Proline enhances cowpea resilience to salinity by maintaining water content, regulating sodium ions, and reducing hydrogen peroxide

Salinity poses a significant threat to crop productivity worldwide, prompting the need for effective strategies to enhance plants' tolerance. The cowpea plant is known for its nutritional value and capacity to withstand salinity to a certain level. Proline, an amino acid known for its role in salt tolerance in various crops, has been hypothesized to mitigate salinity-induced damage by regulating some plant's physiological processes. Therefore, this study aims to investigate the potential use of proline in cowpea plantations in saline soils. Cowpea seeds were planted in pots and watered with varying concentrations of NaCl and proline solutions to determine the optimal proline concentration for alleviating salinity stress. The effects of salinity and proline application were assessed by measuring the plant's biomass, Na+ and K+ ion content, hydrogen peroxide (H2O2) levels, and enzymatic and non-enzymatic antioxidants following proline treatment under salinity to elucidate the mechanism underlying proline's mitigation effects. The results indicate that cowpea plants exhibited the highest salinity tolerance at 150 mM NaCl and 30 mM proline treatment. Compared to water-irrigated plants, proline irrigation under salinity insignificantly (P ≤ 0.05) showed changes in the plant biomass, where salinity only reduced the average shoot height by 5.5%, maintained fresh weight at about 100%, and increased dry weight by over 15.1%. Proline application reduced the average Na+ content by 71.5%, helping to maintain the Na+/K+ balance under salinity compared to plants grown under salinity without proline. Proline also helped sustain relative water content by 40.4% but did not significantly enhance endogenous proline accumulation or soluble sugar levels under salinity. Furthermore, proline significantly (P ≤ 0.05) reduced hydrogen peroxide (H2O2), by 47.2% while slightly decreasing total phenolic content (TPC), total flavonoid content (TFC), glutathione, and superoxide dismutase (SOD) activity, under salinity. These findings suggest that proline enhances cowpea resilience to salinity by maintaining water content, regulating ion homeostasis, and mitigating H2O2 oxidative stress. This study highlights the potential application of proline-enriched organic fertilizers as a strategy to alleviate soil salinity and support sustainable cowpea cultivation.