Explain: Water Potential.

(N/A) $\Rightarrow$ To comprehend plant-water relations,an understanding of certain standard terms is necessary.
$\Rightarrow$ Water potential $(\Psi_{w})$ is a concept fundamental to understanding water movement.
$\Rightarrow$ Solute potential $(\Psi_{s})$ and pressure potential $(\Psi_{p})$ are the two main components that determine water potential.
$\Rightarrow$ Explanation: Water molecules possess kinetic energy. In liquid and gaseous forms,they are in random,rapid,and constant motion.
$\Rightarrow$ The greater the concentration of water in a system,the greater is its kinetic energy or 'water potential'. Hence,pure water has the greatest water potential.
$\Rightarrow$ If two systems containing water are in contact,water molecules move from the system with higher water potential to the one with lower water potential. This process is called diffusion.
$\Rightarrow$ Water potential is denoted by the Greek symbol $\Psi$ and is expressed in pressure units such as pascals $(Pa)$.
$\Rightarrow$ By convention,the water potential of pure water at standard temperatures,not under any pressure,is taken to be zero.
$\Rightarrow$ If a solute is dissolved in pure water,the concentration of free water molecules decreases,reducing its water potential. Hence,all solutions have a lower water potential than pure water.
$\Rightarrow$ The magnitude of this lowering due to the dissolution of a solute is called solute potential $(\Psi_{s})$. $\Psi_{s}$ is always negative. The more the solute molecules,the lower (more negative) is the $\Psi_{s}$.
$\Rightarrow$ For a solution at atmospheric pressure,$\Psi_{w} = \Psi_{s}$.
$\Rightarrow$ If pressure greater than atmospheric pressure is applied to pure water or a solution,its water potential increases. This increases the pressure potential $(\Psi_{p})$.
$\Rightarrow$ Pressure potential is usually positive. In plants,negative potential or tension in the xylem plays a major role in water transport.
$\Rightarrow$ The relationship between these components is: $\Psi_{w} = \Psi_{s} + \Psi_{p}$.