TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for reinforcing fresh ornamental plant parts, such as cut flowers. More particularly, the invention provides a method of reinforcing fresh ornamental plant parts that comprises the successive steps of:

• introducing dissolved anionic gelling polysaccharide into the fresh ornamental plant parts; and

• inducing gelation of the dissolved anionic gelling polysaccharide within said ornamental plant parts by introducing multivalent cations into the ornamental plant parts.

Examples of anionic polysaccharides that can be employed in the present method include alginate and pectin. When dissolved in water, these anionic polysaccharides will form a gel network upon addition of dissolved multivalent cations such as Ca ²⁺ , Mg ²⁺ and Al ³⁺ .

The present method can suitably be used to, for instance, reinforce the stems and petals of cut flowers. Such reinforcement is particularly useful if plant parts are to be subjected to conditions or further treatments that reduce the turgor pressure within the plant parts.

The invention also provides reinforced ornamental plant parts comprising at least 0.1 mg of crosslinked, undissolved anionic polysaccharide salt per g of dry matter, said anionic polysaccharide being selected from calcium alginate, magnesium alginate, aluminum alginate, calcium pectinate, magnesium pectinate and combinations thereof.

BACKGROUND OF THE INVENTION

It is customary around the world to offer bouquets of cut flowers as a gift to express sentiments. Furthermore, flower bouquets are widely used as an attractive decor for homes, public buildings etc. Unfortunately, however, cut flowers have a very limited shelf life, and are adversely affected by elevated temperatures, drafts, low humidity, vibration, and other environmental factors. Many cut flowers must be disposed or unsold due to spoilage in transit and while they are on display prior to sale. Attempts have been made to prolong the usable life of cut flowers during transport, so that they arrive at their destination while still looking fresh and viable and in condition for display and sale. Flower vendors also attempt to prolong the usable life of cut flowers as much as possible while they are on display prior to sale to attract a good customer response. Consumers expect at least 3-4 days for the shelf life of purchased cut flowers. Consumers are displeased when they bring home cut flowers, unwrap and place them in fresh water, only to have them unrecoverably wilt and/or turn brown on the first or second day. Wilting will inevitably occur in cut flowers or cut leaves as, due to the lack of supply of nutrients, the cells in these cut plant parts do not have enough energy to actively transport salts and sugars into the vacuoles. The resulting loss of turgor pressure causes the flowers and leaves to wilt. In cut flowers, even before the occurrence of browning, wilting may cause loss of flower petals and/or leaves, or breaking of the flower stem.

Loss of turgor pressure also poses a major problem in the preservation of cut plant parts. Most preservation techniques involve dehydration of the plant parts. This dehydration is accompanied by a substantial loss of turgor pressure that renders the plant parts very fragile. Turgor is the cause of rigidity in living plant tissue. It causes the stiffening cell walls. Turgidity is caused by the osmotic flow of water from area of low solute concentration outside of the cell into the cell's vacuole, which has a higher solute concentration. The high solute concentration within the vacuole is maintained by a special mechanism that use energy to actively transport chemicals across the membrane. As long as the solute concentration inside the vacuole is higher than the concentration outside, more water will enter than leave the vacuole via osmosis, thus creating a strong osmotic pressure that expands the vacuole. Pressure from the expanding vacuole in turn creates a "hydrostatic" pressure (hydro = water) of the protoplasm on the cell walls, called turgor. Attempts have been made in the past to prevent loss of turgor pressure in cut plant parts or to reinforce cut plant parts so that the adverse effects of loss of turgidity are reduced and/or delayed. US 2,971,292 describes a method of preserving flowers in fresh state comprising keeping said flower's moisture supplying organs in an aqueous gel made from a colloid selected from a large group of colloids, including alginates and pectin.

US 4,906,276 describes a method of increasing the yield of a transplanted crop comprising insertion of a stem or root of the crop into a gel including water and a crosslinked, water- absorbent, mixed salt polyacrylate. The aqueous gels are said to exhibit sufficient gel strength and rigidity to support the plants in the absence of inert solid aggregates.

US 5,464,456 describes method of maintaining the freshness of cut flowers, comprising the steps of: creating a regularly pulsed electrical current in a plant development medium, said medium including a liquid capable of conducting electricity, said current delivered in a range of -0.0001 to 5 volts to said medium; contacting said plant or part thereof with said medium; and, regularly pulsing the electrical current through the medium by alternating turning the electrical current on and off. In the US patent it is stated that this method promotes maintenance of cellular activity in harvested material.

EP-A 1 095 563 describes a method to lengthen the post-harvest life of cut flowers comprising the following procedures:

a) plunging the stems of cut flowers into a solution comprising 10-95% of a linear or ramified aliphatic alcohol with 2-5 carbon atoms, and 0,01-35% of myrrh for a period of time between 10 seconds and 2 hours;

b) drying the flower stems by exposure to outer environment, so that the flowers are transported in a dry status.

The myrrh solution is said to influence the physiology of cut flowers, not as antimicrobial, but as retarder in the senescence process, by influencing both product breathing (C0 ₂ production, ethylene and other gaseous metabolites), and the water loss from tissues.

WO 95/24828 describes a method of making preserved plant material substantially free from curling or distortion, the method comprising the step of immersing plant material in an aqueous solution comprising from 40% to 95% by volume of one or more C3-C6 dihydric alcohols at a temperature of from 40°C to 95°C. The results shown in the Examples of the international application indicate that this treatment delayed discolouration and turgor loss in carnations, ivy leaves etc. Example 4 of the international application describes a method of preserving white carnations by dipping them in an aqueous suspension containing carboxy methyl cellulose, polyvinyl acetate and liquid surfactant for a few seconds followed by drying.

JP62265202 describes a method for producing a dry flower having sustained color, shape and fragrance close to those of original natural flowers. In this method an aqueous solution of a viscous agent, e.g. sodium carboxymethyl cellulose, sodium alginate, polyvinyl alcohol or glycerol, is taken up and absorbed in natural flowers blooming 70-80% for 1-3 days. Furthermore, this aqueous solution is sprayed onto the flowers to give dry flowers. The dry flowers obtained by this method are not fragile and can be readily handled, decorated, etc.

JP2000119102 describes a method of treating cut flowers with a solution of gel-forming substance, and dissolved wax, glycerin or wood vinegar, which are absorbed through their stems. The gel-forming substance is not limited, so long as it is dissolved, when heated, to form a sol which is gelled as it is cooled, e.g. agar, gelatin and pectin, of which agar and gelatin are more preferable.

SUMMARY OF THE INVENTION The present inventors have developed a very simple and effective method for reinforcing fresh ornamental plant parts. In accordance with this method, as a first step, dissolved anionic gelling polysaccharide is introduced into the fresh ornamental plant parts. Once the anionic gelling polysaccharide is distributed throughout the plant parts, gelation of the anionic polysaccharide is induced by introducing multivalent cations into said plant parts. The resulting gel network provides additional rigidity to the plant parts. This improved rigidity protects the plant part against the loss of turgidity that occurs shortly after the plant parts have been cut or that is caused by further treatments such as dehydration.

The present method is particularly suitable for treating cut flowers and foliage. Examples of anionic polysaccharides that may suitably be employed include alginate (e.g. sodium alginate) and pectin (e.g. low methoxy pectin). Gelation of these anionic polysaccharides can be induced by addition of, for instance, Ca ²⁺ , Mg ²⁺ and/or Al ³⁺ cations. Both the anionic polysaccharides and the multivalent cations can be introduced into plant parts by simply immersing a stem, stalk or twig into an aqueous solution of the polysaccharide or the multivalent cation. Alternatively, the anionic polysaccharides and/or the multivalent cations may be introduced by injection, e.g. by injecting directly into a flower's peduncle or receptacle. DETAILED DESCRIPTION OF THE INVENTION

Accordingly, a first aspect of the present invention relates to a method for reinforcing fresh ornamental plant parts, such as cut flowers, said method comprising the successive steps of:

• introducing dissolved anionic gelling polysaccharide into the fresh ornamental plant parts; and

• inducing gelation of the dissolved anionic gelling polysaccharide within said ornamental plant parts by introducing multivalent cations into the ornamental plant parts, thereby producing reinforced ornamental plant parts. The term "ornamental plant parts" as used herein refers to parts of plants that have ornamental qualities that allow these plant parts to be used for decorative purposes. Examples of ornamental plant parts include cut flowers, twigs, petals, leaves and combinations thereof. The plant parts comprise a stem, a twig or a stalk that has been severed from the root system of the plant. The severing of the stem, twig or stalk generates a cross sectional surface at the bottom of the stem, twig or stalk.

The term "anionic gelling polysaccharides" refers to polysaccharides with carboxyl groups that are deprotonated and therefore have a negative charge (eg. carboxylic groups). Anionic polysaccharides that can be employed in the present method include alginate, pectin or combinations thereof. In a preferred embodiment the anionic polysaccharides that can be employed in the present method include sodium alginate, low methoxy pectin or combinations thereof.

The term "xylem" refers to one of the two types of transport tissue in vascular plants (phloem is the other). Its basic function is to transport water, but it also transports some nutrients through the plant. Xylem is found throughout the plant. The most distinctive xylem cells are the long tracheary elements that transport water. Tracheary elements develop a thick lignified cell wall, and at maturity the protoplast has broken down and disappeared. The term "multivalent cations" refers to metal ions with a positive charge greater than one. Multivalent cations that can be used in the present method are selected from Ca ²⁺ , Mg ²⁺ , Al ³⁺ or combinations thereof. In a preferred embodiment the multivalent cations that can be used in the present method are selected from Ca ²⁺ , Mg ²⁺ and combinations thereof.

Typical examples of cut flowers that may suitably be reinforced by the present method include: Gerbera, Chrysanthemum, Anthurium, Calla, Zantedeschia, Lilium, Phalaenopsis, Alstromeria, Gladiolus, Strelitzia, Celosia and Rosa. The present method may also be used to reinforce foliage that may be used in flower arrangements. Typical examples of such foliage include: Aspidistra, Ruscus, Hedera, Gaultheria (salal), Crassula ovate.

The anionic polysaccharide is suitably introduced in aqueous solution into the plant parts either by uptake via the tracheary elements or by injection into the plant part or a combination thereof. Uptake via the tracheary elements causes the anionic polysaccharide to be introduced into the xylem of the plant parts and is achieved by exposing the cross sectional surface of the stem, twig or stalk to the aqueous solution of the anionic polysaccharide. Preferably, the aqueous solution of anionic polysaccharides is introduced by means of injection.

Typically, the cross sectional surface of the stem, twig or stalk of the plant parts is exposed to the anionic polysaccharide solution for at least 5 minutes, more preferably for 0.3-24 hours and most preferably for 1-5 hours.

Typically, the volume of the anionic polysaccharide solution injected into the plant parts is at least 1 ml per plant part, more preferably 2-10 ml anionic polysaccharide solution per plant part and most preferably 3 -6 ml anionic polysaccharide solution per plant part.

Typically, the temperature of the anionic polysaccharide solution during exposure does not exceed 40°C. Preferably, the temperature of the anionic polysaccharide solution does not exceed 30°C, more preferably said temperature is in the range of 2-25°C. Typically, the anionic polysaccharide solution contains 0.2-5 wt.%, more preferably 0.5-3 wt.% and most preferably 1.3-3 wt.% of the anionic polysaccharide. The anionic polysaccharide is typically introduced into the plant parts in an amount of at least 0.1 mg per g of dry matter, preferably of at least 0.5 mg per g of dry matter, and most preferably of 1-50 mg per g of dry matter. The multivalent cations are suitably introduced in aqueous solution into the plant parts either by uptake via the tracheary elements, by immersion of the plant parts, by injection or a combination thereof. Uptake via the tracheary elements causes the multivalent cations to be introduced into the xylem of the plant parts and is achieved by exposing the cross sectional surface of the stem, twig or stalk to the aqueous solution of the multivalent cations (e.g. by immersing the cross sectional surface in said aqueous solutions). Preferably, the multivalent cations are introduced by immersing the plant parts in the aqueous solution of the multivalent cations.

Typically, the cross sectional surface of the stem, twig or stalk of the plant parts is exposed to the multivalent cation solution for at least 5 minutes, more preferably for 0.3 to 24 hours and most preferably for 1-5 hours.

Typically, the temperature of the multivalent cation solution during exposure does not exceed 40°C. Preferably, the temperature of the multivalent cation solution does not exceed 30°C, more preferably said temperature is in the range of 10-25°C.

Typically, the solution of multivalent cations contains at least 5 mmol/1, more preferably 7- 200 mmol/1 and most preferably 10-80 mmol/1 of dissolved multivalent cations. The multivalent cations are typically introduced into the plant parts in an amount of at least 80 mg per kg of dry matter, preferably of at least 200 mg per kg of dry matter, and most preferably of 300-2,000 mg per g of dry matter.

The time between the successive steps of introducing dissolved anionic gelling polysaccharide into the plant parts and inducing gelation of the dissolved anionic gelling polysaccharide within said plant parts by introducing multivalent cations should be less than 12 hours, preferably less than 5 hours and more preferably less than 2 hours. After the reinforcement treatment the plant parts are more rigid and this protects the plant parts against the loss of turgidity that occurs shortly after the plant parts have been severed from their root system. According to a particularly advantageous embodiment, the reinforcement of the plant parts is a pre-treatment step followed by dehydration of the reinforced plant parts. In a even more preferred embodiment, the reinforcement step of the plant parts is followed by dehydration and impregnation with a non-volatile preservative agent of the plant parts.

Dehydration of the reinforced plant part may be carried out before, during or after the impregnation of the reinforced plant part with the non- volatile preservative agent. Preferably, the dehydration is carried out before or during the impregnation with the non-volatile preservative agent. Most preferably, the dehydration is carried out simultaneous with the impregnation with the non-volatile preservative agent. The term "non-volatile" refers to a preservation agent having a vapour pressure of less than 0.1 mm Hg at 20°C, preferably of less than 0.01 mm Hg at 20°C.

Typically, the non-volatile organic preservation agent employed in the impregnation step is a humectant. The humectant employed preferably is a liquid under ambient conditions (20°C, 1 atm). More preferably, the humectant is a polar liquid. Examples of humectants that can suitably be employed in the present method include diols, triols, polyols and combinations thereof. More preferably, the humectant is selected from glycerol, ethylene glycol, propylene glycol and polyethylene glycol (H-(0-CH ₂ -CH ₂ ) _n -OH with n>2). Even more preferably, the humectant is selected from glycerol and polyethylene glycol. Most preferably, the humectant is polyethylene glycol.

The polyethylene glycol employed in the impregnation step preferably contains 2-34, more preferably 2-20 and most preferably 2-11 ethylene oxide units. The molecular weight of the humectant typically lies in the range of 62-1500 g/mol. More preferably, said molecular weight is in the range of 62-800 g/mol, most preferably in the range of 62-500 g/mol.

The dehydration of the plant parts may suitably be achieved by using one or more of the following dehydration techniques: solvent extraction, freeze drying, air drying, drying by burying them in a plant-drying desiccant powder (e.g. borax, silica gel, sand or mixtures thereof). More preferably, the plant parts are dehydrated by means of one or more dehydration techniques selected from solvent extraction and freeze drying. Most preferably, dehydration is achieved by solvent extraction. Examples of solvents that can be used to dehydrate the plant parts include supercritical fluids, liquefied gases and polar solvents. Supercritical fluids are the preferred solvent for dehydration by solvent extraction.

Typically, the fresh ornamental plant parts that are preserved by the present method have a water content at the start of the preservation process in the range of 55-92 wt.%, more preferably of 70-91 wt.% and most preferably of 75-90 wt.%.

Advantageously, the dehydration step is employed to remove a substantial fraction of the water that is contained in the fresh plant parts. Typically, at least 60%, more preferably at least 65%), even more preferably at least 70% and most preferably 75-99%> of the water contained in the fresh plant part is removed by the dehydration step.

In the dehydration step, the plant part is typically dehydrated to produce a dehydrated plant part having a water content of less than 50 wt.%>. The preserved plant part obtained by the dehydration step preferably has a water content of less than 45 wt.%, more preferably of 4-35 wt.%) and most preferably of 5-25 wt.%.

The impregnation of the plant parts may suitably be achieved by partially or completely immersing the plant parts in preservation medium, said preservation medium containing dissolved humectants. In a preferred embodiment the preservation medium is pressurized. According to a particularly preferred embodiment, the reinforced plant part is impregnated by contacting the plant part with a preservation medium containing a liquefied or supercritical solvent and dissolved humectant, said preservation medium having a pressure of at least 30 bar.

The use of a liquefied or supercritical solvent at a pressure of at least 30 bar enables very efficient substitution of water contained within the plant part by the humectant and without significant detrimental effect on the appearance and mechanical properties of the plant parts. Although the inventors do not wish to be bound by theory, it is believed that in the present method the preservation medium readily enters the plant parts to replace at least a part of the water contained therein. The water that is extracted from the plant parts can be removed from the preservation medium during the contacting with the medium and the plant parts, e.g. by contacting the preservation medium with a water-absorbent. When the pressure is reduced to ambient pressure, preserved plant parts are obtained in which a significant part of the water has been replaced by the preservation agent.

The use of the high pressure preservation medium also offers the advantage that little or no shrinkage occurs even though the weight loss induced by the method may be as high as 80%. Furthermore, the treatment effectively sterilizes the plant parts, thereby contributing to the overall effectiveness of the preservation.

The contacting of the reinforced plant parts with the preservation medium results in preserved plant parts that typically contain the humectant in a concentration of at least 4%, more preferably of at least 6% and most preferably of at least 10% by weight of the dry matter contained in the preserved plant parts. Typically, the amount of humectant does not exceed 80%) by weight of the dry matter contained in the reinforced plant parts. The contacting of the reinforced plant parts with the preservation medium results in preserved plant parts with an amount of humectant that is typically at least 3%, more preferably 4-65%>, more preferably 5-55% and most preferably 8-50%>.

The liquefied or supercritical solvent that is contained in the preservation medium typically comprises at least 0.6 wt.%, more preferably 0.8-40 wt.%, even more preferably 0.9-20 wt.% and most preferably 1-10 wt.% of substances that are in a gaseous state at atmospheric pressure and a temperature of 20°C. According to a preferred embodiment, the solvent contains at least 0.5 wt.%, more preferably 0.8-40 wt.%, even more preferably 0.9-20 wt.% and most preferably 1-10 wt.% of one or more gaseous substances selected from carbon dioxide, nitrogen and combinations thereof. Most preferably, the solvent contains at least 0.5 wt.%), more preferably 0.8-40 wt.%, even more preferably 0.9-20 wt.% and most preferably 1- 10 wt.%) of carbon dioxide.

The contact time between the plant parts and the preservation medium as defined herein typically lies in the range of 5-200 minutes. More preferably, said contact time lies in the range of 15-120 minutes, most preferably in the range of 35-90 minutes.

The preservation medium employed in the preservation method preferably has a pressure of 40-300 bar and a temperature of 5-70°C. More preferably, the pressure of the preservation medium is in the range of 50-200 bar, most preferably of 70-190 bar. The temperature of the preservation medium preferably is in the range of 35-60°C, most preferably of 40-50°C.

In accordance with a particularly preferred embodiment of the preservation method, the solvent contained in the preservation medium is in a supercritical state. The use of a solvent in supercritical state enables very effective dehydration and simultaneous impregnation with the humectant.

The preservation medium utilized in the preservation method typically contains 0.5-97 wt.% of liquefied or supercritical solvent, 1-50 wt.% of dissolved humectant, and 0-92 wt.% of co- solvent. The co-solvent that is an optional component of the preservation medium preferably is a Ci-8 monohydric aliphatic alcohol. Naturally, also mixtures of these aliphatic alcohols may be employed. The combination of the solvent, the preservation agent and the optional co- solvent preferably represents at least 80 wt.% of the preservation medium

According to a particularly advantageous embodiment of the preservation method, the solvent contains 30-90 wt.%, more preferably 50-85 wt.% and most preferably 60-80 wt.% of a co- solvent selected from C _1-8 monohydric aliphatic alcohols and combinations thereof. The use of such a co-solvent offers the advantage that the humectant can be included in the preservation medium in very high concentrations. Also, the co-solvent can improve the water- extraction capability of the preservation medium.

The co-solvent employed in the preservation method preferably is selected from C _1-4 monohydric aliphatic alcohols, more preferably from C _1-3 monohydric aliphatic alcohols, even more preferably from methanol, ethanol and combinations thereof. Most preferably, the co- solvent is ethanol.

Particularly good results can be achieved with the present preservation method if the co- solvent and the humectant are miscible under the conditions employed during the contacting of the plant parts and the preservation medium, i.e. under these conditions the two components are liquid and can be mixed in all proportions, forming a homogeneous solution. The co-solvent and humectant are typically contained in the preservation medium in a weight ratio of 100: 1 to 1 :3, more preferably in a weight ratio of 10: 1 to 1 :2, and most preferably in a weight ratio of 4: 1 to 1 : 1. It is further preferred to employ a preservation medium that comprises the liquefied or supercritical solvent, the humectant and the co-solvent, wherein the solubility of the humectant in the co-solvent at the temperature employed during the contacting of the fresh plant parts with the preservation medium exceeds 200 g/1, more preferably exceeds 400 g/1 and most preferably exceeds 500 g/1; and wherein the solubility of the humectant in carbon dioxide at the temperature and pressure employed during the contacting of the fresh plant part with the preservation medium is less than 50 g/1, more preferably less than 20 g/1 and most preferably less than 10 g/1.

In accordance with a preferred embodiment of the preservation method the preservation medium contains 0.5-40 wt.% of the liquefied or supercritical solvent, 1-50 wt.% of the humectant and 40-85 wt.% of the co-solvent. Even more preferably, the preservation medium contains 1-20 wt.% of the liquefied or supercritical solvent, 5-30 wt.% of the humectant and 60-80 wt.%) of the co-solvent. Typically, the solvent, the humectant and the co-solvent together represent at least 85 wt.%, more preferably at least 90 wt.% and most preferably at least 97 wt.% of the preservation medium.

The weight of the plant parts is typically reduced by at least 50% by the preservation method as the amount of water that is removed by the method usually substantially exceeds the amount of humectant that is introduced by the same method. Typically, the weight of the plant parts is reduced by at least 55%, more preferably at least 60% and most preferably by at least 65% by the present method.

In case the preservation method employs a pressurized preservation medium as described herein before, the preserved plant parts can be recovered by depressurizing the vessel in which the contacting with the preservation medium takes place and by removing the preserved plant parts therefrom. In case the preservation medium employed contains a co- solvent, residual co-solvent may be removed from the preserved plant parts by leaving the plant parts in the open air, especially if the co-solvent is a volatile substance, e.g. methanol or ethanol. Removal of the residual co-solvent may be aided by applying vacuum and/or by elevating temperature, e.g. to up to 35°C. The evaporated co-solvent is suitably condensed and re-used.

In accordance with another preferred embodiment of the preservation method, the preservation medium employed in the present method contains a dye, preferably a reactive dye. The inventors have discovered that the preservation medium not only is capable of effectively impregnating the plant parts with humectant, but also that it can be used to dye the plant parts. Thus, the present method can suitably be used to simultaneously preserve and dye plant parts. Dyeing may be advantageous in those cases where the preservation method causes colour changes in the plant parts. These colour changes may be negated by dyeing the plant parts using the present method.

The efficiency of the preservation method of the present invention can be maximized by removing water from the preservation medium during the contacting with the fresh plant parts and/or by replacing at least a part of the preservation medium with preservation medium having a lower water content during said contacting. This may be achieved, for instance, by contacting the preservation medium with a water-absorbent or a water-adsorbent during the contacting with the plant parts, e.g. by recirculating the preservation medium across a water- absorbent or water-adsorbent material. Examples of materials that can suitable be used to dry the preservation medium include zeolites and ionic liquids.

In a particularly advantageous embodiment of the present method, wherein reinforcement of the plant part is combined with the dehydration step, besides the introduction of the aqueous solution of anionic polysaccharide, an aqueous solution of biopolymer is introduced into the xylem of the plant parts. Loading of the xylem with biopolymer preferably takes place before the introduction of the multivalent cations into the plant parts. The loading of the xylem with biopolymer functions as an additional reinforcement step. The subsequent dehydration step after the reinforcement step will induce gelation of the biopolymer, as the concentration of the biopolymer is increased by dehydration to a level that exceeds the concentration (gel point) at which the biopolymer starts forming a gel.

Examples of biopolymers that can suitably be employed in the biopolymer solution include polysaccharides and proteins. Most preferably, the biopolymer is a polysaccharide biopolymer, Examples of polysaccharide biopolymers that can be employed include starches (including native and modified starches), glycogen, chitin, rosin gum, xanthan gum, arabinoxylans and combinations thereof. Most preferably, the biopolymer is selected from the group of starch, chitin and combinations thereof. The xylem of the plant parts can suitably be loaded with biopolymer by partially or completely immersing the plant parts in the biopolymer solution. Preferably at least a part of the stem, but not the flower petals, is immersed in the biopolymer solution. Thus, efficient uptake of the biopolymer solution is achieved and no biopolymer deposits are formed onto the flower petals.

Typically, the introduction of biopolymer into the xylem of the plant parts takes place before, during or after the introduction of aqueous solution of anionic polysaccharide into the xylem of the plant parts. More preferably, the loading of biopolymer into the xylem of the plant parts takes place before the introduction of the aqueous solution of anionic polysaccharide into the xylem of the plant parts.

Typically, the plant parts are partly or completely immersed in the biopolymer solution for at least 5 minutes, more preferably for 0.3 minutes to 24 hours and most preferably for 1-5 hours. The temperature of the biopolymer solution typically does not exceed 60°C. Preferably, the temperature of the biopolymer solution is in the range of 2-40°C, more preferably in the range of 16-35°C when the plant parts are immersed therein.

Typically, the biopolymer solution contains 0.1-6 wt.%, more preferably 0.3-4 wt.% and most preferably 0.5-3 wt.% of the biopolymer.

The biopolymer is typically introduced into the plant parts in an amount of at least 5 mg per kg of dry matter, preferably of at least 10 mg per kg of dry matter, and most preferably of 20- 2,000 mg per kg of dry matter. Another aspect of the invention relates to a reinforced ornamental plant part comprising at least 0.1 mg of crosslinked, undissolved anionic polysaccharide salt per g of dry matter, preferably at least 0.5 mg, and more preferably at least 1 mg of crosslinked, undissolved anionic polysaccharide salt per g of dry matter. Typically, the reinforced plant part contains not more than 50 mg of crosslinked, undissolved anionic polysaccharide salt per g of dry matter. Preferably, said anionic polysaccharide is selected from calcium alginate, magnesium alginate, aluminum alginate, calcium pectinate, magnesium pectinate and combinations thereof. The reinforced plant parts typically contain at least 3%, more preferably 4-65%, more preferably 5-60% and most preferably 8-55% of humectants selected from glycerol, polyethylene glycol, ethylene glycol and combinations thereof.

The water content of the reinforced and preserved plant part preferably does not exceed 45 wt.%. More preferably, the water content of the preserved plant part is in the range of 4-35 wt.%, most preferably in the range of 5-25 wt.%.

The reinforced plant part of the present invention typically contains traces of the co-solvent as defined herein before. Typically, the co-solvent is contained in the reinforced and preserved plant part in a concentration that exceeds 10 ppm, more particularly exceeds 100 ppm. Typically this concentration does not exceed 1 wt.%.

In another preferred embodiment, the tracheary elements of the reinforced and preserved plant parts are filled with exogenous biopolymer selected from starch, glycogen, chitin, rosin gum, xanthan gum, arabinoxylans. The plant part typically contains at least 5 mg of the exogenous biopolymer per kg of dry matter. More preferably, the exogenous biopolymer is contained in the preserved plant part in a concentration of at least 10 mg per kg of dry matter, and most preferably of 20-2,000 mg per kg of dry matter. According to a particularly preferred embodiment, the plant parts are obtained by the methods as described herein before.

The invention is further illustrated by the following non-limiting examples. EXAMPLES Example 1

The stems of fresh cut Gerbera flowers were placed into an aqueous solution containing 2.65%) white or yellow potato starch and kept overnight. Subsequently, the flowers were injected directly into the receptacle with an aqueous alginate solution (1.5 wt%) and a couple of minutes after the injection the stems were almost completely immersed into an aqueous CaCl ₂ solution (0.71 wt.%) for 2 hours. Two treated flowers were placed into an autoclave that was filled with 500. g of a solution comprising PEG 400/Ethanol (weight ratio 1 :3) in which 0.1% yellow reactive dye (2-[2-[(4- methoxyphenyl)amino]vinyl]-1.3-trimethyl-3H-indolium chloride (CAS no:25717-55-9)) was dissolved. Carbon dioxide was added to create a preservation medium having a pressure of 180 bar and a temperature of 40°C. The preservation medium contained approximately 5 wt.% supercritical carbon dioxide.

The preservation medium was kept at 180 bar and 40°C for 1 hour, after which the autoclave was gradually depressurized in 30 minutes. After depressurization the flowers were taken out of the autoclave and left for a couple of hours at ambient conditions.

The preserved Gerbera flowers so obtained had an appearance that was very similar to that of fresh Gerbera flowers.

A bouquet of the preserved Gerbera flowers was put into a vase (without vase water). After 4 months under ambient conditions the bouquet still had a fresh, appealing appearance.

Table 1 depicts the composition of the Gerbera flowers before and after the preservation treatment. Table 1

Comparative Example A

Example 1 was repeated, except that this time the flowers were neither injected with an aqueous alginate solution nor immersed in an aqueous CaCl ₂ solution. The preserved Gerbera flowers so obtained had an attractive appearance, but were mechanically not very robust, i.e. the flower petals came off quite easily.

Table 2 depicts the composition of the Gerbera flowers before and after the preservation treatment.

Table 2

Example 2

The stems of freshly cut roses (Avalanche) were placed into an aqueous solution containing 2.65% white or yellow potato starch and kept overnight. Subsequently, the flowers were injected directly into the receptacle with an aqueous alginate solution (1.5 wt%) and a couple of minutes after the injection the stems were almost completely immersed into an aqueous CaCl ₂ solution (0.71 wt.%) for 2 hours.

Two of the thus treated roses were immersed into the preservation mixture and carbon dioxide was added to create a preservation medium having a pressure of 180 bar and a temperature of 40°C. The preservation medium contained approximately 5 wt.% supercritical carbon dioxide. The preservation medium was kept at 180 bar and 40°C for 1 hour, after which the autoclave was gradually depressurized in 30 minutes. After depressurization the flowers were taken out of the autoclave and left for a couple of hours at ambient conditions.

The preserved flowers so obtained were found to keep their attractive appearance over a period of more than 4 months.

Table 3 depicts the composition of the Rose flowers before and after the preservation treatment. Table 3

Comparative Example B

Example 2 was repeated, except that this time the roses were neither injected with an aqueous alginate solution nor immersed in an aqueous starch and CaCl ₂ solutions.

The preserved flowers so obtained had an attractive appearance. However, the flowers were mechanically not very robust Table 4 depicts the composition of the rose flowers before and after the preservation treatment.

Table 4

Fresh Preserved

Water content (wt.%) 70-90 12-15

PEG content (wt.%) 0 20-25