Xylem sap

Some roots, however, display considerable ability to acclimate to subopti-mal temperatures. For example, in rye or barley acclimated by previous exposure to 8°C, net transport of K, Ca-, and H PO into the xylem sap of detached roots was enhanced by factors of 2 to 3 as compared to controls maintained at 20°C (193,194), with a threefold increase in flux of water to the xylem. These changes almost completely compensated for the smaller root systems that devel-... 

D. T. Clarkson. The influence of temperature on exudation of xylem sap from detached root systems of rye (Secaie cereale) and barley (Hordeum vidgare), Planta 752 297 (1976). 

D. T. Clark.son. M. G. T. Shone, and A. V. Wood, The effect of pre-treatment temperature on the exudation of xylem sap by detached barley root systems, Planta I213 (1974). 

Soar, C. J., J. Speirs et al. (2004). Gradients in stomatal conductance, xylem sap ABA and bulk leaf ABA along canes of Vitis vinifera cv. Shiraz Molecular and physiological studies investigating their source. Fund. Plant Biol. 31(6) 659-669. 

McCully ME. Root xylem embolisms and refilling. Relation to water potentials of soil, roots, and leaves, and osmotic potentials of root xylem sap. Plant Physiol 1999 119 1001-1008. 

In ordinary plants the evidence is for the involvement of anionic complexes of Cu and Ni for uptake and transport. This is generally taken as resulting from citrate rather than amino add complexation [as neutral bis(amino acidato)metal(II) complexes]. However, this conclusion ignores the presence of aspartic add, which is often found in exudates in relatively large amounts and would form bis-anionic spedes. A metal binding role for aspartic acid in xylem sap was proposed in some earlier work.37... 

While in the xylem, sap generally is considered to be sterile. Immediately upon being exposed to taphole conditions, it is acted upon by a wide variety of microorganisms, including bacteria, fungi, and yeasts. The major impact of this contamination on the syrup making process is the conversion of a small quantity of sucrose by invertase. 

A root-derived suppressor negatively regulating the formation of roots has been hypothesized.497 This proposed mechanism, termed root apical dominance, is analogous to the means for apical dominance in shoots. The cytokinin, tZR, in root xylem sap is hypothesized to play the role of the main suppressor, and tZR transported from roots to shoots via the transpiration stream negatively regulates the formation of adventitious roots.365... 

The movement of water and nutrients from the soil to the upper portions of a plant occurs primarily in the xylem. The xylem sap usually contains about 10 mol m-3 (10 mM)2 inorganic nutrients plus organic forms of nitrogen that are metabolically produced in the root. The xylem is a tissue of various cell types that we will consider in more detail in the final chapter (Section 9.4B,D), when water movement in plants is discussed quantitatively. The conducting cells in the xylem are the narrow, elongated tracheids and the vessel members (also called vessel elements), which tend to be shorter and wider than the tracheids. Vessel members are joined end-to-end in long... 

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.

A mobile, water-soluble portion-, it includes cations which are mobile in the xylem sap. Up to 10% of the total Cd belong to this portion, whereas no Pb was detected. 

The characterisation of binding partners/sites for cations in the xylem sap (liquid phase) as well as in walls of xylem vessels or tracheids (solid phase). 

Figure 3. Structures of the major nitrogenous constituents of the xylem sap of Ng fixing plants...

Dambrine, E., Martin, E, Carisey, N., Granier, A., Hallgren, J.-E., Bishop, K., 1995. Xylem sap composition A tool for investigating mineral uptake and cycling in adult spruce. Plant Soil 168-169, 233-241. 

Most of the sulfate obtained through the root system is transported in the xylem (Pate, 1965 Tolbert and Wiebe, 1955) to leaf tissue where it is reduced and assimilated into cysteine in light-dependent reactions (Schmidt and Trebst, 1969 Schwenn and Trebst, 1976 Trebst and Schmidt, 1969). Some sulfate reduction occurs in nonphotosynthetic tissue (Ellis, 1%3 Ferrari and Renosto, 1972) but this is of minor importance. In the field pea for example Pate (1965) found that only 0.2-0.4% of the S label of plants supplied with p S]sulfate via their root systems was present in xylem sap as organic sulfur. 

Photosynthetic tissue is a relatively more important site for the reduction of sulfate than nitrate. Nitrate, as a proportion of the total nitrogen in the xylem sap, can vary greatly depending on the species (Wallace and Pate, 1%7), the age of the plant and the amount of exogenous nitrate supplied (Wallace and Pate, 1965). 

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