Phloem hydrostatic pressure

The apparent orientation of insect stylets to the vascular bundles caused by hydrogen ion concentrations was first reported by Fife and Frampton ( ). The leafhoppers fed mainly in the phloem tissue, which had a substantially higher pH than the surrounding plant cells. Other discriminating criteria for phloem-feeding insects have been indicated, including carbohydrate concentration (10) and positive hydrostatic pressure (11). 

It has been nearly a century and a half since Boussingault (1868) presented the hypothesis that the accumulation of assimilates in an illuminated leaf may be responsible for a reduction in the net photosynthetic rate of that leaf. According to the Munch hypothesis for phloem transport, the greater the sink strength, the greater the depression in solute concentration in the phloem at the sink. This increases the concentration differential between the source and sink, creating the hydrostatic pressure head that drives the system. 

Because of their rigid cell walls, large hydrostatic pressures can exist in plant cells, whereas hydrostatic pressures in animal cells generally are relatively small. Hydrostatic pressures are involved in plant support and also are important for the movement of water and solutes in the xylem and in the phloem. The effect of pressure on the chemical potential of water is expressed by the term VWP (see Eq. 2.4), where Vw is the partial molal volume of water and P is the hydrostatic pressure in the aqueous solution in excess of the ambient atmospheric pressure. The density of water is about 1000 kg m-3 (1 g cm-3) therefore, when 1 mol or 18.0 x 10-3 kg of water is added to water, the volume increases by 18.0 x 10-6 m3. Using the definition ofV,., as a partial derivative (see Eq. 2.6), we need to add only an infinitesimally small amount of water (dnw) and then observe the infinitesimal change in volume of the system (dV). We thus find that Vw for pure water is 18.0 x 10-6 m3 mol-1 (18.0 cm3 mol-1). Although Vw can be influenced by the solutes present, it is generally close to 18.0 x 10-6 m3 mol-1 for a dilute solution, a value that we will use for calculations in this book. 

In the current case, slightly more than half of the hydrostatic pressure drop along the phloem is necessary to overcome the resistance of the sieve plate pores. When the end walls are steeply inclined to the axis of the sieve element, the pores of the sieve plate can occupy an area that is greater than the cross section of the sieve tube. This causes Jv in the pores to be less than in the lumen and tends to reduce the resistance to flow in the phloem. 

The fact that the decrease in hydrostatic pressure, not the change in water potential, represents the driving force along the phloem deserves special emphasis. By Equation 3.39, Jv equals LP(AP — oAII), where the reflection coefficient a along the phloem is zero because no membranes intervene between sequential members of a sieve tube (Fig. 9-17a). Differences in the osmotic pressure at various locations in the phloem, which help... 

Figure 9-18. Idealized representation of xylem and phloem flow driven by gradients in hydrostatic pressure.

Such a large osmotic pressure, caused by the high concentrations of sucrose and other solutes, suggests that active transport is necessary at some stage to move certain photosynthetic products from leaf mesophyll cells to the sieve elements of the phloem. From the definition of water potential, = P — II + pwgh (Eq. 2.13a), we conclude that the hydrostatic pressure in the phloem of a leaf that is 10 m above the ground is... 

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