Nanometer composite antifreezing agent suitable for pear flowering period, its preparation method and application

Abstract

This application discloses a nanocomposite antifreeze agent suitable for pear blossom season, its preparation method, and its application. The antifreeze agent uses the nanoscale metal-organic framework material MOF-801 as a porous carrier, loading an antifreeze solution containing 2.5 mg / L abscisic acid (ABA), 1 g / L calcium gluconate, 0.75 g / L complex amino acids, 1.5 g / L potassium dihydrogen phosphate, 2.5% (v / v) polysorbate-80, 2.5% (v / v) ethylene glycol, and water. It is then formed into a controllable sustained-release formulation through ultrathin chitosan coating. The preparation method includes steps such as MOF-801 synthesis and activation, antifreeze mother liquor preparation, vacuum negative pressure impregnation loading, and chitosan self-adsorption coating. The nanocomposite antifreeze agent of this invention has a minimum tolerance temperature of -4℃. Applying it twice during the pear blossom season, with a 24-hour interval, can significantly reduce the incidence of frost damage to flowering organs. The preparation process of this invention is green and environmentally friendly, and it is suitable for protecting pear trees from late spring frost damage.

Description

Technical Field

[0001] This application relates to the field of frost protection technology for fruit tree flowering period, specifically to a nanocomposite antifreeze agent suitable for pear flowering period, its preparation method and application. Background Technology

[0002] Pear trees are the third largest cultivated fruit tree in my country, with a large-scale industry and significant economic benefits. The pear tree flowering period is concentrated in spring, from March to April, which is a time of drastic temperature fluctuations and frequent late spring frosts. Pear flower organs are extremely vulnerable to low temperatures, and even short-term exposure to low temperatures can cause symptoms of frost damage such as petal browning, stigma necrosis, pollen inactivation, and flower drop. This is the core adverse factor restricting the high quality and high yield of pear trees.

[0003] Currently, the mainstream methods for frost protection of fruit trees during the flowering period in agricultural production mainly include fumigation, irrigation, mulching, and spraying chemical agents. Among these, foliar spraying of antifreeze has become the most commonly used protection method due to its ease of operation, low cost, and suitability for large-scale orchards. Existing commercial antifreeze agents mainly include single formulations such as inorganic salts, sugar alcohols, and amino acids, which have the following main drawbacks: traditional antifreeze agents have weak adhesion of their macromolecular components, making them easily washed away by spring winds and rain after spraying, resulting in short-lasting protection; most formulations are highly versatile but lack specificity, suitable for field crops but not for the physiological characteristics of young pear blossoms, and high-concentration spraying can easily clog stomata and burn the stigma, affecting pollen germination and pollination and fertilization; conventional antifreeze agents rely solely on a single osmotic regulation mechanism for frost resistance, forming a relatively small amount of stable protective layer on the surface of the blossoms, resulting in limited low-temperature protection and difficulty in resisting sudden extreme late spring frosts.

[0004] In recent years, nanomaterials have been increasingly applied in the field of agricultural stress-resistance adjuvants due to their small size effect, high specific surface area, and excellent film-forming and adhesion properties. Metal-organic frameworks (MOFs) are a class of novel porous crystalline materials obtained by coordination assembly of metal nodes (metal ions or clusters) and organic ligands. MOF-801, a MOF-801 material, possesses characteristics such as large specific surface area, uniform pore structure, good biocompatibility, and excellent low-temperature stability, making it an ideal nanoparticle drug loading carrier that enables efficient adsorption and storage of active pharmaceutical ingredients. Therefore, developing a nanocomposite antifreeze agent suitable for pear blossoms, using MOF-801 as a porous carrier to load various composite formulations, is of great significance for solving the problem of late spring frost damage to pear trees and stabilizing the yield and quality of pear fruit. Summary of the Invention

[0005] To address the problem that pear trees are susceptible to low-temperature frost damage during the flowering period, leading to reduced fruit set rate, yield, and quality, this invention proposes a novel chitosan ultrathin-coated nanocomposite antifreeze agent suitable for pear flowering period, loaded with MOF-801 and various composite preparations, along with its preparation and application methods.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a nanocomposite antifreeze suitable for pear blossoming, wherein the antifreeze uses a nanoscale metal-organic framework material MOF-801 as a porous carrier, the pores of which are loaded with antifreeze solution, and the outer layer of the carrier particles is coated with an ultrathin chitosan coating layer (slow-release membrane).

[0008] Preferably, the antifreeze solution contains the following components: abscisic acid (ABA) 2.5 mg / L, calcium gluconate 1 g / L, compound amino acids (Yuan Ye, product number S20168, 0.75 g / L), potassium dihydrogen phosphate 1.5 g / L, polysorbate-80 (Tween 80) 2.5% (v / v), ethylene glycol 2.5% (v / v), with the balance being deionized water. The components work synergistically, providing multiple benefits including stress induction, cell protection, nutrient replenishment, penetrating moisture, and uniform dispersion, thus meeting the antifreeze protection needs of pear trees during their flowering period.

[0009] Preferably, the MOF-801 porous carrier has a particle size of 50–200 nm and a drug loading of 20%–40%.

[0010] Preferably, the thickness of the chitosan ultrathin coating layer is 50-200 nm, and the coating layer can achieve controlled sustained release of the drug by relying on pH and low temperature response.

[0011] Preferably, the MOF-801 porous support needs to undergo activation pretreatment before use, and the activation process includes two steps: solvent exchange and vacuum drying.

[0012] (1) Solvent exchange: The synthesized but not dried nanocrystalline MOF-801 was immersed in anhydrous methanol, anhydrous ethanol or acetone, and the solvent was replaced with fresh solvent every 12 h. The exchange was repeated 3 to 5 times to completely replace the high-boiling-point solvent (such as N,N-dimethylformamide) remaining in the pores.

[0013] (2) Vacuum drying: The product after solvent exchange is placed in a vacuum drying oven and dried for 10 to 12 h under vacuum conditions of -0.08 to -0.09 MPa and temperature of 80 to 100 °C to obtain activated nano MOF-801 porous carrier.

[0014] Preferably, the antifreeze solution is loaded into the pores of MOF-801 using a vacuum negative pressure impregnation method. The impregnation conditions are: a material-to-liquid mass-to-volume ratio of 1 g : (5-10) mL. The specific operation is as follows: the activated MOF-801 powder is added to the antifreeze mother liquor, placed in a vacuum dryer, and vacuumed to -0.06 to -0.07 MPa and maintained for 30-40 min to fully expel air from the carrier pores. Then, the pressure is restored to normal, and the mixture is impregnated in a sealed, dark environment for 12-24 h with stirring. After impregnation, the mixture is centrifuged and rinsed quickly 1-2 times with a small amount of deionized water to remove any unloaded solution from the particle surface.

[0015] Preferably, the chitosan ultrathin coating process is as follows: MOF-801 particles loaded with drug solution are dispersed in a chitosan acetic acid solution with a mass fraction of 0.5% to 1.0% (chitosan is dissolved in a 1% volume fraction aqueous acetic acid solution), stirred at low speed at room temperature for 2 to 3 hours, and a 50 to 200 nm chitosan ultrathin sustained-release film is formed on the particle surface through molecular self-adsorption. Then, the particles are separated by centrifugation and dried at a low temperature of 40 to 45°C under vacuum to obtain the finished sustained-release micro powder.

[0016] A second aspect of the present invention provides a method for preparing the above-mentioned nanocomposite antifreeze suitable for pear blossoming, comprising the following steps:

[0017] Preparation of S1 and Nano MOF-801: Zirconium oxychloride octahydrate and fumaric acid were dissolved in a mixed solvent of N,N-dimethylformamide and formic acid at a molar ratio of 1:1, wherein the volume ratio of N,N-dimethylformamide to formic acid in the mixed solvent was 1:1. After stirring until completely dissolved, the solution was transferred to a high-pressure reactor and reacted solvothermically at 120–130 °C for 24–48 h. After the reaction was completed, the solution was allowed to cool naturally to room temperature, and the white precipitate was collected by centrifugation. The precipitate was washed three times each with N,N-dimethylformamide and anhydrous methanol to remove unreacted raw materials and residual solvent in the pores.

[0018] S2, MOF-801 Activation: The nano MOF-801 powder (or undried wet powder) obtained in step S1 is activated by solvent exchange: it is soaked in anhydrous methanol, and fresh methanol is replaced every 12 h, and the exchange is repeated 3 to 5 times; then it is exchanged twice with anhydrous ethanol; then it is placed in a vacuum drying oven and vacuum dried for 10 to 12 h under the conditions of vacuum degree -0.08 to -0.09 MPa and temperature 80 to 100 ℃, and then sealed and cooled for later use.

[0019] S3. Preparation of antifreeze mother liquor: Add abscisic acid, calcium gluconate, compound amino acids, and potassium dihydrogen phosphate in sequence. After complete dissolution, add polysorbate-80 (Tween 80) and ethylene glycol. Stir at room temperature for 10-15 min and adjust the pH to 4.5-6.0 to obtain the antifreeze active mother liquor.

[0020] S4. Vacuum impregnation loading: Add the activated MOF-801 to the antifreeze mother liquor at a material-to-liquid ratio of 1 g: (5-10) mL, place it in a vacuum filtration device or vacuum dryer, evacuate to -0.06 to -0.07 MPa, maintain for 30-40 min, and remove the air from the pores; then slowly restore to normal pressure, and impregnate in a sealed, light-proof environment at room temperature for 12-24 h with stirring; after impregnation, centrifuge at 8000 r / min for 8 min, and rinse quickly 1-2 times with a small amount of deionized water to remove free drug solution on the surface.

[0021] S5. Chitosan ultrathin coating: Disperse the rinsed solid particles in a dilute acetic acid-chitosan aqueous solution and stir at low speed at room temperature for 2 hours to allow chitosan to spontaneously adsorb and coat the particle surface.

[0022] S6. Low-temperature drying: Vacuum drying at 40 ℃ to obtain nanocomposite antifreeze @MOF-801 slow-release powder, which is a nanocomposite antifreeze for pear blossom period.

[0023] A third aspect of the present invention provides a method for applying the above-mentioned nanocomposite antifreeze agent in the protection against low-temperature freezing damage during the pear blossom period, comprising the following steps:

[0024] (1) Application period: Spraying should be carried out during the budding and flowering periods of pear trees;

[0025] (2) Spraying method: Dilute the antifreeze product 100 to 300 times, spray evenly, and spray twice with an interval of 24 hours. Spray evenly on the pear tree inflorescence, petals, stigma, anthers and tender new shoots until the surface of the flower is moist but not dripping water.

[0026] (3) Minimum tolerance temperature: When this antifreeze is sprayed during the flowering period of fragrant pear, its minimum tolerance temperature can reach -4 ℃.

[0027] In summary, compared with the prior art, the solution of the present invention has the following technical advantages:

[0028] (1) The nanocomposite antifreeze agent of the present invention achieves the compounding and nano-loading of a variety of antifreeze active ingredients (such as ABA, amino acids, calcium ions, etc.) and achieves sustained release through chitosan coating, thereby improving the utilization efficiency of the agent.

[0029] (2) The present invention uses a vacuum negative pressure impregnation method to ensure the efficient and uniform loading of the agent in the MOF-801 nanopores; the chitosan self-adsorption coating process is mild and does not damage the carrier structure and the activity of the agent, and the preparation process is green and environmentally friendly.

[0030] (3) When the antifreeze agent of the present invention is sprayed during the budding and flowering stages of pear trees, it can effectively reduce the incidence of frost damage to flower organs, protect key reproductive organs such as stigma and anthers, and significantly improve the fruit setting rate and fruit yield of pear trees. It is of great significance to solve the problem of frost damage to pear trees in late spring and stabilize the yield and quality of pear fruit industry.

[0031] Other features and advantages of this application will be set forth in detail in the following description, or will become apparent through the implementation of the relevant technical solutions of this application. The objectives and other advantages of this application can be achieved through the technical features and means explicitly pointed out in the description, claims, and drawings, and will be obtained through the implementation of these technical contents. Attached Figure Description

[0032] To more clearly illustrate the technical solution of this application, the accompanying drawings involved in the description of this invention will be briefly introduced below. It should be noted that the drawings only show some embodiments of the invention. For those skilled in the art, other related drawings can be derived from these drawings without creative effort.

[0033] Figure 1 Phenotypic diagram of petals, buds and stigmas affected by freezing.

[0034] Figure 2 The diagram shows the structure of MOF-801. MOF-801 contains three types of cavities: two are tetrahedral cavities (pink and green), with diameters of approximately 4.8 and 5.6 Å, respectively; the other is an octahedral cavity (yellow), with a diameter of approximately 7.4 Å.

[0035] Figure 3 shows the morphology and freezing condition of flower buds and petals before and after antifreeze application, where A: no antifreeze was applied, and B: antifreeze was applied.

[0036] Figure 4 The study investigated the effects of different spraying times on the morphological damage of pear blossom organs to frost under low-temperature stress. Treatment A consisted of one spraying, while treatment B consisted of two sprayings.

[0037] Figure 5 This is a flowchart illustrating the preparation method of the nanocomposite antifreeze of the present invention. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only some embodiments of this application, and this application can also be implemented or applied through other different specific implementation methods. The details in this specification can also be modified or changed based on different viewpoints and applications.

[0039] At the same time, it should be understood that the scope of protection of this application is not limited to the specific implementation schemes described below; it should also be understood that the terminology used in the embodiments of this application is for describing specific implementation schemes, and not for limiting the scope of protection of this application.

[0040] When numerical ranges are given in the embodiments, it should be understood that, unless otherwise stated in this application, both endpoints of each numerical range and any value between the two endpoints may be selected. Unless otherwise defined, all technical terms used in this application have the same meaning as commonly understood by those skilled in the art. In addition to the specific methods, equipment, and materials used in the embodiments, based on the knowledge of those skilled in the art and the description in this application, any prior art methods, equipment, and materials similar to or equivalent to those described, used, and materials in the embodiments of this application may be used to implement this application. In this application, unless otherwise specified, all parts and percentages are in units of weight, and unless otherwise specified, all instruments and reagents are commercially available or commonly used in the industry. Unless otherwise specified, the experimental methods and operations in the following embodiments are conventional experimental methods and operations in the art.

[0041] Example 1: Screening of components for nanocomposite antifreeze agents suitable for pear blossom period

[0042] In order to screen out compound formulations suitable for pear blossoming, obtain agents that can effectively improve the frost resistance of pear blossoms and reduce the impact of adverse climatic conditions on the pear industry, the present invention conducted the following component screening experiments.

[0043] 1) Set up a control group and 16 treatment combinations to explore the concentration and ratio of hormones (abscisic acid and brassinolide), proteins, sugars and minerals (phosphorus and potassium) and screen the optimal concentration and ratio (Table 1).

[0044] Table 1. Composition and Proportion of Different Antifreeze Treatment Combinations

[0045]

[0046] 2) Using 3-year-old Korla fragrant pear trees as test materials, antifreeze was sprayed during the flowering period, avoiding the period of high temperature and strong light.

[0047] 3) Collect branches 12 hours after spraying and subject them to low-temperature treatment.

[0048] 4) During sampling, select 5-6 flower branches with consistent canopy and orientation and consistent degree of openness for each treatment. Insert the collected branches into moist floral foam and place them in an intelligent low-temperature incubator to simulate a low-temperature environment.

[0049] 5) Low-temperature test: The intelligent low-temperature incubator was set to 0℃ for 2 hours. After the low-temperature simulation test, the samples were removed and placed at room temperature for 2 hours to recover. The browning of the flower organs (petals, buds, stigmas) was observed and recorded. The main phenotypes were: petals browning and curling after freezing; pedicels bending and drooping; buds browning or even turning blackish-brown; styles and stigmas turning blackish-brown and wilting (freeze-dried). Figure 1 ).

[0050] 6) To screen the optimal compound pesticide formulation for alleviating low-temperature freezing damage to pear (fruit tree) flower organs and flower buds, using water (CK) as a control, 16 groups of compound pesticides with different concentrations of ABA, brassinolide, and calcium gluconate were sprayed and field low-temperature antifreeze experiments were conducted. The percentage of frozen flowers and flower buds in each treatment was statistically analyzed, and the percentages of six freezing damage indicators—normal flowers, completely frozen flowers, partially frozen flowers, completely frozen buds, partially frozen buds, and normal buds—were statistically analyzed to screen the most suitable components and concentration ratios. The results showed that the control group had no normal flowers or buds, and the percentage of completely frozen buds was as high as 65.08%; the percentage of completely frozen flowers and buds in treatment 5 was relatively low, while the percentage of normal buds (38.18%) was significantly higher than other treatment groups (Table 2). Figure 3 Treatment 5 (ABA 2.5 mg / L, calcium gluconate 1 g / L, compound amino acids 0.75 g / L, potassium dihydrogen phosphate 1.5 g / L, polysorbate-80 (Tween 80) 2.5% (v / v), ethylene glycol 2.5% (v / v), with the remainder being deionized water) was the optimal ratio for improving the frost resistance of flower buds and preserving the normal number of flowers and fruits in this experiment.

[0051] Table 2. Statistics and comparison of frost damage rate of pear blossoms after spraying with different antifreeze ratios.

[0052]

[0053] Example 2: Preparation of nanocomposite antifreeze during pear blossom season ( Figure 5 )

[0054] 1) Preparation of nano MOF-801: Based on the structural diagram of MOF-801 ( Figure 2 Weigh 3.22 g of zirconium oxychloride octahydrate and 1.16 g of fumaric acid, dissolve them in a mixed solvent of 80 mL N,N-dimethylformamide and 80 mL formic acid, and stir until dissolved. Transfer the solution to a 200 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene and react at 120 ℃ for 48 h. After natural cooling, collect the white precipitate by centrifugation, wash it three times each with N,N-dimethylformamide and methanol, and dry it under vacuum at 60 ℃ and -0.08 MPa for 12 h to obtain MOF-801 powder with a particle size of approximately 200 nm.

[0055] 2) MOF-801 Activation: Take 5 g of the above-mentioned undried MOF-801 wet powder (after washing with methanol), add it to 100 mL of anhydrous methanol, and let it soak at room temperature. Every 12 h, discard the supernatant and replace it with fresh anhydrous methanol, repeating this process 3 times. Then, exchange the powder twice with anhydrous ethanol (12 h each time). Collect the exchanged product by centrifugation, place it in a vacuum drying oven, and vacuum dry it for 12 h at 80 ℃ and -0.09 MPa to obtain the activated MOF-801 powder, which is then sealed for later use.

[0056] 3) Preparation of antifreeze stock solution: Take room temperature deionized water, and accurately weigh and add 2.5 mg / L abscisic acid, 1 g / L calcium gluconate, 0.75 g / L compound amino acids, and 1.5 g / L potassium dihydrogen phosphate in sequence, and stir until the solids are completely dissolved; then add 2.5% polysorbate-80 (Tween 80) and 2.5% ethylene glycol by volume, and stir to mix evenly to obtain a transparent antifreeze stock solution.

[0057] 4) Vacuum impregnation loading: Add the activated MOF-801 to the antifreeze mother liquor at a material-to-liquid ratio of 1 g:10 mL, place it in a vacuum filtration device, and extract it for 30 min under a negative pressure of 0.07 MPa to remove air from the pores; then turn off the vacuum system, and impregnate it in a sealed container at room temperature in the dark for 15 h to ensure that the agent is fully loaded into the carrier pores; after impregnation, centrifuge at 8000 r / min for 8 min, and rinse once quickly with a small amount of deionized water to remove free agent on the surface.

[0058] 5) Chitosan ultrathin coating: Prepare a 1% (w / w) dilute chitosan solution (using acetic acid to dissolve chitosan), disperse the rinsed MOF-801 composite particles in the solution, stir at low speed at room temperature for 2 h, and form a uniform ultrathin coating layer with a thickness of 50-200 nm through molecular self-adsorption.

[0059] 6) Low-temperature drying: The coated material is dried in a vacuum oven at 40 ℃ to obtain a light yellow and uniform antifreeze @MOF-801 sustained-release micro powder with a particle size of 80-150 nm and a drug loading of 28%-35%, which meets the technical parameter requirements.

[0060] Example 3: Determination of the minimum critical temperature for the application of nanocomposite antifreeze

[0061] To determine the minimum critical temperature at which the nanocomposite antifreeze agent can effectively exert its antifreeze effect after application, seven temperature gradients were set: 1℃, 0℃, -1℃, -2℃, -3℃, -4℃, and -5℃. At each temperature, a control (CK) and a nanocomposite antifreeze treatment were established, and the frost damage to petals and buds was statistically analyzed. The results showed that at -4℃, the complete frost damage rate of petals in the control group was 82%, and the complete frost damage rate of buds was 81%, while in the treatment group, the complete frost damage rates were 78% for petals and 52% for buds, showing a significant difference. At -5℃, the complete frost damage rates of petals in the control group and the treatment group were 98% and 95%, respectively, and the complete frost damage rates of buds were 99% and 94%, respectively. The agent treatment only slightly reduced the proportion of completely frost-damaged petals, indicating that the antifreeze effect of the agent was significantly weakened under extreme cold conditions, with no significant difference between the two groups (Table 3). Therefore, it is shown that the application of antifreeze to the treatment group significantly reduced the frost damage rate of flower buds at -4℃, and the minimum tolerance temperature for the nano-composite antifreeze applied during the flowering period of Pear Blossom is -4℃.

[0062] Table 3. Statistical analysis of the incidence of frost damage to petals and buds of fragrant pear under different low-temperature stresses compared to the control group after spraying with antifreeze.

[0063]

[0064] Example 4: Spraying frequency of nanocomposite antifreeze

[0065] Pear branches in full bloom were selected as the experimental subjects. The antifreeze prepared in Example 1 was diluted 300 times, and the spraying standard was to thoroughly moisten the inflorescence, petals, buds, stigma, and young shoots without dripping water. A control group and a treatment group were set up: the control group was sprayed once, and the treatment group was sprayed twice, with a 24-hour interval between sprays. The branches of both groups were placed in an intelligent low-temperature incubator for low-temperature treatment, with a minimum critical temperature of -4 ℃, and each group was subjected to three biological replicates. The results showed that after spraying the antifreeze once, the entire branch exhibited browning, drying, wilting, and shedding of most petals and buds, with almost no normally open flowers, and severe frost damage to the flower organs. After spraying the antifreeze twice, most of the branches were frozen, but some retained plump buds, and several normally open healthy flowers were present. The proportion of browned and dead buds was significantly lower than in the control group, and the damage from low-temperature freezing was significantly reduced. Figure 4 This indicates that applying the antifreeze twice significantly improves the frost resistance of the pear blossom organs compared to applying it once.

[0066] The above description is merely a preferred embodiment of this application and is not intended to limit this application in any way. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to obtain equivalent embodiments without departing from the scope of the technical solution of this application. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of this application without departing from the content of the technical solution of this application should be included within the scope of protection of the claims of this application.

Claims

1. A nanocomposite antifreeze agent suitable for pear blossoming period, characterized in that, The invention includes a nanoscale metal-organic framework material MOF-801 porous carrier, an antifreeze solution loaded in the pores of the carrier, and a chitosan ultrathin sustained-release membrane coated on the outer layer of the carrier particles; wherein the antifreeze solution contains abscisic acid, calcium gluconate, complex amino acids, potassium dihydrogen phosphate, polysorbate-80, and ethylene glycol.

2. The nanocomposite antifreeze agent according to claim 1, characterized in that, The contents of each component in the antifreeze solution are as follows: abscisic acid 2.5 mg / L, calcium gluconate 1 g / L, compound amino acids 0.75 g / L, potassium dihydrogen phosphate 1.5 g / L, polysorbate-80 2.5% (v / v), ethylene glycol 2.5% (v / v), and the balance is deionized water.

3. The nanocomposite antifreeze agent according to claim 1, characterized in that, The MOF-801 porous carrier has a particle size of 50–200 nm and a drug loading of 20%–40%.

4. The nanocomposite antifreeze agent according to claim 1, characterized in that, The MOF-801 porous carrier undergoes an activation pretreatment before use, the activation pretreatment including: (1) Solvent exchange: The synthesized but not dried nano MOF-801 was immersed in anhydrous methanol, anhydrous ethanol or acetone, and the solvent was replaced with fresh solvent every 12 h, and the exchange was repeated 5 to 7 times. (2) Vacuum drying: The product after solvent exchange is placed in a vacuum drying oven and dried for 10 to 12 h under vacuum conditions of -0.08 to -0.09 MPa and temperature of 80 to 100 ℃.

5. The nanocomposite antifreeze agent according to claim 1, characterized in that, The thickness of the chitosan ultrathin sustained-release membrane is 50–200 nm.

6. The nanocomposite antifreeze agent according to claim 1, characterized in that, The antifreeze solution was loaded into the pores of the MOF-801 porous carrier using a vacuum negative pressure impregnation method. The mass-to-volume ratio of the material to the liquid in the vacuum negative pressure impregnation method is 1 g : (5-10) mL. The impregnation conditions are as follows: the activated MOF-801 powder is added to the antifreeze mother liquor, the vacuum is drawn to -0.06 to -0.07 MPa and maintained for 30 to 40 min, then the pressure is restored to normal, and the powder is impregnated in a sealed, dark environment for 12 to 24 h with stirring. After impregnation, the powder is centrifuged and rinsed with deionized water 1 to 2 times.

7. The nanocomposite antifreeze according to claim 1, characterized in that, The chitosan ultrathin sustained-release membrane is made by a molecular self-adsorption coating process, specifically: MOF-801 particles loaded with drug solution are dispersed in a chitosan acetic acid solution with a mass fraction of 0.5% to 1.0%, stirred at low speed at room temperature for 2 to 3 hours, then centrifuged and dried at low temperature and vacuum at 40 to 45 °C.

8. A method for preparing a nanocomposite antifreeze agent suitable for pear blossom season as described in any one of claims 1 to 7, characterized in that, Includes the following steps: Preparation of S1 and nano MOF-801: Zirconium oxychloride octahydrate and fumaric acid were dissolved in a mixed solvent of N,N-dimethylformamide and formic acid at a molar ratio of 1:

1. After stirring until completely dissolved, the solution was transferred to a high-pressure reactor and reacted solvothermically at 120-130 °C for 24-48 h. After the reaction was completed, the solution was allowed to cool to room temperature, and the precipitate was collected by centrifugation. The precipitate was washed three times each with N,N-dimethylformamide and anhydrous methanol. S2, MOF-801 Activation: The nano MOF-801 obtained in step S1 was activated by solvent exchange by immersing it in anhydrous methanol, replacing it with fresh methanol every 12 h, and repeating the exchange 3 to 5 times; then it was exchanged twice with anhydrous ethanol; then it was placed in a vacuum drying oven and vacuum dried for 10 to 12 h at a vacuum degree of -0.08 to -0.09 MPa and a temperature of 80 to 100 ℃, and then sealed and cooled for later use. S3. Preparation of antifreeze stock solution: Add abscisic acid, calcium gluconate, compound amino acids and potassium dihydrogen phosphate to deionized water in sequence. After complete dissolution, add polysorbate-80 and ethylene glycol. Stir at room temperature for 10-15 min and adjust the pH to 4.5-6.0 to obtain the antifreeze stock solution. S4. Vacuum impregnation loading: Add the activated MOF-801 to the antifreeze mother liquor at a mass-to-volume ratio of 1 g : (5-10) mL, evacuate to -0.06 to -0.07 MPa, and maintain for 30-40 min; then slowly restore to normal pressure, and impregnate in a sealed, light-protected environment at room temperature for 12-24 h with stirring; after impregnation, centrifuge and rinse with deionized water 1-2 times. S5. Chitosan ultrathin coating: Disperse the rinsed solid particles in an acetate chitosan solution and stir at low speed at room temperature for 2-3 hours to allow chitosan to spontaneously adsorb and coat the particle surface. S6. Low-temperature drying: Vacuum drying at 40-45 ℃ to obtain the finished nanocomposite antifreeze.

9. The preparation method according to claim 8, characterized in that, In step S1, the volume ratio of N,N-dimethylformamide to formic acid in the mixed solvent is 1:

1. The contents of each component in the antifreeze mother liquor mentioned in step S3 are as follows: abscisic acid 2.5 mg / L, calcium gluconate 1 g / L, compound amino acids 0.75 g / L, potassium dihydrogen phosphate 1.5 g / L, polysorbate-80 2.5% (v / v), and ethylene glycol 2.5% (v / v). The chitosan acetic acid solution in step S5 has a mass fraction of 0.5% to 1.0%, and is prepared by dissolving chitosan in an aqueous acetic acid solution with a volume fraction of 1%.

10. A method for applying the nanocomposite antifreeze agent for pear blossom season as described in any one of claims 1 to 7, characterized in that, For protection against low-temperature frost damage during the flowering period of pear trees, the following steps are included: (1) Application period selection: Spray during the budding and flowering stages of pear trees; (2) Spraying method: Dilute the antifreeze product 100 to 300 times, spray it twice in a uniform spraying manner, with a spraying interval of 24 hours. Spray it evenly on the pear tree inflorescence, petals, stigma, anthers and tender new shoots, and the standard is that the surface of the flower is moist but not dripping water.

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