Effect Of New Type Zinc Fertilizer On The Growth Of Oilseed Rape Seedlings And The Function In Alleviating Cadmium Stress

Zinc is an essential trace element for plants.The application of zinc fertilizer can increase crop yield and improve the zinc content in grains.However,traditional zinc fertilizers（Zn SO₄,EDTA-Zn,etc.）have high solubility,are easily fixed by the soil,and have low zinc utilization efficiency,making it difficult to meet the demand for zinc during the crop growth cycle.In recent years,new zinc fertilizers such as amino acid chelated zinc and nano zinc oxide（ZnO NPs）have gradually been widely used in the agricultural field.Research has shown that compared to traditional zinc fertilizers such as Zn SO₄ and EDTA-Zn,new zinc fertilizers can more effectively improve crop agronomic traits,promote crop photosynthesis,and increase crop yield.Rapeseed is an important oil crop in China.This study used nutrient solution and pot experiments,using Zn SO₄,EDTA-Zn,Gly-Zn,and ZnO NPs as experimental materials,to study the effects of different forms of zinc fertilizer on the growth of Brassica napus seedlings.The aim is to provide a theoretical basis for the application of zinc fertilizer in agricultural production.In addition,ZnO NPs,as a new type of zinc fertilizer,play an important role in enhancing plant cadmium（Cd）tolerance.This study used ZnO NPs as materials to explore the physiological role of ZnO NPs in alleviating Cd stress,providing a theoretical basis for improving rapeseed yield under Cd stress.The main results obtained in this study are as follows:1.The effect of new zinc fertilizer on the growth of rapeseed seedlingsGly-Zn and ZnO NPs treatments significantly increased the biomass and leaf area of rapeseed seedlings,decreased the root to shoot ratio,and compared with Zn SO4 and EDTA Zn,the leaf area of new leaves increased by 86%,81%and 45%,42%,respectively,and the biomass increased by 59%,37%and 65%,42%,respectively.Zinc application promoted root development,and the root morphology of ZnO NPs treatment was better than that of Zn SO₄,EDTA Zn,and Gly-Zn treatments.There is a significant difference in the absorption rate of zinc by plants under different forms of zinc fertilizer treatment.The absorption rate of zinc is ZnO NPs>Gly-Zn>Zn SO₄>EDTA-Zn.However,the differences in zinc content and zinc absorption rate among different forms of zinc fertilizer treatments are different.Among them,ZnO NPs,Gly-Zn,and Zn SO₄ treatments had the highest zinc content,while EDTA-Zn treatment had the lowest.Overall,the application of different forms of zinc fertilizer significantly promoted the absorption of zinc by rapeseed,with the most significant effect being achieved when ZnO NPs were applied.In addition,the application of ZnO NPs can significantly improve the transport of zinc from roots to aboveground parts,promoting the distribution of zinc to new leaves.Mineral ions have a significant impact on plant growth and development,and zinc application significantly affects the absorption of other mineral ions.Compared with Zn SO₄ and EDTA-Zn,the application of ZnO NPs and Gly-Zn significantly increased the accumulation of B,Ca,and Mg in leaves.Compared with EDTA-Zn,ZnO NPs and Gly-Zn increased net photosynthetic rate（Pn）by 15%and 11%,respectively.Gly-Zn increased stomatal conductance（Gs）and transpiration rate（Tr）.The treatments of Zn SO₄,Gly-Zn,and ZnO NPs all significantly increased the maximum net photosynthetic rate(A_n,max),maximum carboxylation rate(V_c,max),maximum electron transfer rate(J_max),and trisaccharide phosphate utilization rate（TPU）and other photosynthetic parameters.The application of different forms of zinc fertilizer also affected the content of soluble sugar,sucrose,and fructose in leaves.Zn SO₄ treatment had the highest content of soluble sugar and fructose in the aboveground parts,at 522.44 mg/g and 195.07 mg/g,while ZnO NPs treatment had the highest content of soluble sugar and sucrose in the roots,increasing by 52%-85%and 24%-47%,respectively.However,zinc form had no significant effect on starch content.Different forms of zinc affect the activity of antioxidant enzymes in rapeseed roots.EDTA-Zn,Gly-Zn and ZnO NPs treatments decreased the content of MDA in shoots,the activity of superoxide dismutase（SOD）and peroxidase（POD）in roots.In addition,zinc can also affect soil physicochemical properties and enzyme activity.Zinc application increases the content of available phosphorus in the rhizosphere soil,reduces the content of alkaline hydrolyzed nitrogen,and shows an upward trend in p H value,available potassium,and available zinc.In summary,compared with Zn SO₄ and EDTA-Zn,the application of Gly-Zn and ZnO NPs can promote the absorption of zinc by rapeseed,promote the accumulation of B,Ca,and Mg in the aboveground,enhance photosynthesis and carbohydrate synthesis,improve the activity of major enzymes in the antioxidant system in the roots,and thus promote the growth of rapeseed seedlings.2.Study on the role of ZnO NPs in alleviate Cd stress in rapeseedCd stress leads to a decrease in the biomass,chlorophyll content,and photosynthesis of rapeseed,while the application of ZnO NPs can significantly alleviate the symptoms of Cd stress and improve the biomass,chlorophyll content,and photosynthetic capacity of rapeseed.Under Cd stress,there was no significant difference in the concentration,absorption,and transport of Zn and Cd in the roots and shoot parts of plants when Zn SO₄ and ZnO NPs were applied,but there was a significant difference in Zn and Cd content.The application of ZnO NPs significantly reduced the number of cell death and the content of H₂O₂ and O^2-in rapeseed root tip cells under Cd stress,decreased cell membrane permeability,and reduced the degree of cell membrane damage.Meanwhile,ZnO NPs can alleviate the adverse effects of Cd stress on rapeseed growth by improving the antioxidant system.The study found that the application of ZnO NPs significantly increased the activity of shoot catalase（CAT）and POD under Cd stress,and decreased the activity of SOD,CAT,and POD in the shoot and root.ZnO NPs did not affect the content of available phosphorus and available phosphorus in the soil when alleviating Cd toxicity,but increased the content of alkali-hydrolyzed nitrogen and decreased the content of available potassium;Further research found that under Cd stress,the application of ZnO NPs affected the activities of urease and sucrase in the soil,with urease activity increasing by 16%and sucrase activity decreasing by49%.In summary,compared with Zn SO₄,ZnO NPs increased the content of chlorophyll in leaves under Cd stress,enhanced photosynthesis,reduced the content of root ROS and cell death,improved the activities of SOD,POD,and CAT,thereby increasing the biomass of rapeseed under Cd stress and enhancing its tolerance to Cd.