A method for preparing a nutrient solution for soilless cultivation of strawberries

Abstract

The present application relates to the technical field of organic fertilizer, and particularly relates to a preparation method of a nutrient solution for soilless cultivation of strawberries. The method first performs a multi-step grafting reaction on chitosan by using multiple functional groups such as gallic acid, 2-acrylamide-2-methylpropanesulfonic acid and N-vinylimidazole to prepare a porous graft copolymer carrier, then loads melatonin on the carrier to form a controlled-release composite powder, and finally adds macroelements and microelements at two different times to prepare the nutrient solution together with the controlled-release powder. The present application can significantly improve the pH stability of the nutrient solution and the bioavailability of iron ions, optimize the rhizosphere microenvironment, realize slow release of melatonin, strengthen antioxidant protection, and promote carbon and nitrogen metabolism balance through organic acid salts. Comparative experiments show that the nutrient solution can significantly promote the growth of strawberries, improve the fruit hardness and quality, and effectively support the efficient production of strawberries.

Description

Technical Field

[0001] This invention relates to the field of organic fertilizer technology, and in particular to a method for preparing a nutrient solution for hydroponics of strawberries. Background Technology

[0002] As an important economic crop, strawberry hydroponics plays a crucial role in facility agriculture. However, in actual production, especially under the low temperature and low light conditions of winter and spring, strawberry hydroponics faces numerous technical challenges, severely restricting the sustainable development of the industry.

[0003] Traditional hydroponic strawberry nutrient solution formulations are mainly based on modifications of Hoagland or Yamazaki formulas. While these can meet basic nutritional needs under normal temperature conditions, they reveal significant technical shortcomings under low-temperature and low-light stress. Firstly, the pH stability of the nutrient solution is insufficient; the pH value easily drifts under the influence of temperature fluctuations and light changes, affecting the normal absorption function of plant roots. Secondly, iron ions, a key micronutrient for plant growth, are prone to hydrolysis and precipitation under low-temperature conditions, leading to a sharp decrease in available iron content and iron deficiency chlorosis in plants.

[0004] While commonly used chelated iron fertilizers such as Fe-EDTA perform well under normal temperature conditions, their stability decreases significantly in low-temperature and low-light environments. Studies have shown that when the temperature drops below 12℃, the stability constant of Fe-EDTA complexes decreases significantly, and they are more prone to coordination competition inactivation under pH fluctuations. Simultaneously, low-light conditions limit plant photosynthesis, and the lack of sufficient organic acid secretion in the rhizosphere further exacerbates the difficulty in iron ion utilization.

[0005] In terms of plant physiological regulation, traditional methods mainly rely on the exogenous addition of growth regulators or changes in environmental conditions. However, directly added growth regulators are prone to degradation in nutrient solution environments, making it difficult to maintain a stable physiological activity concentration. Melatonin, as an emerging plant growth regulator, can enhance plant stress resistance and promote root development, but it has poor stability in aqueous solutions and is easily inactivated under light and oxidative conditions.

[0006] The regulation of ion balance in nutrient solutions is also a key issue. Under low-temperature stress, the ion absorption capacity of plant roots decreases, and the conductivity of the rhizosphere solution is prone to abnormal fluctuations, affecting the plant's osmotic regulation function. Existing buffer systems are mainly based on phosphate or carbonate systems, but these inorganic buffers are prone to side reactions with metal ions in complex nutrient solution systems, reducing their buffering effect.

[0007] Coordinating carbon and nitrogen metabolism is another technical challenge. Under low light and low temperature conditions, the synthesis of carbon skeletons in plants is limited, affecting the normal assimilation of nitrogen. Although traditional nitrogen source ratios take into account the ratio of nitrate nitrogen to ammonium nitrogen, they lack consideration for carbon skeleton supply, leading to an imbalance in carbon and nitrogen metabolism and slow plant growth.

[0008] Most strawberry-specific nutrient solutions currently on the market use traditional formulas, which perform poorly under extreme climatic conditions, leading to decreased strawberry yield and quality, and consequently, economic losses. Therefore, there is an urgent need to develop a novel nutrient solution preparation technology for hydroponics of strawberries that can maintain stable performance under low temperature and low light conditions, achieve precise nutrient supply, and possess physiological regulatory functions. Summary of the Invention

[0009] In view of this, the purpose of this invention is to provide a method for preparing nutrient solution for hydroponic strawberry cultivation, so as to solve the problems of poor stability of nutrient solution, easy inactivation of iron ions, and limited plant growth in hydroponic strawberry cultivation under low temperature and low light conditions.

[0010] To achieve the above objectives, the present invention provides a method for preparing a nutrient solution for hydroponics of strawberries, comprising the following steps:

[0011] (1) Preparation of graft copolymerization carrier: Chitosan was dissolved in a mixed solution of glacial acetic acid and deionized water, and gallic acid, N-hydroxysuccinimide and 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride were added for grafting reaction. The pH was adjusted to 5.0 with potassium hydroxide solution. Then 2-acrylamide-2-methylpropanesulfonic acid and potassium persulfate were added for the first graft copolymerization reaction. Then N-vinylimidazolium and potassium persulfate were added for the second graft copolymerization reaction. The graft copolymerization carrier powder was obtained by dialysis and freeze drying.

[0012] (2) Preparation of melatonin-loaded composite powder: The grafted copolymer carrier powder was dispersed in an ethanol aqueous solution, melatonin was added, the mixture was stirred, the ethanol was removed by rotary evaporation and vacuum drying to obtain melatonin-loaded composite powder.

[0013] (3) Preparation of nutrient solution: Chelated iron, α-ketoglutarate disodium salt dihydrate, boric acid, manganese sulfate monohydrate, zinc sulfate heptahydrate, copper sulfate pentahydrate, and sodium molybdate dihydrate are dissolved in deionized water to prepare a trace element solution. Melatonin-loaded compound powder is added and stirred and allowed to stand. Then, calcium nitrate tetrahydrate, potassium nitrate, potassium dihydrogen phosphate, and magnesium sulfate heptahydrate are added in sequence. After adjusting the pH value, melatonin-loaded compound powder is added again. Finally, water is added to make up the volume to obtain the nutrient solution for soilless strawberry cultivation.

[0014] Preferably, the degree of deacetylation of chitosan in step (1) is 75% or higher.

[0015] Preferably, the pH value of the mixed solution of glacial acetic acid and deionized water in step (1) is 2-4.

[0016] Preferably, in step (1), dialysis is performed for 48 hours using a dialysis membrane with a molecular weight cutoff of 3500 Da.

[0017] Preferably, in step (1), the weight ratio of chitosan, gallic acid, 2-acrylamide-2-methylpropanesulfonic acid and N-vinylimidazole is 4500-5500:450-550:810-990:198-242.

[0018] Preferably, the volume fraction of ethanol in the aqueous ethanol solution in step (2) is 95%.

[0019] Preferably, the mass fraction of melatonin in the melatonin-loaded composite powder in step (2) is 6.5%-10.0%.

[0020] Preferably, the chelated iron in step (3) is Fe-EDDHA6.

[0021] Preferably, in step (3), the pH of the nutrient solution is adjusted to 5.8-6.0.

[0022] Preferably, in step (3), each 1000 mL of nutrient solution contains the following components: 31.5-42.6 mg chelated iron, 162-218 mg α-ketoglutarate disodium salt dihydrate, 1570-2128 μg boric acid, 1437-1944 μg manganese sulfate monohydrate, 1224-1656 μg zinc sulfate heptahydrate, 213-288 μg copper sulfate pentahydrate, 103-139 μg sodium molybdate dihydrate, 703-951 mg calcium nitrate tetrahydrate, 430-582 mg potassium nitrate, 116-156 mg potassium dihydrogen phosphate, 315-426 mg magnesium sulfate heptahydrate, 61625-83375 μg melatonin-loaded compound powder, and the remainder deionized water.

[0023] Preferably, in step (3), the weight ratio of the first melatonin-loaded composite powder and the second melatonin-loaded composite powder is 1:1.

[0024] This invention achieves multiple technological breakthroughs in hydroponic strawberry cultivation nutrient solution under low temperature and low light conditions through a unique multi-component graft copolymer carrier design and a timed dosing process, resulting in the following significant beneficial effects:

[0025] Nutrient solution stability is significantly improved: The N-vinylimidazolium group in the porous graft copolymer carrier exhibits reversible protonation properties, forming an effective biological buffer system within the pH range of 5.8-6.0, significantly inhibiting pH drift in the nutrient solution. This buffering mechanism does not rely on traditional inorganic salt buffers, avoiding interference from metal ion side reactions and ensuring the long-term stability of the nutrient solution system.

[0026] The bioavailability of iron ions is significantly improved: the phenolic hydroxyl groups provided by the gallic acid graft structure have a moderate coordination effect on iron ions, providing additional stabilizing effects without interfering with the function of the main chelating agent. At the same time, the sulfonic acid groups in the support structure create a microenvironment conducive to the stability of the iron complex, maintaining a high effective iron retention rate even at low temperatures.

[0027] Precise regulation of the rhizosphere microenvironment: The strong hydrophilicity and ionic compatibility of the 2-acrylamide-2-methylpropanesulfonic acid group ensures the uniform dispersion of the carrier in the nutrient solution, forming a stable colloidal system. This system can regulate the conductivity of the rhizosphere solution, avoid drastic fluctuations in ion concentration, and create a suitable absorption environment for the roots.

[0028] Controlled release of physiologically active substances: The porous carrier structure enables the slow release and spatially uniform distribution of melatonin, avoiding the concentration peaks and rapid degradation problems associated with direct addition. The physical confinement of the carrier pores and the chemical interactions of the functional groups jointly regulate the release kinetics, ensuring that melatonin maintains a stable physiologically active concentration in the rhizosphere environment.

[0029] Synergistic Promotion of Carbon and Nitrogen Metabolism: The introduction of disodium α-ketoglutarate provides essential carbon skeleton support for nitrogen assimilation in plants under low-light conditions, breaking the constraint of carbon source limitation on nitrogen metabolism. The synergistic effect of this organic acid salt and the carrier enhances the plant's low-temperature adaptability and promotes the efficient utilization of nutrients.

[0030] Enhanced antioxidant protection mechanism: The polyphenolic hydroxyl groups in the gallic acid structure possess strong antioxidant activity, capable of scavenging reactive oxygen free radicals generated in the rhizosphere and reducing the damage of oxidative stress to the root system. This protective mechanism is particularly important under low-temperature stress conditions, effectively maintaining the physiological functions of the root system.

[0031] Process operability optimization: The sequential two-stage addition design fully considers the role mechanism of the functional groups of the carrier under different ionic strength environments. The first addition forms a protective carrier film in a low-salt environment, and the second addition provides a continuous stabilizing effect in a high-salt environment, thereby maximizing the functional effect.

[0032] The system exhibits outstanding synergistic effects: the integration of multiple functional groups within the same carrier framework avoids the mutual interference that may occur when adding multiple auxiliaries individually, resulting in a synergistic effect of multiple functions such as pH buffering, metal stabilization, antioxidant protection, and controlled-release supply. The overall effect surpasses the simple sum of the functions of each component. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in this invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0034] Figure 1 The infrared spectra of gallic acid-grafted chitosan, double-grafted chitosan, and porous grafted copolymer carrier in Example 2 of the present invention are shown. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0036] Example 1

[0037] A: Preparation of graft copolymerization support

[0038] Step A1 (Gallic Acid Grafting): Dissolve 4500 mg of chitosan (degree of deacetylation ≥75%, Sigma-Aldrich, catalog number C299272) in a mixed solution of 4500 mg glacial acetic acid and 495 mL deionized water, stir for 30 min to obtain an acidic chitosan solution, then add 450 mg gallic acid, 315 mg N-hydroxysuccinimide and 540 mg 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride in sequence, stir at 25 °C for 3 h, adjust the pH to 5.0 with 1 mol / L potassium hydroxide solution to obtain gallic acid-grafted chitosan solution;

[0039] Step A2 (2-Acrylamide-2-methylpropanesulfonic acid free radical grafting): Dissolve 810 mg of 2-acrylamide-2-methylpropanesulfonic acid in 10 mL of deionized water to prepare solution A, and dissolve 135 mg of potassium persulfate in 5 mL of deionized water to prepare solution B. Add solutions A and B dropwise simultaneously to the gallic acid-grafted chitosan solution from step A1 at 40 °C for 40 min, and continue the reaction at 40 °C for 70 min to complete the double-grafted chitosan solution.

[0040] Step A3 (N-vinylimidazolium grafting): Dissolve 198 mg N-vinylimidazolium in 5 mL of deionized water to prepare solution C, and dissolve 54 mg potassium persulfate in 5 mL of deionized water to prepare solution D. Add solutions C and D dropwise to the double-grafted chitosan solution in step A2 at 40 °C and continue the reaction for 22 min. Put all the reaction solution into a dialysis bag with a molecular weight cutoff of 3500 Da and dialyze with deionized water for 42 h (replace the deionized water every 8 h). After dialysis, pre-freeze at -40 °C for 3 h and freeze-dry under vacuum for 36 h to obtain porous grafted copolymer carrier powder.

[0041] B: Preparation of melatonin-loaded composite powder

[0042] Step B1 (Loading Preparation): 1000 mg of porous graft copolymer carrier powder was dispersed in 20 mL of 95 wt% ethanol aqueous solution, 72 mg of melatonin was added, and the mixture was stirred at room temperature in the dark for 45 min. The ethanol was removed by rotary evaporation at 40 °C, and the mixture was vacuum dried at room temperature for 10 h to obtain melatonin-loaded composite powder with a melatonin loading of 6.5 wt% (differential gravity method).

[0043] C: Preparation of working nutrient solution

[0044] Step C1 (Preparation of Trace Element Solution): Dissolve 31.5 mg of chelated iron fertilizer Fe-EDDHA6 (6% based on elemental iron) and 162 mg of α-ketoglutarate disodium salt dihydrate in 200 mL of deionized water. Then add 1570 μg of boric acid, 1437 μg of manganese sulfate monohydrate, 1224 μg of zinc sulfate heptahydrate, 213 μg of copper sulfate pentahydrate, and 103 μg of sodium molybdate dihydrate in sequence. After stirring and dissolving thoroughly, adjust the pH to 6.1 with 1 mol / L potassium hydroxide solution to obtain the trace element solution.

[0045] Step C2 (Carrier and Macro-element Addition): Add 61625 μg of melatonin-loaded composite powder to the micro-element solution prepared in step C1, stir at 25°C for 5 min, let stand for 10 min, then add 703 mg of calcium nitrate tetrahydrate, 430 mg of potassium nitrate, 116 mg of potassium dihydrogen phosphate, and 315 mg of magnesium sulfate heptahydrate in sequence, stir until completely dissolved, adjust the pH to 5.8 with 1 mol / L potassium hydroxide solution, add 61625 μg of melatonin-loaded composite powder, stir at 25°C for 10 min, and finally add deionized water to make up to 1000 mL to obtain the nutrient solution for hydroponics of strawberries.

[0046] Example 2

[0047] A: Preparation of graft copolymerization support

[0048] Step A1 (Gallic Acid Grafting): Dissolve 5000 mg of chitosan (degree of deacetylation ≥75%, Sigma-Aldrich, catalog number C299272) in a mixed solution of 5000 mg glacial acetic acid and 495 mL deionized water, stir for 30 min to obtain an acidic chitosan solution, then add 500 mg gallic acid, 350 mg N-hydroxysuccinimide and 600 mg 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride in sequence, stir at 25 °C for 4 h, adjust the pH to 5.0 with 1 mol / L potassium hydroxide solution to obtain gallic acid-grafted chitosan solution;

[0049] Step A2 (2-Acrylamide-2-methylpropanesulfonic acid free radical grafting): Dissolve 900 mg of 2-acrylamide-2-methylpropanesulfonic acid in 10 mL of deionized water to prepare solution A, and dissolve 150 mg of potassium persulfate in 5 mL of deionized water to prepare solution B. Add solutions A and B dropwise simultaneously to the gallic acid-grafted chitosan solution from step A1 at 40 °C for 45 min, and continue the reaction at 40 °C for 80 min to complete the double-grafted chitosan solution.

[0050] Step A3 (N-vinylimidazolium grafting): Dissolve 220 mg N-vinylimidazolium in 5 mL of deionized water to prepare solution C, and dissolve 60 mg potassium persulfate in 5 mL of deionized water to prepare solution D. Add solutions C and D dropwise to the double-grafted chitosan solution in step A2 at 40 °C and continue the reaction for 25 min. Put all the reaction solution into a dialysis bag with a molecular weight cutoff of 3500 Da and dialyze with deionized water for 48 h (replace the deionized water every 6 h). After dialysis, pre-freeze at -40 °C for 4 h and freeze-dry under vacuum for 48 h to obtain porous grafted copolymer carrier powder.

[0051] B: Preparation of melatonin-loaded composite powder

[0052] Step B1 (Loading Preparation): 1000 mg of porous graft copolymer carrier powder was dispersed in 20 mL of 95 wt% ethanol aqueous solution, 87 mg of melatonin was added, and the mixture was stirred at room temperature in the dark for 60 min. The ethanol was removed by rotary evaporation at 40 °C, and the mixture was vacuum dried at room temperature for 12 h to obtain melatonin-loaded composite powder with a melatonin loading of 7.8 wt% (differential gravity method).

[0053] C; Preparation of working nutrient solution

[0054] Step C1 (Preparation of Trace Element Solution): Dissolve 37 mg of chelated iron fertilizer Fe-EDDHA6 (6% based on elemental iron) and 190 mg of α-ketoglutarate disodium salt dihydrate in 200 mL of deionized water. Then, add 1850 μg boric acid, 1690 μg manganese sulfate monohydrate, 1440 μg zinc sulfate heptahydrate, 250 μg copper sulfate pentahydrate, and 121 μg sodium molybdate dihydrate sequentially. After thorough stirring and dissolution, adjust the pH to 6.2 with 1 mol / L potassium hydroxide solution to obtain the trace element solution.

[0055] Step C2 (Carrier and Macro-element Addition): Add 72500 μg of melatonin-loaded composite powder to the micro-element solution prepared in step C1, stir at 25°C for 5 min, let stand for 10 min, then add 827 mg of calcium nitrate tetrahydrate, 506 mg of potassium nitrate, 136 mg of potassium dihydrogen phosphate, and 370 mg of magnesium sulfate heptahydrate in sequence, stir until completely dissolved, adjust the pH to 5.9 with 1 mol / L potassium hydroxide solution, add 72500 μg of melatonin-loaded composite powder, stir at 25°C for 10 min, and finally add deionized water to make up to 1000 mL to obtain the nutrient solution for hydroponics of strawberries.

[0056] Example 3

[0057] A: Preparation of graft copolymerization support

[0058] Step A1 (Gallic Acid Grafting): Dissolve 5500 mg of chitosan (degree of deacetylation ≥75%, Sigma-Aldrich, catalog number C299272) in a mixed solution of 5500 mg glacial acetic acid and 495 mL deionized water, stir for 30 min to obtain an acidic chitosan solution, then add 550 mg gallic acid, 385 mg N-hydroxysuccinimide and 660 mg 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride in sequence, stir at 25 °C for 5 h, adjust the pH to 5.0 with 1 mol / L potassium hydroxide solution to obtain gallic acid-grafted chitosan solution;

[0059] Step A2 (2-Acrylamide-2-methylpropanesulfonic acid free radical grafting): Dissolve 990 mg of 2-acrylamide-2-methylpropanesulfonic acid in 10 mL of deionized water to prepare solution A, and dissolve 165 mg of potassium persulfate in 5 mL of deionized water to prepare solution B. Add solutions A and B dropwise simultaneously to the gallic acid-grafted chitosan solution in step A1 at 40 °C for 50 min, and continue the reaction at 40 °C for 90 min to complete the double-grafted chitosan solution.

[0060] Step A3 (N-vinylimidazolium grafting): Dissolve 242 mg N-vinylimidazolium in 5 mL of deionized water to prepare solution C, and dissolve 66 mg potassium persulfate in 5 mL of deionized water to prepare solution D. Add solutions C and D dropwise to the double-grafted chitosan solution in step A2 at 40 °C and continue the reaction for 30 min. Put all the reaction solution into a dialysis bag with a molecular weight cutoff of 3500 Da and dialyze with deionized water for 54 h (replace the deionized water every 4 h). After dialysis, pre-freeze at -40 °C for 6 h and freeze-dry under vacuum for 60 h to obtain porous grafted copolymer carrier powder.

[0061] B: Preparation of melatonin-loaded composite powder

[0062] Step B1 (Loading Preparation): Disperse 1000 mg of porous graft copolymer carrier powder in 20 mL of 95 wt% ethanol aqueous solution, add 110 mg of melatonin, stir at room temperature in the dark for 90 min, remove ethanol by rotary evaporation at 40 °C, and vacuum dry at room temperature for 16 h to obtain melatonin-loaded composite powder with a melatonin loading of 10.0 wt% (differential gravity method).

[0063] C; Preparation of working nutrient solution

[0064] Step C1 (Preparation of Trace Element Solution): Dissolve 42.6 mg of chelated iron fertilizer Fe-EDDHA6 (6% based on elemental iron) and 218 mg of α-ketoglutarate disodium salt dihydrate in 200 mL of deionized water. Then add 2128 μg of boric acid, 1944 μg of manganese sulfate monohydrate, 1656 μg of zinc sulfate heptahydrate, 288 μg of copper sulfate pentahydrate, and 139 μg of sodium molybdate dihydrate in sequence. After stirring and dissolving thoroughly, adjust the pH to 6.3 with 1 mol / L potassium hydroxide solution to obtain the trace element solution.

[0065] Step C2 (Carrier and Macro-element Addition): Add 83375 μg of melatonin-loaded composite powder to the micro-element solution prepared in step C1, stir at 25°C for 5 min, let stand for 10 min, then add 951 mg of calcium nitrate tetrahydrate, 582 mg of potassium nitrate, 156 mg of potassium dihydrogen phosphate, and 426 mg of magnesium sulfate heptahydrate in sequence, stir until completely dissolved, adjust the pH to 6.0 with 1 mol / L potassium hydroxide solution, add 83375 μg of melatonin-loaded composite powder, stir at 25°C for 10 min, and finally add deionized water to make up to 1000 mL to obtain the nutrient solution for hydroponics of strawberries.

[0066] Comparative Example 1:

[0067] The difference between Comparative Example 1 and Example 2 is that the N-vinylimidazol grafting in step A3 is omitted, that is, the free radical grafting modification of N-vinylimidazol is not carried out, and the double-grafted chitosan solution obtained in steps A1 and A2 is directly used for subsequent treatment; the other conditions are the same as those in Example 2.

[0068] Comparative Example 2:

[0069] The difference between Comparative Example 2 and Example 2 is that the gallic acid grafting in step A1 is omitted, and the gallic acid grafting based on 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide activation is not performed. The ungrafted chitosan directly enters steps A2 and A3; the other conditions are the same as in Example 2.

[0070] Comparative Example 3:

[0071] The difference between Comparative Example 3 and Example 2 is that the 2-acrylamide-2-methylpropanesulfonic acid radical grafting in step A2 is omitted, while the other conditions are the same as in Example 2.

[0072] Comparative Example 4:

[0073] The difference between Comparative Example 4 and Example 2 is that in step C2, the melatonin-loaded composite powder was added in two separate additions instead of in a single addition of 145,000 μg of melatonin-loaded composite powder (the total amount added is the same as in Example 2), while the subsequent stirring and volume adjustment steps remained unchanged; the other conditions were the same as in Example 2.

[0074] Comparative Example 5

[0075] The difference between Comparative Example 5 and Example 2 is that sodium α-ketoglutarate is not added in step C1, while the other conditions are the same as in Example 2.

[0076] Comparative Example 6:

[0077] The difference between Comparative Example 6 and Example 2 is that melatonin is not loaded in step B, that is, porous graft copolymer carrier powder with the same mass and pore structure parameters is used directly; the other conditions are the same as in Example 2.

[0078] Comparative Example 7:

[0079] The difference between Comparative Example 7 and Example 2 is that the melatonin-loaded complex powder in step C2 was replaced with 11310 μg of melatonin.

[0080] Performance testing:

[0081] Common experimental arrangements (applicable to all of the following items):

[0082] Sample preparation and cultivation: 12 hydroponic strawberry seedlings per group (examples and comparative examples), 3 groups in parallel; nutrient solution replenishment and climate settings were the same. Low light and low temperature stress conditions: 12±1°C, photosynthetically active radiation 100 μmol / m². 2 / second, photoperiod 10h / day, continuous for 7 days; standard operating conditions: 22±1°C, 300μmol / m 2 / second, 12h / day.

[0083] Infrared spectral characterization: Infrared spectrometer was used for testing, and the results are as follows: Figure 1 As shown.

[0084] Physicochemical stability of nutrient solution: The nutrient solutions of each group were placed under weak light and low temperature and under normal working conditions, and the pH was measured at t=0 and 72h. The pH drift ΔpH was calculated and the results are shown in Table 1.

[0085] Effective iron retention rate: Under the conditions of 1.0 klx visible light, pH=5.80-6.00, and 12°C for 72 consecutive hours, samples were taken at t=0 and 72 hours. The iron content in the solution was determined by inductively coupled plasma atomic emission spectrometry, and the effective iron retention rate was calculated. The results are shown in Table 1.

[0086] Evaluation of strawberry growth performance: Healthy strawberry seedlings (variety Red Cheek, 3-4 true leaves) were selected. A hydroponic experiment was conducted in a controlled-environment greenhouse. After 7 days of continuous treatment under low light and low temperature stress, the plants were transferred to conventional conditions and continued cultivation until flowering and fruiting. Plant height, number of leaves, root length, fresh weight, and dry weight were measured regularly. Plant height and root length were measured using calipers (accuracy 0.1 mm), and weight was measured using an electronic balance (accuracy 0.01 g). The plants were dried in an oven at 65℃ until constant weight, and the dry weight was determined. The results are shown in Table 1.

[0087] Fruit quality analysis: 20 mature fruits were randomly selected from each group, with 3 replicates. The firmness of the fruit was measured using a texture analyzer with a probe diameter of 2 mm, a test speed of 1 mm / s, and a compression deformation of 10%. The results are shown in Table 1.

[0088] Rhizosphere microenvironment analysis: The rhizosphere solutions of strawberries treated with nutrient solution in each group were analyzed. The conductivity EC value was measured using a conductivity meter. Twenty strawberry rhizosphere solutions were randomly selected from each group, with three replicates. The mean and standard deviation were calculated. The results are shown in Table 1.

[0089] Table 1 Performance Test Results

[0090]

[0091] Data Analysis:

[0092] As can be seen from the data in Examples 1-3 in Table 1, the hydroponic strawberry nutrient solution prepared by this invention exhibits significant advantages in synergistic effects. Example 2 demonstrates the best overall performance, with a 72-hour available iron retention rate of 96.8%, pH drift of only 0.12 and 0.09 under low light, low temperature, and conventional conditions, respectively, plant height growth of 5.8 cm, root length of 6.4 cm, aboveground fresh weight of 10.5 g, fruit firmness of 0.82 N, and rhizosphere solution EC value controlled at 1.31 mS·cm. -1 These superior properties stem from the synergistic effect of the protonation buffering mechanism of the N-vinylimidazolium ring and the ionic compatibility of the 2-acrylamido-2-methylpropanesulfonic acid group in the multi-component graft copolymer structure, forming a stable rhizosphere microenvironment. The antioxidant phenolic hydroxyl groups provided by the gallic acid graft structure can effectively scavenge rhizosphere free radicals, while also exhibiting resistance to Fe... 3+ The weak coordination of the carrier and the carbon skeleton supply of disodium α-ketoglutarate form a dual promoting mechanism of iron stabilization and carbon-nitrogen assimilation. The controlled release of melatonin achieves spatial uniform distribution through the porous structure of the carrier, avoiding potential root damage from concentration peaks and maintaining plant physiological activity during low-temperature stress. The gradient changes presented in the data from the three examples reflect the fine-tuning effect of the carrier component ratio on the final effect. In Example 2, the moderate carrier dosage and appropriate functional group density achieved the optimal coordination balance among multiple mechanisms, demonstrating a significant synergistic effect.

[0093] Comparative Example 1 omitted the N-vinylimidazolium grafting, thus lacking the protonation buffering capacity of the imidazole ring, leading to a decrease in the pH stability of the nutrient solution. The reversible protonation / deprotonation reaction of the N-vinylimidazolium group in the pH range of 5.8-6.0 is a key mechanism for maintaining the stability of the rhizosphere microenvironment. Its absence makes the Fe-EDDHA complex more susceptible to coordination competition inactivation during pH fluctuations, and the EC value of the rhizosphere solution increases to 1.43 mS·cm. -1 This reflects a disruption of ion balance. This difference reveals the decisive role of the imidazole buffer system in the stability of iron ions.

[0094] Comparative Example 2, lacking gallic acid grafting, lost the antioxidant protection of the phenolic hydroxyl groups and the weak coordination stabilizing effect on metal ions. The electron-conjugated system formed by the three ortho- and ortho-phenolic hydroxyl groups in the gallic acid structure can effectively scavenge hydroxyl radicals and superoxide anions generated in the rhizosphere, while also exhibiting resistance to Fe... 3+ The weak coordination of the ions can slow down the hydrolysis and precipitation process of iron ions. The rhizosphere EC value of Comparative Example 2 increased to 1.44 mS·cm. -1 This indicates that roots lacking antioxidant protection have reduced ion osmotic pressure regulation capacity under low temperature stress, and the cumulative effect of this physiological stress is ultimately reflected in significant differences in plant growth performance.

[0095] Comparative Example 3, omitting the 2-acrylamide-2-methylpropanesulfonic acid grafting, resulted in the loss of strong hydrophilicity and ion compatibility of the carrier. The absence of the sulfonic acid group fundamentally altered the dispersion stability of the carrier in the nutrient solution and the coordination environment of metal ions. The strong electronegativity and high hydrophilicity of the AMPS group are key factors in maintaining the stability of the carrier-nutrient solution interface; its absence led to abnormal melatonin release kinetics and disruption of the iron complex microenvironment. The rhizosphere EC value increased to 1.50 mS·cm. -1 This reflects the most severe ion balance disorder, where the synergistic effect caused by this systemic functional defect is completely lost.

[0096] Comparative Example 4, which involved a single application of melatonin-loaded compound powder, showed the same total dosage, but the effective iron retention rate decreased to 94.0%, and the plant growth indicators were significantly worse than in Example 2. The single application resulted in an excessively high instantaneous concentration of the carrier in the nutrient solution, potentially causing aggregation and sedimentation of carrier particles and disrupting the uniform release kinetics of melatonin. The sequential design of two-stage application allowed the carrier containing functional groups to exert stabilizing effects under different ionic strength environments. The first application formed an initial protective film under low ionic strength, while the second application provided continuous buffering and complexing capabilities under high salinity. The decrease in fruit firmness to 0.64 N reflects the negative impact of this improper application strategy on fruit quality, demonstrating the technical necessity of the sequential application process.

[0097] Comparative Example 5, lacking disodium α-ketoglutarate, exhibited decreased available iron retention and restricted plant growth. α-Ketoglutarate, a key intermediate in the tricarboxylic acid cycle, provides essential carbon skeleton support for nitrate assimilation under low light and low temperature conditions; its deficiency leads to decreased nitrogen use efficiency and carbon-nitrogen metabolic imbalance. The rhizosphere EC value increased to 1.48 mS·cm. -1 This reflects a weakened ability of plant roots to regulate ion absorption balance under carbon-deficient conditions. This metabolic limiting factor affects root vigor and nutrient absorption efficiency, ultimately resulting in a decline in overall plant growth performance, highlighting the crucial role of organic acid salts in nutrient management under low-temperature stress.

[0098] Comparative Example 6, without melatonin loading, significantly lagged behind Example 2 in all indicators, particularly in plant height growth (only 3.8 cm) and aboveground fresh weight (only 7.4 g). Melatonin, as an endogenous signaling molecule in plants, plays a crucial role in regulating antioxidant enzyme activity, maintaining cell membrane stability, and promoting root development under low-temperature stress. The carrier-controlled release of melatonin maintains a stable physiological activity concentration in the rhizosphere, avoiding rapid degradation and concentration fluctuations associated with direct addition. The rhizosphere EC value increased to 1.46 mS·cm. -1This indicates that roots lacking melatonin regulation have a reduced ability to maintain ion balance under stress conditions, and the cumulative effect of this lack of physiological regulation leads to a significant reduction in the overall stress resistance and growth potential of the plant.

[0099] Comparative Example 7, with the direct addition of an equal amount of melatonin, while superior to other comparative examples in some indicators, was still significantly inferior to the carrier-controlled release system of Example 2. The effective iron retention rate decreased to 93.9%, and both plant growth and fruit quality indicators showed varying degrees of decline. Directly added melatonin is prone to photolysis and oxidative degradation in the nutrient solution environment, making it difficult to maintain a consistently stable physiological activity concentration, and it lacks the spatial localization effect provided by the carrier. The carrier-controlled release system, through the physical encapsulation of porous structures and the chemical interaction of functional groups, achieves the slow release and localized enrichment of melatonin. This micro-regional supply method is more in line with the absorption and utilization characteristics of physiological regulators by plant roots. The rhizosphere EC value was 1.42 mS·cm. -1 The slight increase reflects the impact of the lack of a carrier buffer system on the stability of the rhizosphere microenvironment, demonstrating the technical advantages of carrier-based melatonin supply.

[0100] from Figure 1 It can be seen that: compared with chitosan grafted with gallic acid alone, the double-grafted sample showed better performance at 1212 cm⁻¹. -1 With 1040cm -1 A significant enhancement of -SO3 was observed at the site. - The anti / symmetric stretching peaks confirm the successful introduction of 2-acrylamide-2-methylpropanesulfonic acid; based on this, the porous graft copolymerization support was observed at 1565 cm⁻¹. -1 A new imidazole ring characteristic band is generated at 1450 cm⁻¹. -1 With 1228cm -1 The appearance of a ring backbone and a synaptic / shoulder peak of CN indicates that the high-frequency OH / NH broadband has further broadened and slightly redshifted, suggesting that the hydrogen bonds within the network are more compact; all three remain at ~1660 cm⁻¹. -1 (Amide I) and ~1540cm -1 (Amide II) and 1150-1020cm -1 The polysaccharide COC / CO region characteristics provide overall evidence for the successful implementation of stepwise grafting.

[0101] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing a nutrient solution for hydroponically grown strawberries, characterized in that, Includes the following steps: (1) Preparation of graft copolymerization carrier: Chitosan was dissolved in a mixed solution of glacial acetic acid and deionized water, and gallic acid, N-hydroxysuccinimide and 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride were added for grafting reaction. The pH was adjusted to 5.0 with potassium hydroxide solution. Then 2-acrylamide-2-methylpropanesulfonic acid and potassium persulfate were added for the first graft copolymerization reaction. Then N-vinylimidazolium and potassium persulfate were added for the second graft copolymerization reaction. The graft copolymerization carrier powder was obtained by dialysis and freeze drying. (2) Preparation of melatonin-loaded composite powder: The grafted copolymer carrier powder was dispersed in an ethanol aqueous solution, melatonin was added, the mixture was stirred, the ethanol was removed by rotary evaporation and vacuum drying to obtain melatonin-loaded composite powder. (3) Preparation of nutrient solution: Chelated iron, α-ketoglutarate disodium salt dihydrate, boric acid, manganese sulfate monohydrate, zinc sulfate heptahydrate, copper sulfate pentahydrate, and sodium molybdate dihydrate are dissolved in deionized water to prepare a trace element solution. Melatonin-loaded compound powder is added and stirred and allowed to stand. Then, calcium nitrate tetrahydrate, potassium nitrate, potassium dihydrogen phosphate, and magnesium sulfate heptahydrate are added in sequence. After adjusting the pH value, melatonin-loaded compound powder is added again. Finally, water is added to make up the volume to obtain a nutrient solution for soilless strawberry cultivation. In step (1), the weight ratio of chitosan, gallic acid, 2-acrylamide-2-methylpropanesulfonic acid and N-vinylimidazol is 4500-5500:450-550:810-990:198-242. The mass fraction of melatonin in the melatonin-loaded composite powder in step (2) is 6.5%-10.0%.

2. The method for preparing the nutrient solution for hydroponics of strawberries according to claim 1, characterized in that, In step (1), the degree of deacetylation of chitosan is above 75%.

3. The method for preparing the nutrient solution for hydroponics of strawberries according to claim 1, characterized in that, The pH value of the mixed solution of glacial acetic acid and deionized water in step (1) is 2-4.

4. The method for preparing the nutrient solution for hydroponics of strawberries according to claim 1, characterized in that, In step (1), dialysis is performed for 48 hours using a dialysis membrane with a molecular weight cutoff of 3500 Da.

5. The method for preparing the nutrient solution for hydroponics of strawberries according to claim 1, characterized in that, In step (2), the volume fraction of ethanol in the aqueous ethanol solution is 95%.

6. The method for preparing the nutrient solution for hydroponics of strawberries according to claim 1, characterized in that, In step (3), the chelated iron is Fe-EDDHA6.

7. The method for preparing the nutrient solution for hydroponics of strawberries according to claim 1, characterized in that, In step (3), the pH of the nutrient solution is adjusted to 5.8-6.

0.

8. The method for preparing the nutrient solution for hydroponics of strawberries according to claim 1, characterized in that, In step (3), each 1000 mL of nutrient solution contains the following components: 31.5-42.6 mg chelated iron, 162-218 mg α-ketoglutarate disodium salt dihydrate, 1570-2128 μg boric acid, 1437-1944 μg manganese sulfate monohydrate, 1224-1656 μg zinc sulfate heptahydrate, 213-288 μg copper sulfate pentahydrate, 103-139 μg sodium molybdate dihydrate, 703-951 mg calcium nitrate tetrahydrate, 430-582 mg potassium nitrate, 116-156 mg potassium dihydrogen phosphate, 315-426 mg magnesium sulfate heptahydrate, 61625-83375 μg melatonin-loaded complex powder, and the remainder deionized water.

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