For cell expansion to continue, cells must continuously take up water from their surroundings. The movement of water occurs in response to several forces, including its concentration, pressure, position in the gravity field, degree of association with soil particles, etc.
Kinds of water flow
The driving force for much of water’s movement at the cellular level is diffusion, the random intermingling of molecules as they collide with like molecules and change directions. Because of these collisions, over time, diffusion tends to result in the movement of molecules from an area of higher concentration to an area of lower concentration. The force behind such a directional movement is therefore the concentration gradient of the molecule. While we are accustomed to thinking about the concentration of a solute, such as a salt, dissolved in a solvent, it is also just as appropriate to speak in terms of the concentration of the solvent itself, as we will see.
Diffusion is a very effective means of movement, but only across short distances or within small volumes. For example, a small molecule in aqueous solution can diffuse 50 μm in about 0.6 s. However, for that same molecule to diffuse 1 m would require 2.5 x 108 seconds, or about 8 yrs! Thankfully, plants are not limited to diffusion as a means of water transport, but also employ bulk flow, which is the movement of a mass of molecules, often in response to a pressure gradient. This kind of transport is employed by the vascular tissues of the plant to accomplish transport across great distances.
A special kind of diffusion called osmosis occurs in living systems, when a selectively permeable membrane is involved. Very often, forces besides solvent concentration influence osmotic flow, with the sum of all such forces represented by the free energy of the solvent. If diffusion is in response to a concentration gradient, and bulk flow a pressure gradient, osmosis is in response to the sum of both of these forces and other influences, all of which together describe water potential.
Water potential is a term denoting the chemical potential of water, which is to say the free energy of water — the energy available to do work. This energy is expressed in units of energy per unit volume J m-3, which is equivalent to pressure (in MPa). There are many contributing factors to water potential, as we’ve seen above. Always keep in mind that pure water has a water potential of 0 (Ψw = 0)
The solute potential (Ψs) represents the effect of any solutes on the free energy of water. The addition of a solute to water decreases the concentration of water, thus reducing its free energy. The degree of this reduction is a function of the final concentration of solute particles:
where c is in units of osmolality (moles of total dissolved solutes per L). Note the negative sign, a reminder that solutes lower water potential.
The pressure potential (Ψp) represents the effect of pressure on the free energy of water; positive pressure increases free energy, and negative pressure reduces it. Pressure within a typical leaf or root cell is positive, while that within the xylem is negative. A cell with Ψp = 0 is flaccid.
For some tall plants, gravity becomes an important consideration in predicting total water movement, thus it is necessary to consider the gravity potential (Ψg) in such a plant. Each 10 m in vertical distance translates to a 0.1 MPa change in water potential.
Under some conditions, the matric potential (Ψm) plays a significant role in determining the flow of water. When water is bound to a surface in a very thin layer, that water is less available to do work, reducing its free energy. This parameter plays an important role during seed germination and under drought conditions.
The effect of all of these factors on the overall water potential is additive, and can be expressed as follows:
Under most circumstances, the first two terms, corresponding to solute and pressure potentials, are sufficient to determine plant water potential. Plant water potential governs transport of water across membranes, with water always moving from an area of high to low water potential.