ASHS 2025 Conference

Calcium phosphate [Ca₃(PO₄)₂] is an nonlabile form of phosphorus (P) commonly found in substrates with a basic pH and it is not available for plant uptake. Phosphate solubilizing bacteria (PSB) are plant-associated microorganisms that can solubilize calcium phosphate by secreting organic acids. PSB inoculation alleviates P deficiency symptoms including leaf purpling, and PSB inoculum can be amended with substances that may improve PSB performance (i.e. glucose). Our goal was to evaluate the efficacy of PSB isolated from the rhizosphere of greenhouse ornamentals (Pantoea trifolii C2B11, Pantoea formicae C8D10, Pantoea sp. C2G6 and Enterobacter soli C4A1) in substrate with pH 6.0 or 6.5, and with an inoculum amended with glucose (10 g·L-1), Luria Bertani (LB) broth (111 mL·L-1), or humic and fulvic acids (20 g·L-1). An experiment was conducted using French marigold 'Durango Yellow' (Tagetes patula) fertilized once per week with 200 mg·L-1 N from a 15N-0P-12.5K-2.9Ca-1.2Mg fertilizer. P was supplemented to the fertilizer solution as Ca3(PO4)2 at 10 mg·L-1 P. Bacillus velezensis was included as a positive control. Digital biomass, Normalized Pigment Chlorophyll Ratio Index (NPCI), Plant Senescence Reflectance Index (PSRI) and canopy hue (color) were measured with the TraitFinder, a digital phenotyping workstation. NPCI and PSRI values from healthy foliage are low but increase under stress. At pH 6.0, all marigolds receiving PSB had increased growth regardless of the inoculum amendments. PSB amended with glucose resulted in marigolds with a lower NPCI than non-inoculated plants. Additionally, PSB-treated marigolds had a higher proportion of green canopy, and a lower proportion of red canopy compared to non-inoculated marigolds when the inoculum was amended with glucose or not amended. At pH 6.5, all PSB resulted in increased marigold growth regardless of inoculum amendments. PSB treated marigolds were healthier than non-inoculated plants, as indicated by their lower NPCI and PSRI values, regardless of the inoculum amendments. Accordingly, PSB treated marigolds had a higher proportion of green canopy and a lower proportion of red canopy compared to non-inoculated marigolds. Bacterial effects were more consistent when inoculum was supplemented with LB. A greater benefit was observed in plants grown at pH 6.5. Microbiome analysis of the rhizosphere showed that inoculation with C2B11, C2G6, and C8D10 increased the relative abundance of the genus Pantoea, and C4A1 enriched the genus Enterobacter. Careful determination of optimal inoculum supplements is key to developing efficient inoculants that promote growth in greenhouse ornamentals.