Jasmonic acid and engineered dog manure biochar improve salinity tolerance in Phaseolus vulgaris through coordinated hormonal, antioxidant, and root responses

Salinity is a major abiotic stress that disrupts plant growth by altering hormone balance, oxidative homeostasis, and root development. However, the combined effects of jasmonic acid and engineered biochar on these responses remain poorly understood. A two-year glasshouse pot experiment was conducted during the 2023 and 2024 growing seasons using a randomized complete block design with four replications to investigate the effects of JA (0, 1, and 2 mM) and dog-manure-derived biochar (raw, hydrochloric acid-modified, and hydrogen peroxide-modified) on the morphological, root anatomical, physiological, biochemical, and hormonal responses of Phaseolus vulgaris under three salinity levels (0, 3, and 6 dS m^-1 NaCl). Salinity significantly inhibited shoot and root growth, root cortical parenchyma and vascular cylinder development, relative water content, and endogenous indole-3-acetic acid (IAA), while promoting oxidative stress. In contrast, JA and biochar jointly improved salinity acclimation, as shown by changes in endogenous ABA, IAA, and ACC concentrations, antioxidant enzyme activities, oxidative damage markers, osmotic adjustment, and root traits. Under saline conditions, biochar treatments reduced the ACC and ABA concentrations while increasing the IAA and the IAA to ABA and IAA to ACC ratios, indicating a shift in hormone balance associated with improved root growth and plant water status. The combined treatments also strengthened the antioxidant capacity through increased catalase activity and reduced oxidative damage. Root anatomical traits remained largely unaffected by biochar under non-saline conditions, suggesting that biochar-induced structural remodeling primarily occurred during stress adaptation. The strongest osmoprotective response was observed with raw biochar plus 2 mM JA under moderate salinity, whereas hydrochloric acid-modified biochar combined with 1 mM JA produced the greatest catalase activity. To our knowledge, this study provides the first integrated evaluation of dog manure-derived biochar and JA across hormonal, antioxidant, oxidative, and root anatomical, and growth responses in salt-stressed common bean. These findings show that the combined treatments are associated with coordinated changes in hormone balance, antioxidant capacity, oxidative damage, and root traits, while direct signaling mechanisms require molecular and pathway-based validation. These findings identify a potential treatment approach for improving salinity tolerance in P. vulgaris under controlled glasshouse conditions and provide a basis for future field evaluation.