Water Potential (Ψw)



Plant Water Relations :

Before discussing  the absorption  and transportation  of water in plants, it is desirable to known some basic facts and forces involved in the process. These are discussed  below: 

Importance  of water in Living  Systems:

Water forms about 71% of the earth's  surface  and represents the most extensive   habitat for the organisms. In side the plant tissue, nearly 80 per cent of the fresh weight is comprised of water. It exists as liquid at relatively  low temperature. The molecules of water ( H2O) are polar due to unequal sharing of electrons between  oxygen and each hydrogen atom. Most of the important  properties  of water  are due to weak hydrogen  bonds formed between  a slight positive  hydrogen atom of one molecule  and a slightly  negative  atom of the  same or another molecule.  

Water plays a significant role in the living systems. It is a universal solvent and acts as a reagent  in many biochemical  reactions. A large number of substances enter and are distributed  within the body of organisms only when they are dissolved  in water. These substances  are usually  held in true solutions, suspension  and colloidal solutions in the cells. Within the protoplasm ( the term refers to the living contents of cells and consists of cytoplasm and nucleus), small molecules unite to form larger molecules with the removal of water (i.e., dehydration synthesis) and larger molecules split into smaller molecules by addition of water ( i.e., hydrolytic  splitting). Water is also required for maintaining the turgidity of cells, functioning  of the protoplasm and regulating  a constant body temperature. 

Since water is essential  for all physiological activities of plants some physiological processes concerned with the plant_ water relations are discussed  below: 

 Water Potential (Ψw) 

Since plant is a multiphase system  as far as water movement  is concerned, the movement of water, as in osmosis, cannot be accurately explained  in terms of differences in concentration or in any other linear expressions. The best way to express spontaneous  movement of water from one region  to another is in terms of the differences in free energy of the water between  the two regions. Water will flow in the energetically  down hill directions because the free energy of water molecules is greater in one area than in the other. The free energy is the thermodynamic parameter that determines the direction in which physical and chemical changes must occur ( Free energy  is the sum of an energy of a  system capable of doing work). 




The difference  between  the free energy of water molecules in pure water and the energy of water in any other system (e.g., water in a solution  or in a plant cell or tissue) is termed the water potential.  

The Greek  letter psi, the symbol  for which is Ψ designates the water potential  in a system. The water potential is expressed in pressure  units such as pascals ( Pa), bars or atmosphere; a bar is a pressure  unit, which equals 14.5 lbIn², 750 mm Hg or 0.987 atm. ( 1 milli bar = 10² Pascals) 

  The water potential  of a solution  can be determined  using pure water as the standard  of reference. The pure water, at atmospheric  pressure, has a water potential  of zero (0) . The presence of solute particles  reduces the free energy of water and thus decreases the water potential  ( negative  value). Therefore, the water potential of a solution  is always less than  zero. 

 If a differences  in water potential exists between  the two regions spontaneous movement of water will take place, provided the movement can occur between  them. The direction of flow will be energetically  down hill, that is, from the region of higher water potential (as  in pure water) to the region of lower water potential ( as in a solution). The movement  of water continues  until the water potential  of the two regions become equal.At this point of equilibrium , net transfer  of water will cease. 




The Components  of Water Potential: 

The typical  plant cell consists of a cell wall, a vacuole filled with an aqueous solution and a layer of cytoplasm between the vacuole and the cell wall. When such a cell is subjected to  the movement  of water, many factors begin to operate which ultimately  determine the water potential of cell sap. For solutions, such as contents of cells, water potential is determined  by three major sets of internal factors viz., matric potential (Ψm), solute potential (Ψs) and pressure  potential (Ψp). A fourth factor_ gravity  potential  (Ψg) also sometimes influences the water potential of cell sap when the contents face tension due to force of gravity. The water potential  ( Ψ) in a plant cell or tissue  can be written  as the sum of the matric potential  ( Ψm) due to binding  of water to cell walls and cytoplasm, the solute potential ( Ψs) due to concentration of dissolved solutes, which by its effect  on the entropy components reduces the water potential  and the pressure  potential  ( Ψp) due to hydrostatic pressure,  which by its effect on the energy components increases the water potential: 

                     Ψw= Ψm + Ψs +Ψp              ...1
Matric potential (Ψm) is significant in case of dry seeds, young cells and dry seeds. In case of mature cells where cell walls are well hydrated, the matric potential  is almost negligible. The fourth factor i.e., force of gravity ( gravity  potential, Ψg) becomes operative as the water tends to move upwards and gravitational force pulls downwards. The above equation (1) may be accordingly modified as following: 

                  Ψg= Ψs +Ψp +Ψg                ...2
Gravity Potential (Ψg ) is the effect of gravitational force on the water potential. It depends on the height of water above the reference state,(h), the density of water (Pw) and the acceleration due to gravity (g): 

                   Ψg= Pwgh
The value of Pwg is 0.01 Megapascals (0.1 bar) per meter. Therefore, Ψg is negligible if the vertical distance is small (less than 5 meters). After ignoring matric potential and gravity potential the equation (1) may be modified as follows: 

                                 Ψw=Ψs +Ψp                    ...3
Each component potential is discussed separately below : 

Matric Potential (Ψm) : Matric is the term used for the surface(such as, soil particles ; cell walls; protoplasms etc.) to which  water molecules  are absorbed. The matric  potential (Ψm) is the component of water potential influenced by the presence  of a matric. It has got a negative  value. In case of plant cells and tissues, the matric potential is often  disregarded because it is not significant  in osmosis. The above equation (1) may be simplified  as follows: 

                        Ψw=Ψs +Ψp                       ...4
Solute Potential (Ψs): Sollute potential  is also know  as Osmotic Potential . It is defined as 
The amount by  which the water potential  is reduced as a result of the  presence of solute is called solute potential  or osmotic potential.  

 Solute potentials or osmotic potential  (Ψs) are always  in negative  values ( number). The term solute potential takes the place of osmotic pressure, expressed  in bars with a negative  sign. 




Pressure Potential (Ψp) : Plant cell wall is elastic and it exerts a pressure  on the cellular  contents. As a results of inward wall pressure, hydrostatic pressure is developed in the vacuole termed as turgor pressure.  The pressure  potential is usually positive and operates on plant cells as wall pressure  and turgor pressure











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