Selection of lime-tolerant azaleas based on seed germination responses to pH regimes

In the natural habitat, azaleas grow on acid soils. Depending on the species, the optimal value of pH of the soil ranges between 4.5-6.0. High content of calcium compounds in soil has adverse effect on the development of these plants. Being strongly alkalic, the hydrogen carbonate ions have very toxic influence on the root system and, in consequence, the growth of the plant is inhibited. Thus, the alkalization of rhizosphere constitutes the main factor limiting the possibility of azalea cultivation in calcareous soils. With a long-term goal of selecting genotypes tolerant to pH higher than optimum, seed germination responses of five azalea species (R. japonicum, R. kiusianum, R. kaempferi, R. macrosepalum, R. ripense) to nine pH regimes (2, 2.5, 3, 3.5, 5, 7, 7.5, 8, 8.5) were investigated. Wild azaleas showed different adaptability to pH conditions, according to the species and habitat environment.