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This study examined the effect of inoculation with the plant-growth-promoting rhizobacterium (PGPR) Pseudomonas mendocina Palleroni, alone or in combination with an arbuscular mycorrhizal (AM) fungus, Glomus intraradices (Schenk & Smith) or Glomus mosseae (Nicol & Gerd.) Gerd. & Trappe, on antioxidant enzyme activities (superoxide dismutase, catalase and total peroxidase activities), phosphatase and nitrate reductase activities and solute accumulation in leaves of Lactuca sativa L. cv. Tafalla affected by three different levels of water stress. At moderate drought, bacterial inoculation and mycorrhizal inoculation with G. intraradices, alone or in combination, stimulated significantly nitrate reductase activity. At severe drought, fertilisation and P. mendocina inoculation, alone or in combination with either of the selected AM fungi, increased significantly phosphatase activity in lettuce roots and proline accumulation in leaves. Total peroxidase (POX) and catalase (CAT) activities increased in response to drought, whereas superoxide dismutase activity decreased. Inorganic fertilisation and both combined treatments of PGPR and AM fungus showed the highest values of leaf POX activity under severe drought. The highest CAT activity was recorded in the fertilised plants followed by the P. mendocina-inoculated plants grown under severe stress conditions. These results support the potential use of a PGPR as an inoculant to alleviate the oxidative damage produced under water stress.

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A field-plot experiment was undertaken to compare the effect of inorganic fertilizer with that of inoculation with an arbuscular mycorrhizal fungus, Glomus intraradices, or with a plant-growth-promoting rhizobacterium, Pseudomonas mendocina, alone or in combination with inorganic fertilizer, on plant growth and nutrient uptake by lettuce. The influence of the microbial inoculation treatments on soil physical, biochemical and biological properties was also assessed. Two months after planting, fertilizer and inoculation with G. intraradices or P. mendocina had significantly increased shoot and root biomass and foliar nutrient contents (P, Fe). The inoculation with G. intraradices or P. mendocina both increased the soil water-soluble carbohydrates and the percentage of stable aggregates. In this study, we provide the first evidence of the beneficial effect of a plant-growth-promoting rhizobacterium on soil aggregate stabilization under field conditions. Only inoculation with P. mendocina had a significant effect on the dehydrogenase and phosphatase activities, 21 and 89%, respectively, compared with the control. Inorganic fertilization alone did not increase aggregate stability or enzyme activities in soil, even though this treatment produced the largest increases in mass of lettuce.