Evaluation of variability in Japanese wild azaleas and application of lime-tolerant genetic resources for breeding

Evergreen azaleas (genus Rhododendron L. section Tsutsusi Sweet) are horticulturally important ornamental plants, with Asian origins and a long breeding history. Especially in Japan, wild evergreen azalea species are distributed widely and abundantly. Rhododendron species grow in nature in humus soils with low pH values. Azalea cultivars, performing best when the pH of the medium is between 4.5 and 6.0 (acidic), often show strong iron-deficiency chlorosis symptoms if they are grown on calcareous soil. This stress is one of the most serious difficulties in their cultivation. At present, no evergreen azalea cultivar has complete resistance to iron deficiency chlorosis. However, genetic variability, which permits improvement by plant breeding, exists even within species. In this study we investigated the biodiversity of Japanese wild azaleas during the flowering season looking for genetic resources tolerant to abiotic stresses. In particular, in order to select lime-tolerant species, we analysed the soil taken from their root zone in natural habitats. Wild azaleas showed different adaptability to soil pH, according to the species and habitat environment. Results on germination and seedling growth of these species, carried out at different pH conditions, also reflected this different aptitude.