Transport in Plants
| 165
8
7
6
Dry seeds and woody parts also contain little water.
A mature corn plant absorbs 3 litres of water in a day.
A mustard plant absorbs water equal to its own weight
in about 5 hours.
WÅTËR PØTËÑTÏÅL (ΨW)
It is the potential energy of water.
It is the measure of the ability of water molecules to move freely in
solution.
Solute potential (Ψs) and pressure potential (Ψp) are the 2 main
components that determine water potential.
As the concentration of water in a system increases, its kinetic energy
(‘water potential’) also increases. Hence, pure water will have the
greatest water potential.
Water potential of pure water at standard temperatures, which is not
under any pressure, is zero.
It is expressed in pressure units such as Pascals (Pa).
Water molecules have kinetic energy.
In liquid & gaseous form, they show random, rapid and constant motion.
Water molecules move from higher energy system (higher water
potential) to lower energy system (lower water potential). (AIPMT 2007)
Such movement of substances down a gradient of free energy is called
diffusion.
If some solute is dissolved in pure water, the concentration of water
decreases, reducing its water potential. Hence, water potential of
solutions is lower than pure water. Magnitude of this lowering due to
dissolution of a solute is called solute potential (Ψs).
(NEET 2017)
Ψs is always negative. The more the solute molecules, the lower (more
negative) is the Ψs.
For a solution at atmospheric pressure, water potential (Ψw) = solute
potential (Ψs).