Zinc Oxide Nanoparticles Alleviate Salt Stress in Cotton (Gossypium hirsutum L.) by Adjusting Na+/K+ Ratio and Antioxidative Ability

Abstract

Soil salinization poses a threat to the sustainability of agricultural production and has become a global issue. Cotton is an important cash crop and plays an important role in economic development. Salt stress has been harming the yield and quality of many crops, including cotton, for many years. In recent years, soil salinization has been increasing. It is crucial to study the mechanism of cotton salt tolerance and explore diversified materials and methods to alleviate the salt stress of cotton for the development of the cotton industry. Nanoparticles (NPs) are an effective means to alleviate salt stress. In this study, zinc oxide NPs (ZnO NPs) were sprayed on cotton leaves with the aim of investigating the intrinsic mechanism of NPs to alleviate salt stress in cotton. The results show that the foliar spraying of ZnO NPs significantly alleviated the negative effects of salt stress on hydroponic cotton seedlings, including the improvement of above-ground and root dry and fresh weight, leaf area, seedling height, and stem diameter. In addition, ZnO NPs can significantly improve the salt-induced oxidative stress by reducing the levels of MDA, H₂O₂, and O₂^- and increasing the activities of major antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). Furthermore, RNA-seq showed that the foliar spraying of ZnO NPs could induce the expressions of CNGC, NHX2, AHA3, HAK17, and other genes, and reduce the expression of SKOR, combined with the CBL-CIPK pathway, which alleviated the toxic effect of excessive Na⁺ and reduced the loss of excessive K⁺ so that the Na⁺/K⁺ ratio was stabilized. In summary, our results indicate that the foliar application of ZnO NPs can alleviate high salt stress in cotton by adjusting the Na⁺/K⁺ ratio and regulating antioxidative ability. This provides a new strategy for alleviating the salt stress of cotton and other crops, which is conducive to the development of agriculture.

Keywords: Na/K ratio; ZnO NPs; antioxidative system; cotton; soil salinization.

Figure 1

ZnO NPs alleviate salt stress in cotton. The phenotypes (A), shoot fresh weights (B), root fresh weights (C), leaf areas (D), and seedling heights (E) of cotton seedlings under salt stress and subjected to 0, 50, 100, 150, and 200 mg/L of ZnO NPs. Scale bar = 100 mm. The data given are the averages of three replicates, with the standard deviation (SD) shown by the error bars. Different letters above or below the error bars show the differences at p < 0.05.

Figure 2

Effects of ZnO NPs on chlorophyll content and chlorophyll fluorescence parameters of cotton seedlings under salt stress. (A) SPAD value; (B) F_V/F_M, maximum photochemical efficiency, which can be used as a reliable index of photochemical activity of photosynthetic apparatus; (C) F_O/F_M, quantum ratio for heat dissipation; (D) ABS/CS, light energy absorbed per unit leaf area. Control (CK), CK treated with 100 mg/L ZnO NPs (CZ), CK treated with 150 mM NaCl (NK), and CZ treated with 100 mg/L ZnO NPs (NZ). The data given are the averages of three replicates, with the standard deviation (SD) shown by the error bars. Different letters above or below the error bars show the differences at p < 0.05.

Figure 3

Effects of ZnO NPs on elemental contents of cotton seedlings under salt stress. (A) Nitrogen content; (B) phosphorus content; (C) potassium content; (D) sodium content; (E) Na⁺/K⁺ Ratio; (F) zinc content; (G) calcium content; (H) sulfur content. Control (CK), CK treated with 100 mg/L ZnO NPs (CZ), CK treated with 150 mM NaCl (NK), and CZ treated with 100 mg/L ZnO NPs (NZ). The data given are the averages of three replicates, with the standard deviation (SD) shown by the error bars. Different letters above or below the error bars show the differences at p < 0.05.

Figure 4

Effects of ZnO NPs on antioxidant enzymes and other biochemical indicators of cotton seedlings under salt stress. (A) superoxide dismutase activity (SOD); (B) peroxidase activity (POD); (C) catalase activity (CAT); (D) MDA content; (E) H₂O₂ content; (F) O₂⁻ content; (G) proline content; (H) protein content; (I) amino acid content. Control (CK), CK treated with 100 mg/L ZnO NPs (CZ), CK treated with 150 mM NaCl (NK), and CZ treated with 100 mg/L ZnO NPs (NZ). The data given are the averages of three replicates, with the standard deviation (SD) shown by the error bars. Different letters above or below the error bars show the differences at p < 0.05.

Figure 5

Heat maps of differentially expressed genes after spraying ZnO NPs on cotton seedlings under salt stress. (A) Differential expression of genes related to ion stress; (B) differential expression of genes related to osmotic stress.

Figure 6

The mechanism model of ZnO NPs alleviating salt stress in cotton seedlings.