Methyl Jasmonate and γ-Aminobutyric Acid as Attenuators of Salt Stress in Sunflower

Abstract Foliar application of stress-mitigating biomolecules, such as methyl jasmonate (MJ) and γ-aminobutyric acid (GABA), is a promising strategy to protect plant physiology under salinity stress. However, it remains unclear whether their combined application provides benefits beyond those of each compound alone in oilseed crops. This study evaluated MJ and GABA, applied alone and in combination, as attenuators of salt stress in sunflower (Helianthus annuus L., cv. BRS 323). A greenhouse experiment was conducted in a randomized block design with a 3 × 4 factorial arrangement: three electrical conductivities of the irrigation water (ECw: 0.5, 3.5, and 6.5 dS m–1) and four foliar treatments (control, MJ at 1 mM, GABA at 2 mM, and MJ + GABA). We also decomposed the attenuator factor into a 2 × 2 factorial (MJ × GABA) to test for synergistic effects. Salinity at 6.5 dS m–1 reduced plant height by 29%, total dry mass by 41%, and net CO2 assimilation (A) by 37%; in untreated plants, it also reduced the maximum quantum efficiency of photosystem II (Fv/Fm) by 26% and nearly doubled leaf Na+ content. Regardless of salinity, GABA increased stem dry mass by 16%, total dry mass by 15%, and A by 20%. In contrast, MJ + GABA increased capitulum dry mass by 21%, A by 22%, and water-use efficiency by 20% relative to untreated plants. At 6.5 dS m–1, GABA and MJ + GABA increased Fv/Fm by 7–9% and relative water content by 38–40%, and all attenuators reduced leaf Na+ content by 29–47%, with MJ alone being the most effective. The MJ × GABA interaction was not significant for most traits. Responses to the combined treatment were largely driven by GABA, with no consistent evidence of synergistic benefits. Therefore, GABA, alone or combined with MJ, is the most promising option among those tested for mitigating salt stress in sunflower, although field validation, including seed and oil yield, is still required.