Water Potential (ψ)

According to the principle of thermodynamics, every component of a physical system possesses a certain amount of free energy that is available to do work; the free energy per molecule of a substance in a chemical system is called its chemical potential. Water potential is simply the chemical potential of water, and it governs the osmotic movement of water throughout the plant.

Water potential is represented by the Greek letter psi (ψ) and is measured in units of pressure — bars, pascals, or atmospheres. It is numerically equal to the old Diffusion Pressure Deficit but of opposite sign: DPD, once positive, becomes water potential with a negative sign. Pure water is defined to have a water potential of exactly zero; dissolving any solute in it always lowers (makes more negative) its water potential. In the same reworking of terminology, D.P.D. is now called water potential, O.P. (osmotic pressure) is now called osmotic potential, and T.P. (turgor pressure) is now called pressure potential — and pressure potential is always a positive value, since it is generated by the cell's own turgor pushing outward.

The single governing rule of water movement in plants is that water always flows from a region of higher (less negative) water potential to a region of lower (more negative) water potential. This is exactly what decides how water moves from one living cell to its neighbour through the connecting plasmodesmata: whichever of the two adjacent cells has the more negative water potential will draw water from the other.

Factors affecting water absorption:

i. The presence of capillary water in the soil is essential — without it, there is nothing available to absorb (section 6.3).

ii. The rate of water absorption is maximum at a soil temperature between 20 and 30 °C.

iii. A high concentration of dissolved solutes in the soil water reduces the rate of water absorption (because it raises the soil water's own osmotic concentration, narrowing the gradient favouring the root). …