| Summary: | <i>Azospirillum</i> spp. are plant growth-promoting rhizobacteria (PGPR) that exert beneficial effects on plant growth and yield of agronomically important plant species. The aim of this study was to investigate the effects of a root treatment with <i>Azospirillum baldaniorum</i> Sp245 on hormones in xylem sap and physiological performance in purple basil (<i>Ocimum basilicum</i> L. cv. Red Rubin) plants grown under well-watered conditions and after removing water. Treatments with <i>A. baldaniorum</i> Sp245 included inoculation with viable cells (1·10<sup>7</sup> CFU mL<sup>–1</sup>) and addition of two doses of filtered culture supernatants (non-diluted 1·10<sup>8</sup> CFU mL<sup>–1</sup>, and diluted 1:1). Photosynthetic activity, endogenous level of hormones in xylem sap (salicylic acid, jasmonic acid, and abscisic acid), leaf pigments, leaf water potential, water-use efficiency (WUE), and drought tolerance were determined. Fluorescence and gas exchange parameters, as well as leaf water potential, showed that the highest dose of filtered culture supernatant improved both photosynthetic performance and leaf water status during water removal, associated with an increase in total pigments. Moreover, gas exchange analysis and carbon isotope discrimination found this bacterial treatment to be the most effective in inducing an increase of intrinsic and instantaneous WUE during water stress. We hypothesize that the benefits of bacterial treatments based on <i>A. baldaniorum</i> Sp245 are strongly correlated with the synthesis of phytohormones and the induction of plant-stress tolerance in purple basil.
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