Rooting and acclimatization to ex vitro conditions of regenerants of frost-resistant members of Rhododendron

Rooting and acclimatization are limiting stages in clonal micropropagation, especially of woody plant species. This study aimed to evaluate effect of different treatments with indole-3-butyric acid (IBA) on in vitro and ex vitro rooting and acclimatization of microshoots of six winter-hardy rhododendron genotypes. Two methods of IBA applications: direct cultivation on Anderson's medium (AM). supplemented with 25.0 nM IBA and 4-h liquid -pulse treatment with solution of 148.0 I'M IBA were tested. After IBA pulse treatment microshoots were transferred either to in vitro on hormone-free AM or ex vitro in a hydroponic system or mixture of peat and sand (1:1). Maximum in vitro rooting after IBA pulse treatment was observed for evergreen cultivars 'Helsinki University' (75 %) and cl-laaga' (67 %). whereas Far Eastern wild species R. sichotense, R. macronttlatunt, and R. schlippenbachii showed 30-40 % rhizogenesis only. Er vitro rooting in hydroponic system was efficient for evergreen rhododendrons, but R. sichotense microshoots formed roots poorly. The best results for all genotypes studied were obtained under IBA liquid-pulse treatment with ex vitro rooting in the mixture of peat and sand ('Helsinki University' 100 %. R. mucronulatum 89 %, R. sichotense- 84 %, and R. schlippenhachil- 60 %). Our research revealed genotype differences in rooting percent under all IBA treatment studied. The developed protocol demonstrated some advantages of ex vitro rooting including high quality of root and shoot systems and reduction of acclimatization period. Thus proposed method can be used for breeding and conservation of rhododendron gennplasm, as well as for large-scale commercial propagation.