IntroductiomPlant–water relations concern how plants control the hydration of their cells, including the collection of water from the soil, its transport within the plant and its loss by evaporation from the leaves. 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. Flow of water through plant and soil over macroscopic distances is driven by gradients in hydrostatic pressure. Over microscopic distances (e.g. across semipermeable membranes) it is driven by gradients in water potential. Evaporation of water from leaves is primarily controlled by stomata, and if not made good by the flow of water from soil through the plant to the leaves,

results in the plants wilting .

The relationships between plants and water, including the hydration of plant cells and the transport of water within a plant. Water is the most abundant constituent of all physiologically active plant cells. Leaves, for example, have water contents that lie mostly within a range of 55–85% of their fresh weight. Other relatively succulent parts of plants contain approximately the same proportion of water, and even such largely nonliving tissues as wood may be 30–60% water on a fresh-weight basis. The smallest water contents in living parts of plants occur mostly in dormant structures, such as mature seeds and spores. The great bulk of the water in any plant constitutes a unit system. This water is not in a static condition. Rather it is part of a hydrodynamic system, which in terrestrial plants involves absorption of water from the soil, its translocation throughout the plant, and its loss to the environment, principally in the process known as transpiration

but also through flow across cells in the roots and leaves.

Well-watered plants are turgid. Their cells, which are enclosed in a strong but slightly elastic wall, are distended by an internal pressure that may be as high as 1 MPa, 5 times the pressure in a car tyre and 10 times the pressure of the atmosphere. Plants perform best when they are turgid. Many of the structures of higher plants serve to maintain their cells sufficiently hydrated to function – to grow, to photosynthesize and to respire – even though most of these cells are in the shoots of the plants and so are not only remote from the supply of water in the soil, but are also exposed to a dry environment.

A well-hydrated leaf may transpire several times its own volume of water during a day. Water evaporates from wet cell walls into the internal gas spaces of the leaf. It then flows away as vapour, largely through stomata, which are variable pores in the surface of the leaf. The loss is an unavoidable consequence of the stomata being open, as they must be to allow carbon dioxide to enter the leaf. The relative humidity inside a leaf is typically greater than 99%, and thus there is usually a large difference of absolute humidity across the stomata that induces rapid diffusion of water vapour out of the leaf. See also Leaf and Internode

Although a leaf may lose much water by evaporation, its net loss of water is usually small. Evaporation from cell walls creates in them a large suction that replenishes water

by drawing it from the soil, principally not only via the plant's vascular system ,

Functions of Water in PlantsIt has been said that the study of plant physiology is, for the most part, the study of plant waterrelations. This is understandable, given the central role water plays in a large number of plantprocesses. Among the many functions of water in plants are the following:

·it serves as a medium (and sometimes substrate) for biochemical reactions in cells, sincemany enzymes are dissolved in the cell water

·structural support – water provides the “turgor pressure” that gives many cells theirshape; thus, many tissues will lose their structure and wilt when water availability isinadequate

·cell enlargement – turgor pressure provides the physical force needed to expand cellsduring growth

·transport of solutes between organs, via the xylem and phloem vessels ·evaporative cooling of leaves during transpiration

Next to light, water availability is probably the single most important environmental factoraffecting plant growth. Accordingly, plants have evolved with complex physiological strategiesfor regulating water use, including, but not limited to, minutes timescaleregulation of stomatalapertures in response to sudden changes in environmental conditions. In cropping situationsworldwide, water deficits constitute the single largest cause of crop failure.

Properties of WaterPROPERTY 1: DipolarityWater is a strongly dipolar molecule. The two hydrogen atoms are not attached to the oxygenatom in a straight line. Instead, the two oxygenhydrogencovalent bonds are at an angle ofapproximately 105° to one another.

Because the electrons associated with the covalent bonds are, on average, closer to the oxygennucleus than to the hydrogen nuclei, the molecule is left with a slight negative charge near the Oend, and a slight positive charge at the two H ends.

In liquid water, these charge differences cause a tendency for adjacent molecules to alignthemselves such that a H of one molecule is in close proximity to the O of another molecule. Theelectrical interactions that produce this tendency are called hydrogen bonds.

All of the other physiologically important properties of water arise as a consequence of this basicproperty of diploarity.

The dipolar nature of water. In the liquid state, hydrogen bonding tends tocause adjacent molecules of water to align themselves with the O of onemolecule in close proximity to an H of another molecule. (From Salisburyand Ross, 1992)

PROPERTY 2: Liquid at Physiological TemperatureBecause the hydrogen bonds of water are stronger than the Van der Waals attractive forces thatact between molecules of nonpolarliquids, water has a higher boiling point than manysubstances with much higher molecular weights. For example, ethane, with a MW of 30, is a gasat room temperature, while water (MW = 18) is a liquid. It is water’s strongly dipolar nature thatcauses it to remain in a liquid state at physiological temperatures, even though it has a very lowmolecular weight.

PROPERTY 3: Adhesion and CohesionThe hydrogen bonds between molecules of liquid water are responsible for the phenomenon ofsurface tension. This bonding between adjacent, identical molecules is termed cohesion.

Cohesion between water molecules is thought to be critical for the maintenance of continuouscolumns of water in the xylem of tall plants, so that the water can effectively be "pulled" fromthe 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, waterhas a tendency to wet surfaces. This tendency has important implications for plant waterrelations, as we shall see.

PROPERTY 4: High Latent Heat of EvaporationBecause water molecules adhere to one another so strongly, an unusually large amount of heatenergy 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 tissuewhen transpiration rates are high. We will consider this phenomenon further in a futurediscussion of the energy balance of leaves.

PROPERTY 5: IncompressibilityWater, like all liquids, is essentially incompressible. As such, the laws of hydraulics are relevantto many plant processes. For example, tugor pressure resulting from the elastic cell wallpressing against the incompressible cell contents is largely responsible for the structural rigidityof herbaceous tissues. Also, the turgor pressure of cells provides the force to drive cell expansionduring growth.

IMBIBITION:

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.

ABSORPTION OF WATERWater in the soil is mostly and abundantly, under normal conditions, is available in the form of Capillary water. In the soil the space in between soil particle forms a network of spaces, which normally is filled with water. The water that is present in such spaces is called capillary water.

Most of the water is absorbed by the plants is through root hair zone.

Absorption of Water by Plants

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.

Water in the roots move by two pathways. They can be classified as1) Apoplast pathway2) Symplast pathway

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 pathwayHere 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.

Mechanism of Water Absorption

Water can be absorbed by two methods:Active absorptionPassive 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