Intercellular air spaces

Diffusion of gases in the air surrounding and within leaves is necessary for both photosynthesis and transpiration. For instance, water vapor evaporating from the cell walls of mesophyll cells diffuses across the intercellular air spaces (Fig. 1-2) to reach the stomata and from there diffuses across an air boundary layer into the atmosphere (considered in detail in Chapter 8,... 

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. 

We will now show that the availability of water adjacent to that in wettable cell walls affects P0611 wal1 and the contact angle in the interstices. Suppose that pure water in mesophyll cell wall interstices 10 nm across is in equilibrium with water vapor in the intercellular air spaces where the relative humidity is 99%. As we calculated previously, VFWV for this relative humidity is -1.36 MPa at 20°C. Hence, at equilibrium the (pure) water in the cell wall interstices has a hydrostatic pressure of —1.36 MPa ( F = P - n -I- pwgh Eq. 2.13a). Using Equation 2.25 we can calculate the contact angle for which P can be -1.36 MPa for pores 5 nm in radius ... 

The resistances and the conductances that we will discuss in this section are those encountered by water vapor as it diffuses from the pores in the cell walls of mesophyll cells or from other sites of water evaporation into the turbulent air surrounding a leaf We will define these quantities for the intercellular air spaces, the stomata, the cuticle (see Fig. 1-2 for leaf anatomy), and the boundary layer next to a leaf (Fig. 7-6). As considered later in this chapter, CO2 diffuses across the same gaseous phase resistances or conductances as does water vapor and in addition across a number of other components in the liquid phases of mesophyll cells. 

Another conductance encountered by the diffusion of substance j in plant leaves is that of the intercellular air spaces, gjas. The irregular shape of these air spaces (Fig. 1-2), which usually account for about 30% of the leaf volume, makes gjas difficult to estimate accurately. However, the intercellular air spaces do act as an unstirred air layer across which substances must diffuse hence we will describe gjas by a relation similar to Equation 8.3 (gbl = Dj/8bl) ... 

The effective length Sias, including all the factors just enumerated, ranges from 100 pm to 1 mm for most leaves. Equation 8.6 indicates that the water vapor conductance across the intercellular air spaces then ranges from an upper value of... 

The H2O lost from a leaf during transpiration evaporates from the cell walls of mesophyll cells (Figs. 1-2 and 8-4), the inner sides of guard cells, and the adj acent subsidiary cells. If the cell walls were uniform and wet, then most of the water would evaporate from the immediate vicinity of the stomatal pores. However, the waxy material that occurs on the cell walls within a leaf, especially on guard cells and other nearby cells, causes much of the water to evaporate from the mesophyll cells in the leaf interior. We can imagine that the water vapor moves in the intercellular air spaces (area Aias) toward the leaf surface by diffusing down planar fronts of successively lower concentration. Our imaginary planar fronts are parallel to the leaf surface, so the direction for the fluxes is perpendicular to the leaf surface. When we reach the inner side of a stomatal pore, the area for diffusion is reduced from... 

Water vapor that evaporates from cell walls of mesophyll cells or the inner side of leaf epidermal cells (Fig. 1-2) diffuses through the intercellular air spaces to the stomata and then into the outside air. We have already introduced the four components involved—two are strictly anatomical (intercellular air spaces and cuticle), one depends on anatomy and yet responds to metabolic as well as environmental factors (stomata), and one depends on leaf morphology and wind speed (boundary layer). Figure 8-5 summarizes the symbols and arranges them into an electrical circuit. We will analyze resistances and conductances for these components, some of which occur in series (i.e., in a sequence) and some in parallel (i.e., as alternatives). 

The conductance gj and the resistance include all parts of the pathway from the site of water evaporation to the leaf epidermis. Water can evaporate at the air-water interfaces of mesophyll cells, at the inner side of epidermal cells (including guard cells), and even from cells of the vascular tissue in a leaf before diffusing in the tortuous pathways of the intercellular air spaces. The water generally has to cross a thin waxy layer on the cell walls of most cells within a leaf. After crossing the waxy layer, which can be up to 0.1 pm thick, the water vapor diffuses through the intercellular air spaces and then through the stomata (conductance = g, resistance = Fig. 8-5)... 

Because the water vapor conductance of the intercellular air spaces and the stomata in series is g g /fe +g ) (see Eq. 8-13a and Footnote 3), which in turn is in parallel with g, the water vapor cond uct ance of the leaf is... 

Let us represent the difference in water vapor concentration across the intercellular air spaces by AcJ, that across the stomatal pores by Ac, and that across the air boundary layer adjacent to the leaf surface by Ac. Then the overall drop in water vapor concentration from the cell walls where the water evaporates to the turbulent air surrounding a leaf, Ac Jal, is... 

The commonly used expression Vapor Pressure Deficit or VPD is the partial pressure of water vapor in the leaf intercellular air spaces, minus the partial pressure of water vapor in the turbulent air outside the boundary layer, P 0,. Often P af is calculated as the saturation water vapor partial pressure at the temperature of the leaf (for a leaf water potential of -1.4 MPa at 20°C, this leads to an error of only 1% in Pj. af Table 2-1). P a, equals the air relative humidity times the saturation water vapor partial pressure (P. w) at the air temperature (values of P , in kPa, which can be used to calculate P f and P 0, are given at the end of Appendix I). 

We will now reconsider the values of the various conductances affecting the diffusion of water vapor through the intercellular air spaces, out the stomata, and across the air boundary layer at the leaf surface. Usually gjas is relatively large, is rarely less than 500 mmol m-2 s-1, but g,s, is generally less than... 

We next consider the main function of a leaf, photosynthesis, in terms of the conductances and the resistances encountered by CO2 as it diffuses from the turbulent air, across the boundary layers next to the leaf surface, through the stomata, across the intercellular air spaces, into the mesophyll cells, and eventually into the chloroplasts. The situation is obviously more complex than the movement of water vapor during transpiration. Indeed, CO2 not only must diffuse across the same components encountered by water vapor moving in the opposite direction5 but also must cross the cell wall of a mesophyll cell, the plasma membrane, part of the cytosol, the membranes surrounding a chloroplast, and some of the chloroplast stroma. Resistances are easier to deal with than are conductances for the series of components involved in the pathway for CO2 movement, so we will specifically indicate the resistance of each component. 

The area of the mesophyll cell walls across which CO2 can diffuse is considerably larger than the surface area of the leaf (Figs. 1-2 and 8-4). For the constricting effect caused by the stomata, we used Ast/A, the fraction of the leaf surface area that is occupied by stomatal pores. Here we will use the ratio Am s/A to indicate the increase in area available for CO2 diffusion into cells within a leaf compared to the leaf surface area, where Ames is the total area of the cell walls of mesophyll cells that is exposed to the intercellular air spaces, and A is the area of one side of the same leaf. More conveniently, A mesM can refer to the internal and the external areas of a part of the leaf that is examined microscopically. 

Figure 8-11. Schematic cross section near the periphery of a mesophyll cell (Fig. 1-1), indicating the sequential anatomical components across which CO2 diffuses from the intercellular air spaces to the carboxylation enzymes in the chloroplast stroma.

We begin our discussion of the newly introduced C02 resistances by evaluating the components of r, the resistance encountered by C02 as it diffuses through the water-filled interstices between the cellulose microfibrils in the cell wall (Fig. 1-13). In this way, C02 moves from the interface with the intercellular air spaces on one side of the cell wall to the plasma membrane on the other side (Fig. 8-11). We will use an appropriate form of Equation 8.22 to describe this resistance ... 

As we have indicated, stomatal conductance has a greater influence on transpiration (Section 8.2G) than on photosynthesis (Section 8.4E), for which both gas-phase and liquid-phase conductances must be considered. For instance, transpiration increases more rapidly than photosynthesis with increases in gj so WUE then decreases (Eq. 8-39). Thus we need specific criteria to predict optimal stomatal behavior. Specifically, to maximize WUE, stomatal opening must be synchronized with the capability for CO2 fixation. As indicated above (Section 8.IB), stomatal opening can be regulated by the CO2 level in the intercellular air spaces, a decrease in caused by photosynthesis leading to an increase in, which then lets more... 

CO2 uptake by C4 plants is C02 saturated at a relatively low C02 level in the intercellular air spaces. For instance, an of 150 pmol mol-1 usually leads to over 90% of the maximum Jcoz for C4 plants, so increasing the ambient CO2 level above 350 pmol mol-1 usually has little effect on their quantum yield (Fig. 8-18). However, the quantum yield for CO2 fixation by C3 plants progressively increases as the ambient C02 level is raised, and at an... 

In the literature, plots of net C02 up take versus the C02 level in the intercellular air spaces are often referred to as A -c curves, where A stands for carbon assimilation (= Jco ancl ci refers to the C02 concentration in the intercellular air spaces, usually expressed as a mole fraction... 

E. What is gja in the two units in D if the effective path length in the intercellular air spaces equals the leaf thickness ... 

A. What is Ames/A if essentially the entire surface area of the mesophyll cells is exposed to the intercellular air spaces ... 

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