Physiological and Molecular Insights into the Development of Single and Double Flowers in Syringa vulgaris L.

The double-flowering phenotype is crucial for improving the ornamental value of flowering plants; this trait substantially enhances the varieties of Syringa vulgaris L. To date, no studies have examined the transcriptomics and metabolomics of key nodes in the flower bud differentiation of the single and double flowers of Syringa. This study investigated both the single and double flowers of S. vulgaris using a comprehensive, multifaceted analytical approach, including physiological assessments, transcriptomics, and metabolomics. The floral bud differentiation process can be divided into six distinct stages. Compared with the single flowers, the double flowers of S. vulgaris presented significant developmental delays during floral bud differentiation. Although there was no significant difference in the soluble protein content between the two flower types during this process, the soluble sugar content varied during pistil primordium differentiation and as temperature increased. The antioxidant enzyme activity was significantly greater in the double flowers than in the single flowers during most differentiation stages, while the malondialdehyde (MDA) level gradually increased. The levels of endogenous hormones, such as indole-3-acetic acid (IAA), gibberellin (GA3), and abscisic acid (ABA), differed between the two flower types. The transcriptomics and metabolomics results indicated that during pistil primordium differentiation and subsequent development, the double flowers exhibited increased antioxidant enzyme activity and secondary metabolite accumulation. These secondary metabolites not only contributed to the vibrant coloration of the double flowers but also increased their cellular metabolic stability and stress tolerance through their antioxidant properties. Conversely, the rapid differentiation mechanism of the single flowers of S. vulgaris relied more on efficient primary metabolism to meet simpler structural demands. These findings not only provide scientific guidance for S. vulgaris breeding programs but also expand its potential in horticultural and landscape applications, offering a new theoretical foundation for studies on floral organ development in Oleaceae species.