Changes in xylem architecture modulate physiological responses in coexisting tropical dry forest plants under salinity

Plant mortality under increasing abiotic stress is associated with xylem vulnerability and the ability to adjust hydraulic mechanisms. Understanding these processes in saline environments can reveal the future survival dynamics of coexisting species. This study investigated how adjustments in the xylem traits of different functional plant groups from the Caatinga dry forest influence adaptation under saline conditions. Deciduous species with high wood density (HWD) and low wood density (LWD) were analyzed. The plants were subjected to different electrical conductivities: 0.15 (control), 2.5, 3.5, and 4.5 dS m⁻¹ for 90 days in a nursery (70 % shading) under a completely randomized design (CRD). The following parameters were evaluated: stomatal conductance (gs), xylem water potential (Ψ_xylem), Na⁺/K⁺ ratio in leaf, stem, and root, leaf number, xylem anatomical characteristics, potential hydraulic conductivity (Kp), and the xylem vulnerability index of the stem and root. Data were analyzed using polynomial regression and Tukey’s test (P = 5 %). The HWD group showed higher xylem trait resistance values, higher gs (298 mmol m⁻² s⁻¹), leaf drop with a higher Na⁺/K⁺ ratio (1.1–1.9) at the highest level of EC, stomatal density (by 31.2 %), and leaf renewal. The LWD group exhibited a more significant reduction in xylem vessel lumen diameter (60 %), vulnerability index (90 %), and Kp (80 %) with salinity, higher Ψ_xylem (by ca. -1 MPa), and Na⁺/K⁺ ratio in the root (ca. 160 %). HWD species showed resistance, with physiological stability and slight anatomical adjustment, while LWD species adjusted the xylem more to cope with salinity. Future studies should explore the limits of these adjustments in different environments.