Morphological and Physiological Responses to Drought Stress of Three Species of the Zygophyllum Genus and a Comprehensive Evaluation of Their Drought Resistance

Drought stress triggers significant adaptive responses in plant morphology and physiology. As a representative xerophytic genus, Zygophyllum possesses distinct drought-adaptive traits in morphology, anatomical structure and physiological metabolism. This study examined the responses of three Zygophyllum species (Zygophyllum fabago, Zygophyllum brachypterum, and Zygophyllum obliquum) to progressive osmotic stress induced by 20% PEG-6000 over 28 days under controlled conditions. Among the three species, Z. brachypterum displayed the greatest tolerance, as evidenced by its lowest wilting rate, longest root elongation, and more stable leaf anatomical traits under stress, despite a general decrease in all measured structural parameters across species. Physiologically, Z. brachypterum showed the smallest reduction in leaf water content (5.99% vs. 13.51% in Z. fabago and 20.31% in Z. obliquum), the smallest increase in relative electrolyte leakage (126.72% vs. 172.84% and 162.91%, respectively), lower excised-leaf water loss, and sustained higher photosynthetic rates during later stress stages. It also accumulated the lowest levels of total reactive oxygen species (ROS) and malondialdehyde (MDA), alongside the highest proline content and the highest activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). Based on a comprehensive evaluation integrating multiple morphological, anatomical, photosynthetic, antioxidant, and osmotic-adjustment indicators through principal component analysis and membership function analysis, the drought-resistance ranking at the seedling stage was determined as Z. brachypterum > Z. fabago > Z. obliquum. The superior tolerance of Z. brachypterum was associated with its more stable leaf structure, enhanced water retention, sustained late-stage photosynthesis, and stronger antioxidant and osmotic adjustment capacities. These findings provide physiological insights into PEG-induced osmotic stress tolerance in Zygophyllum seedlings.