In vitro embryo germination and interploidy hybridization of rose (Rosa sp)

In roses the problems associated with inter-specific breeding include low percent of seed set and lack or low percent of seed germination. Low seed set is usually due to non-amenable parents, which may have different ploidy level or other divergences that result in embryo abortion at early stages of development. Lack of seed germination is mostly attributed to the mechanical restrictions such as thick pericarp or the regulatory mechanisms such as the hormonal control of dormancy. The aims of the present investigation were to optimize in vitro embryo germination technique in rose and study the ploidy of progenies resulted from interploidy hybridizations. To optimize embryo germination, seeds were surface sterilized, whole pericarp and testa were removed and embryos were placed on half strength Murashige and Skoog media supplemented with different concentrations (0, 1, 2.5, 5 mg l⁻¹) of benzyladenine (BA) in combination with different concentrations (0, 0.5, 1, 1.5 mg l⁻¹) of gibberellic acid. The maximum percent of in vitro embryo germination (93.40 %) was observed on medium containing 2.5 mg l⁻¹ BA. In order to select the most fertile seed parents which could be used in interspecific hybridization, 11 commercial rose cultivars (R. × hybrida) were employed in 36 reciprocal crosses. Three rose cultivars including ‘Golden Celebration’, ‘Tess of the d’Urbervilles’ and ‘Molineux’ were selected as the maternal parents. The selected seed parents were employed in crosses with one rose species from Gallicanae section [R. damascena (2n = 4x)] and four rose species form Caninae section [R. orientalis (2n = 5x), R. iberica (2n = 5x) R. canina (2n = 5x) and R. pulverulenta (2n = 6x)]. The highest percent of hip set and in vitro embryo germination were observed in crosses between tetraploid rose cultivars and R. damascena. In all of the crosses with R. canina, the percent of hip and seed set was 0 %. However, in the crosses between tetraploid rose cultivars and other pentaploid or hexaploid rose species from Caninae section both triploid and tetraploid offsprings were attained. Future morphological analysis of the progenies is necessary to show to what extent progenies demonstrate the characteristics of the pollen parents from the Caninae section. Nevertheless, progenies from interploidy hybridizations would be beneficial in future breeding programs in order to expand the relatively small gene pool of roses.