Coordinated responses of leaf water status, anatomical structure, and photosynthetic performance in different Bougainvillea cultivars under low-temperature stress

Systematic studies on coordinated regulation of leaf water status, anatomical structure, and photosynthetic characteristics of Bougainvillea under cold stress are limited. Here, we aimed to investigate the physiological response mechanisms of Bougainvillea under low-temperature stress; two Bougainvillea species with contrasting levels of cold tolerance—Bougainvillea glabra “Brasiliensis” (cold-tolerant) and B. spectabilis “Auratus” (cold-sensitive)—were selected as experimental materials. Plantlets were subjected to low-temperature stress for 0, 1, 3, and 7 days, and leaf water status, anatomical structure, and photosynthetic parameters were analyzed. Principal component and orthogonal partial least squares discriminant analyses were used to identify the major response indicators, and correlation analysis was conducted to explore the potential synergistic regulation among key physiological traits. Low-temperature stress significantly affected leaf water metabolism, photosynthetic performance, and structural stability in Bougainvillea. Brasiliensis exhibited higher relative water content and water use efficiency, lower water saturation deficit, and higher photosynthetic activity than did Auratus, under low-temperature conditions; its leaf anatomical structure was relatively stable, with significantly high thickness of epidermal and palisade tissues. Furthermore, intercellular CO₂ concentration (Ci), and leaf, palisade tissue, and spongy tissue thickness were key physiological–structural indicators differentiating cold tolerance between the cultivars, with Ci negatively correlated with the above structural parameters. Overall, the differences in cold tolerance of Bougainvillea cultivars under low-temperature stress were primarily attributed to the integrated regulation of water retention capacity, anatomical stability, and photosynthetic efficiency. This study provides theoretical and technical basis for selecting cold-tolerant Bougainvillea cultivars and elucidating the mechanisms of cold resistance.