Superabsorbent bioactive polymer, method for producing same, and use thereof

Abstract

The present invention relates to a superabsorbent polymer obtained by crosslinking at least one polycarboxylic acid. The invention also relates to a method for producing said superabsorbent polymer, and to its uses and applications, in particular in the field of agronomy for diffusing water or active substances.

Description

[0001] Bioactive superabsorbent polymer, its manufacturing process and its uses

[0002] Field of invention

[0003] The present invention relates to a superabsorbent polymer obtained by the crosslinking of at least one polycarboxylic acid. The invention also relates to a method for manufacturing said superabsorbent polymer and its uses and applications, in particular in the field of agronomy for the controlled diffusion of water and active substances.

[0004] The present invention is therefore in the field of polymer chemistry and their applications.

[0005] State of the art

[0006] The most efficient superabsorbent products currently available on the market are obtained by chemical crosslinking with monomers that can be toxic to the environment because they are derived from petrochemicals and are not biodegradable. These products can release molecules into the environment that are not systematically controlled.

[0007] These superabsorbent products are mainly derived from chemical reactions incompatible with the current objectives of developing green chemistry, environmental protection, crop health, as well as animal and human health. Currently known superabsorbent products, such as cellulose, starch, and chitosan, have the advantage of meeting the green chemistry objectives mentioned above and of being biodegradable. However, to date, their technical performance, and in particular their liquid absorption capacity, is inferior to that of synthetic superabsorbent products (Chang et al., “Superabsorbent polymers used for agricultural water retention,” Polymer Testing 94, 107021, 2021).

[0008] There is therefore a need to develop new high-performance superabsorbent products, produced from natural and biodegradable components.

[0009] Description of the invention

[0010] The inventors have now designed a crosslinked superabsorbent polymer that provides an alternative to chemically produced superabsorbent products. Such a polymer is produced from natural polymers that are crosslinked using a process involving divalent and trivalent cations. The inventors have surprisingly demonstrated that a crosslinking reaction of a polycarboxylic acid in the presence of a low concentration of divalent and trivalent cations produces a crosslinked polymer with an ultrapure water absorption capacity of between 500 and 800 grams of water per gram of polymer, which is comparable to currently available superabsorbent products manufactured by chemical synthesis.

[0011] A superabsorbent crosslinked polymer that is the subject of the invention therefore constitutes a biodegradable matrix capable of storing and gradually releasing a large quantity of water. These properties make it of great interest for various applications, including in particular agronomy, health and possibly hygiene. The presence of a chelating agent during the crosslinking process further increases the performance of said biodegradable matrix when it is placed in reference saline conditions. A superabsorbent crosslinked polymer that is the subject of the invention has a positive effect on the hydration and growth of plants.

[0012] The inventors therefore developed a process for manufacturing said crosslinked polymer.

[0013] The inventors have also developed a method for manufacturing a crosslinked polycationic polymer, said crosslinked cationic polymer being capable of forming particles within the biodegradable matrix of the crosslinked polymer according to the invention. When said biodegradable matrix of crosslinked polymer comprises cationic polymer particles encapsulating an active agent, the treatment of plants with this matrix enriched with active ingredients leads to the prolonged release of said active ingredient. Finally, the effectiveness on the treated plants of said active ingredient was characterized by the demonstration of the overexpression by the plants of plant compounds involved in their response against environmental stresses.

[0014] In addition, due to the biodegradation of the superabsorbent product, the microbiota and the plant are stimulated, notably activating acquired systemic defense systems of the SAR type (“Systemic Acquired Resistance” in English).

[0015] Finally, when the natural polymers are polycarboxylates, said crosslinked polycarboxylates are capable of chelating mineral species traditionally unavailable to the plant in order to mobilize them and increase the bioavailability of these mineral species for the treated plant.

[0016] The present invention also relates to an ionotropic gelation process for preparing superabsorbent polymer matrices, and in particular so-called "bioactive" superabsorbent polymer matrices comprising biologically active molecules. Said bioactive superabsorbent polymer matrices constitute a combination of cationic biopolymers and oligomers, the latter belonging to the chitosan or protein families, and comprising, for example, phenolic compounds and phytohormones. A crosslinked polymer according to the invention therefore has numerous advantages: its use results in a significant increase in the water reserve available for the growth of plants under water stress conditions, the controlled release over time of biostimulants and fertilizers, encapsulated within said crosslinked polymer, during plant growth and the specific stimulation of the plant and its rhizosphere.In addition, a crosslinked polymer according to the invention makes it possible to capture inorganic nutrients essential for plant growth from the soil or the plant microenvironment. Finally, the biodegradation of a polysaccharide polymer generates oligosaccharides that are elicitors for the plant.

[0017] The superabsorbent crosslinked polymer according to the invention, due to the low concentration of crosslinking agent during the production process which induces a low crosslinking rate, also has the advantage of being in the form of a gel and not in the form of granules or microbeads. Said gel can thus be dried, then crushed / ground to obtain a dispersible superabsorbent powder. The invention thus finds a particular application in the dispersion of a superabsorbent powder on a plant growing soil or as an addition to a potting soil in which a plant grows. The dispersive aspect of the powder allows a better distribution of the crosslinked polymer according to the invention and therefore better efficiency.

[0018] Detailed description of the invention

[0019] The present invention relates firstly to a crosslinked polymer obtained by the crosslinking of at least one polycarboxylic acid in solution in a solvent, in the presence of a crosslinking agent chosen from divalent cations and trivalent cations, said polymer being characterized in that during said crosslinking the initial concentration of said crosslinking agent in said solvent is between 1 and 30 mM, said crosslinked polymer of polycarboxylic acid being capable of absorbing at least 200 grams of water per gram of crosslinked polymer.

[0020] The term "polycarboxylic acid" means a molecule comprising at least two carboxyl groups, a carboxyl group being a functional group composed of a carbon atom linked by a double bond to an oxygen atom and by a single bond to a hydroxyl group. Said at least two carboxyl groups are linked to a functional group R, R being an organic group comprising 2, 3, 4 or n carbon atoms. The group of carboxylic acids includes in particular dicarboxylic acids, tricarboxylic acids, tetracarboxylic acids and polymeric structures consisting of assemblies of monomers bearing one or more carboxyl groups.

[0021] “Polycarboxylic acid” means polycarboxylic acid and its conjugate bases, which are polycarboxylate ions, particularly present in the form of alkali metal polycarboxylate, more particularly sodium polycarboxylate, potassium polycarboxylate, calcium polycarboxylate and magnesium polycarboxylate.

[0022] According to a particular embodiment, in a crosslinked polymer according to the invention, the molar mass of said polycarboxylic acid is between 1,000 Da and 1,000,000 Da.

[0023] A crosslinked polycarboxylic acid polymer according to the invention is characterized by its capacity to retain and absorb a liquid, in particular water, this capacity is expressed in grams of liquid per gram of crosslinked polymer.

[0024] More particularly, a crosslinked polycarboxylic acid polymer according to the invention is characterized by its swelling index (SI) expressed in grams (g) of water (or of a reference saline solution in which the crosslinked polymer is placed) per gram (g) of crosslinked polymer. The swelling index of a crosslinked polymer according to the invention is notably measured using the method described by Dai et al (“Multifunctional self-assembling hydrogel from guar gum”, Chemical Engineering Journal, 330, pp 1044-1051, 2017). The swelling index can be measured in the presence of ultrapure water or a reference saline solution, for example a 0.9% sodium chloride solution.

[0025] A crosslinked polycarboxylic acid polymer according to the invention is characterized by a capacity to absorb pure water, in g of pure water / g of crosslinked polymer greater than or equal to 200, greater than or equal to 300, greater than or equal to 400, greater than or equal to 450, greater than or equal to 500, greater than or equal to 550, greater than or equal to 600, greater than or equal to 650, greater than or equal to 700, greater than or equal to 750, or greater than or equal to 800.

[0026] A crosslinked polycarboxylic acid polymer according to the invention is characterized by a capacity to absorb a 0.9% sodium chloride solution, expressed in g of 0.9% sodium chloride solution / g of crosslinked polymer, greater than or equal to 20, greater than or equal to 30, greater than or equal to 40, greater than or equal to 45, greater than or equal to 50, greater than or equal to 55, greater than or equal to 60, greater than or equal to 65, greater than or equal to 70, greater than or equal to 75, or greater than or equal to 80.

[0027] A crosslinked polycarboxylic acid polymer according to the invention can further be characterized according to the following criteria:

[0028] - its gelling rate,

[0029] - its viscosity,

[0030] - its density, expressed in g / cm 3

[0031] - its shear rate, expressed in seconds -1, - its dimensional stability, when placed in variable environmental conditions,

[0032] - its biodegradability, particularly expressed by the percentage of mass loss during application in the soil, as well as the assimilation by microorganisms of the released monomers (microbial biostimulation),

[0033] - its mechanical resistance properties,

[0034] - its crosslinking density,

[0035] - the diameter of the polymer fibers.

[0036] These characteristics can be measured using methods well known to those skilled in the art.

[0037] By "crosslinking agent" is meant a chemical agent capable of creating chemical bonds between macromolecular chains, thus forming a three-dimensional network. According to a particular embodiment of a polymer according to the invention, said crosslinking agent comprises as active agent, or is constituted by, a divalent metal or a trivalent metal. More particularly, said crosslinking agent comprises calcium ions, in particular calcium chloride.

[0038] According to a particular embodiment, a crosslinked polymer according to the invention is obtained by crosslinking at least one polycarboxylic acid in solution in a solvent, in the presence of a crosslinking agent chosen from divalent cations and trivalent cations, characterized in that during said crosslinking the initial concentration of said crosslinking agent in said solvent is between 1 and 30 mM, between 1 and 20 mM, between 1 and 15 mM, between 1 and 10 mM, between 2 and 7.5 mM, between 2 and 6 mM, between 3 and 6 mM, between 3.5 mM and 6 mM, and preferably 5 mM.

[0039] In particular, during said crosslinking, the concentration of the polycarboxylic acid in said solvent is from 0.1% to 6% by mass per volume of solvent (m / v). In particular, the concentration of the polycarboxylic acid is from 0.1 to 4% m / v, particularly from 0.1 to 3% m / v, in particular from 0.2 to 4% m / v, in particular still from 0.2 to 3% m / v, in particular from 0.2 to 2% m / v, particularly from 0.2 to 1% m / v, in particular still from 0.2 to 0.8% m / v, in particular from 0.3 to 0.7% m / v, for example 0.5% m / v.

[0040] Among polycarboxylic acids, linear polycarboxylic acids can be distinguished from branched polycarboxylic acids. Furthermore, natural polycarboxylic acids can be distinguished from synthetic or modified polycarboxylic acids. Finally, polysaccharides constitute an important group among polycarboxylic acids.

[0041] Among the polysaccharides, alginic acid and alginate derivatives (CAS No. 9005-32-7), extracted from brown algae or bacteria, are well known. Alginate is a linear polymer consisting of two types of uronic acid: D-mannuronic acid, or mannuronate, and L-guluronic acid, or guluronate, whose proportion and distribution vary depending on the alginate. The number of monomers constituting alginic acid varies between 100 and 3000. Alkali metal alginates, including sodium alginates, potassium alginates, calcium alginates, and magnesium alginates, are known.

[0042] According to a particular aspect, a crosslinked polymer according to the invention is obtained by the crosslinking of at least one polycarboxylic acid in solution, said polycarboxylic acid being biosourced alginic acid, more particularly alginic acid extracted from a brown algae.

[0043] According to a particular embodiment, the subject of the invention is a polymer obtained by the crosslinking of alginic acid comprising mannuronic acid, in a proportion of between 30 and 70% of the weight of alginic acid, and guluronic acid, in a proportion of between 30 and 70% of the weight of alginic acid.

[0044] More particularly, a crosslinked polymer according to the invention comprises alginic acid characterized by the ratio between the number of mannuronic acid monomers and the number of guluronic acid monomers, also referred to as the “M / G ratio”. This ratio may be between 0.1 and 2.

[0045] Even more particularly, a crosslinked polymer according to the invention comprises alginic acid characterized by an “M / G ratio” of between 0.2 and 1.8, more particularly between 0.4 and 1.8, in particular between 0.6 and 1.8, in particular from 0.7 to 1.6, in particular from 1.2 to 1.6, in particular from 1.3 to 1.5, for example 1.4. According to a particular embodiment, a crosslinked polymer according to the invention comprises alginic acid characterized by an “M / G ratio” of 0.1; 0.2; 0.3; 0.4; 0.5; 0.6; 0.7; 0.8; 0.9; 1; 1.1; 1.2; 1.3; 1.4; 1.5; 1.6; 1.7; 1.8; 1.9 or 2.

[0046] Even more particularly, a crosslinked polymer according to the invention comprises alginic acid characterized by a molar mass of between 50 and 400 kDa, preferably between 100 and 350 kDa.

[0047] According to a first embodiment, a crosslinked polymer according to the invention is obtained by the crosslinking of alginic acid characterized by an M / G ratio of 0.7 and a molar mass of 300 kDa ± 10%, designated here by “Al M / G 0.7”.

[0048] According to a second embodiment, a crosslinked polymer according to the invention is obtained by the crosslinking of alginic acid characterized by an M / G ratio of 1.4 and a molar mass of 230 kDa ± 10%, designated here by “Al M / G 1.4”.

[0049] According to another aspect, a crosslinked polymer according to the invention is characterized in that during the crosslinking step a chelating agent is added to the solvent. Among the suitable chelating agents, mention may be made of: - n-carboxylic acids, in which "n" is equal to 1, 2, 3 or 4, among which are citric acid and ascorbic acid,

[0050] - phenolic compounds, including pyrocatechol, catechol, elagitannins and gallotannins,

[0051] - salicylates,

[0052] - EDTA and EGTA.

[0053] The chelators are added at an appropriate concentration such that, when the crosslinked polymer according to the invention, obtained by crosslinking in the presence of a chelating agent, said crosslinked polymer has better water retention capacities when placed in a saline medium. The inventors have shown that when citric acid is used as a chelating agent, or counterion, the optimal concentration of citric acid in the solvent during the crosslinking step is between 0.1 and 5 mM, between 0.5 and 2 mM and preferably is 1 mM.

[0054] The present invention also relates to a crosslinked polymer, optionally characterized in that during the crosslinking step a chelating agent is added to the solvent, characterized in that it comprises small particles, or nanoparticles, comprising, or consisting of, a crosslinked cationic polymer. A crosslinked polymer according to this subject of the invention comprises from 0.01 to 10%, expressed by volume of polymer, of small particles comprising, or consisting of, a crosslinked cationic polymer.

[0055] More particularly, a crosslinked polymer according to this subject of the invention comprises from 0.1 to 5%, from 0.1 to 2%, from 0.1 to 1%, from 0.1 to 0.5%, or 0.18% of small particles comprising, or consisting of, a crosslinked cationic polymer.

[0056] More particularly, said crosslinked cationic polymer is obtained by crosslinking a product chosen from: proteins, chitosan (CAS No. 9012-76-4), ureic compounds, monoamine compounds, polyamine compounds, inorganic polymers, and a combination thereof. Even more particularly, said crosslinked cationic polymer is obtained by crosslinking chitosan.

[0057] According to a particular embodiment, said crosslinked cationic polymer is obtained by crosslinking in the presence of a crosslinking agent chosen from: a phosphate salt, citric acid, tannic acid, phenolic compounds and clay materials.

[0058] More particularly, in a cationic polymer present in a polymer according to the invention, said phosphate salt is chosen from: a solution of a phosphate derivative, a solution of n-polyphosphates, and a phosphate-rich sludge. According to a particular embodiment, in a cationic polymer present in a polymer according to the invention, said inorganic polymer is chosen from: a clay material, bentonite, laponite, regolith, apatite and hydroxyapatite derivatives and diatomaceous earth.

[0059] According to a particular aspect, a crosslinked polymer according to the invention comprises nanoparticles of crosslinked cationic polymer, said nanoparticles comprising an active agent. In particular, said active compound may be encapsulated in said nanoparticles.

[0060] An active agent included in said crosslinked cationic polymer particles may in particular be chosen from:

[0061] - phenolic compounds,

[0062] - oligosaccharides, in particular chitosan,

[0063] - phytohormones,

[0064] - amino acids, non-proteinogenic amino acids, including the family of "glycine-like" amino acids and aminobutyric acid

[0065] - nutrients,

[0066] - microelements,

[0067] - osmoprotectors, including mannitol, sorbitol, trehalose and glycine betaine and

[0068] - a combination of these.

[0069] According to a particular embodiment, the term "bioactive crosslinked polymer" means a crosslinked polymer according to the invention comprising a crosslinked cationic polymer comprising a bioactive agent, the term "Al M / G 0.7 Bioactive" means an alginate polymer with a M / G ratio of 0.7 according to the invention comprising a crosslinked cationic polymer comprising a bioactive agent.

[0070] The second subject of the invention is a process for preparing a polymer according to the invention, comprising at least the following steps: a) the addition to a polycarboxylic acid solution of a crosslinking agent chosen from divalent cations and trivalent cations, at a final concentration of crosslinking agent of between 1 and 30 mM in said polycarboxylic acid solution, b) a crosslinking reaction to obtain a gel, c) drying the gel obtained in b).

[0071] According to one embodiment of the invention, step b) does not include a droplet formation step, for example carried out by emulsification or by dripping. According to a particular embodiment, said physical crosslinking is chosen from: sonication, heat treatment, microwave treatment.

[0072] According to a particular embodiment, said method further comprises a step of crosslinking a cationic polymer and of associating said crosslinked cationic polymer with said crosslinked polymer according to the invention.

[0073] According to a particular embodiment, a method according to the invention comprises a step of adding to an alginate solution a crosslinking agent chosen from divalent cations and trivalent cations, at a final concentration of crosslinking agent of between 1 and 30 mM in said alginate solution.

[0074] According to a further preferred embodiment, a method according to the invention comprises, at room temperature, a step of adding to a 1% carboxylic acid solution a crosslinking agent chosen from divalent cations and trivalent cations between 3 and 10 mM, in a volume / volume proportion of polymer and crosslinking agent of 3:1.

[0075] According to a particular aspect of a method according to the invention, the step of drying the gel obtained in b), is carried out either at room temperature, or at a temperature between 40 and 80°C, preferably at 60°C, or by lyophilization.

[0076] "Lyophilization" means a drying step that results in a product with a moisture content of between 0.1 and 5%. For example, the gel formed can be spread over a 1 cm thick layer and placed in an oven at 60°C for 24 hours. The gel formed can be spread over a 1 cm thick layer and placed in an oven at 40°C for 48 hours. This process ensures uniform drying. When the polymer is dry, it is ground and stored away from moisture.

[0077] More particularly, a process for preparing a polymer according to the invention comprises the following steps: a) the addition to a solution of alginic acid, or its sodium form, with a molar mass of between 50 and 400 kDa, of calcium chloride at a final concentration of between 1 and 30 mM, preferably 5 mM; this addition can in particular be carried out as follows: the alginate is stirred at a concentration of 20 gL -1in water and a dilution is carried out in water to three-quarters of the final volume, in a volume equivalent to one-quarter of the final volume the calcium chloride is diluted to 20 mM, to reach a concentration of 5 mM in the final volume, b) the crosslinking reaction by stirring for 10 minutes, to obtain a homogeneous gel, c) drying the gel obtained in b). In particular, step c) can be followed by a step d) of grinding the gel in order to obtain a powder. This aspect of the invention makes it possible to obtain a dispersible powder.

[0078] The invention also relates to a process for preparing a crosslinked polymer further comprising a step of preparing a cationic polymer, intended to be integrated into the crosslinked polymer according to the invention.

[0079] A process for preparing a cationic polymer comprises the following steps:

[0080] - a cationic biopolymer is dispersed in a suitable solvent, meanwhile a crosslinking agent solution is prepared

[0081] - after complete dispersion of the cationic polymer, the pH of the polymer solution is adjusted,

[0082] - then the crosslinking agent solution is added with medium stirring (300 rpm),

[0083] - once the addition of the crosslinker is complete, the solution is centrifuged,

[0084] - the base is rinsed and reconditioned for later use.

[0085] More specifically, this process includes the following steps:

[0086] - five grams of cationic biopolymer are dispersed in one liter of 1% acetic acid solution, meanwhile, a 0.25% phosphate solution is prepared,

[0087] - after complete dispersion of the cationic polymer, the pH of the polymer solution is adjusted to pH 4.5,

[0088] - then the phosphate solution is added in a ratio of 5:3 (cationic polymer-crosslinker) with medium stirring (300 rpm),

[0089] - once the addition of the crosslinker is complete, the solution is centrifuged at 10,000 g for 15 minutes at 4°C,

[0090] - the base is rinsed twice with ultra-pure water and reconditioned for further use.

[0091] Finally, a method according to the invention may further comprise a step of encapsulating molecules of active substance in said cationic polymer, according to the following steps:

[0092] - in a ratio of 3:1 (cationic biopolymer: active substance) the active substance is dispersed until a homogeneous solution is obtained.

[0093] - the dispersion is then crosslinked with the crosslinker in the same way as the production of the polymer support.

[0094] The third subject of the present invention is a product comprising, in a proportion of between 0.01 and 99% by weight, relative to the weight of said product, a crosslinked polymer according to the invention, or a crosslinked polymer obtained by a process according to the invention. More particularly, a product according to the invention comprises a crosslinked polymer according to the invention, or a crosslinked polymer obtained by a process according to the invention in a proportion of between 0.01 and 99%, between 0.1 and 99%, between 1 and 99%, between 5 and 99%, between 10 and 99%, between 15 and 95%, between 20 and 90%, or between 30 and 80%, by weight, relative to the weight of said product. Said product may comprise, in addition to said crosslinked polymer, an excipient or an additive.

[0095] According to a particular embodiment, a product according to the invention is chosen from:

[0096] - a cell culture support,

[0097] - a water or nutrient supply agent, usable in agronomy,

[0098] - a plant biostimulation agent,

[0099] - an absorbent product for medical and / or hygienic application, notably chosen from: dressings, active detoxifying hydrogel patch,

[0100] - an agent for hydrogel and bio-ink technologies in 3D printing,

[0101] - a decontamination agent for soil and wastewater (elimination of heavy metals, recalcitrant molecules, including fertilizers, pesticides, medicines, and hydrocarbons),

[0102] - encapsulation of microorganisms for agriculture, health and water and soil decontamination.

[0103] The fourth subject of the present invention is the use of a crosslinked polymer according to the invention for the water supply and / or biostimulation of plant growth. A biostimulant is considered to be any component, substance or microorganism capable of reducing the use of fertilizing agents with a view to stimulating plant health without having harmful effects a posteriori. Their methods of application are varied: it can be applied directly to the plant or to the soil, which will allow effective nutrition, better tolerance to abiotic stresses and optimal plant growth.

[0104] A product according to the invention can in particular be used for:

[0105] - provide additional support to the soil to increase retention capacity and increase crop survival in the face of water stress,

[0106] - biostimulate the plant using targeted molecules and increase their defense system against abiotic stress,

[0107] - generate prebiotic nutrition at the rhizosphere level in order to increase plant-microorganism interactions. The present invention also relates to the use of a crosslinked polymer obtained by a process according to the invention for the water supply and / or biostimulation of plant growth.

[0108] The present invention also relates to the use of a product comprising a crosslinked polymer according to the invention, or of a product comprising a crosslinked polymer obtained by a process according to the invention, for the water supply and / or biostimulation of plant growth.

[0109] The present invention also relates to the use of a product comprising a crosslinked polymer according to the invention, for sanitation; adsorption and chelation of metals. By following the method according to the invention and generating films in the form of a membrane (monolayer or multilayer), the superabsorbent hydrogel can adsorb metals.

[0110] Brief description of the figures:

[0111] Figure 1 of Example 1 represents the swelling index (in g of water per g of polymer) of crosslinked polymers in the presence of varying amounts of crosslinker. The conditions represented are from left to right: I) Al M / G 0.7 with 0.05 M Ca 2+ , II) Al M / G 0.7 with 0.1 M Ca 2+ , III) Al M / G 0.7 with 0.5 M Ca 2+ , IV) Al M / G 1 ,4 with 0.05 M Ca 2+ , V) AI M / G 1.4 with 0.1 M Ca 2+ , VI) Al M / G 1 ,4 with 0.5 M Ca 2+The white and black bars correspond to 0.5 or 24 hours of drying respectively.

[0112] Figure 2 of Example 1 shows a histogram expressing the mass of swollen gel from AI M / G 1.4 (in grams) as a function of the calcium chloride concentration during crosslinking (in mM) and a curve showing the gelation rate (in percentage) as a function of the calcium chloride concentration (in mM).

[0113] Figure 3 of Example 1 shows a thick line curve representing the mass (in grams) of swollen gel from AI M / G 1 ,4 as a function of calcium concentration (left y-axis) and a thin line curve representing the mass (in grams) of dry gel from AI M / G 1 ,4 as a function of calcium concentration (right y-axis).

[0114] Figure 4 of Example 1 shows the viscosity of AI M / G 1 ,4 as a function of time. The first plateau shows the viscosity of the AI ​​M / G 1 ,4 solution in water for 50 minutes. The second plateau corresponds to the crosslinked polymer from AI M / G 1 ,4 upon addition of 5 mM calcium chloride.

[0115] Figure 5 of Example 1 represents the swelling index (in g of water per g of polymer) measured for 0.5 hours or for 21 hours, of alginic acid gels or their sodium salts crosslinked in the presence of 5 mM calcium and then dried at room temperature for 48 hours or lyophilized for 48 hours. The conditions tested are: a) Al M / G 0.7 dried at room temperature, b) Al M / G 1.4 dried at room temperature, c) Al M / G 0.7 lyophilized, d) Al M / G 1.4 lyophilized.

[0116] Figure 6 of Example 1 represents the swelling index (in g of water per g of polymer) measured for 30 minutes or 21 hours, of alginic acid gels or their sodium salts crosslinked in the presence of 10 mM calcium and then dried at room temperature for 48 hours or lyophilized for 48 hours. The conditions tested are: a) Al M / G 0.7 dried at room temperature, b) Al M / G 1.4 dried at room temperature, c) Al M / G 0.7 lyophilized, d) Al M / G 1.4 lyophilized.

[0117] Figure 7 of Example 1 represents the water absorption kinetics, in g of water per g of polymer, as a function of time, in minutes, of a crosslinked polymer comprising alginate Al M / G 1.4 (white squares) and a crosslinked polymer comprising alginate Al M / G 0.7 (diamonds).

[0118] Figure 8 of Example 2 represents the swelling index (in g of water per g of polymer) as a function of time (in minutes) of a crosslinked polymer prepared from Al M / G 1.4 (curve with black circles) and the commercial superabsorbent polymer (potassium polyacrylate or PAK) (curve with cross) under saline stress (0.9% NaCl solution).

[0119] Figure 9 of Example 2 is a histogram representing the swelling index under saline stress (0.9% NaCl solution), in g of water per g of polymer, of 7 different polymers. The white and black histograms represent the conditions after 30 minutes or 1440 minutes of swelling, respectively, with from left to right: 1) potassium polyacrylate (PAK); 2) Al M / G 1 ,4 prepared without counterions (Cl); 3) Al M / G 1 ,4 prepared in the presence of 1 mM citric acid; 4) Al M / G 1 ,4 prepared in the presence of 2.5 mM citric acid; 5) Al M / G 1 ,4 prepared in the presence of 5 mM citric acid; 6) Al M / G 1 ,4 prepared in the presence of 7.5 mM citric acid; 7) Al M / G 1,4 prepared in the presence of 10 mM citric acid.

[0120] Figure 10 represents the water absorption kinetics under saline stress (0.9% NaCl solution), expressed in g of water per g of polymer, as a function of time in minutes of a crosslinked polymer comprising Al M / G 1 ,4 Bioactive (black squares) supplemented with counterions (1 mM citric acid) and a commercial superabsorbent polymer PAK (white diamonds).

[0121] Figure 11 of Example 3 represents the absorption and desorption kinetics expressed in g of water per g of polymer, as a function of time in minutes of a crosslinked polymer comprising Al M / G 1 ,4 Bioactive (black circles) and a commercial superabsorbent polymer PAK (cross). Figure 12 of Example 4 is a histogram representing the swelling index in ultrapure water (in g of water per g of polymer), according to the proportion of calcium alginate (CaAl) in the Al M / G 1 ,4 alginate with, from left to right, 30%, 40%, 50% or 70% calcium alginate loading. For each percentage of calcium alginate, the values ​​120, 990, 1200 and 2460 represent the swelling time, in minutes.

[0122] Figure 13 of Example 5 represents the length of roots treated by root canal by (from left to right): untreated roots, roots treated with a polymer prepared from alginate with M / G ratio 0.7, and alginate with M / G ratio 1.4; the different letters show significant effects according to the Tukey test (p-value=0.01).

[0123] Figure 14 of Example 5 represents the length of root-treated seedlings as follows (from left to right): untreated roots, roots treated with a polymer prepared from AI M / G 1.4, and AI M / G 0.7; the different letters show significant effects according to the Tukey test (p-value=0.01).

[0124] Figure 15 of Example 5 represents the length of roots treated by the root route as follows (from left to right): untreated roots (Control); roots treated with a bioactive component (cationic polymer: ChNPs 10 mg); roots treated with a bioactive crosslinked polymer according to the invention comprising Al M / G 1,4 enriched with counterions and bioactive molecules (Al M / G 1,4 Bioactive); the different letters show significant effects according to the Tukey test (p-value=0.01).

[0125] Figure 16 shows the stomatal conductance of winter wheat plants (1 month of growth) as a function of the volumetric water content in the soil (VWC) during a 16-day water stress period and as a function of the treatment they receive, consisting of: no treatment (black curve), crosslinked polymer according to the invention prepared with Al M / G 1 ,4; crosslinked polymer according to the invention prepared with Al M / G 1 ,4 comprising a bioactive molecule (Al M / G 1 ,4 Bioactive); commercial chemical compound (PAK).

[0126] Figure 17 represents the relative water content of winter wheat plants (1 month of growth) treated as follows (from left to right): no treatment, crosslinked polymer according to the invention prepared with Al M / G 1 ,4; crosslinked polymer according to the invention prepared with Al M / G 1 ,4 comprising a bioactive molecule (Al M / G 1 ,4 Bioactive); commercial superabsorbent polymer (PAK), after 14 days of high intensity water stress by water deprivation; the different letters show significant effects according to the Tukey test (p-value=0.01).

[0127] Figure 18 represents the relative water content of winter wheat plants after 1 month of growth, treated as follows (from left to right): no treatment, crosslinked polymer according to the invention prepared with Al M / G 1 ,4; crosslinked polymer according to the invention prepared with Al M / G 1 ,4 comprising a bioactive molecule (Al M / G 1 ,4 Bioactive); commercial superabsorbent polymer (PAK), after 16 days of high intensity water stress by water deprivation; the different letters show significant effects according to the Tukey test (p-value=0.01).

[0128] Figure 19 is a histogram representing the water potential (MPa) in leaves of winter wheat plants after 3 months of growth at zenith, in kinetics up to 11 days of high intensity water stress by water deprivation, the plants are treated as follows: no treatment, commercial superabsorbent polymer (PAK), crosslinked polymer according to the invention prepared with Al M / G 1,4; crosslinked polymer according to the invention prepared with Al M / G

[0129] 1.4 Bioactive); the different letters show significant effects according to the Tukey test (p-value=0.01).

[0130] Figure 20 is a histogram representing the relative water content (% water) in leaves of winter wheat plants after 3 months of growth in kinetics up to 11 days of high intensity water stress by water deprivation, the plants are treated as follows: no treatment, commercial superabsorbent polymer (PAK), crosslinked polymer according to the invention prepared with Al M / G 1,4; crosslinked polymer according to the invention prepared with Al M / G

[0131] 1,4 comprising a bioactive molecule (Al M / G 1,4 Bioactive), the different letters show significant effects according to the Tukey test (p-value=0.01).

[0132] Figure 21 is a histogram representing the electrolyte leakage (%) in leaves of winter wheat plants after 3 months of growth, in kinetics up to 11 days of high intensity water stress by water deprivation, the plants are treated as follows: no treatment, commercial superabsorbent polymer (PAK), crosslinked polymer according to the invention prepared with Al M / G 1,4; crosslinked polymer according to the invention prepared with Al M / G

[0133] 1.4 including a bioactive molecule (Al M / G 0.7 Bioactive), the different letters show significant effects according to the Tukey test (p-value=0.01).

[0134] Figure 22 shows the solution-to-gel transition of several alginates by quantifying the swollen gel and dry gel content as a function of calcium concentration. Gels formed from alginate Al M / G 0.7 bis (A) are shown by the solid line with empty diamonds (swollen gel) and the solid line with filled diamonds (dry gel). Gels from alginate Al M / G 1.4 bis (B) are shown by the broken lines with empty circles (swollen gel) and filled circles (dry gel).

[0135] Figure 23 represents the gelation percentage and swollen gel content on two types of alginates with different proportions of mannuronic and guluronic acid, Al M / G 0.7 (A) bis (62% G and 38% G) and Al M / G 1.4 bis (B) (57% G and 43% M) by quantifying the swollen gel and % gelation as a function of calcium concentration. Gels formed from Sigma alginate are shown by the dark solid line (% gelation) and the black bar (swollen gel). Gels from S900-NS are shown by the light solid line (% gelation) and the gray bar (swollen gel).

[0136] Figure 24 shows the quantification of swollen gel and dry gel content as a function of calcium concentration for AL M / G 1.4 alginate. The evolution of swollen gel is represented by the dark solid line and the evolution of dry gel mass is represented by the light line. The points represent the mean values ± SE.

[0137] Figure 25 represents the expression of genes selected as biomarkers of the immunostimulatory response in wheat (7-day-old seedlings treated for 7 days with the alginates Al M / G 1,4 bis (A), Al M / G 0,7 bis (B), Al M / G 1,4 (C) and XPC (D) (XPC: highly cross-linked calcium alginate). PR-1, PR-2 and PR-3 correspond to the genes encoding pathogen response proteins (Pathogenesis Related Protein) types 1, 2 and 3. PAL and CAT represent the genes encoding Phenylalanine Ammonia-lyase and Catalase, involved in phenylpropanoid metabolism and hydrogen peroxide catabolism, respectively.

[0138] EXAMPLES

[0139] Example 1: Preparation and characterization of superabsorbent crosslinked polymers according to the process according to the invention and comparison with crosslinked polymers not forming part of the invention

[0140] 1.1 Materials and methods

[0141] The products used are as follows:

[0142] - chitosan oligomer (Water-soluble fungal oligochitosan, 1.28 kDa, Chibio Biotech Co.),

[0143] - chitosan polymers (acid-soluble fungal polymeric chitosan, 150 kDa and 98% DDA; water-soluble chitosan polymer of fungal origin 32.4 kDa and 98% DDA, Chibio Biotech Co.),

[0144] - sodium alginate ratio M / G 1.4 (denoted Al M / G 1.4), molar mass Mw 300 kDa (Algaia, Lannilis, France)

[0145] - alginate with a sodium ratio of M / G 1.4 (denoted Al M / G 1.4 bis), molal mass 161.7 kDa (Algaia, Lannilis, France)- alginate with a sodium ratio of M / G 0.7 (denoted Al M / G 0.7), molal mass Mw 230 kDa (Algaia, Lannilis, France)- alginate with a sodium ratio of M / G 0.7 (denoted Al M / G 0.7 bis), molal mass Mw 222 kDa (Sigma Aldrich, lot W201502)- alginate with a calcium ratio of M / G 1.6 (denoted Al M / G 1.6) (Algaia, Lannilis, France),

[0146] - calcium chloride (Sigma-Aldrich, (CAS 10035-04-8),

[0147] - calcium citrate (Sigma-Aldrich, CAS 5785-44-4),

[0148] - sodium citrate dihydraté (Sigma-Aldrich, CAS 6132-04-3 ),

[0149] - potassium citrate (Sigma-Aldrich, CAS 6100-05-6),

[0150] - mono-, di-, tri- polyphosphate de sodium (Sigma-Aldrich, CAS 7558-80-7 ; CAS 7722-88- 5 ; CAS 7758-29-4),

[0151] - diatomaceous earth (Sigma-Aldrich, CAS 68855-54-9),

[0152] - Commercial superabsorbent polymer PAK (Alquera, CAS 25608-12-2).

[0153] 1.2 Preparation of crosslinked polymers

[0154] The anionic polymers used are:

[0155] - sodium alginate in which the ratio between mannuronic acid and guluronic acid is 0.7, in other words "at the ratio M / G 0.7" with a molecular weight around 230 kDa, designated by "Al M / G 0.7",

[0156] - sodium alginate in which the ratio between mannuronic acid and guluronic acid is 1.4, in other words at the ratio M / G 1.4, with a molecular weight around 300 kDa, designated by “Al M / G 1.4”.

[0157] The solvent is pure water. The crosslinking agent is calcium chloride, present in the solvent at a concentration between 0 and 500 mM during the crosslinking reaction.

[0158] Comparative crosslinked polymers not forming part of the invention were prepared according to the reaction conditions presented in Table 1 below:

[0159] Table 1 Superabsorbent crosslinked polymers in accordance with the present invention were also prepared according to the reaction conditions presented in Table 2 below:

[0160] Table 2

[0161] The crosslinked polymers were prepared as follows:

[0162] - mixing of a polymer solution, at 0.5% w / v of polymer, and the crosslinking agent solution in a ratio of 1:3, for 10 minutes by Ultraturrax used at maximum speed,

[0163] - magnetic stirring for 30 minutes at room temperature,

[0164] - rinse repeated once with ultrapure water,

[0165] - drying at 40°C for 24-48 hours,

[0166] - homogenization by grinding at maximum speed for 3 minutes.

[0167] The swelling index was then determined following the method described by Dai et al (2017) and Zhang et al. (“Protocol efficiently measuring the swelling rate of hydrogels”, MethodsX 7, 100779, 2020).

[0168] 1.3 Characterization of shear rate-viscosity and gelation rate

[0169] Rheological analyses were performed using an AR-2000 rheometer (TA Instruments, UK) equipped with a cone (52 μm gap) equipped with a Peltier heater for precise control. TA Instrument Rheology Advantage software (V5.7.0) was used to collect and analyze rheological data. Flow rate measurements were performed by applying a shear rate of 0.001 to 1000 1 / s.

[0170] The gelation time of the alginate was measured by a peak holding step by applying a shear rate of 1 1 / s per step measurement. The measurement was carried out on the polymer without crosslinking and then for 60 s during crosslinking. 1.4 Characterization of the swelling index of the crosslinked polymer

[0171] The swell index (SI) of the crosslinked polymer was determined in water according to the protocol described by Dai et al (2017) and expressed in grams of water absorbed per gram of polymer. The swell index was measured in water and then in a 0.9% NaCl saline solution.

[0172] The swelling index of the dry hydrogel was determined by the tea bag method with ultrapure water and saline (0.9% NaCl). 50–200 mg of dry powdered polymer was placed in the tea bag and sealed. The tea bag was placed on the water / saline solution and the time was recorded. The swelling index was then calculated by mass differences.

[0173] 1.5 Swelling index vs. dry method

[0174] After preparation of the crosslinked polymer, it was then dried in a ventilated oven at 40°C or freeze-dried / desiccated to achieve a dry matter content of 90-95%. After drying, the swelling index was measured by the tea bag method.

[0175] 1.6 Behavior of counterions under ionic stress

[0176] The process is identical to the process described in Example 1.2, taking a calcium concentration of 5mM with slight modifications in the dispersion step: counterions, here citrate ions, were added at a concentration between 0 and 20%. Then the process according to Example 1.2 was continued until a superabsorbent hydrogel powder was obtained. Then, the swelling index of the polymer was evaluated using the tea bag method and under an ionic strength of 0.9%, a method which allows an approach on ionic activities for various applications (e.g. salinity of different agricultural soils).

[0177] 1.7 Results

[0178] Crosslinking agent concentration and swelling index

[0179] The swelling index of crosslinked polymers as a function of crosslinking agent concentration is shown in Figure 1. Crosslinked polymers not in accordance with the present invention, i.e. prepared with high calcium ion concentrations (0.05 to 0.5M) result from structurally consolidated gelation but are, however, rigid and not very flexible. High calcium concentrations are therefore not suitable for allowing appropriate and progressive swelling.

[0180] Crosslinking agent concentration and gelling rate Figure 2 includes a histogram expressing the mass of swollen gel, expressed in grams, as a function of the calcium chloride concentration during crosslinking, expressed in mM; a curve superimposed on the histogram shows the gelling rate as a percentage as a function of the calcium chloride concentration for each of the crosslinked polymers in accordance with the present invention.

[0181] Figure 3 represents, on the one hand, the mass of swollen gel as a function of the calcium concentration (thin line curve and left-hand y-axis) and, on the other hand, the mass of dry gel as a function of the calcium concentration (thick line curve, right-hand y-axis) for each of the crosslinked polymers in accordance with the present invention.

[0182] These results show that the 5 mM calcium concentration is optimal to obtain a stable continuous gel and an appropriate swelling capacity.

[0183] Figure 4 represents the gelation of an AI M / G 1.4 solution (first plateau) in the presence of 5 mM calcium ions (second plateau) on a “peak hold doublestep” experiment, allowing confirmation of gelation after the addition of 5 mM calcium salts which results in an increase in viscosity at low shear rate (shear rate of 0.5 Hz at 25 °C).

[0184] Drying mode and absorption capacity

[0185] Figure 5 represents, in g of water per g of polymer, the swelling index, obtained by swelling for 30 minutes (0.5 hours) or 21 hours, of sodium alginate gels crosslinked in the presence of 5 mM calcium and dried at room temperature or lyophilized. The results obtained with sodium alginates Al M / G 1.4 and Al M / G 0.7 are similar under the different preliminary drying conditions. These results show that a low concentration of calcium allows the perfect stabilization of a partially crosslinked continuous gel, corresponding to a low crosslinking density compared to a rigid gel.

[0186] Figure 6 represents, in g of water per g of polymer, the swelling index, obtained by swelling for 30 minutes (0.5 hours) or 21 hours, of sodium alginate gels crosslinked in the presence of 10 mM calcium and dried at room temperature or lyophilized.

[0187] A gel according to the invention therefore has the advantages of ease of production, the low number of inputs required, and in that drying at room temperature under air flow does not reduce the swelling capacity, compared with freeze-drying.

[0188] Absorption kinetics in water

[0189] The absorption kinetics of a polyanionic matrix derived from two types of alginate (Al M / G 1.4 and Al M / G 0.7) crosslinked in the presence of 5 mM calcium are compared. Figure 7 shows the absorption kinetics of the two polymers prepared as indicated above in point 1.2. The water absorption capacity of two types of crosslinked polymers in accordance with the present invention is comparable and greater than 500 g of water per gram of polymer.

[0190] Example 2: Preparation of a crosslinked polyanionic matrix in the presence of counterions, swelling index under high ionic strength

[0191] In the presence of a 0.9% saline solution (23.8 mS / cm 2 ) (ISO 17190-5:2001), a crosslinked polyanionic matrix prepared as described above in Example 1.2 from an Al M / G 1.4 alginate in the presence of crosslinker (5 mM calcium) and in the absence of counterions tends to collapse (white squares) unlike a matrix of the commercial superabsorbent PAK (black diamonds) (Figure 8). To overcome this, a counterion was added during the preparation of the crosslinked polymer. The counterion used is citric acid, added at a concentration of 1 mM; 2.5 mM; 5 mM; 7.5 mM or 10 mM.

[0192] Behavior under ionic stress in the presence of counterions

[0193] The process for preparing the gel in the presence of counterions is similar to the process previously described in Example 1.2, with slight modifications in the dispersion step: counterions, here citrate ions, were added at a concentration between 0.001 and 20%, more precisely between 0.01 and 10%. Then the process described in Example 1.2 was continued until a superabsorbent hydrogel powder was obtained. Then, the swelling index of the polymer was evaluated using the tea bag method and under an ionic strength of 0.9% according to the method described above in Example 1.4.

[0194] A crosslinked polymer according to the invention was prepared as follows:

[0195] - mixing of the Al M / G 1.4 alginate solution, the 5 mM crosslinking agent solution and the counterion solution, for 10 minutes by Ultraturrax® used at maximum speed,

[0196] - magnetic stirring for 30 minutes,

[0197] - rinse repeated once with ultrapure water,

[0198] - drying,

[0199] - homogenization.

[0200] The swelling index of the crosslinked polymer comprising a counterion was determined in the presence of a 0.9% saline solution (23.8 mS / cm 2 ) (ISO 17190-5:2001) and for 24 hours.

[0201] The results show that the presence of citric acid at a concentration of at least 1 mM improves the matrix's capacity in saline environments and prevents its collapse over time (Figure 9). The addition of a counterion of the citric acid / sodium citrate couple type therefore allows the increase of the liquid retention capacity in saline environments. Such a counterion acts as a chelator within the crosslinked polymer matrix.

[0202] Kinetics of the swelling index under saline conditions

[0203] In the presence of a saline solution consisting of 0.9% NaCl, a crosslinked polyanionic polymer in the presence of a 5 mM calcium crosslinking agent and a 1 mM chelator, such that citric acid does not collapse (Figure 10, curve with black squares). This crosslinked polyanionic polymer comprises a bioactive substance, it is designated Al M / G 1 ,4 Bioactive in the present description and is prepared as indicated in Example 4. Its absorption in saline medium is comparable to that of the commercial superabsorbent PAK (diamonds) (Figure 10, diamonds).

[0204] Kinetics of water absorption and desorption

[0205] Figure 11 shows that the water absorption capacity of a crosslinked polymer according to the invention (black circles) is greater than that of the commercial superabsorbent polymer PAK (cross). A crosslinked polymer according to the invention has the advantage of being biosourced and biodegradable, whereas PAK is obtained by chemical synthesis, and its superabsorbent properties are maintained during re-swelling and drying. A crosslinked polymer according to the invention and PAK gradually dry out in 6 days at room temperature.

[0206] Example 3: Mass production of superabsorbent matrix

[0207] In order to increase the volume of superabsorbent matrix that can be produced, the inventors developed a process in which the crosslinked polymer comprises sodium alginate and calcium alginate.

[0208] Preparation of crosslinked polymer

[0209] The crosslinked polymer was prepared as follows: a sodium alginate paste was prepared by the addition of 200 mL per 100 grams (500 g / L sodium alginate) of sodium alginate polymer, under high shear rate. A calcium alginate suspension was prepared by the addition of 200 mL of water per 100 grams (500 g / L calcium alginate) of calcium alginate polymer. The proportions of sodium alginate (NaAl) and calcium alginate (CaAl) are modulated according to the addition of calcium alginate in mass proportion: 30% CaAI-70% NaAl; 40% CaAI-60% NaAl; 50% CaAI-50% NaAl; 70% CaAI-30% NaAl (Figure 12).

[0210] The paste and suspension were mixed according to the desired proportions, as well as the addition or absence of chelating agent or active substance. The gel formed was spread on a non-stick plate and subjected to drying for 48 hours at 40°C.

[0211] Example 4: Production of crosslinked polymer comprising crosslinked cationic polymer nanoparticles

[0212] The cationic polymer corresponds to highly deacetylated (> 99% deacetylation) high molar mass polymeric chitosan.

[0213] Five grams of cationic polymer were dispersed in one liter of 1% acetic acid solution. Meanwhile, a 0.25% phosphate solution was prepared. After the cationic polymer was completely dispersed, the pH of the polymer solution was adjusted to 4.5. Then, the phosphate solution was added in a ratio of 5:3 (cationic polymer-crosslinker) under moderate stirring. After the crosslinker addition was complete, the solution was centrifuged at 10,000 g for 15 minutes and 4°C. The pellet was rinsed twice with ultrapure water and repackaged for further use.

[0214] Preparation of nanoparticles loaded with active molecules

[0215] In a ratio of 3:1 (cationic polymer: active substance) the active substance was dispersed until a homogeneous solution was obtained. The dispersion was then crosslinked with the crosslinker in the same manner as below for the preparation of the cationic polymer support.

[0216] Absorbent properties of a superabsorbent polymer according to the invention

[0217] Crosslinked polymers blended with cationic polymers and / or diatoms and / or anionic polymers and crosslinked with di-tri-valent cations following process 1 have a swelling index of between 250 and 800 g of water per gram of polymer.

[0218] In the presence of a 0.9% NaCl saline solution, the absorption capacity of a superabsorbent polymer according to the invention is comparable with that of the commercial superabsorbent polymer PAK (Potassium Polyacrylate), i.e. 70 g of water per gram of polymer.

[0219] In ionic strength, the hydrogel mixed with citrates and / or phosphates and / or cationic polymers and / or anionic polymers, crosslinked with di-trivalent cations according to method 1 can have a swelling index of 40 to 60 g of solution per g of polymer.

[0220] The results presented in Figure 12 show the performance of the different mixtures of sodium alginate and calcium alginate. It is possible to produce around 1 kg of gel. The drying time is reduced compared to a polymer comprising only sodium alginate. These results show that it is possible to mass produce a crosslinked polymer according to the invention.

[0221] Example 5: Role of a cross-linked polymer enriched with bioactive molecule on plant growth

[0222] Low molar mass chitosan can be used as a biostimulant, as well as sodium and calcium alginates.

[0223] Crosslinked polymer matrices were prepared from sodium alginate Al M / G 0.7, sodium alginate Al M / G 1.4 or calcium alginate, in the presence of a concentration of CaCh 5 mM and citric acid 1 mM, as indicated in Example 2.

[0224] Wheat seedlings received a root treatment comprising the application of a cross-linked polymer matrix at a concentration of 1 g / L. The length of roots, leaves, and treated wheat seedlings were measured and compared with the lengths measured for untreated wheat.

[0225] The results in Figures 13 and 14 show that the raw materials (Al M / G 0.7; Al M / G

[0226] 1,4 and calcium alginate) have an intrinsic capacity to increase seedling growth at both root and foliar levels.

[0227] A bioactive cross-linked polymeric matrix was prepared from sodium alginate M / G

[0228] 1.4 in the presence of a concentration of calcium ions at 5 mM and citric acid at a concentration of 1 mM, and nanoparticles containing oligomeric chitosan at a concentration of 10 mg / L.

[0229] Wheat seedlings received a root treatment comprising the application at a concentration of 3 g / L of a cross-linked and bioactive polymeric matrix comprising nanoparticles containing oligomeric chitosan at a concentration of 10 mg / L (Al M / G

[0230] 1,4 Bioactive), other wheat seedlings received a root treatment including the application of chitosan nanoparticles at 10 mg / L (ChNPs 10 mg).

[0231] After 7 days of growth under root treatment, the root length of wheat treated under different conditions was measured and compared. The results presented in Figure 15 show the synergistic effect of the intrinsic effects of the components in a combined formulation according to the invention.

[0232] The polymer matrix prepared from sodium alginate and including chitosan makes it possible to activate root growth and increase the growth of seedlings and leaves in the wheat model. Example 6: Role of the crosslinked polymer matrix, enriched or not with bioactive molecule, under conditions of environmental constraints

[0233] A crosslinked polymer matrix was prepared from sodium alginate, Al M / G 1,4, crosslinked with calcium chloride at a concentration of 5mM and 1mM citric acid counterion, dried at 40°C for 24-48 hours in a ventilated oven, and then ground for 3 min at high speed, until granules were obtained, as indicated in Example 2.

[0234] The crosslinked and bioactive polymeric matrix was then prepared as indicated in Example 4.

[0235] The crosslinked and bioactive polymeric matrix was applied at 3 g / L (comprising sodium alginate (Al M / G 1.4), calcium chloride (5 mM CaCh), chitosan nanoparticles (high molecular weight polymeric chitosan crosslinked with TPP) and citric acid (1 mM). The effect on plant growth of the treatment of seedlings with this crosslinked and bioactive polymeric matrix was compared with the effect on seedlings of the polymeric matrix (according to Example 1), potassium polyacrylate (positive control of a superabsorbent matrix) and a negative control (absence of superabsorbent matrix).

[0236] The effect was evaluated on two growth stages in separate experiments; the first at 1 month of growth (tillering stage) and the second at 3 months of growth (end of bolting, beginning of flowering).

[0237] Stomatal conductance and relative water content in wheat leaves at one month of growth

[0238] The results in Figure 16 show the variation in stomatal conductance of winter wheat plants treated with a polymeric matrix according to the invention, said matrix being optionally enriched with bioactive molecule (chitosan) as a function of the soil water content. Treatment of plants with Al M / G 1,4 Bioactive results in partial stomatal closure which results in the maintenance of tissue water content on plants of 1 month of growth.

[0239] The results in Figures 17 and 18 show that under water stress, treatment of plants with the chitosan-enriched matrix, designated “Al M / G 1,4 Bioactive”, allows the maintenance of a water content 6% higher than the control water content.

[0240] Figure 19 shows the evolution of the water potential of wheat leaves subjected to water stress depending on the treatment applied. It is possible to observe that the water potential on the negative control evolves more quickly. In addition, by comparing the relative content (Figure 20), it is observed that the relative tissue water content decreases drastically, while the polymeric treatments allow maintenance of turgor even on very low water potentials. The inventors observed that during 11 days of stress the bioactive matrix (Al M / G 1 ,4 Bioactive) allows maintenance of tissue water at around 50% comparable to the superabsorbent potassium polyacrylate matrix. Potassium polyacrylate is widespread in the agricultural field to increase absorption in the soil and infiltration of runoff.

[0241] Figure 21 shows the evolution of electrolyte leakage during the stress episode. The percentage of leakage is a stress marker indicating damage to cell membranes. The inventors observed that the Al M / G 1,4 Bioactive treatment has the ability to reduce cell damage caused by severe water stress and, compared to the control, a significant 50% reduction in cell damage expressed by electrolyte leakage is observed.

[0242] Example 7: Quality and stability

[0243] The viscous solution transition of the alginates Al M / G 1 ,4, Al M / G 1 ,4 bis and Al M / G 0,7 bis to a gel was quantified (Figures 22 to 24). Alginate solutions at 5g / L were prepared and a range of calcium was tested on the different alginate types. For the alginate Al M / G 1 ,4, the 5g / L solutions were crosslinked with calcium at 1 mM, 2.5 mM, 5 mM, 7.5 mM and 10 mM. For the alginates Al M / G 1 ,4 bis and Al M / G 0,7 bis, three calcium concentrations were tested (2.5 mM, 5 mM and 10 mM).

[0244] As shown in Figure 24, the results show that the transition for a swollen gel of alginate Al M / G 1,4 goes from 3.5mM up to 7mM to obtain a gel that retains water.

[0245] The gelation rates of the alginates Al M / G 1,4 bis and Al M / G 0,7 bis shown in Figure 22 and 23 are similar to those found with the gelation of the alginate Al M / G 1,4, with a maximum rate of swollen gel at 5 mM. The same behavior was visualized with the concentrations at 10 mM for which the gel is slightly swollen but with a high gelation rate.

[0246] Example 8: Biological activities

[0247] As shown in Figure 25, alginates with a mannuronic acid ratio higher than 55% (Al M / G 1,4 and Al M / G 1,4 bis) show interesting biological activity in plants. Alginate Al M / G 1,4 bis with a mannuronic acid ratio of 57% showed effects on the activation of the “Pathogenesis-Related” PR-2 proteins. Alginate Al M / G 1,4 follows this same trend, while alginate Al M / G 0,7 which has a lower mannuronic acid ratio (40%) does not lead to as significant an induction as that with a mannuronic acid level higher than 55%. In addition, the alginate very strongly crosslinked with calcium (XPC), the biological activity is less pronounced.

Claims

Claims 1. Crosslinked polymer obtained by crosslinking at least one polycarboxylic acid in solution in a solvent, in the presence of a crosslinking agent chosen from divalent cations and trivalent cations, said polymer being characterized in that during said crosslinking the initial concentration of said crosslinking agent in said solvent is between 1 and 30 mM, said crosslinked polymer of polycarboxylic acid being capable of absorbing at least 200 grams of water per gram of crosslinked polymer.

2. Crosslinked polymer according to claim 1, characterized in that said crosslinking agent is a divalent metal, including calcium, or a trivalent metal.

3. Crosslinked polymer according to claim 1 or 2, characterized in that said polycarboxylic acid is alginic acid comprising mannuronic acid, in a proportion of between 30 and 70% of the weight of alginic acid, and guluronic acid, in a proportion of between 30 and 70% of the weight of alginic acid.

4. Crosslinked polymer according to any one of the preceding claims, characterized in that during said crosslinking a chelating agent is added to said solvent, said chelating agent being chosen from: n-carboxylic acids for which “n” is equal to 1, 2, 3 or 4, phenolic compounds, salicylates, EDTA and EGTA.

5. Crosslinked polymer according to any one of the preceding claims, characterized in that it comprises nanoparticles comprising, or consisting of, a crosslinked cationic polymer.

6. Crosslinked polymer according to claim 5, characterized in that said nanoparticles comprise an active agent.

7. Crosslinked polymer according to claim 6, characterized in that said active agent is chosen from: phenolic compounds, oligosaccharides, phytohormones, amino acids, non-proteinogenic amino acids, nutrients, microelements, osmoprotectants and a combination thereof.

8. Process for the preparation of a crosslinked polymer as defined in any one of claims 1 to 7, said process being characterized in that it comprises at least the following steps: a) the addition to a polycarboxylic acid solution of a crosslinking agent chosen from divalent cations and trivalent cations, at a final concentration of between 1 and 30 mM in said polycarboxylic acid solution, b) the crosslinking reaction to form a gel, c) drying the gel obtained in b).

9. Product comprising, in a proportion of between 0.01 and 99% by weight, relative to the weight of said product, a polymer as defined in any one of claims 1 to 7, or a polymer obtained by a process according to claim 8.

10. Use of a polymer as defined in any one of claims 1 to 7, or of a polymer obtained by a process according to claim 8, or of a product as defined in claim 9, for the water supply and / or biostimulation of plant growth.

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