Arbuscular Mycorrhizal Fungi and Molybdenum Act on Different Components of the Cowpea Plant–Soil System Under Salinity

This study evaluated the combined effects of arbuscular mycorrhizal fungi (AMF) and two molybdenum (Mo) doses on cowpea (Vigna unguiculata L., cv. Karnıkara) grown under saline and non-saline soil conditions. A greenhouse experiment was conducted using a 2 × 6 factorial design with three replications, and morphological, physiological, biochemical and mycorrhizal traits were measured. The Mo doses were deliberately set above the agronomic range in order to test how the symbiosis behaves under Mo excess. Salinity reduced fresh root weight by 53% and root length by 30%, whereas stomatal conductance and soil catalase activity increased by 87% and 379%, respectively. None of the treatments changed plant height, plant fresh weight, stomatal conductance, root length or root fresh weight: the main treatment effect and the treatment × soil interaction were not significant for any plant growth trait. Under saline conditions, AMF applied alone produced the highest mycorrhizal density (3.7%), and the addition of Mo reduced it, indicating a dose-dependent suppression of colonization rather than a synergistic enhancement. The AMF × Mo interaction was significant for soil enzyme activities but not for plant biomass, indicating that the combination acted mainly on the soil biochemical compartment. Overall, AMF and Mo influenced largely separable components of the cowpea plant–soil system under salinity. Under the experimental conditions, AMF application under salinity was associated with more favorable responses in terms of root development and stomatal characteristics, whereas the combined application of supra-optimal Mo doses with AMF did not provide a marked improvement in these responses. These results should be interpreted within the context of the application doses and experimental conditions used in this study. Further field studies encompassing different soil properties, salinity levels, Mo doses, and growing conditions are needed before these findings can be directly generalized to broader agronomic conditions.