<p>It is unclear how plant adapts to salt stress via mediating the water and ion balance between below- and above-ground parts of maize under the operation of naphthaleneacetic acid (NAA), an important hormone protecting plants against salt-induced osmotic and oxidative injury. In the current work, maize (<i>Zea mays</i> L.) seedlings, treated with or without exogenous NAA (80&#xa0;mg/L), were exposed to salinity (120&#xa0;mM NaCl) stress in hydroponic conditions. Our results displayed that salinity stress increased Na⁺ buildup and reactive oxygen species (ROS) activity in leaves and roots, while increased MDA and H₂O₂ contents resulted in oxidative stress. These oxidative indicators’ fluctuations led to a reduction in&#xa0;below- and above-ground maize biomass, impaired the amount of relative water content (RWC), and disturbed osmotic potential across maize roots and leaf tissues. Further, salinity negatively affected photosynthetic and gas exchange attributes by altered the accumulation of (soluble proteins, proline, and soluble sugars), causing osmotic imbalance. In contrast, NAA-treated maize seedlings significantly displayed increased proline accumulation, which regulated the osmotic potential, Na⁺/K⁺ ratio in response to osmotic stress. This mechanism enhanced the photosynthetic ability and increased fresh (24.6%) and dry (18.5%) weights in the salt-stressed maize plants. Likewise, NAA-treated salt-stressed plants displayed enhanced water uptake, decreased Na⁺ toxicity, and lowered levels of MDA and H₂O₂ contents in leaf tissues by improving enzymatic antioxidant activities to maintain osmotic balance. Taken together, NAA supplementation proved to be effective in promoting the growth of salt-stressed maize plants, which was found to be associated with enhanced regulation of the hydrological and oxidative defense system to control cellular homeostasis. The rebuilt tradeoff of water and ion levels across below- and above-ground parts under the NAA appeared to be a key physiological driver improving biomass accumulation and adaptation to salt stress. Additionally, correlation and PCA analysis presented strong evidence and confirmed a direct contribution of NAA phytohormones in the growth, hydrological, and physio-biochemical traits of maize seedlings under severe saline stress. This finding suggests that exogenous NAA supplementation could offer valuable strategies for maize cultivation in salt-contaminated environments.</p> Graphical Abstract <p>Graphical abstract of the potential regulatory mechanistic function of NAA in the mitigation of NaCl toxicity in maize seedlings. The Na⁺ toxicity inhibited plant growth characteristics, and a higher Na⁺ concentration was accumulated in the roots and shoots of maize cultivars. In contrast, the application of NAA significantly alleviated Na⁺ toxicity, improved root and shoot growth, and alleviated Na⁺-induced inhibition of gaseous exchange attributes. NAA addition regulated the antioxidant defense system while reducing the oxidative stress and lipid peroxidation of the membranes. The current study demonstrated that NAA could relieve Na⁺ toxicity in maize cultivars by reducing Na⁺ uptake at the root surface and its translocation (root to shoot) and regulating proficient antioxidant coordination in the leaves of maize.</p> <p></p>