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Plant developments in Mediterranean ecosystems are affected by a characteristic multiple stress situation, which mainly derives from water-deficit in these areas. Drought stress conditions restrict nutrient and water acquisition capability of plants thus affecting the sustainability of such ecosystems. Because mycorrhizal fungi are known to enhance the ability of plants to establish and cope with stress situations (nutrient deficiency, drought, etc.), the use of these fungi as plant inoculants, is currently being investigated to help plants to thrive in degraded arid/semi and Mediterranean areas. To assess the role of arbuscular mycorrhizal (AM) fungi on nutrient acquisition by plant under drought stress, this study used a biochemical marker, i.e., the nitrate reductase (NR) activity, both in roots and shoots, because this is the first enzyme involved in nitrogen (N) assimilation by plants. Measurement of the "intrinsic water use efficiency" was used to evaluate the effect of AM fungi on some physiological activities related to water relation in plants under drought stress. Three experimental variables were tested including: (i) the use of two Mediterranean-autochthonous shrub host species to the target ecosystem, i.e., Olea europaea subsp. sylvestris and Retama sphaerocarpa L.; (ii) mycorrhizal inoculation with AM fungi either autochthonous or alochthonous from the target area; and (iii) an imposed drought stress to nursery-produced plants. Plant growth, nutrient acquisition, NR activity, both in shoot and root, photosynthesis rate (A) and stomatal conductance (g(s)), were measured before transplanting to the target sites. AM inoculation improved plant growth, NR activity (only autochthonous fungi), nutrient acquisition and the "intrinsic water use efficiency" (A/g(s)) in drought stressed O. europaea plants, while in stressed R. sphaerocarpa plants AM inoculation benefited only NR activity (with alochthonous fungi) and N acquisition. The advantages of mycorrhizal inoculation to help plant development under Mediterranean climate conditions were therefore demonstrated. The differential plant species responses are discussed in terms of mycorrhizal dependency/functional compatibility criteria.