Ameliorating Drought Effects in Wheat Using an Exclusive or Co-Applied Rhizobacteria and ZnO Nanoparticles

Abstract

Drought is a major abiotic factor and affects cereal-based staple food production and reliability in developing countries such as Pakistan. To ensure a sustainable and consistent food supply, holistic production plans involving the integration of several drought mitigation approaches are required. Using a randomized complete block design strategy, we examined the drought-ameliorating characteristics of plant growth-promoting rhizobacteria (PGPR) and nanoparticles (NPs) exclusively or as a combined application (T₄) through three stages (D₁, D₂, and D₃) of wheat growth (T₁, control). Our field research revealed that Azospirillum brasilense alone (T₂) and zinc oxide NPs (T₃) improved wheat plant water relations, chlorophyll, proline, phenolics and grain quality, yield, and their allied traits over the stressed treatments. Specifically, the best outcome was observed in the combined treatment of PGPR and ZnO NPs (T₄). Interestingly, the combined treatment delivered effective drought mitigation through enhanced levels of antioxidants (15% APX, 27% POD, 35% CAT, 38% PPO and 44% SOD) over controls at the grain-filling stage (GFS, D₃ × T₁). The 40% improvements were recorded under the combined treatment at GFS over their respective controls. Their combined usage (PGPR and ZnO NPs) was concluded as an effective strategy for building wheat resilience under drought, especially in arid and semi-arid localities.

Keywords: Azospirillum; antioxidants; biostimulant; crop resilience; drought; nanoparticles; plant growth promoting rhizobacteria.

Figure 1

The effects of PGPR and NPs application on foliar chlorophyll contents on wheat under drought. (a) Chlorophyll a, (b) Chlorophyll b, (c) Total chlorophyll content. DTS = drought at tillering, DGS = drought at grain filling, T₁ = control, T₂ = single use of Azospirillum brasilense, T₃ = single use of ZnO NPs, and T₄ = co-application of PGPR and NPs. Error bars indicates standard error (n = 3). (Note: Different lowercase letters indicate significant differences between different drought levels and treatments by the LSD test at p < 0.05).

Figure 2

Effects on relative water content modifications RWC (a), transpirational rates (b), foliar water and osmotic potentials (c,d) in drought-subjected wheat by single and co-applied ZnO NPs and Azospirillum brasilense (PGPR). DTS = drought at tillering, DGS = drought at grain filling, T₁ = control, T₂ = single use of Azospirillum brasilense, T₃ = single use of ZnO NPs, and T₄ = co-application of PGPR and NPs. Error bars indicates standard error (n = 3). (Note: Different lowercase letters indicate significant differences between different drought levels and treatments by the LSD test at p < 0.05).

Figure 3

Phosphorus (a), potassium (b), and nitrogen (c) uptake in wheat as affected by Azospirillum brasilense and ZnO NPs under drought conditions. DTS = drought at tillering, DGS = drought at grain filling, T₁ = control, T₂ = single use of Azospirillum brasilense, T₃ = single use of ZnO NPs, and T₄ = co-application of PGPR and NPs. Error bars indicates standard error (n = 3). (Note: Different lowercase letters indicate significant differences between different drought levels and treatments by the LSD test at p < 0.05).

Figure 4

Effects of PGPR and ZnO NPs on zinc (a) and iron (b) uptake in wheat under different drought levels. DTS = drought at tillering, DGS = drought at grain filling, T₁ = control, T₂ = single use of Azospirillum brasilense, T₃ = single use of ZnO NPs, and T₄ = co-application of PGPR and NPs. Error bars indicates standard error (n = 3). (Note: Different lowercase letters indicate significant differences between different drought levels and treatments by the LSD test at p < 0.05).

Figure 5

Effects of PGPR and ZnO NPs on foliar proline (a) and total phenolics (b) in drought-stressed wheat. DTS = drought at tillering, DGS = drought at grain filling, T₁ = control, T₂ = single use of Azospirillum brasilense, T₃ = single use of ZnO NPs, and T₄ = co-application of PGPR and NPs. Error bars indicates standard error (n = 3). (Note: Different lowercase letters indicate significant differences between different drought levels and treatments by the LSD test at p < 0.05).

Figure 6

Effects of Azospirillum brasilense and ZnO NPs on wheat catalase (a) and peroxidase (b) activities under drought conditions. DTS = drought at tillering, DGS = drought at grain filling, T₁ = control, T₂ = single use of Azospirillum brasilense, T₃ = single use of ZnO NPs, and T₄ = co-application of PGPR and NPs. Error bars indicate standard error (n = 3). (Note: Different lowercase letters indicate significant differences between different drought levels and treatments by the LSD test at p < 0.05).

Figure 7

Effects of Azospirillum brasilense and ZnO NPs applications on ascorbate peroxidase (a), polyphenol oxidase (b), and superoxide dismutase (c) activities in wheat under drought. DTS = drought at tillering, DGS = drought at grain filling, T₁ = control, T₂ = single use of Azospirillum brasilense, T₃ = single use of ZnO NPs, and T₄ = co-application of PGPR and NPs. Error bars indicate standard error (n = 3). (Note: Different lowercase letters indicate significant differences between different drought levels and treatments by the LSD test at p < 0.05).

Figure 8

(a) The Principal Component Analysis (PCA) was based on different parameters of wheat under different treatments of Azospirillum brasilense, ZnO NPs, and drought stress. (b) The PCA was performed using variables with contribution of different parameters from wheat under PGPR, ZnO NPs, and drought stress. NFT = Non fertile tillers (m⁻²); LWP = Leaf water potential (MPa); LOP = Leaf osmotic potential (MPa); POD = Peroxidase (units mg⁻¹ pro. mint⁻¹); PPO = Polyphenol oxidase (units g⁻¹ pro. mint⁻¹); CAT = Catalase (units mg⁻¹ pro. mint⁻¹); SOD = Superoxide dismutase (units mg⁻¹ pro. mint⁻¹); APX = Ascorbate peroxidase (units mg⁻¹ pro. mint⁻¹); N, P, and K = Nitrogen, phosphorous and potassium foliar uptakes (mg g⁻¹ FW); Zn and Fe = zinc and iron foliar uptakes (mg kg⁻¹ FW); Gprot. = Grain protein (%); Gzn = Grain zinc content (mg Kg⁻¹ DW); Chl. a, b and a + b = Chlorophyll a, b and a + b(mg g⁻¹ FW); TP = Total phenolics (mg g⁻¹ Fw); PH = Plant height (cm), NS⁻¹ = Nodes per stem; SL = spike length (cm); Int.L = Inter-nodal length (cm); NSPS = Number of spikelets per spike; SY = Straw yield (T ha⁻¹); RWC = Relative water contents (%); BY = Biological yield (T ha⁻¹); PT = Number of productive tillers (m⁻²); NGS = Number of grains per spike; GY = Grain yield (T ha⁻¹); SpWt. = Grain weight per spike (g); 1000 Gwt. = 1000 grain weight (g); HI = Harvest index (%); Tn = Transpiration rate (µmol m⁻² s⁻¹); Dim1 = First principal component; Dim2 = Second principal component; DTS = Drought at tillering stage; DGS = Drought at grain-filling stage; Contrib = Contribution.