Evergreen azaleas tolerant to neutral and basic soils: breeding potential of wild genetic resources.

Evergreen azaleas (Rhododendron subg. Tsutsusi) are ornamental plants which usually grow in acidic soils. They perform best when the pH ranges between 4.5 and 6.0. On calcareous soils, they often show strong iron-deficiency chlorosis symptoms and their growth is inhibited. Therefore their cultivation area is limited. However, wild azaleas were seen to show different adaptability to soil pH, according to species and habitat environment. With a long-term goal of selecting commercial genotypes tolerant to pH higher than optimum, we collected plant materials in the wild and in historical gardens and we performed specific crosses. To evaluate their neutral-basic pH tolerance, seeds were placed to germinate at nine different pH regimes ranging from 2 to 8.5. Ultra acid pH inhibited seed germination especially in R. macrosepalum, R. ripense and R. japonicum. However a clear correlation between germination capacity and pH was not observed. Genotype dependent differences were better observed by a screening, in which rooted cuttings were grown in nutritive solutions at three pH regimes (6.0, 7.5, and 9.0). R. indicum showed serious chlorosis symptoms in all conditions. By contrast, R. macrosepalum var. hanaguruma and R. scabrum were tolerant to neutral-basic pH, showing very limited leaf damage. As confirmed by molecular analyses, these two latter species belong to the subsection Macrosepala and are close related to R. ripense, whose elevated pH tolerance was already known. These wild genetic resources appear promising for pH tolerance breeding efforts.