Objective This study aimed to investigate the phosphate-solubilizing characteristics of a highly efficient phosphate-solubilizing bacterium isolated from the rhizosphere soil of safflower (Carthamus tinctorius L.), as well as its regulatory effects on safflower growth, accumulation of active components, and the rhizosphere microenvironment. The goal was to provide a scientific basis for improving sustainable and high-efficiency safflower cultivation. Method Rhizosphere soil of C. tinctorius was used to screen for dominant phosphate-solubilizing strains on inorganic phosphorus selective medium. The colonization ability of the selected strain C9 was evaluated using resistant strain markers. The strain was further characterized by physiological and biochemical analysis, phosphate solubilization assays, growth curve fitting, and 16S rDNA molecular identification. Pot experiments were conducted using three safflower varieties with white, yellow, and red flowers. Bacterial fertilizer was applied via root irrigation, and plant biomass, major active components in the florets, physicochemical properties of the rhizosphere soil, and plant growth-promoting indexes were dynamically monitored. Result A highly efficient phosphate-solubilizing bacterium, designated C9, was isolated from safflower rhizosphere soil and identified as the Gram-negative bacterium Pseudomonas fluorescens. Strain Rif-C9 showed strong solubilizing activity against calcium phosphate and zinc phosphate, and the amount of phosphate solubilized was negatively correlated with a decrease in culture medium pH. Pot experiments demonstrated that strain Rif-C9 stably colonized the safflower rhizosphere and significantly promoted the growth of all three flower-color varieties (white, yellow, and red). Specifically, the number of effective fruiting heads per plant increased by 28.24%, 37.36%, and 26.98%, respectively. Meanwhile, the contents of hydroxysafflor yellow A (HSYA) in the florets increased by 51.5%, 34.7%, and 13.4%, and the contents of kaempferol (KF) increased by 183.3%, 300.0%, and 133.3%, respectively, for the white, yellow, and red flower varieties. Application of the Rif-C9-based bacterial fertilizer led to a decrease in rhizosphere soil pH and significant increases in total nitrogen, organic matter, and available phosphorus contents by 7.64%–18.56%, 15.74%–29.94%, and 4.34%–8.32%, respectively. Furthermore, strain Rif-C9 was capable of secreting indole-3-acetic acid (IAA), which synergistically improved the rhizosphere microecology, thereby promoting safflower growth and the accumulation of active components. Conclusion P. fluorescens Rif-C9 possesses highly efficient phosphate-solubilizing capacity. It significantly promotes safflower growth and development, enhances the content of medicinal components, and improves soil fertility by acidifying the rhizosphere microenvironment, releasing soluble phosphorus, and secreting IAA.
