Deoxygenation process by stripping nitrogen or other gaseous mixture containing nitrogen and hydrogen to promote plant cultures in the presence of salt water

Abstract

Process for growing plants, where the plants are supplied with water during irrigation phases of a fixed duration, the water used being salt or brackish water, water which has a given dissolved oxygen content, characterized in that, before the water arrives at the point of use by the plant growing installation, a step is carried out of stripping all or part of the dissolved oxygen with nitrogen or a mixture of nitrogen and hydrogen.

Description

Title of the invention: Process for deoxygenation by stripping nitrogen or other gaseous mixture containing nitrogen and hydrogen to promote plant cultivation in the presence of salt water

[0001] The present invention relates to the field of plant cultivation and in particular fruit and vegetables.

[0002] As we know, global agriculture is currently facing a reduction in cultivable areas caused by the advance of sea water into the land, an advance which is leading to salt enrichment of certain soils.

[0003] Salt-rich soils become unsuitable for agriculture due to several phenomena, including: - The fact that salt makes water and its nutrients difficult for plants to access (water pressure / osmotic pressure). - The fact that salt causes oxidative stress on plants.

[0004] This problem has given rise to a great deal of research worldwide, based on plant genetics and aimed at combating this phenomenon by developing new varieties resistant to salty soils.

[0005] As will be seen in more detail below, the present invention seeks to propose a new technical solution for cultivation in saline environments, constituting an alternative or a complement to the genetic solutions mentioned above.

[0006] This solution must be able to be applied alone or in addition to other techniques.

[0007] One of the objectives underlying the present invention is not necessarily to obtain a solution allowing, for example, the cultivation of tomatoes in sea water (approximately 35 g / l NaCl) but to push back the limits of salt tolerance of plants and thus to bring back into cultivation areas currently considered unsuitable for agriculture.

[0008] The present solution then consists in particular of combating the oxidative effect created by salt water by deoxygenating the irrigation water by injecting an appropriate gas, in particular by injecting gaseous nitrogen, or hydrogen or a mixture of these two gases.

[0009] A process called "stripping" of dissolved oxygen is then applied by injecting nitrogen or another gas, to deoxygenate brackish or salty irrigation water. In French, according to current classical dictionaries, we say "stripping" or “striping” to describe these operations of sweeping, of training, of dissolved oxygen in the water.

[0010] It is of course possible to consider gases other than nitrogen, such as argon or helium, but for obvious reasons of cost, other gases are not preferred in the context of the present invention.

[0011] The use of hydrogen and its reducing effect, or of mixtures containing hydrogen, will make it possible to go further in the fight against the oxidative effect.

[0012] Let's recall the context. Agriculture is known to be very water-intensive. However, if there is one water expenditure that we can hardly do without, it is this one, which feeds entire populations.

[0013] Hence the well-known question that arises: to reduce water stress and continue to cultivate during droughts, could we use sea water which represents 97.5% of our water resources?

[0014] Farmers, entrepreneurs and scientists have long been interested in growing edible plants capable of growing in salt water.

[0015] Paradoxically, a plant growing with its feet in sea water is in the same situation as one growing in an arid zone: it lacks water. Present in too great a quantity, salt limits the absorption of water by the roots. It should be noted that the doses at which it becomes toxic vary from one plant to another. We thus find certain plants, called halophytes, which for their part offer a remarkable adaptation to salty environments.

[0016] The current water stress in many regions of the world (northern China, India and Pakistan, Middle Eastern countries, western United States, Sahel, etc.) and the worsening of freshwater pollution by agriculture and industry can only lead to increasingly frequent and widespread water shortages.

[0017] Experts estimate that one in ten people worldwide lives in a region of high or critical water stress.

[0018] So in Africa, when we talk about water, it is in terms of need or abundance. A single productive well can make all the difference between a living village and a dying one.

[0019] And so a village whose well only delivers salt water forces most of its residents to abandon it and relocate closer to a safe water source.

[0020] Furthermore, brackish water is considered to be water whose salt content is lower than that of sea water. The total concentration of dissolved salt is generally between 1 and 10 g / l, whereas it is (on average) 35 g / l for sea water. Anglo-Saxon countries consider water to be brackish from a salinity level of between 0.05% and 1.8%, or in certain cases up to 3%.

[0021] It is known that the Agrocenter in the Negev Desert in Israel is researching to find the best crops and varieties for production in this desert using brackish water that is available in the region. The goal of this agricultural research center is to share their knowledge with farmers around the world, and especially with strong cooperation of this center with African countries.

[0022] For vegetable production, brackish water is mixed with fresh water from the Ashkelon desalination plant. This fresh water does not contain any nutrients or minerals. Brackish water is loaded with minerals, but also contains a lot of salt, ten times more than fresh water. They mix the two to find out what works best for the plants. Being able to use more brackish water means cheaper production, which is better for the farmer.

[0023] Plants, regardless of their type, absorb water from the soil through their roots. This occurs through a process known as osmosis, which is controlled by the salt levels in the soil water and the water within the plant. If the salt content of the soil is higher than that of the plant, water passes from the plant to the soil. As a result, the plant dehydrates and slowly dies. Of course, each plant has different needs, which means different salt levels. Using seawater for irrigation is possible, but with caution. Plant roots can absorb water when it is moderately salty, but they have to work hard to do so. If salt is used in high concentrations, it stresses the plants and leads to yield loss. It is also important to note that all soil types contain salts in different amounts. They are essential for the growth of each plant.Cereals are tolerant of salts in the soil. The only crops in this family that do not like it are corn and rice.

[0024] Watering tomatoes with salt water is also possible. In fact, they benefit from an increase in salinity. Carrots, eggplant, peppers, and asparagus are other vegetables that can be watered with seawater. Other vegetables are moderately sensitive, so caution should be exercised with them or completely avoid adding seawater around their stems.

[0025] When it comes to watering lawns, it is generally considered that at low concentrations (up to about 50%), seawater stimulates lawn growth, making it thicker and greener. But once this concentration is exceeded, there is a risk of drying it out.

[0026] Despite its toxicity in plants, the sodium ion (Na+) is also a nutrient, especially when the potassium ion (K+) is in low concentration in the soil. The concentration of K+ in the soil in the millimolar range allows for optimal plant growth. The availability of K+ ions in the soil solution, slowly released by soil particles and clays, is often limiting for optimal growth in most natural ecosystems. When the K+ concentration in the soil is very low, in the micromolar range, Na+ can substitute it in certain vital functions, such as its role as a solute to maintain osmolarity in the cell.

[0027] About 90% of K+ is stored in the vacuole where it plays an osmotic role. Confined in the vacuole, Na+ can play this same role; the cell then mobilizes K+ into the cytosol where it plays its metabolic role. A Na+ transport system specifically expressed when the K+ concentration in the soil is low would allow Na+ to be absorbed into the plant for this beneficial use. Na+ can thus stimulate plant growth at low concentrations.

[0028] Halophilic plants may need certain concentrations of NaCl to grow properly, such as Atriplex vesicaria (Australian shrub, close cousin of the Halimione portulacoides found in salt meadows in France), Echinochloa utilis (Japanese millet) or Portulaca grandiflora (purslane).

[0029] Salt-tolerant plants are therefore called "halophile," which means "salt-loving." A "halophyte" is any plant that lives in contact with abnormally high concentrations of salt. These are typically plants found on seashores, deserts, marshes, or salt lakes, for example. In salt marshes, we find glasswort (Salicornia sp.), which includes around thirty edible species, and on the dunes, sea spurge (Euphorbia paralias), sand couch grass (Elymus farctus), or the magnificent sea lily (Pancratium maritimum).

[0030] In other latitudes in humid tropical and equatorial climates, mangrove vegetation which grows roots in brackish water illustrates well the salinity tolerance capacities of halophilic grasses or shrubs.

[0031] Halophytes can tolerate salt concentrations of the order of 500 mM to 1 M, exacerbating the effectiveness of the mechanisms for managing the toxicity of the sodium ion Na+. Some plants such as samphire require this ion to be able to grow; these are strict halophytes.

[0032] These ungrateful ecosystems are suitable for agriculture; thus, salt meadow pastoralism provides an example of an agricultural practice with sheep raised and fed with halophilic flora adapted to the salinity of the soil, to submersion and drought, including puccinellia (Puccinellia maritima, a grass), troscart (Triglochin maritima) or obione (Halimione portulacoides).

[0033] The presence of salt in the soil impacts its water potential. This potential is the energy that must be applied to the soil to release 1g of water. It is always negative, and is lower the stronger the bond between water and soil. Pure water has a water potential of 0, but in soil, the water is not pure and also contains solutes, responsible for the drop in water potential. It is therefore defined: • by its water content. Thus, a well-hydrated soil will have a water potential with values ​​of approximately -0.1 MPa, while a dry soil will have values ​​of approximately -1 MPa. • by the concentration of solutes in the soil. Thus, this value can reach approximately -0.4 MPa in a soil contaminated by a 150 mM NaCl saline solution.

[0034] To understand the importance of these physicochemical parameters in salinity problems, the water potential of the root cells must be taken into account. Under normal conditions, the latter have a water potential value of approximately -0.5 MPa.

[0035] The movement of water goes from the highest potential to the lowest potential (in other words from the least negative to the most negative). The difference in water potentials between the soil and the plant cells (0.4 MPa) will allow water to go from the soil (-0.1 MPa) to the root cells (-0.5 MPa).

[0036] When the soil is contaminated by a 150 mM NaCl saline solution, this difference in water potentials is reduced to 0.1 MPa. This difference in water potential is one of the driving forces for water flow across the cell membrane. It can be estimated that, apart from any adaptive cellular response to salinity, this motor is 4 times less efficient in transferring water from the soil to the inside of the roots under salinity conditions compared to normal conditions.

[0037] In an extreme situation where the soil salinity is higher, we could theoretically see water leaving the root cells and moving into the saline soil, and dehydrating the plant through its roots. This phenomenon is reminiscent of water stress, which can have several causes (drought, frost, etc.) and occurs when the soil is unable to provide enough liquid water to the roots to ensure tissue hydration and evaporation through the leaves. Damage due to the osmotic effect of salinity not only impacts cell turgor, but induces metabolic changes similar to those caused by water stress. For example, osmotic stress has an immediate effect on the growth rate of plants.

[0038] In addition to undergoing osmotic stress preventing normal water absorption at the root cell level, the plant must face disorders in its leaf parts. In particular, photosynthesis is altered, due to the closure of the stomata, a phenomenon controlled by the hormone abscisic acid, and the inhibition of CO2 fixation.

[0039] The immediate consequence of these disorders in photosynthesis is the production of activated forms of oxygen and the expression of enzymes involved in the management of oxidative stress to prevent damage to photosystems, lipids, proteins and nucleic acids. However, one of these forms, hydrogen peroxide (H2O2), also has a cell signaling role in salt tolerance. Thus, there is a coordination mechanism between the production of activated forms of oxygen, their elimination by enzymes and a sufficient amount required for cell signaling.

[0040] The fact remains that plants have developed several biochemical and molecular mechanisms to resist the harmful effects of soil salinity.

[0041] The components of saline stress can be grouped into two categories: - Oxidative stress encountered during saline stress must be managed at the level cellular by mechanisms of protection and repair of damage. - The response to osmotic stress helps maintain water homeostasis through biosynthesis of compatible solutes and the involvement of aquaporins (water channels).

[0042] These mechanisms involve the function and regulation of the Na+ and / or K+ transport systems involved in the response to ionic stress.

[0043] Thus, oxidative stress, caused by salt stress, in corn seedlings was observed mainly in the roots and mature leaves, and to a lesser extent in the young leaves. Various detoxification strategies were then implemented: • Increase in H2O2 content and markers of oxidative damage to cell membranes (electrolyte leakage and lipid peroxidation). • Accumulation in cells of antioxidant molecules (polyphenols, flavonoids, ascorbate, etc.) and antioxidant enzymatic activities (catalase, superoxide dismutase, peroxidase).

[0044] Protective mechanisms for activated forms of oxygen can thus be activated throughout the plant, as is the case in many other stress situations.

[0045] To summarize: • The sodium ion (Na+) is the main cause of salt toxicity by disrupting: - water and nutrient absorption by the roots; - photosynthesis in the leaves; - but also by accumulating activated forms of oxygen leading to oxidative stress. • Due to their similar physicochemical properties, Na+ competes with the potassium ion (K+), a major nutrient in plants. • Plants react to the presence of Na+ in several stages: - By protecting against oxidative stress; - By accumulating solutes to counteract the osmotic effect exerted by a too much Na+ in the soil; - By limiting the absorption of Na+ in the root, increasing its expulsion from the root cells, confining it in the vacuole, and managing its transport and exclusion from the leaves. The plant also improves its K+ nutrition.

[0046] The present invention therefore seeks to propose new conditions for growing plants, particularly in hydroponic conditions.

[0047] But it is understood that if hydroponic conditions are particularly interesting for the implementation of the present invention, the present invention finds its application much more widely in other types of crops including field cultivation, in the open ground, with very varied watering methods, and in particular gravity irrigation (very easy to set up and inexpensive in terms of infrastructure) and drip irrigation which uses water very efficiently by directly supplying water in drops to the roots of the plants, and which reduces losses by evaporation and can be automated.

[0048] As will be seen in more detail below, it is proposed according to the present invention to carry out a deoxygenation of the water irrigating the crops, by sweeping (also called "stripping") using preferably nitrogen or a gas mixture comprising nitrogen, preferably nitrogen and hydrogen, this to promote and allow this cultivation to be carried out in the presence of salt water (typically ranging from 1 to 3.5% salt).

[0049] It is obviously desirable to be able to approach the salt content of sea water, but reasonably it is preferred according to the present invention to be between 50% and 100% of the content of sea water, knowing that the average salinity of the oceans is of the order of 35 g / l, and generally remains between 30 g / l and 40 g / l, it will therefore be preferred according to the present invention to be in waters not exceeding 17 to 35 g / l of salt.

[0050] It is thus desired, thanks to the invention, to eliminate or substantially limit the dissolved oxygen present in the sea water or brackish water used for the culture in question, by a stripping process: This involves extracting the gases dissolved in the water to pass them into the gaseous phase in order to obtain water with low concentration of dissolved oxygen gas. These gases extracted from the liquid phase are carried by a significant flow of water (at a speed preferably greater than 2m / s).

[0051] In a way, if as we have seen the major problem of brine or salt water is linked to oxidative stress generated on the plant, in the presence of too much oxidative element it is advantageous to try to remove the oxygen naturally present in the water by a replacement process (stripping), certainly the activated forms of oxygen will always be present, but there will then not be the disadvantage of an accumulation of the two forms.

[0052] The application of the transfer laws shows that to obtain a very low level of dissolved gas, the molar fraction of the gas considered in the entrainment gas phase must be lowered: hence the fact of favoring stripping of oxygen by nitrogen or a mixture of nitrogen and hydrogen.

[0053] For reasons of ease of management of the safety aspect, gas mixtures not exceeding 3.5% hydrogen will be preferred according to the invention.

[0054] As has therefore been seen above, the present invention favors application to hydroponic crops.

[0055] It should be remembered here that hydroponics is the cultivation of plants without soil, and in water. The roots of the plants are immersed in an inert substrate irrigated by a nutrient solution, or are simply soaked in it. The substrate serves, where appropriate, only as a support, and can be made of pozzolan, perlite, vermiculite, clay balls, synthetic foam, sphagnum, coconut fiber, or even rock wool.

[0056] The system typically operates in a closed circuit: the nutrient solution is pumped and recycled, and renewed regularly.

[0057] It should be noted that now, a good number of vegetable crops are grown hydroponically: this is particularly the case for fruit vegetables and leafy vegetables such as tomatoes (70% of tomatoes produced in France are said to be "hydroponic") and lettuce mainly, but also cucumbers, aubergines, peppers, various salads and aromatic herbs, etc.

[0058] Strawberries are also good candidates for soilless cultivation.

[0059] Hydroponics offers two major advantages: • be able to produce even without soil (of particular interest to countries where fertile land is rare and where drought is too severe, where there is a need to be self-sufficient in certain food crops, urban agriculture, etc.) and • gain in productivity (rapid growth, high yields, possible vertical cultivation, hence saving space, etc.).

[0060] Furthermore, with hydroponic cultivation, the plants do not compete with weeds (therefore not requiring weeding or herbicide), and the Pest pressure is low (no germs or unwanted insects carried by the soil), so the use of insecticides and fungicides is theoretically very reduced. And since plants do not grow in the soil, the earth and groundwater are not in principle polluted by fertilizers or pesticides.

[0061] The present invention then relates to a method for growing plants, where the plants are supplied with water during irrigation phases of determined duration, the water used being salty or brackish water, water which has a given dissolved oxygen content, characterized in that, before the arrival of the water at the point of use by the plant growing installation, a step of stripping all or part of the dissolved oxygen with nitrogen or a mixture of nitrogen and hydrogen is carried out.

[0062] According to one of the embodiments of the invention, said plant cultivation is a cultivation in a controlled environment, among the following methods: in a greenhouse, in a cellar protected from light, in soilless mode, in hydroponics, aquaponics, aeroponics, or even bioponics or other mode.

[0063] According to one of the embodiments of the invention, a mixture of nitrogen and hydrogen is used with a content not exceeding 3.5% hydrogen.

Claims

Claims

1. A method of growing plants, where the plants are supplied with water during irrigation phases of a determined duration, the water used being salt or brackish water, water which has a given dissolved oxygen content, characterized in that, before the water arrives at the point of use by the plant growing installation, a step of stripping all or part of the dissolved oxygen with nitrogen or a mixture of nitrogen and hydrogen is carried out.

2. A method according to claim 1, characterized in that the salt content of said water is in the range from 1 to 10 g / l of salt.

3. A method according to claim 1, characterized in that the salt content of said water is in the range from 17 to 35 g / l of salt.

4. Method according to one of the preceding claims, characterized in that said cultivation of plants is a cultivation in a controlled environment, among the following methods: in a greenhouse, in a cellar protected from light, in soilless mode, in hydroponics, aquaponics, aeroponics, or even bioponics or other mode.

5. Method according to one of claims 1 to 3, characterized in that said cultivation of plants is a cultivation carried out in fields, in open ground, using gravity irrigation or drip irrigation.

6. Method according to one of the preceding claims, characterized in that a mixture of nitrogen and hydrogen is used with a content not exceeding 3.5% hydrogen.