90 Chapter 3

tive pressures occur within plants. The positive hydro- Three major factors contribute static pressure within cells is referred to as turgor pressure. to cell water potential Negative hydrostatic pressures, which frequently develop The major factors influencing the water potential in plants in xylem conduits, are referred to as tension. As we will are concentration, pressure, and gravity. Water potential is see, tension is important in moving water long distances symbolized by Y (the Greek letter psi), and the water through the plant. The question of whether negative pres- potential of solutions may be dissected into individual sures can occur in living cells is considered in WEB TOPIC components, usually written as the following sum: 3.5. Hydrostatic pressure is often measured as the deviation Y = Y + Y + Y (3.2) s p g from atmospheric pressure. Remember that water in the

The terms Ys and Yp and Yg denote the effects of solutes, reference state is at atmospheric pressure, so by this defi-

pressure, and gravity, respectively, on the free energy of nition Yp = 0 MPa for water in the standard state. Thus,

water. (Alternative conventions for expressing the compo- the value of Yp for pure water in an open beaker is 0 MPa, nents of water potential are discussed in WEB TOPIC 3.3.) even though its absolute pressure is approximately 0.1 Energy levels must be defined in relation to a reference, MPa (1 atmosphere). analogous to how the contour lines on a map specify the distance above sea level. The reference state most often GRAVITY Gravity causes water to move downward used to define water potential is pure water at ambient unless the force of gravity is opposed by an equal and

temperature and standard atmospheric pressure. The ref- opposite force. The gravitational potential (Yg) depends erence height is generally set either at the base of the plant on the height (h) of the water above the reference-state

(for whole plant studies) or at the level of the tissue under water, the density of water (rw), and the acceleration due examination (for studies of water movement at the cellular to gravity (g). In symbols, we write the following: level). Let’s consider each of the terms on the right-hand Y = r gh (3.4) side of Equation 3.2. g w –1 where rwg has a value of 0.01 MPa m . Thus, raising water

SOLUTES The term Ys , called the solute potential or the a distance of 10 m translates into a 0.1 MPa increase in osmotic potential, represents the effect of dissolved sol- water potential.

utes on water potential. Solutes reduce the free energy of The gravitational component (Yg ) is generally omit- water by diluting the water. This is primarily an entropy ted in considerations of water transport at the cell level, effect; that is, the mixing of solutes and water increases because differences in this component among neighbor- the disorder or entropy of the system and thereby low- ing cells are negligible compared with differences in the ers the free energy. This means that the osmotic potential osmotic potential and the pressure potential. Thus, in is independent of the specific nature of the solute. For dilute these cases Equation 3.2 can be simplified as follows: solutions of nondissociating substances such as sucrose, Y = Y + Y (3.5) the osmotic potential may be approximated by: s p

Ys = –RTc s (3.3) Water potentials can be measured where R is the gas constant (8.32 J mol–1 K–1), T is the abso- Cell growth, photosynthesis, and crop productivity are all

lute temperature (in degrees Kelvin, or K), and cs is the strongly influenced by water potential and its components. solute concentration of the solution, expressed as osmolar- Plant scientists have thus expended considerable effort in ity (moles of total dissolved solutes per volume of water devising accurate and reliable methods for evaluating the [mol L–1]). The minus sign indicates that dissolved solutes water status of plants. reduce the water potential of a solution relative to the ref- The principal approaches for determining Y use psy- erence state of pure water. chrometers, of which there are two types, or the pres- Equation 3.3 is valid for “ideal” solutions. Real solu- sure chamber. Psychrometers take advantage of water’s tions frequently deviate from the ideal, especially at high large latent heat of vaporization, which allows accurate concentrations—for example, greater than 0.1 mol L–1. measurements of (1) the vapor pressure of water in equi- Temperature also affects water potential (see WEB TOPIC librium with the sample or (2) the transfer of water vapor

3.4). In our treatment of water potential, we will assume between the sample and a solution of known Ys. The that we are dealing with ideal solutions. pressure chamber measures Y by applying external gas pressure to an excised leaf until water is forced out of the

PRESSURE The term Yp, called the pressure potential, living cells.

represents the effect of hydrostatic pressure on the free In some cells, it is possible to measure Yp directly by energy of water. Positive pressures raise the water poten- inserting a liquid-filled microcapillary that is connected

tial; negative pressures reduce it. Both positive and nega- to a pressure sensor into the cell. In other cases, Yp is

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