The below mentioned article provides a brief account of structure, physical properties and importance of water to plant life.
Structure of Water:
Water (H2O) is normal oxide of hydrogen in which the two hydrogen atoms are joined to oxygen atom by covalent bonds forming an angle of 105° (Fig. 2.1 A). Since, oxygen atom is more electronegative than hydrogen atom; the electrons of the covalent bonds tend to be attracted towards oxygen atom. This results in partial negative charge (δ–) on oxygen and equal partial positive charges (δ+) on each hydrogen in water molecule. Because the partial negative and positive charges are equal, water molecule carries no net charge and is neutral.
However, partial negative and positive charges on two sides of water molecule make it a polar molecule with the result that positive side of one water molecule is attracted towards negative side of another water molecule forming a weak electro static chemical bond between the polar water molecules which is called as a hydrogen bond and is represented by dotted line (Fig. 2.1 B). The hydrogen bonds present in between the water molecules provide water with unique physical properties.
Hydrogen bond is a weak electrostatic chemical bond formed between covalently bonded hydrogen atom and a strongly electronegative atom with a lone pair of electrons such as nitrogen or oxygen and is represented by dotted line. The energy of hydrogen bond is lesser than ionic or covalent bond but higher than Van der Waals forces and it varies from 8-42 kilojules/ mols of bonds (kJ mol–‘).
In plants, hydrogen bonds may also be formed between water and other substances especially those which contain electronegative O or N atom with lone pairs of electrons. The hydrogen bonds are of tremendous biological importance, especially the N-H…N bond that enables complex proteins and nucleic acids to be built up.
Physical Properties of Water:
Natural water (rain, spring, river etc.) is never pure and contains dissolved substances in it. However, pure water is colourless, odorless liquid with mol. wt. 18 Dalton, m.p. 0°C, b.p. 100°C and maximum density of 1 gm. per cm3 at 4°C.
The amount of heat energy required to raise the temperature of unit mass of a substance 1°C is called as specific heat. For 1 gm. of pure water, this value is 1 calorie (4.184 joules). The specific heat of water is higher than other liquids (except liquid ammonia).
It is due to the presence of hydrogen bonds between water molecules. When temp, of water is raised, the water molecules vibrate faster and absorb large quantities of energy to break the hydrogen bonds. Therefore, relatively large input of energy is required to raise its temp, in comparison to other liquids. The high specific heat of water is of great importance to plants in protecting them from potentially harmful temperature fluctuations.
Latent Heat of Vaporization:
It is the energy required to convert liquid into gas (vapour) phase at constant temperature. For water, the latent heat of vaporization is 44 kJ mol-1 at 25°C and is highest known value among all the liquids. Most of this energy is needed to break the hydrogen bonds between water molecules. The higher latent heat of vaporization of water enables the plants to cool themselves by dissipating heat through foliar transpiration.
Latent Heat of Fusion:
It is the heat energy required to convert unit mass of a solid to a liquid at the same temperature. To melt 1 gm of ice at 0°C, 80 Cal. (335 J) of energy is needed which again is a very high value caused due to the presence of hydrogen bonds, even though ice has fewer hydrogen bonds per molecule than liquid water. In ice, each water molecule is joined to four others by H-bonds, forming a tetrahydral structure (Fig. 2.1 C). The tetrahydrons are arranged in such a way that ice crystals are basically hexagonal.
When ice melts to liquid water, the water molecules move farther apart. However, it is noteworthy that its volume actually decreases during melting. The reason lies in the fact that water molecules are more efficiently packed in liquid water than in ice. In water, each molecule is joined to 5 or more others by H-bonds.
Water Expansion and Density:
Water has a tendency to expand as it freezes and its density is decreased. Therefore, ice has lower density than water and it floats on top of oceans, lakes, rivers etc. in winters and provides a shield to life forms growing underneath it. On cooling, water reaches is maximum density of 0.999973 gm/cm3 at 3.98°C (or app 1 gm/cm at 4°C). Water expands as the temp, falls to 0°C. Its density at 0°C being 0.999841 gm/cm3. When water freezes, it expands still further forming ice with a density of 0.9168 gm/cm3 at 0°C. This expansion of water in freezing temperatures often causes bursting of water pipes in winters.
Cohesive and Adhesive Properties:
Mutual force of attraction between like molecules such as in water (due to H-bonds) is called as cohesion. On the other hand, attraction of water to a solid phase such as cell wall or glass surface is called as adhesion. Cohesive and adhesive properties of water are of great significance in ascent of sap in plants.
Surface tension results due to forces of attraction existing between the molecules of a liquid at the open boundary surface of that liquid and is measured by the force per unit length (newton/metre) acting in the surface at right angle to any line drawn in the surface.
With reference to water, the water molecules at the air-water interface are continuously being pulled into liquid due to cohesion than to the gas (vapour) phase on the other side of the surface. This unequal attraction of water molecules tends to minimise surface area at air- water interface and exerts a force or surface tension on the latter.
The surface tension of water is relatively higher than most of other liquid (except hydrazine and most metals in liquid state such as mercury). Surface tension is responsible for ‘transpiration pull’ that facilitates ascent of sap in higher plants.
It is the ability to resist pulling without breaking and is measured as force per unit area e.g., newton’s per square metre of dynes per square centimetre. Cohesion of water molecules gives water a high tensile strength which enables water column in xylem elements of stem to be pulled to the top of tall trees without breaking.
Water as a Solvent:
The polarity of water makes it an excellent solvent. Water dissolves greater amounts and wider variety of substances than any other common solvent. Water is especially powerful solvent for electrolytes and other substances such as sugars, proteins etc. which have polar -OH or -NH2 groups. Water forms a shield around charged ions or charged surface of solvents due to its polar nature, thereby decreasing the electrostatic interaction between charged substances and increasing their solubility.
Importance of Water to Plant Life:
Life is unconceivable without water and plants are no exceptions. Water constitutes 80-95% of the total weight of growing plant tissues. The seeds which are driest plant tissues still contain 5-15% water content and must absorb considerable amount of water before they germinate.
i. Water is best known solvent and provides medium for the movement of molecules within and in between the cells.
ii. Almost all molecules of protoplasm owe their specific biochemical activities to water environment (milieu) in which they exist.
iii. The structures of macromolecules such as proteins, nucleic acids, polysaccharides and other cell constituents are greatly influenced by water.
iv. Water takes direct part in many biochemical reactions in the cells such as hydrolysis, hydration, and dehydration. Water is also one of the raw materials in photosynthesis.
v. Through transpiration, water plays an important role in controlling temperature of plants.
vi. Contrary to animals, the plant cells contain large central vacuole filled with cell sap and develop large intracellular pressure called as turgor pressure.
Turgor pressure is essential for many physiological processes in plants such as:
i. Cell enlargement,
ii. Stomatal movements,
iii. Transport of solutes in phloem,
iv. Transport processes in cell membranes,
v. Maintaining shape or form of the plant tissues,
vi. Emergence of young seedling from the soil etc.
vii. Water is most important factor for agricultural productivity.
viii. Water is an essential factor in completing the life cycles of lower forms of plant life and aquatic higher plants.