Question

In which of the following options would transpirational pull cause water to move from root parenchyma to root xylem? (Values are in kPa)

• A. $\begin{array}{ccccc}& \text{Root Parenchyma}& & \text{Root Xylem}& \\ & {\psi }_P& {\psi }_S& {\psi }_P& {\psi }_S\\ \left(a\right)& 200& -190& -200& 5\end{array}$
• B. $\begin{array}{ccccc}& \text{Root Parenchyma}& & \text{Root Xylem}& \\ & {\psi }_P& {\psi }_S& {\psi }_P& {\psi }_S\\ \left(b\right)& -200& 220& 65& -5\end{array}$
• C. $\begin{array}{ccccc}& \text{Root Parenchyma}& & \text{Root Xylem}& \\ & {\psi }_P& {\psi }_S& {\psi }_P& {\psi }_S\\ \left(c\right)& 200& -220& 65& -5\end{array}$
• D. $\begin{array}{ccccc}& \text{Root Parenchyma}& & \text{Root Xylem}& \\ & {\psi }_P& {\psi }_S& {\psi }_P& {\psi }_S\\ \left(d\right)& 200& -220& -65& -5\end{array}$

Answer 1

The correct option is D $\begin{array}{ccccc}& \text{Root Parenchyma}& & \text{Root Xylem}& \\ & {\psi }_P& {\psi }_S& {\psi }_P& {\psi }_S\\ \left(d\right)& 200& -220& -65& -5\end{array}$

Water potential $\left({\psi }_w\right)$ is a concept fundamental to understanding water movement. Solute potential $\left({\psi }_s\right)$ and pressure potential $\left({\psi }_p\right)$ are the two main components that determine water potential.

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 or $\left({\psi }_s\right)$. $\left({\psi }_s\right)$ is always negative. The more the solute molecules, the lower (more negative) is the $\left({\psi }_s\right)$.

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, it makes the cell turgid and 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 $\left({\psi }_p\right)$.

Water potential of a cell is affected by both solute and pressure potential. The relationship between them is as follows:

$({\psi }_w={\psi }_p+{\psi }_s)$

And water moves from higher $\left({\psi }_w\right)$ to lower $\left({\psi }_w\right)$ and only in option d all of the above conditions are fulfilled.