Mesophyll resistance

Figure 2.21 schematically depicts the dry deposition of a pollutant to a typical surface in the form of resistances (Lovett, 1994 Wesely and Hicks, 1999). In this case, the surface resistance rsurf has been broken down even further into a combination of parallel and series resistances (rs, rm, rct, rsoil, rwa(cl, etc.). Since leaves may absorb pollutants either through stomata or through the cuticles, the absorption into the leaf is represented by two parallel resistances, rcl for the cuticular resistance and rs for the stomatal resistance, which is in series with a mesophyllic resistance rm. Also shown are resistances for uptake into the lower part of the plant canopy and into water, soil, or other surfaces. 

As indicated in Figure 8-8, five additional resistances are involved in CO2 movement compared to water vapor movement. The new components of the pathway are the nongaseous (i.e., liquid phase) parts of the cell wall of a mesophyll cell (resistance = r ), a plasma membrane (r ), the cytosol (r X the chloroplast limiting membranes (r ), and the interior of the chloroplasts (r ). For convenience we will divide these five resistances into two parts, the mesophyll resistance, and the chloroplast resistance,... 

Figure 5 A simplified cross-section in a leaf indicating the CO2 diffusion pathway between the atmosphere and the chloroplast. Changes in CO2 concentrations and the and values along this pathway are indicated for the atmosphere, c, the leaf boundary layer, Cb, substomatal internal air spaces, Cj, the chloroplast surface, Ci and the center of the chloroplast, c, are indicated (pathway between Cj and constitutes the internal, mesophyll, resistance to CO2 diffusion, inadequately constrained at present). Values for are based on of hquid water at the evaporating surfaces (near the chloroplast membranes) of +6.4%o. The arrows indicate the one-way CO2 fluxes into the leaf (controlled by cj, and retro-diffusion out of the leaf (controlled by and Ci in the case of and respectively). The difference between the two one-way fluxes constitutes CO2 assimilation into sugars (values for...

In Wesely s (1989) formulation what we have called the stomatal pore resistance rp is called just the stomatal resistance and given the symbol rst. As in Figure 19.4, it is assumed to act in series with the mesophyll resistance. 

Outside this range, the stomata are assumed to be closed and rst is set to a large value. The combined minimum stomatal and mesophyll resistance is calculated from... 

The foliar resistance consists again of two parallel resistances the cuticular resistance and the stomatal resistance which is in series with the mesophyll resistance r . 

Beardmore et al (30) and Tiburzy (31) showed that epidermal (31) and mesophyll (30,31) cells of resistant wheat plants, penetrated by haus-toria of an avirulent race of the fungus, can be stained with phlorogluci-nol/HCl (31) and chlorine/sulfite (30,31). These cells show yellow autofluorescence under UV-light (30,31), the emission spectrum is identical to that of lignified tracheary elements (31). 

The next figure (Figure 10) demonstrates that the intercellular mycelium in the treated tissue advanced to the mesophyll cells where it elicited a hypersensitive response within colonized, as well as in adjacent cells. The fungicide treatment induced in the susceptible wheat variety a reaction (hypersensitivity) which resembled that of a highly resistant host plant after rust attack, as described earlier for powdery mildew and barley. 

One can expect the same types of mechanisms, which can operate at other levels, to explain thresholds and their variability. The numbers and condition of the stomata will determine how much of the pollutant will reach the mesophyll cells of the leaf. Indeed, the reflex-type resistance (4) of the plant is attributed to the closure of stomata in response to 03, which thereby impedes further entrance of these pollutants. Young or old tobacco leaves are very resistant to oxidants. When young, a leafs resistance may be attributed to the density of the cells, which limits penetration of gases into the mesophyll. In the mature and aging leaf, suberization of the cell walls blocks the penetration of the pollutants (28). The cells of the leaf and their susceptibility to oxidants illustrate another aspect of the threshold and response—they are determined by the stage of development of the receptor, and different mechanisms operate at different times. 

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. 

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)... 

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. 

Figure 8-8. Principal conductances and resistances involved in the movement of C02 from the turbulent air surrounding a leaf, across the lower epidermis, and then to the enzymes involved in the fixation of C02 into photosynthetic products in the chloroplasts of mesophyll cells.

Consequently, C02 diffusing from the turbulent air up to the cell walls of mesophyll cells encounters a resistance that is 60% higher than does water vapor diffusing in the opposite direction over the same pathway (Eq. 8.20). Likewise, the gas phase conductance is (100%)/(1.60) or only 63% as great... 

The resistance to diffusion of a molecular species across a barrier equals the reciprocal of its permeability coefficient (Chapter 1, Section 1.4B). In this regard, we will let f COi be the permeability coefficient for CO2 diffusion across barrier j. To express the resistance of a particular mesophyll or chlo-roplast component on a leaf area basis, we must also incorporate Am sIA to allow for the actual area available for diffusion—the large internal leaf area acts like more pathways in parallel and thus reduces the effective resistance (Fig. 8-4). Because the area of the plasma membrane is about the same as that of the cell wall, and the chloroplasts generally occupy a single layer around the periphery of the cytosol (Figs. 1-1 and 8-11), the factor AmesIA applies to all of the diffusion steps of CO2 in mesophyll cells (all five individual resistances in Eq. 8.21). In other words, we are imagining for simplicity that the cell wall, the plasma membrane, the cytosol, and the chloroplasts are all in layers having essentially equal areas (Fig. 8-11). Thus, the resistance of any of the mesophyll or chloroplast components for CO2 diffusion,, is reduced from 1 /P co, by the reciprocal of the same factor, Ames/A ... 

Let us now estimate r. We will assume that the mesophyll cells have a typical cell wall thickness, Ax0 , of 0.3 pm, that the diffusion coefficient in the cell walls for CO2 or HCO3-, >ov s x 10-1° m2 s 1, and that Kq is 1. The magnitude of also depends on the relative surface area of the mesophyll cells compared with the leaf area. We will let Ames/A be 20, a reasonable value for mesophytes. Using Equation 8.25, the resistance of the cell walls to the diffusion of CO2 then is... 

This is a small value for a CO2 resistance (Table 8-4) and indicates that the cell walls of the mesophyll cells generally do not represent a major barrier to the diffusion of the various forms of CO2 into cells. 

We next examine rj, the resistance of the plasma membrane of mesophyll cells to the diffusion of the various forms of CO2. Although we do not know the actual permeability coefficient of the plasma membrane of mesophyll... 

HCO3- or CO2 could be actively transported across the plasma membrane or perhaps could cross by facilitated diffusion (Chapter 3, Section 3.4C). Facilitated diffusion would act as a low-resistance pathway in parallel with the ordinary diffusion pathway and consequently would reduce the effective resistance of the plasma membrane. Unfortunately, the actual mechanism for CO2 or HCC>3 movement across the plasma membrane of mesophyll cells is not known with certainty, although for diffusion of CO2 is low enough to account for the observed CO2 fluxes. 

Our analysis for CO2 fluxes could be carried out using conductances and mole fractions instead of resistances and concentrations. Also, we could divide the C02 pathway into a gas-phase component from the turbulent air up to the mesophyll cells and a liquid-phase component representing the mesophyll cells and their chloroplasts. The drop in CO2 mole fraction across the gas phase, ATVg, can be related to the CO2 conductance for the gas phase, gccv anc CO2 mole fractions as follows ... 

Brassinosteroids (BSs) represent a new group of plant hormones that possess a broad spectrum of physiological activities (1,2). A most Intriguing property of BSs Is their capacity to Increase stress resistance In plants, but the mechanism of such an antistress activity still remains unknown (1). As cell stress resistance Is usually associated with stress protein synthesis (3 4) our aim was to study the BS effect on protein synthesis and ultrastructure of wheat leaf cells at normal temperature and under heat shock conditions. We have also studied the Influence of BSs on mesophyll cell ultrastructure under saline stress. 

Rm = resistance of mesophyll cells containing mainly 3 components (compare Fig. 3) Rcw = cell wall, Rpi = plasmalemma and / Cyt = cytoplasma. Rm = Rqw + Rpi + Rcyt-/ Ch = resistance of chloroplasts containing 2 components Rqe = chloroplast envelope and Rstr = stroma including the photosynthetic apparatus of CO2 incorporation. 

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