Integrated Multi-Trait Analysis of Photosynthetic Carbon Assimilation Pathways and Adaptive Patterns in Dendrobium Species

To analyze the diversity of photosynthetic carbon assimilation pathways and ecological adaptation characteristics in Dendrobium species, this study selected nine native Chinese Dendrobium species. A comprehensive approach was employed, integrating diurnal dynamics of net CO ₂ exchange, carbon isotope ratios (δ ¹³ C), diurnal titratable acid accumulation (ΔH ⁺ ), leaf anatomical structure, and mesophyll succulent index (Sm). The results indicate: that Dendrobium parishii , Dendrobium aphyllum , and Dendrobium anosmum maintain positive net CO ₂ exchange at night, with δ ¹³ C values exceeding − 16‰ and high nocturnal acid accumulation. Combined with their substantial leaf thickness (LT) and Sm, these species are classified as typical CAM plants; Dendrobium cariniferum , Dendrobium gibsonii , and Dendrobium hancockii exhibited no positive CO ₂ exchange throughout the night, with δ ¹³ C values below − 26‰ and low ΔH ⁺ levels, demonstrating physiological characteristics consistent with the C ₃ pathway; Additionally, Dendrobium linawianum , Dendrobium moschatum , and Dendrobium crystallinum exhibited weak nocturnal CO ₂ exchange and moderate to low acid accumulation. However, their δ ¹³ C values remained within the C ₃ range, indicating they are not stable CAM types but may exhibit plastic expression under specific environmental conditions, displaying characteristics of intermediate C ₃ /CAM photosynthetic plants. The LT and SM exhibited partial overlap between C ₃ /CAM and strictly C ₃ species, highlighting the limitations of using morphological traits as sole diagnostic criteria. Correlation analyses further revealed that although LT was significantly associated with Nighttime Net CO ₂ Exchange (NNEE) and δ ¹³ C, the Sm displayed only weak correlations with key physiological indicators. In contrast, NNEE, δ ¹³ C, and ΔH ⁺ were strongly aligned with one another, underscoring that structural characteristics can serve only as supplementary references, while reliable classification must depend on the coordinated variation of physiological metrics. Furthermore, the photosynthetic types of the examined Dendrobium species closely corresponded to moisture availability and light conditions in their native habitats, reflecting the ecological plasticity of their carbon-assimilation strategies. In summary, Dendrobium species exhibit remarkable diversity and environmental plasticity in their photosynthetic carbon assimilation strategies. This study, through multi-indicator integrated assessment, not only clarifies the photosynthetic types of different species but also provides important references for identifying photosynthetic carbon assimilation pathways and cultivating Dendrobium species.