Chapter 6

Water potential ψ

Water potential is a concept which explains the direction of water movement from the soil, through the plant to the air. It is a measure of the energy that water molecules contain and which enables them to move. Water potential is linked to water concentration because the total amount of energy in a given volume of water depends on the number of water molecules present. Water potentials can be changed in two ways:

• By adding a solute. When other molecules are dissolved in water there is less room for the water molecules in a given volume so the total energy decreases.
• By changing the pressure. Increasing the pressure will compress more water molecules into a given volume so total energy increases. Conversely, reducing the pressure means the molecules spread out and there are less in a given volume so total energy decreases.

The water potential of pure water at a standard pressure and temperature is fixed at zero so values of ψ can be either positive (i.e. if water is compressed by a positive (pushing) hydrostatic pressure) or negative (i.e. if a solute is added or if the water is expanded by a negative (pulling) hydrostatic pressure). Water potential measurements are given in units of pressure called megapascals (MPa).

Water always moves from a higher water potential to a lower water potential along a water potential gradient. Since the water potential of the water vapour in the air is almost always less than the water potential of the soil water, water will move from the soil through the plant to the air along a water potential gradient. Along this route, water potentials govern the direction of water flow from the soil into root cells, from cell to adjacent cell, along the xylem and from leaf cells to air, for example.

Cold tolerance

A plant’s ability to withstand different degrees of cold is dependent on its ability to prevent water leaving the cells and consequent freezing and dehydration. Some plants achieve this by maintaining high salt concentrations which, by osmosis, maintain water content. Others are able to recover from water loss. In many cases, the length of time of exposure to cold will determine survival.

Pollution

Gases in the air, which are usually products of industrial processes or burning fuels, can cause damage to plants, often resulting in scorching symptoms of the leaves. Fluoride can accumulate in composts and be present in tap water, so causing marginal and tip scorch in leaves of susceptible species such as Dracaena and Gladiolus. Sulphur dioxide and carbon dioxide may be produced by faulty heat exchangers in glasshouse burners, especially those using paraffin. Scorch damage over the whole leaf is preceded by a reddish discoloration.