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An Acinetobacter calcoaceticus UTMR2 strain was evaluated in tomatillo plants (Physalis ixocarpa Brot.) using a factorial design with different potassium doses (100, 75, 50 and 0% of the recommended dose). In addition to the agronomic parameters, an analysis of the physicochemical, antioxidant, and metabolomic properties of the fruit was performed. The application of the inoculant affected several parameters of the plant (chlorophyll, weight, and contents of several mineral elements) as well as of the fruit (yield, maturity index, FRAP antioxidant capacity, and contents of protein, fiber, and fat). A multivariate analysis was performed by means of a PCA and a heatmap, indicating that the inoculant induced a strong modulating activity in tomatillo plants for the evaluated parameters, with a remarkable effect at low K doses (0 and 50%). The inoculated treatment at 75% of the K dose resulted in similar plant and fruit characteristics to the fully fertilized control. On the other hand, the biofertilized treatment with no K addition resulted in the highest values in the plant and fruit parameters. In addition, from the metabolomics analysis of the fruits at 75% of the K dose, the up-regulation of 4,4″-bis(N-feruloyl)serotonin, salvianolic acid K, and chlorogenic acid was observed, which may have a role in anti-senescence and resistance mechanisms. In conclusion, the rhizobacterial strain had a positive effect on plant growth, nutritional quality, bioactive compounds, and antioxidant activity of tomatillo fruits at reduced doses of K fertilizer, which gives support for its consideration as an effective biofertilizer strain. [ABSTRACT FROM AUTHOR]