Strategies for enhancing alkaline stress tolerance in strawberry (Fragaria × ananassa cv. Sabrina) using zinc oxide nanoparticles and plant biostimulants

Abstract

Strawberry ( Fragaria × ananassa ) is a globally cultivated fruit crop valued for its nutritional quality and short production cycle. However, alkaline stress caused by high soil pH and sodium carbonate/bicarbonate accumulation significantly impairs plant growth, photosynthesis, and nutrient uptake. This study aimed to enhance alkaline stress tolerance in Sabrina strawberry using zinc oxide nanoparticles (ZnO NPs) and plant growth-promoting rhizobacteria (PGPR). A factorial experiment based on a completely randomized design with three replicates was conducted, including two pH levels (6.5 and 8.0), three ZnO NPs concentrations (0, 100, and 200 mg L⁻¹), and Bacillus velezensis inoculation treatments. Results showed that alkaline stress (pH 8.0) significantly reduced shoot and root biomass, photosynthetic rate, water use efficiency, and pigment content. However, the combined application of ZnO NPs and B. velezensis alleviated the adverse effects of alkalinity. Under alkaline conditions (pH 8.0), 200 mg L⁻¹ ZnO NPs and B. velezensis inoculation significantly improved morphological traits, increased Relative Water Content (RWC), gas exchange parameters while reducing Electrolyte Leakage (EL). This treatment led to a 14.92% enhance in RWC, a 47.43% increase in photosynthetic rate, a 44.44% rise in carotenoid content and a 4.8% decrease in EL compared with the control treatment (pH 8.0, 0 mg L⁻¹ ZnO NP and without bacterial inoculation). The highest levels of hydrogen peroxide (H₂O₂) (51.29 µmol L⁻¹) and malondialdehyde (MDA) (135.00 µmol g − 1 f.w) were observed at pH 8.0 in the absence of nanoparticles and bacteria. Conversely, the addition of ZnO NPs and B. velezensis significantly reduced both H₂O₂ and MDA levels under alkaline stress. At pH 8.0, the combined application of bacteria and ZnO resulted in a significant increase in osmotic protectant content (proline, starch) and antioxidant enzyme activity (peroxidase, superoxide dismutase). Under alkaline stress, the highest activities of peroxidase (POD) and superoxide dismutase (SOD) were recorded in plants treated with 100 and 200 mg L⁻¹ ZnO-NPs, respectively, in combination with B. velezensis inoculation. In addition, the maximum enzymatic activities for CAT and APX were achieved at 200 and 100 mg L⁻¹ ZnO NPs, respectively, in the absence of bacterial inoculation. These results suggest that ZnO nanoparticles and B. velezensis may enhance antioxidant defenses and improve tolerance to alkaline stress.