Physiological, biochemical and microstructural responses of the epiphytic orchid Dendrobium chrysanthum to drought...

To elucidate organ-level drought adaptation in epiphytic orchids, this study investigated the endangered species Dendrobium chrysanthum under well-watered control (CK), moderate drought (T1; 14 d of water withholding), and severe drought (T2; 28 d). Water status, photosynthetic physiology, microstructure, osmotic adjustment, antioxidant defense, and lipid peroxidation were analyzed. Relative water content (RWC) declined progressively in roots, pseudobulbs, and leaves, with pseudobulb RWC decreasing more markedly than leaf RWC under severe drought, suggesting that pseudobulbs may contribute to buffering leaf dehydration. Gas exchange was strongly suppressed: net photosynthetic rate (P _n) decreased under T1 and became slightly negative under T2, while intercellular CO₂ concentration (C _i) first decreased and then increased, indicating a shift from stomatal limitation to non-stomatal inhibition. The slightly negative Pn under T2 suggests a transition toward a survival-maintenance state with limited carbon gain. Chlorophyll a, chlorophyll b, total chlorophyll, carotenoids, and the chlorophyll a/b ratio showed no significant differences among treatments, indicating maintenance of the pigment pool. Osmotic adjustment substances and antioxidant enzymes showed organ-specific responses, suggesting coordinated stem-leaf metabolic regulation and ROS-scavenging adjustment under drought. MDA content decreased under T1 but increased markedly under T2 in both leaves and pseudobulbs, indicating enhanced lipid peroxidation under severe drought; however, MDA levels remained lower in pseudobulbs than in leaves. F _v/F _m was maintained under T1 but decreased under T2, and Φ _PSII declined progressively, reflecting partial photoinhibition. Anatomical changes, including increased root cortex thickness, reduced root xylem vessel diameter, and decreased pseudobulb tissue area fraction, further reflected structural adjustment associated with water conservation. Overall, D. chrysanthum appears to adopt a conservative drought-response strategy involving pseudobulb-associated water buffering, photosynthetic down-regulation, osmotic regulation, and organ-specific antioxidant adjustment.