Phosphorus (P) is an essential macronutrient absorbed by plants as orthophosphate (PO4). P availability depends on the pH of the substrate. At high pH, P forms insoluble compounds like Ca3(PO4)2 which is unavailable for plant uptake. Phosphate solubilizing bacteria (PSB) are plant-associated microorganisms that can break down Ca3(PO4)2 by secreting organic acids. PSB have been primarily evaluated as inoculum for crops grown in soil to improve P availability. However, less is known about the application of PSB in ornamentals grown in soilless substrates. Our goal was to identify PSB from a collection of bacteria isolated from the rhizosphere of greenhouse ornamentals. First, the collection was screened in vitro for the bacterial capacity to reduce pH of the media using the bromophenol-blue color assay. Thirty-five isolates were identified to reduce media pH, and their P solubilization capacity was quantified using ion chromatography. Fourteen isolates with the highest P solubilization were selected for whole-genome sequencing, but only two bacterial isolates (C2B11 and C8D10) were advanced to the in-planta evaluation using Marigold (Tagetes patula) 'Durango Yellow' grown in a peat-based substrate (pH = 7). Plants were irrigated with 100 mg·L-1 N from a 15N-0P-15K fertilizer, and P was supplemented weekly as Ca3(PO4)2 via substrate drench applications. Lalrise Vita (Lallemand Plant Care) and Bacillus velezensis (the active ingredient in Lalrise Vita) were included as positive controls. Plant phenotyping was conducted using the TraitFinder automatic system (Phenospex). TraitFinder assessment parameters included digital biomass, bloom area, Green Leaf Index (GLI), Normalized Pigment Chlorophyll Ratio Index (NPCI), and Plant Senescence Reflectance Index (PSRI). Lalrise Vita performed better than the control in all the parameters evaluated. B. velezensis increased bloom area and showed higher GLI and lower NPCI than the control. Pantoea sp. C2B11 significantly increased digital biomass, bloom area, and GLI. Pantoea sp. C8D10 only showed an increase in GLI values. Both C2B11 and C8D10 possess various genes involved in gluconic acid production. We identified two PSB that solubilize P in vitro, promote growth and bloom area, and improve canopy health (high GLI or low NPCI) in marigold plants fertilized with Ca3(PO4)2. Our next step is to validate the growth-promoting capacity of the identified PSB in other ornamentals. PSB can contribute to improving phosphorus nutrition and fertilizer use efficiency in greenhouse ornamental production.
