Inheritance of the narrow leaf mutation in traditional Japanese evergreen azaleas

Morphological mutations have been admired and preserved in the classical cultivars of the Japanese evergreen azalea since the Edo era (1603–1867). One such mutation is the presence of narrow leaves and petals. In this study, we investigated the characteristics and inheritance patterns of the narrow leaf-type mutational form of Rhododendron macrosepalum ‘Seigaiha’, R. kaempferi ‘Kin-kujyaku’, and R. kiusianum Yanagi-ba mutant. The leaves of all cultivars showed weak development in the transverse plane and thickened margins. Subsequently, based on this qualitative character, the inheritance of the narrow leaf mutation was investigated. F1 seedlings of cross combinations between normal and narrow leaf-type plants had normal leaf-type. All F1 progenies of cross combinations between narrow leaf-type plants had narrow leaves like their parents. In addition, seedlings obtained from backcross and sib-cross tests using narrow leaf mutants segregated into normal and narrow leaf-types at a 1:1 and 3:1 ratio, respectively. These results indicate that the narrow leaf trait of three different azalea cultivars, ‘Seigaiha’, ‘Kin-kujyaku’, and R. kiusianum Yanagi-ba mutant, is probably a recessive mutation caused by a single, common gene.