Induced Variation in Tetraploid Rudbeckia subtomentosa ‘Henry Eilers’ Regenerated from Gamma-irradiated Callus

Abstract

Rudbeckia subtomentosa ‘Henry Eilers’ is an adaptable and popular garden plant; however, reduction in height and increased novelty in flower color would be desirable. The effect of gamma radiation dose on in vitro survival and development, and ex vitro fertility, phenology, and morphology of tetraploid Rudbeckia subtomentosa ‘Henry Eilers’ was investigated. In vitro embryogenic callus was treated with gamma radiation (0, 5, 10, 20, or 40 Gy). Rooted microshoots were established ex vitro and evaluated for morphology (plant height, number of flowers, diameter of the terminal flowers, diameter of the secondary flowers, number of stems, number of nodes, and internode length), date of first anthesis, winter survival, and pollen fertility on mature, second-year plants. Callus survival had no response to dose 2 months after treatment; however, microshoot number was significantly reduced with increasing dose. In vitro microshoot survival continued to decline at higher doses up to 4 months after treatment. Plant height, average stem height, number of flowers, flower diameter, percent winter survival, and pollen viability were all reduced with increasing radiation dose. Date of first anthesis was also delayed with increased radiation dose. Several off phenotypes were recorded including increased apical splitting of the ray florets in several plants. Gamma radiation was somewhat effective for reducing the height of R. subtomentosa ‘Henry Eilers,’ but flower morphology, flower number, and plant overwintering survival were often negatively affected with increasing radiation dose. Treating callus with low levels of gamma radiation (5–10 Gy) resulted in relatively high in vitro and field survival while inducing a range of other mutations that could be selected.