Integrated morpho-physiological and transcriptomic dissection of waterlogging tolerance in Hibiscus hamabo

Hibiscus hamabo is an important coastal ornamental plant and a pioneer tree species for coastal protection forests. Prolonged waterlogging severely restricts its ecological expansion toward the intertidal‑zone forefront and limits population regeneration; clarifying its waterlogging‑adaptation mechanisms is essential for coastal wetland ecological restoration. In this study, phenotypic, physiological and transcriptomic responses of H. hamabo under 3-, 9-, and 18-day waterlogging treatments were characterized. Prolonged waterlogging markedly suppressed plant growth and biomass accumulation, and impaired photosynthetic performance via pigment degradation and stomatal closure. To counteract oxidative injury, H. hamabo sequentially triggered osmotic‑adjustment and antioxidant‑defense systems, relying on soluble sugars, proline and ROS‑scavenging enzymes. Transcriptome profiling identified stage‑dependent gene expression changes mainly related to photosynthesis, hormone signaling and phenylpropanoid biosynthesis. WGCNA uncovered two core co‑expression modules with opposite functional roles, representing stress‑defense activation and growth repression, respectively, and hub genes for osmotic homeostasis, ROS detoxification and photosynthesis preservation were screened. Collectively, our findings illustrate the coordinated adaptive strategy of H. hamabo against waterlogging stress and supply promising candidate genes for breeding waterlogging‑tolerant coastal woody plants.