Xylem pressure potentials

Additionally, at the stress site, about a third of the area beneath each tree was trenched to cut roots In an effort to further maximize water stress. Xylem water potentials were measured using a Scholander pressure bomb when the larvae were In their 5th Instar. Budworm larvae were placed on each of the trees at the sites as described above for the Barley Canyon study. Terpenes and nitrogen were analyzed as described above. Polyphenols and protein complexlng capacity were measured as described for the fontana site. 

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. 

Figure 2-15. Relation between the reciprocal of leaf water potential determined with a pressure chamber (Fig. 2-10) and the volume of xylem sap extruded as the air pressure in the chamber is progressively increased. The solid line indicates a typical range for data points for material initially at full turgor (TJ, = 0). The reciprocal of the internal osmotic pressure (1/n1) including the value at full turgor (1 /nj,), the internal hydrostatic pressure (P1), the point of incipient plasmolysis and turgor loss, and the volume of symplastic water (VSympiasm) can all be determined from such a P-V curve.

Many solute properties are intertwined with those of the ubiquitous solvent, water. For example, the osmotic pressure term in the chemical potential of water is due mainly to the decrease of the water activity caused by solutes (RT In aw = —V ri Eq. 2.7). The movement of water through the soil to a root and then to its xylem can influence the entry of dissolved nutrients, and the subsequent distribution of these nutrients throughout the plant depends on water movement in the xylem (and the phloem in some cases). In contrast to water, however, solute molecules can carry a net positive or negative electrical charge. For such charged particles, the electrical term must be included in their chemical potential. This leads to a consideration of electrical phenomena in general and an interpretation of the electrical potential differences across membranes in particular. Whether an observed ionic flux of some species into or out of a cell can be accounted for by the passive process of diffusion depends on the differences in both the concentration of that species and the electrical potential between the inside and the outside of the cell. Ions can also be actively transported across membranes, in which case metabolic energy is involved. 

Water enters and leaves the phloem by passively moving toward regions of lower water potential ( P = P - n + pwgh Eq. 2.13a). The conducting cells of the xylem generally have a low and relatively constant osmotic pressure (here 0.1 MPa). Solutes either diffuse or are actively transported into and out of the sieve elements, leading to a high phloem osmotic pressure of 1.7 MPa in the leaf and a decrease to 0.7 MPa in the root the much lower n in the sink leads to a lower P there, which favors the delivery of more solutes. 

Water is conducted to and across the leaves in the xylem. It then moves to the individual leaf cells by flowing partly apoplastically in the cell walls and partly symplastically (only short distances are involved, because the xylem ramifies extensively in a leaf). The water potential is usually about the same in the vacuole, the cytosol, and the cell wall of a particular mesophyll cell (see values in Table 9-3). If this were not the case, water would redistribute by flowing energetically downhill toward lower water potentials. The water in the cell wall pores is in contact with air, where evaporation can take place, leading to a flow along the cell wall interstices to replace the lost water. This flow can be approximately described by Poiseuille s law (Eq. 9.11), which indicates that a (very small) hydrostatic pressure decrease exists across such cell walls. 

Osmotic pressures in the wall space are believed to explain the observation that nontranspiring plants have negative water potentials. Cosgrove and Cleland conclude that the internal gradient in water potential from the xylem to the epidermis, which sustains cell enlargement, is small. Auxin does not alter the hydraulic conductance of stem tissue either at the cellular or whole tissue level. How extracellular nitrogenous compounds contribute to or modify this response remains unknown. 

Stem potential is measured during the day, on a leaf that has been covered by an opaque, airtight bag for at least one hour before the measurement is made. The leaf stoma close in the dark and the leaf potential balances with that of the xylem in the stem. This measurement gives a close approximation of the water supply of the whole plant during the day. Provided certain conditions are observed (measuring time and weather conditions), stem potential is the most accurate of the three pressure chamber applications (Chone et al, 2001a,b). 

The xylem sap differs in composition and is often more concentrated than the soil solution. This is easy to understand if there is selective secretion of ions into the xylem from the stelar parenchyma. However, selective uptake of ions mediated by the root cortical cells, for example preferential uptake of into the cytoplasm by a system 1 mechanism (see p. 223) with further enhancement of the K tNa" ratio in the symplast by a system 2 secretion into the vacuoles could result in selective supply of ions from the root to the shoot even if the release of ions into the xylem was itself passive and non-selective. From the region where ions enter the xylem they are transferred by a mass flow of sap maintained by transpiration or induced by the difference in osmotic potential between xylem sap and soil solution (root pressure). 

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