Role of Water in Plants

 

Water is the main mineral compound of the living matter, on which practically all the vital processes depend and which maintains the normal physical state of the cell. The physiological function of the plant organs are only possible when the cells are saturated with water.

Water is a most important constituent of plant life due to the physical and chemical properties it possesses.In the process of homeostasis maintenance and cell composition formation, water (Fig. 3.1) fulfills multiple roles:

•  A basic solvent for mineral salts and organic compounds and, at the same time, a dispersion medium for colloidal macro-molecules and a medium for biochemical reaction progression;
• An important factor in maintaining the stability of plant temperature helping to avoid tissue overheating, which could arise due to the heat released during the metabolic processes or under direct sunlight (summer);
• an element of protoplasm structure, which is fixed electrostatically among the long catenae of polypeptides, allowing the physical and chemical properties of the protoplasm, favoring the formation of colloidal systems and determining the conformational structure of the proteins crucial for their functioning and also necessary to ensure the maintenance of the ultrastructure and the functional activity of cell organelles. Protein dehydration leads to coagulation and sediment
  deposition;
• ensures the phenomenon of osmosis and allows turgidity, contributing to stomata movement, to plant orientation in space and to sprout, leave and other organ positioning and orientation;
• serves as a donor of protons and electrons for CO2 reduction in the dark phase of photosynthesis;
• is a component of the redox reactions of the Krebs cycle;
• participates in the reactions of hydrolysis, oxidation and reduction, assimilation and dissimilation;
• structural water in biological membranes ensures the assembly of the phospholipid bilayer, and thus, influences on the permeability of these membranes to electrons and protons;
• represents a universal carrier, ensuring the transport of dissolved substances through the xylem and phloem vessels, as well as the radial transport though the symplast and apoplast;
• ensures the integrity of plant organisms, forming a continuous flow from the root to the leaves, via which mineral salts and organic substances are transported.

Water Content and State in Plants

Total water content in plants is highly variable and depends on plant species and, within the same species,—it depends on the organ, tissue, ontogenetic phase, etc. Thus, algae contain 94–98 % of water, succulent leaves—95 %, reserve organs— 85 %, leaves—80 %, dry seeds—12–14 %.

Environmental factors and the organ type may influence the hereditarily expected values for this index. The variability of water content in this case is determined by the water retention capacity of the plant.
Water retention is caused by osmotic forces, colloidal and capillary imbibition forces, etc.

Protoplasmic colloids have a higher capacity to retain water in young leaves compared to older ones.
Water in plant cells is retained in the cell wall, the protoplasm (up to 90–95 % of water), the vacuole sap (98 %). The amount of water retained by cellular envelopes depends on its thickness, structure and chemical composition.

Approximately 7–8 % water is bound to cellulose polymeric chains and is retained by superficial bonds. The vacuolar sap contains up to 98 % water, which is retained by osmotic, electroosmotic and imbibition forces.
Water is also a structural component of biological membranes—water interacting with the membrane surface, water located in the space between the internal and external chondriosome (mitochondria) membranes. There are 3 water aggregation states that can be found in plants.

Water in liquid state is the basic component of all cells, because it is a component of the membranes (30–35 % of the membrane weight), of the protoplasm and vacuole. In its gaseous state (vapors), water can be found in intercellular spaces and in all the aeriferous tissues.

Water in the solid state of aggregation represents ice crystals, formed during severe frost in intracellular and especially in intercellular spaces. Intracellular crystals break cytoplasmic membranes deteriorating the cells.

Liquid water in the vegetal organism can be free (95 %), representing the basic solvent for mineral and organic substances, ensuring colloid micelle dispersion in the cytoplasm, or bound (4–5 %), retained by hydrogen bonds or by other types of chemical bonds or immobilized in fibrillar structures of macromolecules (Fig. 3.2).

Free water is retained weakly in the plant organism. It circulates very easily in vacuoles, cytoplasm and conducting vessels either inside the cell, or from cell to cell, enabling, at the same time, turgidity. Free water represents the medium where the biochemical processes take place and it often directly participates in these reactions.

Free water freezes at temperatures down to minus 10 °C, so plants with high content of free water are less resistant to low temperatures. Bound water is retained in plants very strongly. This water type is made up of immobile molecules with no possibility for diffusion or evaporation, it is hardly released by the cell.

Bound water freezes at temperatures lower than −10 °C. It does not circulate in the cell or in the entire plant, doesn’t take part in biochemical processes and in dissolving organic or inorganic substances. Due to inability to act as solvent, bound water doesn’t participate in the transfer and circulation of substances.

 

Fig. 3.2 Hydration of NaCl molecules

Bound water is held by:

• osmotic forces, caused by dissolved substances whose dispersed particles retain water. Water retained by osmotic forces is called osmotic water (dissolving water). The elimination of this water form from the tissues (by transpiration) is the more difficult, the more concentrated the vacuolar sap is.
• imbibition forces, caused by hydrophilic colloid soaking. Water retained by these forces is called imbibition water. There can be found numerous hydrophilic colloids in the cell (proteins, mucilage, cellulose), which retain water very strongly. Each mole of protein amino-groups is able to bind 2.6 mol of water and each mole of protein molecules (they can vary greatly in size)—tens of thousands of moles of water.

Forms of Water in the Soil. Accessible and Inaccessible Water

The amount of water absorbed by plants depends not only on the root system size, but also on the amount of water available in the soil and on the forces with which the last retains water. Water retention forces depend on the osmotic pressure of the solution present in the soil. Water can be found in many forms in the soil:

• Constitutional water (crystallization water) enters in the composition of organic or inorganic molecules from the soil as crystallization water: for instance CuSO4·5H2O, Na2CO3·10H2O etc. This water form can’t be used by plants due to its huge retention force.
• Hygroscopic water forms a very thin hydration layer on the surface of soil particles and is retained by very high forces (approximately 10,000–31 atm). This water form can’t be used by plants, its removal from the particle’s surface being possible only by drying at 105 °C.
• Pellicular water from the surface of soil particles is retained by the hygroscopic water film with forces higher than 30 atm, while the external layers of the film—with 0.5–30 atm. Plants can absorb only a certain part of pellicular water (that from the peripheral layers). Exceptions are some halophyte species that absorb water from the deeper layers as well.
• Capillary water is mobile, moving ascendantly, being retained in the soil by forces smaller than 1 atm. It is easily absorbed by plants, contains dissolved minerals and represents their basic water supply.
• Gravitational water is very mobile, moves descendantly, is located in big amounts in the large gaps between soil particles, accumulating after heavy rains or irrigation.
• Water contained in the soil, which is inaccessible for plants, is called physiologically dead water or dead water reserve in the soil. The soil dried up to the limit when it can’t release water contains inaccessible pellicular, hygroscopic and constitutional water.
• The amount of unused water in the soil during plant wilting got the name of wilting point (wilting coefficient). The weaker water is retained by the soil particles and solution the more plants are able to absorb it. Water mobility decreases, retention forces increase and the absorption process complicates as the soil is drying.

The wilting point (θwp) can be calculated according to the formulae proposed by Brigs (θwp = hygroscopic water/0.67) and Bogdanov (θwp = 2·hygroscopic water). Wilting coefficients in different soil types are as follows: θwp = 1.0–1.1 for sandy soil, 6.5–6.9 for clayey-sandy soil and 16.6 for loam-clay soil. The useful water reserve in the soil is the amount of water available for plant growth and development.

Terminologies For Water Relation In Plants

Bound water: Water linked with hydrophilic colloids of the protoplasm after their hydration, being retained with big forces, doesn’t diffuse, freezes at temperatures lower than −10 °C, doesn’t take part in the transformation and circulation of substances.

Free water : Water that preserves all the properties of pure water, moves freely, is retained by relatively small forces, has solvent properties, evaporates via transpiration and freezes at temperatures higher than −10 °C.

Water balance of plants: The ratio between the amount of absorbed water and the amount of water eliminated through transpiration. The value of this ratio depends on environmental factors, A/T > 1 is characteristic for humidity excess, and A/T < 1—for drought conditions. The volumes of transpired and absorbed water should be equal for normal growth and development.

Wilting coefficient The amount of water in the soil expressed as a percentage, that has remained unused by plants during their wilting. For sandy soils wilting coefficient is 0.9 % and for the clayey ones—9.7 %.

Transpiration coefficient The amount of water (g) eliminated by plants through transpiration, necessary for accumulation of 1 g of dry matter. Usually varies from species to species within the limits 300–1,000.

Cohesion : Property of water molecules to remain united due to attraction forces (hydrogen bonds). This phenomena can be observed in xylem tissues enabling water circulation in plants.

Hydropassive stomata movement Osteole closure in conditions of high humidity. Takes place when guard cells are being pressed by the surrounding tissue at high water concentration in leaves (for example, during the long-periods of rain). Stomata open passively when weather is stabilized again.

Osmosis Diffusion of water from a higher water potential to a lower one through semipermeable membranes.

Root pressure The force that causes unilateral (upward) water movement through the root vessels.

Transpiration productivity The value, which indicates the amount of dry matter (g) accumulated by the plant during the evaporation of 1 kg of water through transpiration. The average value of this index is 2–8 g.

Dead water reserve of the soil The amount of water absolutely inaccessible to plants consisting of pellicular, hygroscopic and chemically bound water. The dead water amount depends on the soil type and its mechanical composition. The dead water content of fine sand is 1.3 %, of the sandy-clayey soil—10.2 %, of the silty-clayey soil—14.5 %.

Crude sap Liquid eliminated by plants from the injured tissues of the stem or the root under the action of the root pressure. Chemically, the sap is an aqueous solution containing minerals and organic substances.

Turgidity Water saturation state of the cells. Such a state provides the mechanical rigidity and strength of the tissues, contributing to plant shape maintenance and orientation of plant organs in space.