Preliminary Physiological Mechanism Of L-aspartic Acid Nano-calcium [Ca(L-asp)-NPs] Regulated The Growth And Development Of Rapeseed

Rapeseed is one of the major oilseed crops produced in China.Farmers often apply large amounts of nitrogen（N）,phosphorus（P）and potassium（K）fertilizers during production but neglect the input of essential micronutrients such as calcium（Ca）.Ca is one of the essential intermediate elements for plant growth,and its long-distance transportation can lead to Ca deficiency in the tender parts of plants.Therefore,supplementing Ca fertilizer can alleviate the adverse effects of Ca deficiency on crop growth.With the development of nanotechnology,nanoparticles（NPs）have been applied to nano-fertilizers due to their unique morphology,composition and continuous dissolution characteristics.Understanding the interaction between NPs and plant reactions can improve nutrient utilization and crop yield.L-aspartate nano-calcium（Ca（L-asp）-NPs）is a type of nano-level chelated amino acid Ca with high biological activity.However,it is unknown whether it can be used as a Ca supplement in plant production.Our research aim was to investigate the potential of Ca（L-asp）-NPs as a Ca supplement for Brassica napus to improve understanding about interactions between NPs and plants.Furthermore,the effects of exogenous Ca（L-asp）-NPs on growth,roots,photosynthetic parameters,cell wall metabolism and other indexes were studied through nutrient solution culture;paraffin section and TEM were used to analyze the effects of Ca（L-asp）-NPs and Ca（NO₃）₂·4H₂O on the ultrastructure of rapeseed cells.The effects of Ca（L-asp）-NPs on soil fertility and rapeseed yield were preliminarily discussed through soil culture pot experiment,which provided theoretical support for the application of Ca（L-asp）-NPs as a nano-fertilizer in agricultural production.The main findings from this research are as follows:1 Effects of different concentrations of Ca（L-asp）-NPs on the growth and photosynthetic properties of rapeseedCompared with 0 mg/L Ca（L-asp）-NPs（NPS-0）,25 and 50 mg/L Ca（L-asp）-NPs had no significant effect on rapeseed plant growth.The application of 100 mg/L Ca（L-asp）-NPs showed a marked effect on the growth and development of rapeseed plants,as proved by the following increases in plant height（15.74%）,root length（16.74%）and total fresh weight（43.37%）.Net photosynthetic rate,intercellular CO₂ concentration,stomatal conductance and transpiration rate increased by 295.31%,31.90%,1158.03%and 376.74%respectively,but there was no significant difference among Ca²⁺treatment.2 Effects of Ca（L-asp）-NPs on root growth,Ca morphology and cell wall metabolism in rapeseedCa（L-asp）-NPs improved the root biomass and morphological parameters compared to the treatment without Ca application.Furthermore,Ca（L-asp）-NPs positively impacted the growth of coarse roots in rapeseed.In the treatment with a low concentration of Ca（L-asp）-NPs,plants maintained root growth by increasing root length ratio and root fineness while reducing root mass ratio and root tissue density.The accumulation of Ca,Ca-pectin,Ca-phosphate,Ca-carbonate and Ca-oxalate in roots and leaves of plants were positively correlated with the concentration of Ca（L-asp）-NPs.Still,significantly lower than the 1180mg/L Ca（NO₃）₂·4H₂O treatment.Under different treatments,the proportion of Ca-pectin content in leaves was the highest,and the proportion of Ca-oxalate content in roots was the highest.In addition,Ca（L-asp）-NPs treatment not only increased the content of pectin in the cell wall but also enhanced the activity of cell wall degrading enzymes,such as gelatinase（PME）,cellulase（CE）,polygalacturonase（PG）andβ-Galactosidase（β-Gal）.3 Effects of Ca（L-asp）-NPs and Ca（NO₃）₂·4H₂O on the ultrastructure of both rapeseed roots and leavesComparison to 100 mg/L Ca（NO₃）₂·4H₂O,the application of 100 mg/L Ca（L-asp）-NPs demonstrated a more beneficial effect on the growth of rapeseed.Through paraffin section and TEM results,we found that the root cell structure was complete and the content was clear after applying Ca（L-asp）-NPs and Ca（NO₃）₂·4H₂O.Compared with NPS-0treatment,the root diameter,cortex thickness and column diameter of rapeseed increased by 25.32%,14.89%and 14.80%respectively in 100 mg/L Ca（L-asp）-NPs treatment.Additionally,NPs can be absorbed by roots and accumulated in cells.They can also be adsorbed onto the surface of the root system and influence the root cell wall,causing it to bend.The paraffin section of rapeseed leaves was stained with safranine and solid green,and it was found that after adding Ca,the structure of the leaves was completely developed and the cells were arranged orderly.4 Effect of Ca（L-asp）-NPs on yield and nutrient uptake in rapeseedThe application of 200 mg/kg Ca（L-asp）-NPs significantly increased the plant height,root length,stem weight,branch weight,horn shell weight and root weight of rapeseed plants,and had a positive effect on yield factors such as effective branch height,the effective length of main flowers,the pods number per plant,and yield per plant.Compared to Ca（NO₃）₂·4H₂O,Ca（L-asp）-NPs had a higher Ca nutrient uptake rate,the highest fertilizer agronomic utilization rate and fertilizer bias productivity rate at 200 mg/kg.The application of 200 mg/kg Ca（L-asp）-NPs promoted the absorption of Ca,Mg,and B by rapeseed roots while decreasing the absorption of Fe,Mn,Cu.In summary,Ca（L-asp）-NPs promoted growth and development of the roots by changing the root conformation of plants,which facilitated the uptake and accumulation of Ca in the roots.Ca was converted into different forms of Ca in the plant to meet the demand of plant growth.Ca（L-asp）-NPs increased chlorophyll content and promoted leaf photosynthesis.Ca（L-asp）-NPs application also improved the formation and development of root and leaf cell structure,contributing to the growth and development of plants.Moreover,the utilization of Ca（L-asp）-NPs in the soil proved to be an effective strategy in enhancing the rapeseed yield and nutrient uptake,and had higher Ca nutrient uptake rate,fertilizer agronomic utilization rate and fertilizer bias productivity rate compared to conventional fertilizers.