Plant–water relations
Plant–water relations concern
The water status of plants is usually expressed as ‘water potential’, which has units of pressure, is always negative, and in simple form is the algebraic sum of the hydrostatic pressure and the osmotic pressure of water
Next to light, water availability is probably the single most important environmental factor affecting plant growth. Accordingly, plants have evolved with complex physiological strategies for regulating water use, including, but not limited to, minutes timescale regulation of stomatal apertures in response to sudden changes in environmental conditions. In cropping situations worldwide, water deficits constitute the single largest cause of crop failure.
PROPERTY 1: Dipolarity
Water is a strongly dipolar molecule. The two hydrogen atoms are not attached to the oxygen atom in a straight line. Instead, the two oxygen hydrogen covalent bonds are at an angle of approximately 105° to one another.
Because the electrons associated with the covalent bonds are, on average, closer to the oxygen nucleus than to the hydrogen nuclei, the molecule is left with a slight negative charge near the O
end, and a slight positive charge at the two H ends.
In liquid water, these charge differences cause a tendency for adjacent molecules to align themselves such that a H of one molecule is in close proximity to the O of another molecule. The
electrical interactions that produce this tendency are called hydrogen bonds.
All of the other physiologically important properties of water arise as a consequence of this basic property of diploarity.
The dipolar nature of water. In the liquid state, hydrogen bonding tends to cause adjacent molecules of water to align themselves with the O of one molecule in close proximity to an H of another molecule. (From Salisbury
and Ross, 1992)
PROPERTY 2: Liquid at Physiological Temperature
Because the hydrogen bonds of water are stronger than the Van der Waals attractive forces that act between molecules of nonpolar liquids, water has a higher boiling point than many substances with much higher molecular weights.
For example, ethane, with a MW of 30, is a gas at room temperature, while water (MW = 18) is a liquid. It is water’s strongly dipolar nature that causes it to remain in a liquid state at physiological temperatures, even though it has a very low molecular weight.
PROPERTY 3: Adhesion and Cohesion
The hydrogen bonds between molecules of liquid water are responsible for the phenomenon of surface tension. This bonding between adjacent, identical molecules is termed cohesion.
Cohesion between water molecules is thought to be critical for the maintenance of continuous columns of water in the xylem of tall plants, so that the water can effectively be "pulled" from the soil to the leaves.
Hydrogen bonds also cause adhesion of water to other polar molecules and charged surfaces, including soil particles and the protein and polysaccharide constituents of cell walls. Thus, water has a tendency to wet surfaces. This tendency has important implications for plant water relations.
PROPERTY 4: High Latent Heat of Evaporation
Because water molecules adhere to one another so strongly, an unusually large amount of heat energy is required to convert water from its liquid state to its gaseous state.
At normal pressure, 2452 J are required to convert 1 g of liquid water at 20°C to water vapor at 20°C. This very high latent heat of evaporation is important in regulating the temperatures of leaves; evaporation of water in the substomatal cavity results in significant cooling of the leaf tissue when transpiration rates are high.
PROPERTY 5: Incompressibility
Water, like all liquids, is essentially incompressible. As such, the laws of hydraulics are relevant to many plant processes. For example, tugor pressure resulting from the elastic cell wall pressing against the incompressible cell contents is largely responsible for the structural rigidity of herbaceous tissues. Also, the turgor pressure of cells provides the force to drive cell expansion
during growth.
PROPERTY 6 : Specific Heat
The amount of energy required to raise the temperature of a unit mass of substance by 1degree C is called its specific heat. The Specific heat of water varies slightly equal to entire range higher than that of any other substance except liquid ammonia. Thus water can absorb large quantiles of energy .
Animals consist of largely of water and thus have relatively high temperature and stability even when gaining or loosing energy.
PROPERTY 7 : Surface Tension
Water has the second – highest surface tension of all liquid substances ( mercury has the highest ). Because water molecules want to cling together, the ones at the surface form stronger bond than those below he surface. This leads to surface tension, which becomes a barrier between the water and air above it ( the atmosphere ). Surface tension explains why small objects will float on top of water rather than sinking below the surface.
PROPERTY 8 : Viscosity
The more viscosity a fluid has, the more it resists the flow. Conversely, the less viscous a liquid is, the more freely it flows. Honey is an example of a highly viscous fluid. Water is , of course , not as viscous as honey, but it does have viscosity. The extent to which water is viscous is impacted by temperature. There is an inverse relationship, which means that as the temperature of water increases, its viscosity decreases.
PROPERTY 9 : Universal Solvent
Water is called as the universal solvent because more substance dissolve in water than in any other chemical. This is because of polarity of each water molecule. The hydrogen side of each water molecule carries a slight positive electric charge, while oxygen side carries a slight negative electric charge. This helps water dissociate (separate) ionic compounds into their positive and negative ions. The positive part of an ionic compound is attracted to the oxygen side of water while the negative portion is attracted towards the hydrogen side of water. Water is called as universal solvent because it dissolves the most substances, not because it dissolves every single compound.
PROPERTY 10 : Amphoteric Nature
One of the unique property of water is amphoteric nature. An Amphoteric substance is one which can act as an acid or base. While water is neither acidic or basic it acts as both. This is because of its ability to both donate and accept protons. For acids stronger than water it acts as base. And it acts like a acid to bases stronger than itself. Following reactions shows its Amphoteric nature –
H2O (I)+ HCl (aq) 🡪 H3O++ Cl- H2O (I) + NH3 (aq) 🡪 NH4+ + OH-
PROPERTY 11 : Density
Density is the ratio of mass to volume and dense objects feel heavier and tend to sink, while less dense objects feel lighter and tend to float. The density of water is a special case . Water is more dense at 39 degree F, and it cools or warms from this temperature, the water expands slightly. This means that ice is slightly less dense than cold water, that’s why ice floats on surface of bodies of water.
Hydrophilic substances like polysaccharides, proteins etc. of cell walls and storage tissues attract dipolar water to them. Water molecules in turn bind to the charged surfaces. As a consequences the imbibant swells in volume; such a phenomenon is called imbibition and the pressure generated due to imbibition i.e., in the form of swelling force is called Imbibition pressure. During this process some amount of energy is lost and it is called imbibitional energy.
In many cases the imbibition force developed due to the imbibition of water is very high (ranges from 1000 to 10000 bars). The same can be used for breaking big boulders in queries. Even today this method is in practice.
Water in the soil is mostly and abundantly, under normal
Most of the water is absorbed by the plants is through root hair zone.
Plants absorb water through the entire surface - roots, stems and leaves. However, mainly the water is absorbed by roots. The area of young roots where most absorption takes place is the root hair zone. The root hairs are delicate structures which get continuously replaced by new ones at an average rate of 100 millions per day. The root hairs lack cuticle and provide a large surface area. They are extensions of the epidermal cells. They have sticky walls by which they adhere tightly to soil particles. As the root hairs are extremely thin and large in number, they provide enormous surface area for absorption. They take in water from the intervening spaces mainly by osmosis.
Apoplast pathway
In this pathway the movement of water occurs exclusively through cell wall without the involvement of any membranes. Majority of the amount of water goes through the apoplast pathway. The cortex of the root does not oppose such movement of the water.
Symplast pathway
Here the movement of water molecules is from cell to cell through the plasmodesmata. The plasmodesmata forms a network of cytoplasm of all cells. The Casparian strip separates the cortex and the endodermis. It is composed of a wax like substance called suberin, which blocks water and solute molecules through the cell wall of the endodermis. Now the water is forced to go through the cell membranes of different cells leading to a transmembrane pathway.
Water can be absorbed by two methods:
Active absorption Passive absorption
Active Absorption
Water is absorbed due to activities going on in roots. Absorption of water occurs with the help of energy in the form of ATP, which is released due to metabolic activities of root cells such as respiration. Absorption takes place against concentration gradient - even when the concentration of cell sap is lower than that of soil water.
Passive Absorption
Passive absorption is by osmosis. Passive absorption takes place along the concentration gradient - when the concentration of cell sap is higher than that of soil water. Water is absorbed when transpiration rate is high or soil is dry. Due to high transpiration rate, water deficit is created in transpiring cells. Rapid transpiration removes water and reduces turgor pressure in living cells of root. The suction force thus developed is transmitted to root xylem. It pulls water from surrounding root cells to make up water deficit
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