5.1.5  Solute transport via transpiration

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Xylem sap is very dilute when compared to both phloem sap and the contents of cells lining the transport pathway and for this reason it is often regarded as virtually pure water. Indeed, the osmotic pressure of xylem sap rarely exceeds 0.1 MPa (about 40 mOsmol L–1). This dilute sap effectively transports large amounts of inorganic nutrients by mass flow (see Case study 5.1), providing the supply of water and nutrients to roots can be maintained. In a rapidly growing plant such as an annual herb, powerful mechanisms for concentrating essential ions in the roots (see Case study 4.1) combine with transpiration of water from leaves to sustain nutrient supply to shoots.

(a)  Composition of xylem sap

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While most solutes in xylem sap are inorganic ions (e.g. nitrate, potassium, magnesium and calcium), other important solutes are also present (Table 5.1). Organic molecules in xylem sap can be present in substantial concentrations, sugars reaching 5mM in maize xylem sap (Canny and McCully 1989). Many trees including eucalypts are host to boring insects at particular times of the year when sugar and nitrogen content of the sap is nutritionally valuable. Even though sugar concentrations in xylem sap are much lower than in phloem sap, the high nitrogen to sugar ratios and low osmotic pressures make it a good substrate for many predators. More extreme examples of the carbohydrate content of xylem sap are temperate deciduous trees such as maple which have traditionally been tapped to yield a sugary solution in the period prior to budburst. This indicates that xylem can be a conduit for carbon remobilisation in addition to its central role as a pathway for water and nutrient transport. In secondary tissues, rapid transfer of solutes into and out of the xylem is partly achieved through close association of living ray cells and xylem vessels (Figure 5.2c).

Other organic molecules act to transport inorganic nutrients to the shoots. Nitrate and ammonium are assimilated into organic forms, such as amino acids, in the roots of many plants. In legumes, nodules deliver an even wider selection of nitrogenous compounds to the xylem, including ureides and amides. These often constitute the dominant form of nitrogen reaching shoots and are therefore a major component of the sap. Other examples of complexed forms of inorganic nutrients in xylem sap are metal ions such as zinc, copper and iron which are almost exclusively chelated to organic acids.

Phytohormones are also found in xylem sap but often in concentrations several orders of magnitude lower than those considered necessary to elicit a physiological response. This does not preclude xylem sap as a critical source of these sub-stances because delivery of solutes in the xylem must also take into account rates of sap flow. Mass transfer of phytohormones such as abscisic acid (Jokhan et al. 1996) and cytokinins (Nooden et al. 1990) is significant even though delivered in a dilute solution (Section 9.1.2).

(b)  Modification of xylem contents

Xylem sap composition is highly variable, and modified according to requirements of shoot tissues. Toxic ions can be removed from sap and essential nutrients recycled intensively as described in Case study 5.1. Unidirectional flow of sap in the xylem stream (although not invariably in the same direction) contrasts with bidirectional flow in the phloem and the two consort to deliver resources to where they are most needed. In particular, transfer from xylem to phloem provides a means of diverting essential elements from the main transpiring surfaces, the older leaves, to growing tissues where they are required (see Case study 5.1).

Discovery of transfer cells (Pate and Gunning 1972) showed how these specialised cells differentiate in order to effect a rapid transfer of solutes, in this case into and out of the xylem stream (see Case study 4.2). Transfer cells lie adjacent to xylem vessels and are highly modified to carry out rapid ion exchange of xylem and phloem elements. While exchange can occur in both directions, concentrations of most nutrients and organic solutes are much lower in the dilute xylem sap than in adjacent phloem elements, requiring active transport mechanisms for xylem to phloem transfer (Table 5.1). Such energetically ‘uphill’ transport is thought to be energised by proton pumps in the membranes of transfer cells. Solutes such as potassium in cereals and amino acids in soybeans are known to be transferred to the phloem by this route, particularly through nodes in the stem.

Xylem parenchyma cells, which are believed to participate in loading of ions into xylem vessels in roots (Section 3.6.4(a)), also contribute to modification of ion levels along the xylem pathway. Amino acids, potassium, calcium, magnesium and even micronutrients such as molybdenum are exchanged between xylem vessels and the accompanying parenchyma cells to achieve a xylem sap composition appropriate to shoots. This process is species dependent. For example, molybdenum is sequestered in the roots of beans and participates in nodule function, whereas tomato has less capacity to withdraw molybdenum from xylem sap (Hecht-Buchholz 1973). In secondary tissues, rapid transfer of solutes into and out of the xylem is partly achieved through close association of living ray cells and xylem vessels (Figure 5.2c).

Even weaker associations between ions and cell walls of both xylem vessels and surrounding cells can retard the up-ward movement of inorganic ions like cadmium, copper, zinc and calcium. For ions such as copper, this is not desirable un-less toxic quantities are likely to reach the shoot during brief periods of oversupply. Retarded translocation of cadmium and other heavy metals, on the other hand, might be ad-vantageous to a plant by preventing a rapid supply that cannot be diluted by growth.

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