Effects of thidiazuron on in vitro morphogenic response of Rhododendron sichotense Pojark. and Rhododendron catawbiense cv. Grandiflorum leaf explants

Thidiazuron (TDZ)-induced shoot organogenesis from in vitro-derived leaf explants of winter-hardy Rhododendron sichotense Pojark. and Rhododendron catawbiense cv. Grandiflorum was studied using two TDZ application methods: adding various concentrations to Anderson nutrient medium and a liquid pulse treatment. The highest frequency of shoot regeneration (93%) and maximum number of shoots per explant (24.6) from R. sichotense was obtained after a two-stage culture procedure that included exposure of leaf explants to 1.0 μМ TDZ followed by TDZ-free cultivation. TDZ at concentrations of 0.5 and 1.0 μМ was effective for leaf explants of R. catawbiense cv. Grandiflorum, inducing 60 and 85% regeneration frequencies, and 36.0 and 25.0 shoots per explant, respectively. The liquid-pulse treatment of leaf explants with 30 μM TDZ for 4 h was effective only for R. catawbiense cv. Grandiflorum. Histological analysis revealed that epidermal cells of the adaxial side at the leaf-blade base were involved in de novo shoot regeneration in both genotypes. However, the adventitious shoot buds of R. sichotense developed from protuberances, whereas shoot organogenesis of R. catawbiense cv. Grandiflorum occurred through the formation of embryo-like structures. Optimum root formation for both genotypes was achieved using indole-3-butyric acid liquid-pulse treatment of microshoots followed by ex vitro rooting. An efficient system was developed for multiplication of R. sichotense and R. catawbiense cv. Grandiflorum, which are cold-hardy genotypes promising for future genetic transformation.