From Bench to Greenhouse: The Comparative Nano-Bio System Effects of Green-Synthesized TiO2-NPs and Plant-Growth-Promoting Microorganisms in Capsicum annuum

In this study, titanium dioxide nanoparticles (TiO2-NPs) were produced via green routes using blueberry extracts obtained with isopropanol (I-TiO2-NPs) and methanol (M-TiO2 NPs). HPLC-DAD confirmed phenolic/flavonoid profiles in the extracts, and spectroscopy/microscopy established anatase, polyhedral, mesoporous TiO2-NPs with Eg ≈ 3.0 eV, hydrodynamic sizes ≈ 130–150 nm and negative ζ-potentials (−33 to −50 mV). The in vitro compatibility between TiO2-NPs and the plant-growth-promoting microorganisms (PGPMs) Bacillus subtilis (Bs), Bacillus thuringiensis (B), and Trichoderma harzianum (T) sustained increased growth up to 150 µg/mL without visible negative effects. In greenhouse experimentation of Capsicum annuum exposed to low-moderate TiO2-NPs, an increased leaf number and plant height were observed, while root length did not exceed the controls. I-TiO2 at moderate concentrations, particularly with a single PGPM (B or T), promoted fresh and dry biomass accumulation. Biochemically, peroxidase rose sharply for M-TiO2 at a low dose with consortium, whereas I-TiO2 elicited broader antioxidant responses; total protein increased at higher doses for both formulations, and total chlorophyll was highest with I-TiO2 (high dose with or without PGPMS). Collectively, the nano-bio system shows a formulation- and dose-dependent biphasic behavior: (I) I-TiO2 enhances biomass and photosynthetic pigments; (II) M-TiO2 favors strong POX induction under specific microorganism-dose combinations; and (III) single PGPM co-application with I-TiO2-NPs or M-TiO2 NPs outperforms consortia under our experimental conditions. Green synthesis thus provides surface functionalities that improve dispersion, microbial compatibility, and predictable physiological/biochemical outcomes for precision agriculture.