Seeds of common buckwheat usually mature while leaves are still green, causing problems for harvest machinery and limiting yields. We developed an early leaf-senescence line, Kyushu 1, from spontaneous mutants. To characterize this mutant line physiologically and genetically, we measured the change in leaf chlorophyll content in two regions of Japan with different environments, and performed genetic analyses with self-compatible lines. The chlorophyll content of Kyushu 1 was lower than that of the wild type at seed maturity in both regions. F₁ progeny of the cross between wild-type plants and Kyushu 1 showed a wild-type phenotype. F₂ progeny segregated into wild-type and early leaf-senescence phenotypes in a 3:1 or 13:3 ratio, suggesting that the early leaf senescence of Kyushu 1 is controlled by either a single recessive gene or an epistatic interaction between two loci.