Chapter 4, Problem 1CP

Summary Introduction

To explain: Water potential and the concept of value of water potential for plant physiologist.

Introduction: Water is the most crucial factor for a plant to perform its normal cellular functions. The movement of water from the roots into the shoot can be explained using cohesion tension theory of water transport. The water movement from the roots to the vascular tube (xylem) is facilitated by osmosis.

Explanation of Solution

Water potential (ψw) is defined as the free energy of water or tendency of water to move freely from an area to another. It helps to measure a cell’s ability to absorb water by osmosis.

The free energy of water decreases when solutes are dissolved in water. For the transport of water from the root into the shoot the water potential needs to become more negative in the xylem because of the presence of the dissolved solutes. The solutes induce hydration in which the water molecules surround polar molecules and ions. The association of water molecules with ions and hydrated molecules decreases the motion of water molecules and thus reduces their free energy. Under normal conditions, the water potential of the root is more negative than the water potential of the soil. Thus, water moves from higher (less negative) water potential to lower (more negative) water potential; that is water moves from the soil into the root by osmosis.