuning rice heading dates by utilizing different <i>Hd1</i> alleles in rice breeding programs.
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