WATER POTENTIAL

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Water Potential: Water potential is defined as the difference between the free energy of water molecules in pure water and the free energy of water molecules in a solution. It is represented by φω. It is a concept fundamental to understanding water movement. Solute potential φs& pressure potential φp are the two main components that determine water potential. The term water potential was introduced by Slatyer & Taylor. In old terminology the equivalent word for water potential is diffusion pressure deficit DPD which was introduced by Meyer. Water potential is a relative term, which refers to the chemical potential of pure water to that of chemical potential of a solution. Just as altitude on land is measured to sea level, so water potential in a system is measured to sea level, so water potential in a system is relative to that of a standard and pure water is taken as a standard. The water potential of pure water is taken as a standard. The water potential of pure water is arbitrarily set as zero. It is expressed in pressure units as Pascal's Pa. One Mega Pascal = 10 bars. If a difference in water potential exists between two regions, water moves from the region of higher water potential to the region of lower water potential.Example: A & B systems. The water potential of system A is -2 bars & water potential of system B is -8 bars. The water moves from A φw is -2into system Bφw is -8 bars. The movement of water from A to B continues till the water potential of the two systems becomes equal. At this point of equilibrium, the net movement of water will cease. If some solute is dissolved is pure water, the solution has less free water & the concentration of water decreases, reducing its water potential. Hence all solutions have a lower water potential than pure water; the magnitude of this lowering due to dissolution of a solute is called solute potential φw. Therefore, the water potential of a solution is always less than zero and negative. The more the solute molecules, the lower more negative is the φs. For a solution at atmospheric pressure φw = φs.If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. It is equivalent to pumping water from one place to another, pressure can build up in a plant system when water enters a plant cell due to diffusion causing a pressure built up against the cell wall, and it makes the cell turgid. This increases the pressure potential. Pressure potential is usually positive, though in plants negative potential or tension in the water column in the xylem plays a major role in water transport up a stem. Pressure potential is denoted as φp .Water potential of a cell is affected by both solute and pressure potential. The relationship between them is φw = φs+φp .Water potential is decreased by factors which reduce the relative water vapour i.e. by addition of solutes, negative pressures or tensions, reduction in temperature and by metric forces. Water potential is increased by factors which increase the negative vapour pressure, mechanical pressure & increase of temperature. Leaves of mesophytes usually have a water potential of -10 bars while those of xerophytes have -15 bars.