Foliar application of allantoin and serotonin mitigates salinity stress in bitter melon (Momordica charantia L.)

Bitter melon (Momordica charantia L.) is a climbing species frequently employed in traditional medicine to treat a wide number of diseases. Soil salinity represents a primary abiotic constraint on plant growth, impacting nearly 50% of irrigated farmland and constantly increasing. Previous studies revealed the individual benefits of allantoin and serotonin, in mitigating abiotic stresses. Therefore, in the current study, we investigated their ability, both individually and combination, ameliorating the damaging consequences of salt stress on the development and physiological and biochemical attributes of bitter melon. A factorial experiment arranged within a completely randomized design, replicated three times, was performed. Two variables were examined: (a) three salinity levels (0, 50 and 100 mM NaCl) and (b) foliar application treatments (control (distilled water), allantoin (1000 µmol), serotonin (100 µmol) and allantoin (1000 µmol) +serotonin (100 µmol)). At the end of the experiment, critical parameters related to morphology, biochemistry, and physiology were measured from leaf and root tissues, and fruit secondary metabolites was extracted. Vegetative traits (plant height, number of leaves, leaf area), and fruit-related parameters (fruit mean weight and yield) of bitter melon were suppressed under increasing salinity stress. Exposure to 100 mM NaCl reduced photosynthetic efficiency (SPAD (soil-plant analysis development), Fv/Fm), relative content of water and potassium and phosphorus concentration in bitter melon leaf, while increasing electrolyte leakage, proline, sodium, chloride, hydrogen peroxide, malondialdehyde and antioxidant enzyme activities. However, the combined application of allantoin and serotonin alleviated these effects by increasing antioxidant enzyme activity, proline accumulation, total soluble sugar, total protein, ascorbic acid, relative content of water, potassium and phosphorus, while decreasing malondialdehyde and hydrogen peroxide and sodium and chloride accumulation. In fruit, the content of p-coumaric acid (14.64%), ferulic acid (10.48%), caffeic acid (15.74%), gallic acid (22.04%), and cinnamic acid (37.77%) increased in plants treated with allantoin + serotonin compared to the control. The highest amount of phenolic acids was produced by plants treated with allantoin and serotonin. The application of allantoin and serotonin had a positive effect on the quality characteristics and yield of bitter melon fruit compared to the control. In summary, the findings suggest that treating bitter melon with of allantoin and serotonin, particularly in combination, effectively alleviated the adverse impacts of salinity by inducing critical modifications in bitter melon's physiological and biochemical processes. Further investigation might be useful for translating these results to other salt-stressed crops.