Polyethylene glycol and proline synergistically improve salinity tolerance via physiological and biochemical reprogramming in mango

Abstract Background Mango (Mangifera indica L.) is a globally important fruit crop, but its sensitivity to salt stress poses a serious threat to its sustainable cultivation. Salt stress impairs mango growth through osmotic imbalance, ion toxicity, oxidative damage, and reduced nutrient uptake. This study examined the biochemical, physiological, vegetative, and reproductive responses of the Egyptian mango cultivar ‘Ewais’ under constant salinity stress reflecting the naturally saline irrigation water in the orchard. The research specifically aimed to evaluate the efficacy of polyethylene glycol (PEG) and proline in mitigating the adverse effects of salt exposure. Nine treatments were tested over two consecutive growing seasons (2023 and 2024). The treatments consisted of PEG applied individually at 15 mM (T1) and 30 mM (T2), as well as proline applied at 8 mM (T3) and 13 mM (T4). Four combinations were also examined, including T1 + T3 (T5), T2 + T4 (T6), T1 + T4 (T7), and T2 + T3 (T8). A control group (T9) consisted of trees subjected to the same salinity conditions without any application of PEG or proline. Results The treatments investigated revealed significant improvements in relative water content (RWC), membrane stability index (MSI), peroxidase (POD) and polyphenol oxidase (PPO) activities, chlorophyll concentration, ascorbate accumulation, proline concentration, total soluble sugar metabolism, and the hormonal balance of indole-3-acetic acid (IAA) and abscisic acid (ABA). Also, positive responses were observed in vegetative growth traits such as shoot elongation, number of flushes, and leaf area, as well as in reproductive traits including fruit set percentage, average fruit weight, and yield. Specifically, the combined treatments of PEG and proline, T6 (PEG 30 mM + Proline 13 mM) and T8 (PEG 30 mM + Proline 8 mM), led to sustained improvements in physiological and agronomic performance. Conclusion The findings support a dual-action mechanism in which PEG triggers osmotic signalling, while proline contributes to maintaining antioxidant defences and metabolic stability, thereby establishing PEG-proline co-application as a promising strategy for enhancing mango productivity under saline conditions.