Stress Adaptation Phenomena of Rhododendron Species in Alpine Tundra and Timberline of Changbai Mountain: Physiological Traits and Molecular Evolution.

In the context of climate change, Rhododendron species are pivotal in sustaining the stability of alpine ecosystems. Within alpine tundra (elevation > 2200 m) and timberline (elevation ~ 2000 m) regions of Changbai Mountain, the three studied Rhododendron species (Rhododendron aureum, Rhododendron lapponicum, and Rhododendron redowskianum) are prevalent; their mechanisms of adaptation to high-altitude environments remain insufficiently understood. This study employed an integrative approach, combining soil chemical analysis, physiological assessments, and molecular evolutionary analysis, to investigate phenotypic plasticity and genetic adaptation of these Rhododendron species. Both habitats demonstrated oligotrophic characteristics, with no significant differences (p > 0.05) observed in the concentrations of soil total organic carbon (TOC), ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N), and available phosphorus (AP). Nonetheless, soil nutrient variability was more marked in timberline. Physiological traits, including malondialdehyde (MDA), soluble sugar, proline, and soluble protein, exhibited species-specific patterns; for example, R. redowskianum displayed elevated proline content in the timberline habitat, although no consistent inter-habitat trends were identified. From a total of 1995 orthogroups analysed, we identified 279 positively selected genes (PSGs, dN/dS > 1). These genes were found to be enriched in GO terms associated with DNA replication, amino acid transport, and pathway of nucleocytoplasmic transport. The study highlights tissue development and reproduction as primary evolutionary trajectories, while identifying cold stress as a significant environmental selection pressure. This research elucidates Rhododendron's alpine adaptability and provides insights into alpine plant adaptation mechanisms and species conservation under climate change.