Combined Effect of Methionine and γ-Aminobutyric Acid Reduces Ionic Toxicity and Increases Arugula Tolerance to Salt Stress

This study aimed to evaluate the foliar application of methionine and γ-aminobutyric acid (GABA), alone or in combination, in the mitigation of salt stress and its effects on the physiology and growth of arugula ( Eruca sativa ). The plants were subjected to three levels of electrical conductivity of irrigation water (0.5, 2.5, and 4.5 dS m⁻¹) and four foliar treatments in a greenhouse pot experiment: no application, 0.3 mM methionine, 0.5 mM GABA, and the combined application of both. Severe salt stress (4.5 dS m⁻¹) drastically reduced development and gas exchange in non-supplemented plants. However, the combined application of methionine and GABA significantly mitigated this damage. Under severe salinity, the joint treatment increased the net CO₂ assimilation rate by 44.9% and carboxylation efficiency by 87.4% compared to the control. In addition, the combination protected membrane integrity by reducing electrolyte leakage. Consistent with the preservation of the photosynthetic machinery, structural protection, and partial restriction of Na⁺ accumulation depending on stress intensity, plants receiving the joint application showed significant increases in plant height and a 54.1% increase in root dry mass at the highest saline level evaluated. The combined application of methionine and GABA effectively mitigates the deleterious effects of salinity by preserving photosynthetic performance, maintaining membrane integrity, and sustaining root biomass accumulation. The combined use of these biostimulants therefore represents a promising agronomic strategy for arugula cultivation under saline conditions.