Nitrogen metabolism, plant growth, leaf gas exchange and stomata density under inoculation of Pseudomonas fluorescens and Azospirillum brasilense into hydroponic arugula

Plant growth-promoting bacteria may improve nutrient acquisition and physiological performance in hydroponic crops, but the benefits of increasing inoculant concentration remain poorly understood. Two independent experiments were conducted to evaluate Pseudomonas fluorescens strain CCTB03 and Azospirillum brasilense strains AbV5 and AbV6 in hydroponically grown arugula. In each experiment, inoculants were applied once to the recirculating nutrient solution to obtain nominal initial concentrations of 0, 3.2 × 10 4 , or 6.4 × 10 4 CFU mL −1 . In the A. brasilense experiment, the nominal initial concentration of 3.2 × 10 4 CFU mL −1 increased shoot N accumulation by 144% relative to the corresponding non-inoculated control. In the independent P. fluorescens experiment, the same nominal initial concentration increased shoot N accumulation by 202% relative to its respective non-inoculated control. Nitrate reductase activity increased by 1035% with A. brasilense , while P. fluorescens increased this activity by 2194 and 2063% at 3.2 × 10 4 and 6.4 × 10 4 CFU mL −1 , respectively. Inoculation also reduced shoot NO 3 − accumulation by at least 35% with A. brasilense and 44% with P. fluorescens , while increasing NH 4 + and mineral nutrient accumulation. Both inoculants increased CO 2 assimilation, stomatal conductance, transpiration, intercellular CO 2 concentration, stomatal density, and plant biomass, whereas water-use efficiency remained unchanged. Across several nutritional and growth variables, 3.2 × 10 4 CFU mL −1 produced responses equal to or greater than those obtained with 6.4 × 10 4 CFU mL −1 . Thus, doubling the nominal bacterial concentration did not consistently provide additional benefits under the conditions of this study.