Environmental stress responses and climate vulnerability of Abies beshanzuensis: combined physiological and transcriptomic analysis.

Abies beshanzuensis M. H. Wu is a critically endangered conifer endemic to cool, high-elevation forests; its narrow range renders it vulnerable to climate warming and associated heat and water stress. We simulated climate warming by growing seedlings along an elevational gradient (500-1200 m) to assess physiological and molecular stress responses. Seedlings at mid-high elevation (1000-1200 m) grew optimally, whereas low-elevation (500 m) seedlings showed stress symptoms. At 500 m, Yield and ETR declined sharply; stomatal conductance also dropped, and NPQ collapsed entirely, dropping below the 1550 m baseline, proving quantitatively insufficient to prevent massive photoinhibition. Consequently, leaf chlorophyll content and biomass peaked at 1000 m and declined precipitously toward 500 m. Low-elevation leaves accumulated significantly higher levels of proline, soluble sugars, and proteins. However, this robust osmotic adjustment was physically insufficient to counteract the severe drought and heat stress, culminating in elevated MDA content and severe cellular damage. Antioxidant enzyme activities were highest at 1000 m and decreased at lower elevations. Transcriptome profiling and analysis of DEGs revealed altitude-dependent gene regulation. At 500 m, structural hub genes driving oxidative phosphorylation, glutathione metabolism, flavonoid biosynthesis (CSF7, MDMC, CHS), circadian rhythms, and heat-shock proteins were strongly upregulated. In contrast, genes for photosystems, primary hormone signaling, brassinosteroid biosynthesis, and pathogen defense were suppressed. Simulated warming induced pronounced stress responses, with photosynthesis and growth severely compromised, overwhelming osmotic adjustment and antioxidant defenses. These findings underscore the extreme climate vulnerability of A. beshanzuensis, establish 1000 m as a lower survival threshold, and highlight the urgent need to protect its high-elevation habitat and implement assisted conservation strategies.