Alternative Conventions for Components of Water Potential
Students planning further study of plant water relations should note that the components of water potential defined in the text are sometimes given different names and symbols. In particular, the equation
(textbook Equation 3.5) is often replaced by the following equivalent equation:
In this alternative convention, P is the same as Ψp. It is the hydrostatic pressure of the solution, and may be positive, as in turgid cells, or negative, as in xylem water. The symbol π is called osmotic pressure and is the negative of Ψs. That is, π has positive values, and Ψs has negative values. "Osmotic pressure" is the term that physical chemists, zoologists, and many others use to denote the effect of dissolved solutes on the free energy of water. Most handbooks of physics and chemistry use the term "osmotic pressure" and the symbol π. The negative sign in front of π in the equation above accounts for the reduction in water potential (Ψw) by dissolved solutes. Thus Ψs = –π. A very interesting, if somewhat unconventional, account of the history and physical meaning of osmotic pressure is given by Hammel and Scholander (1976).
Unfortunately, some authors have mixed the conventions for Ψs and π, leading to unnecessary confusion about what is meant by the symbol π. Thus, π is sometimes incorrectly called osmotic potential instead of osmotic pressure, and it may be used either as a positive quantity or as a negative quantity.
Regarding matric potential, it is usually designated by the symbol t.