Phosphorus (P) is an essential macronutrient with low availability for plant uptake. The availability of P is reduced by the formation of insoluble complexes with calcium (Ca) and iron (Fe). Phosphorus solubilizing bacteria (PSB) can enhance P uptake by producing organic acids that acidify the rhizosphere and breakdown the insoluble P compounds. The goal of this research is to identify PSB that can increase P uptake efficiency in soilless production systems. A collection of bacteria isolated from the rhizosphere of greenhouse grown ornamentals was used to identify PSB using both in vitro and in planta evaluations. A malachite green assay optimized for 96-well plates was used to screen 1,056 bacterial isolates for the ability to solubilize phosphorus from both calcium phosphate and iron phosphate. This in vitro assay identified 14 and 24 PSB that solubilized 25% or more of the P from Ca2(PO4)3 and FePO4·2H20, respectively. There was no overlap between the PSB that solubilized Ca2(PO4)3 and those that solubilized FePO4. In planta evaluations were conducted in Marigold ‘Durango Yellow’ (Tagetes patula) and Tomato ‘Bush Beefsteak’ (Solanum lycopersicum) grown in a peat-based substrate (pH=7) and inoculated with individual PSB as a media drench (20 mL; OD595=0.1). Digital phenotyping with the TraitFinder (Phenospex) was used to quantify growth promotion (digital biomass and bloom area), and the severity of P deficiency symptoms [Green Leaf Index (GLI), normalized difference vegetation index (NDVI), Normalized Pigment Chlorophyll Ratio Index (NPCI), and Plant Senescence Reflectance Index (PSRI)]. Only seven FePO4·2H20 solubilizers (Fe-PSB) and six Ca2(PO4)3 solubilizers (Ca-PSB) performed better than the control in at least one of the evaluated parameters. A validation experiment was conducted to confirm the beneficial activity of the seven Fe-PSB (C2F9, C3A8, C11G1, C8D10, C6E7, C3G9, and C3F10) and the six Ca-PSB (C4A1, C2G6, B3A7, C11A5, C4B6, C12F4, and C6H6). The validation experiments were conducted with Marigold ‘Durango Yellow’ and Tomato ‘Early girl’ grown in a peat-based substrate (pH=7) and inoculated with individual PSB as a media drench (20 mL; OD595=0.1). Bacillus megaterium (C3F10), Pseudomonas sp. (C6E7), and two strains of Pantoea rwandensis (C3A8 and C8D10) showed a beneficial response when applied to plants fertilized with FePO4·2H20. Only, Enterobacter soli C4A1 showed a beneficial response when applied to plants fertilized with Ca2(PO4)3. The malachite-green assay and digital phenotyping are suitable tools for high-throughput identification of PSB that can be used to improve phosphorus nutrition in soilless culture systems.
