Water potential is a measure of free energy associated with water per unit volume $\left(\mathrm{JM}^{-3}\right)$. The water potential of pure water $\left(\psi_w\right)$ at atmospheric pressure is zero. The unit of water potential is bars or Pascal ( $1 \mathrm{mPa}=10$ bars).

Solute Potential The addition of solutes reduce water potential (to a negative value). This reduces the concentration of water. Hence, solutions have a lower water potential than pure water, the magnitude of this lowering due to dissolution of a solute is called solute potential or $\psi_{\mathrm{s}}$.

If some solute is dissolved in pure water, solution has fewer free water molecules and the concentration of water decreases, reducing its water potential.

Hence, all the solutions have a lower water potential than pure water. The magnitude of this lowering is due to dissolution of solute is called solute potential or $\psi_s . \psi_s$ is always negative. The more the solute molecules, the lower (more negative) is the solute potential $\psi_s$ water potential of a cell is affected by both solute and pressure potential.

The relationship can be illustrated as

$$\begin{aligned} &\text { Where, }\\ &\begin{aligned} & \psi_w=\psi_s+\psi_p \\ & \psi_w=\text { water potential } \\ & \psi_s=\text { solute potential } \\ & \psi_p=\text { pressure potential } \end{aligned} \end{aligned}$$

(a) Onion peel when placed in salt solution, shrinks as water from cells cytoplasm moves out of the cell i.e., hypertonic solution.

(b) When it is placed again back in distilled water, cell regains it's shape and absorbs water and become turgid (full of water) i.e., hypertonic solution.

Difference between apoplast pathway and symplast pathway is as follows

Apoplast Pathway	Symplast Pathway
It consists of the non-living parts of plant body, i.e., cell walls and inter cellular spaces. There is a little resistance in the movement of water in apoplastic pathway. It is a faster process.	It consist of living parts of plant body, i.e., protoplast connceted by the plasmodesmata. Some resistance occurs in the movement of water through symplast. It is a slightly slower process.

Most of the water flow in the root occur via the apoplast pathway since, the cortical cells are loosly packed and hence offer no resistance to water movement, which is through mass flow. This mass flow of water occurs due to adhesive and cohesive properties of water. However, symplast pathway is also involved in the movement of water molecules within the root (i.e., via endodermis to xylem).