14172 10.30848/PJB2026-12(22) <p> Foliar application of alginate oligosaccharides enhances salinity tolerance, vegetative growth, flowering performance, and physiological responses of <em>Bougainvillea spectabilis</em> under salt stress</p> Mohammed A. Al-Yafrsi, Mohammed Sallam, Thabit Alqardaeai, Akram Rajeh and Ahmed Z. Dewidar <p style="text-align: justify;"> Salinity stress is one of the key environmental stresses that restricts the development, decorative quality and blooming performance of landscape plants. <em>Bougainvillea spectabilis</em> is a common ornamental shrub grown in arid and semi-arid areas for its drought resistance and stunning flower display, however chronic salt exposure severely affects its development and physiological functioning. Marine-derived biostimulants, especially alginate oligosaccharides (AOS) have emerged as powerful eco-friendly agents to improve plant tolerance to abiotic stressors. The current research was carried out to evaluate the effects of foliar applications of AOS on growth, flowering, physiological characteristics, antioxidant defense system and ionic balance of <em>B. spectabilis</em> under salt stress. Plants were watered with 0 or 60 mM NaCl and sprayed with AOS at 0, 50, 100 or 150 mg L^-1. Salinity markedly decreased plant height, branching, leaf production, shoot biomass, flowering features, chlorophyll concentration and relative water content but increased lipid peroxidation, hydrogen peroxide build-up and leaf sodium content. The foliar spray of AOS significantly mitigated the deleterious effects of salt stress. Among all the investigated doses, the maximum vegetative growth, flowering performance, photosynthetic pigments, antioxidant enzyme activities and K⁺/Na⁺ ratio were obtained with 100 mg L^-1 AOS. Treated salt-stressed plants with 100 mg L^-1 AOS showed an increase of around 32% in plant height, 56% in shoot dry weight and 86% in inflorescence output relative to untreated salt-stressed plants. In addition, AOS significantly decreased oxidative damage by reducing malondialdehyde and hydrogen peroxide build-up. The results indicate that AOS may successfully increase the salt tolerance of <em>B. spectabilis</em> by improving physiological performance, antioxidant capacity, osmotic adjustment and ion homeostasis, thereby supporting their prospective use in sustainable ornamental horticulture.</p> inpress 2026 58 12 1 8