Integrated TiO2 nanoparticles and Pseudomonas sp. GM5 lower tissue chromium concentrations and strengthen physiological resilience in wheat under chromium and drought stress

Abstract

Chromium contamination and drought can co-occur in agricultural soils and jointly impair wheat establishment, nutrient balance and cellular integrity. This study evaluated titanium dioxide nanoparticles (TiO₂ NPs), the chromium-tolerant plant growth-promoting rhizobacterium Pseudomonas sp. GM5, and their co-application under control, Cr(VI), drought and combined Cr(VI) + drought conditions. Preliminary germination-stage screening identified 500 mg L−1 Cr(VI) as a severe but measurable exposure and 20 mg L−1 TiO₂ NPs as the concentration producing the most favourable seedling response. In the pot experiment, TiO₂ NPs were applied once as a 50 mL soil drench (1 mg pot⁻¹) at stress initiation, whereas GM5 was applied by seed inoculation. Potassium dichromate supplied a nominal 500 mg Cr(VI) kg−1 dry soil; acid digestion followed by atomic absorption spectrophotometry quantified total Cr in tissues and aqua-regia-extractable soil total Cr at harvest. Complete treatment-matched pot growth records and matched shoot and root dry masses were not retained; therefore, biomass responses and organ-specific or whole-plant Cr contents could not be reported, and elemental interpretation was restricted to tissue concentrations and concentration ratios. Relative to untreated combined stress, co-application was associated with 84.2% and 89.2% lower shoot and root total Cr concentrations, respectively, and 57.2–164.4% higher tissue micronutrient concentrations. It also produced more favourable pigment, protein, osmolyte, membrane-injury and antioxidant-enzyme profiles. Root bioconcentration factor declined, whereas translocation factor was interpreted together with absolute tissue concentrations. Overall, co-application produced the most favourable measured concentration and physiological profile under controlled pot conditions. The underlying rhizosphere, transport, nanoparticle–microbe and Cr-speciation mechanisms require direct validation.