Plant-Air Interface

Water equilibrium across the plant-air interface occurs when the water potential in the leaf cells equals that of the surrounding atmosphere. (This presupposes that the leaf and the air are at the same temperature, an aspect that we will reconsider in Chapter 8.) To measure vFleaf, the leaf can be placed in a closed chamber and the relative humidity adjusted until the leaf does not gain or lose water. Such a determination is experimentally difficult because small changes in relative humidity have large effects on VPVW, as we will show next. 

Extremely large negative values are possible for VPVW. In particular, RT jVw at 20°C is 135 MPa. In the expression for VPVW (Eq. 2.24), this factor multiplies In (% relative humidity/100), and a wide range of relative humidities can occur in the air. By Equation 2.24 with pwgh ignored, a relative humidity of 100% corresponds to a water potential in the vapor phase of 0 (In 1 = 0). This is in equilibrium with pure water at atmospheric pressure, which also has a water potential of 0. For a relative humidity of 99%, VFWV given by Equation 2.24 is 

Going from 100% to 99% relative humidity thus corresponds to a decrease in water potential of 1.36 MPa (Table 2-1). Small changes in relative humidity do indeed reflect large differences in the water potential of air We also 

Plant cells come into contact with air where the cell walls are adjacent to the intercellular air spaces (see Fig. 1-2). Thus, the water potential in the cell walls must be considered with respect to T 1W in the adjacent gas phase. The main contributing term for T in cell wall water is usually the negative hydrostatic pressure arising from surface tension at the numerous ail-liquid interfaces of the cell wall interstices near the cell surface. In turn, Z 11 wal1 can be related to the geometry of the cell wall pores and the contact angles. 

The magnitude of the negative hydrostatic pressure that develops in cell wall water can be estimated by considering the pressure that occurs in a liquid within a cylindrical pore — the argument is similar to the one presented 

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Table 2-1. Magnitudes of Certain Water Vapor Parameters Useful for Understanding Water Movement at a Plant-Air Interface"...

C. Chemical Potential and Water Potential of Water Vapor 84 2.4D. Plant-Air Interface 87... 

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