Final answer:
The measure used to describe the free energy of water is its water potential, denoted by the Greek letter Y. Water potential provides information about the potential energy in water and is expressed in units of pressure called megapascals (MPa). It helps describe water movement in plant systems.
Step-by-step explanation:
The measure used to describe the free energy of water at any point along the soil—plant—atmosphere continuum is called its water potential (Y).
Water potential is a measure of the potential energy in water and is expressed in units of pressure called megapascals (MPa). It is denoted by the Greek letter Y (psi).
Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to pure water (zero water potential), and water moves from an area of higher water potential to an area of lower water potential through processes like osmosis and transpiration.