Salicylic Acid Alleviates Moderate Salinity-Induced Stress in Petroselinum crispum (Mill.), within Reduced Effectiveness under Prolonged High Salinity Conditions

This study elucidates the role of salicylic acid (SA) in modulating salinity stress responses in parsley through a controlled RCBD experiment at Taiz University. Plants exposed to incremental NaCl concentrations (0–200 mM) either alone or in combination with three SA concentrations (0, 0.1, and 0.5 mM) exhibited dose-dependent growth inhibition. With 200 mM NaCl, reduced shoot length by 42% and dry matter by 35%, while concurrently suppressing protein content (28%) and catalase activity (40%) but elevating proline accumulation (69%). SA application (0.1 mM) significantly alleviated moderate stress (100 mM NaCl), restoring physiological parameters and enhancing antioxidant defenses, notably increasing catalase activity by 30–35% and reducing proline by 25–30%. Intriguingly, SA’s efficacy was temporally constrained, with optimal protection at 15 days that diminished under prolonged exposure. Biochemical profiling revealed a stress-threshold response: ascorbate peroxidase activity remained stable at ≤ 150 mM NaCl but increased 20–25% at 50–100 mM NaCl with 0.5 mM SA. Critically, high SA (0.5 mM) synergized with extreme salinity (200 mM NaCl) to induce complete mortality, demonstrating a concentration-dependent phytotoxic shift. These findings establish 0.1 mM SA and 100 mM NaCl as critical thresholds for stress mitigation.