Mn₃O₄ Nanoparticles Improved Rapeseed Salt Tolerance Via Maintaining Ros And Na⁺/K⁺ Homeostasis

Soil salinization limits agricultural production worldwide,and there is an ever-expanding trend,which seriously affects the sustainable development of agricultural economies in various countries.In recent years,nanotechnology has attracted much attention due to its wide application in various fields.As an emerging and rapidly developing research field,nanotechnology shows huge potential in improving crop resistance.Manganese tetroxide（Mn₃O₄）nanoparticles,as a kind of nanoparticles with enzyme-like activity（nanozymes）,have shown the latent capacity to enhance the intrinsic function of plants.In this project,Mn₃O₄ nanoparticles with different water solubility,morphology and particle size were synthesized,and the Brassica napus variety Zhongshuang 11 was used as the tested variety to explore the mechanism of improving the salt tolerance of rapeseed.The results will provide a theoretical basis for enriching Mn₃O₄nanoparticles to improve rapeseed germination and seedling growth under salt stress,which is of great significance for correctly utilizing salinized soil to develop sustainable agriculture.The major finding of this study are as follows:（1）The biological effects of different particle sizes and water-soluble Mn₃O₄nanoparticles（MnA,EMnA,PMnS）were compared.It was determined that PAA-modified Mn₃O₄ nanoparticles（PMnS）could significantly improve the germination ability of rapeseed under salt stress by seed soaking with three different Mn₃O₄ nanoparticles.And compared with MnA and EMnA treatments,the PMnS soaked rapeseed significantly improved the germination rate and superoxide dismutase（SOD）activity of rapeseed under salt stress,and the seedlings had lower ROS（H₂O₂ and^·O₂^-）,PMnS could enhance the salt tolerance of rapeseed by scavenging ROS levels in rapeseed and enhancing antioxidant enzyme activities.（2）Under 200 mmol/L NaCl stress,Mn₃O₄ nanoparticles were applied to rapeseed leaves,among which PMnS exhibited biological effects of improving the growth of rapeseed seedlings that MnA and EMnA did not possess.Foliar application of PMnS increased the activities of SOD and POD to varying degrees,decreased the contents of H₂O₂and^·O₂^-in leaves.Alleviating the oxidative damage of ROS to seedlings.The contents of osmotic regulators such as proline in rapeseed were significantly increased under PMnS treatment,while the content of malondialdehyde（MDA）in leaves and the rate of electrolyte extravasation were significantly decreased.PMnS,which was identified to have the ability to scavenge reactive oxygen species,alleviated the damage to plasma membrane by scavenging reactive oxygen species and maintained the normal growth of rapeseed.（3）After 200 mmol/L NaCl stress for 12 days,the PMnS-treated rapeseed maintained higher Pn,Gs and Tr than the control,MnA and EMnA treatments,which promoted the enhancement of the photosynthetic capacity of rapeseed.The results of chlorophyll fluorescence parameters showed that Fv/Fm,qP and Fv’/Fm’were significantly enhanced,and PMnS alleviated light damage and improved the stability of PSII under salt stress.PMnS protected the photosystem of rapeseed and enhanced the photoadaptability of rapeseed under salt stress.（4）The results showed that PMnS treatment had no significant effect on the changes of K⁺and Na⁺content in leaves under normal growth conditions.PMnS application significantly reduced the Na⁺content in rapeseed leaves under salt stress,while K⁺content was significantly increased.The subcellular localization of K⁺in mesophyll cells of leaves found that PMnS maintained a higher K⁺content in the cytoplasm and kept a higher K⁺/Na⁺ratio.Overall,PMnS enhanced the ability of rapeseed to exclude Na⁺and the ability of mesophyll cells to retain K⁺,thus maintaining a high intracellular K⁺/Na⁺ratio,and finally improved the salt tolerance of rapeseed.In summary,the results show that under salt stress,PAA-modified Mn₃O₄nanoparticles with good water solubility and small particle size can improve the salt tolerance of rapeseed by scavenging ROS and regulating Na⁺/K⁺homeostasis.