Salt stress is a major constraint on cowpea cultivation in semi-arid regions, primarily due to excess salts in irrigation water and soils. We aimed to investigate the effects of foliar magnesium (Mg) application on the production, phenology, and seed vigor of the cowpea landraces “Pingo de Ouro” and “Costela de Vaca” under salt stress conditions. Two experiments were conducted. The first was carried out in a greenhouse using a randomized block design with five replicates, in a 2 × 3 × 4 factorial scheme: two cowpea landraces (“Pingo de Ouro” and “Costela de Vaca”), three irrigation water salinity levels (0.54, 3.50, and 5.00 dS m⁻¹), and four foliar doses of a product (0.0, 1.0, 2.0 and 3.0 mL L⁻¹) containing 8% magnesium. Morphological traits and seed production were evaluated. The second experiment was conducted in a laboratory using a completely randomized design, also in a 2 × 3 × 4 factorial, with four replicates of 25 seeds each. In the first experiment, the 1 mL L⁻¹ dose provided the best results for pod length in the variety “Pingo de Ouro” under an electrical conductivity salinity of 5.00 dS m⁻¹. In the variety “Costela de Vaca”, this same dose increased the number of seeds per pod and the 100-seed weight under the same salinity level. In the second experiment, seedlings of “Pingo de Ouro” grown from seeds produced by plants treated with 2 and 3 mL L⁻¹ doses showed greater shoot length, root length, stem diameter, and shoot fresh mass, particularly under 0.54 dS m⁻¹ salinity. Therefore, “Pingo de Ouro” exhibited superior seedling growth at doses of 2 and 3 mL L⁻¹, particularly under conditions of low salinity. These findings support the use of foliar magnesium fertilization as an effective agronomic strategy to enhance seed production and quality in cowpea landraces under salt stress conditions.

Salinization is one of the main environmental challenges affecting agriculture in semi-arid regions. We evaluated the feasibility of foliar magnesium and its effects at different doses on the acclimation of cowpea varieties under salt stress. The experiment occurred in a greenhouse using a randomized block design in a 2 × 3 × 4 factorial scheme, with five replicates. Two cowpea varieties—‘Pingo de Ouro’ and ‘Costela de Vaca’—were subjected to three salinity levels in irrigation water (0.54, 3.50, and 5.00 dS m⁻¹) and four foliar magnesium (Mg) doses (0, 1, 2, and 3 mL L⁻¹). Under 3.50 dS m⁻¹ salinity, the 1 mL L⁻¹ dose resulted in the highest yield per plant (18.29 g). CO₂ assimilation was highest with 2 mL L⁻¹ Mg at 3.50 dS m⁻¹ for ‘Costela de Vaca’, and with 1 mL L⁻¹ Mg at 5.00 dS m⁻¹ for ‘Pingo de Ouro’. The ‘Pingo de Ouro’ variety was more tolerant to ‘Costela de Vaca’. Foliar Mg fertilization proved to be a promising strategy to mitigate the effects of salt stress in cowpea, especially for ‘Pingo de Ouro’. Magnesium effectively reduces salt stress, but its effect varies by plant variety and irrigation salinity, necessitating customized dose adjustments.