Terminology of cell-water relations.

Use of the term "water potential" in place of "diffusion pressure deficit" would improve communication between botanists and scientists in other fields because the concept of potential is familiar to most scientists. Water potential, expressed as PsiW, is the difference in free energy or chemical potential per unit molal volume between pure water and water in cells at the same temperature. The potential of pure water is set at zero; hence the potential of water in cells and solutions is less than zero, or negative. The water potential of a cell is numerically equal to its diffusion pressure deficit, but has a negative sign.