Define water potential .

Explain pressure potential . How is it build up???

Water potential is the measure of relative tendency of water to move from one region of higher water concentration to another region of lower water concentration through diffusion. It is denoted by ψw. Solute potential and pressure potential are the two main components that determine water potential. 

Solute potential (ψs) − Magnitude of lowering of water potential when a solute is added to the water.

Pressure Potential (ψp) − Magnitude of increase of water potential when pressure greater than atmospheric pressure is applied to pure water or a solution.

ψw = ψs + ψp

Pressure can be build up in a plant system when water enters a plant cell due to diffusion causing a pressure built up against  the cell wall, it makes the cell turgid, this increases the pressure potential.

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Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosisgravity, mechanical pressure, or matrix effects such as surface tension. Water potential has proved especially useful in understanding water movement within plantsanimals, andsoil. Water potential is typically expressed in potential energy per unit volume and very often is represented by the Greek letter Psi.Water potential integrates a variety of different potential drivers of water movement, which may operate in the same or different directions. Within complex biological systems, it is common for many potential factors to be important. For example, the addition of solutes to water lowers the water's potential (makes it more negative), just as the increase in pressure increases its potential (makes it more positive). If possible, water will move from an area of higher water potential to an area that has a lower water potential. One very common example is water that contains a dissolved salt, like sea water or the solution within living cells. These solutions typically have negative water potentials, relative to the pure water reference. If there is no restriction on flow, water molecules will proceed from the locus of pure water to the more negative water potential of the solution; flow proceeds until the difference in solute potential is balanced by another force, for example, pressure potential.

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