Dormancy and germination in Rosa multibracteata Hemsl. & E. H. Wilson

Most of the achenes produced by Rosa multibracteata Hemsl. & E. H Wilson are dormant on maturity and require pretreatment to stimulate germination. To investigate the mechanism of dormancy and to develop effective methods of improving germination, roles of the pericarp, testa, and embryo of R. multibracteata in regulating dormancy were studied by investigating the effect of different pretreatments on germination. The effects of temperature and water stress were also tested with achenes treated by warm plus cold stratification. In freshly harvested achenes, pericarps are permeable and the embryo fully developed, which eliminates the possibility of physical, morphological, or morphophysiological dormancy. Germination percentage remained low (<5%) despite softening the pericarp or even removing it fully. However, fully removing the testa improved germination significantly (39%), indicating the possible presence of germination inhibitors in the testa. Dry storage, scarification with sulphuric acid (H₂SO₄), and warm stratification proved ineffective by themselves but when combined with cold stratification, improved germination and shortened the cold stratification period needed to break dormancy. Dry storage for 68 weeks followed by cold stratification for 16 or 24 weeks resulted in maximum germination (72-79%) among all the treatments. In achenes scarified with H₂SO₄, germination increased with an increase in the duration of cold stratification. Neither gibberellic acid (GA₃) nor 'smoke water' (water through which smoke had been bubbled for 2h) had any positive effect on germination even on seeds that had been mechanically scarified or stratified. Both high temperature and water stress lowered germination in achenes treated with warm plus cold stratification. Our results suggest that R. multibracteata achenes have an intermediate physiological dormancy, and that dry storage for 68 weeks followed by cold stratification for 16 or 24 weeks is the best method for propagating R. multibracteata from seed.