Although a portion of the carbon assimilated on a daily basis is retained by the leaf to support its continued growth and metabolism, the majority is exported out of the leaf to non-photosynthetic organs and tissues. There, it is either metabolized directly or placed in storage for retrieval and metabolism at a later time. The transport of photoassimilates over long distances is known as translocation. Translocation is the movement of assimilates from the source leaves to the sink where assimilate are either stored or metabolised. Translocation occurs in the vascular tissue called phloem.

The small veins of leaves and the primary vascular bundles of stems are often surrounded by a bundle sheath which consists of one or more layers of compactly arranged cells. In the vascular tissue of leaves, the bundle sheath surrounds the small veins all the way to their ends, isolating the veins from the intercellular spaces of the leaf.

Phloem translocation is a highly significant process that functions to ensure an efficient distribution of photosynthetic energy and carbon between organs throughout the organism. This is called carbon partitioning. The conversion of photoassimilates to either sucrose or starch is called carbon allocation. Phloem translocation is also important from an agricultural perspective because it plays a significant role in determining productivity, crop yield, and the effectiveness of applied herbicides and other xenobiotic chemicals.

Pathways of translocation

The two long-distance transport pathways—the phloem and the xylem—extend throughout the plant body. The phloem is generally found on the outer side of both primary and secondary vascular tissues. In plants with secondary growth the phloem constitutes the inner bark. The cells of the phloem that conduct sugars and other organic materials throughout the plant are called sieve elements. Sieve element is a comprehensive term that includes both the highly differentiated sieve tube elements typical of the angiosperms and the relatively unspecialized sieve cells of gymnosperms. In addition to sieve elements, the phloem tissue contains companion cells and parenchyma cells (which store and release food molecules). In some cases the phloem tissue also includes fibers and sclereids (for protection and strengthening of the tissue) and laticifers (latexcontaining cells). However, only the sieve elements are directly involved in translocation.

An experiment conducted by a plant anatomist M. Malpighi in which a ring of bark (containing phloem) from the wood (containing xylem) of young stems was removed by separating the two at the vascular cambium, a technique known as girdling provided the evidence that photossimilates are transported within the phloem. Because the woody xylem tissue remained intact, water and inorganic nutrients continued to move up to the leaves and the plant was able to survive for some time. Girdled plants (Fig. 7.1B), however, developed characteristic swellings of the bark in the region immediately above the girdle

The sieve tube elements of most angiosperms are rich in a phloem protein called P-protein (Clark et al. 1997). (In classical literature, P-protein was called slime.) P-protein is found in all dicots and in many monocots, and it is

Fig. 7.1. The results of girdling on woody stems. (A) The phloem tissue can be removed by separating the phloem (the bark) from the xylem (the wood) at the vascular cambium. (B) The girdle interrupts the downward flow of nutrients and hormones, resulting in a proliferation of tissue immediately above the girdle.

absent in gymnosperms. It occurs in several different forms (tubular, fibrillar, granular, and crystalline) depending on the species and maturity of the cell. In immature cells, P-protein is most evident as discrete bodies in the cytosol known as P-protein bodies. P-protein bodies may be spheroidal, spindle-shaped, or twisted and coiled. They generally disperse into tubular or fibrillar forms during cell maturation.

P-protein appears to function in sealing off damaged sieve elements by plugging up the sieve plate pores. Sieve tubes are under very high internal turgor pressure, and the sieve elements in a sieve tube are connected through open sieve plate pores. When a sieve tube is cut or punctured,

Companion cells aid the highly specialized sieve elements

Each sieve tube element is associated with one or more companion cells. The division of a single mother cell forms the sieve tube element and the companion cell. Numerous plasmodesmata penetrate the walls between sieve tube elements and their companion cells, suggesting a close functional relationship and a ready exchange of solutes between the two cells. The plasmodesmata are often complex and branched on the companion cell side.

Companion cells play a role in the transport of photosynthetic products from producing cells in mature leaves to the sieve elements in the minor (small) veins of the leaf. They are also thought to take over some of the critical metabolic functions, such as protein synthesis, that are reduced or lost during differentiation of the sieve elements (Bostwick et al. 1992). In addition, the numerous mitochondria in companion cells may supply energy as ATP to the sieve elements.

There are at least three different types of companion cells in the minor veins of mature, exporting leaves: “ordinary” companion cells, transfer cells, and intermediary cells. All three cell types have dense cytoplasm and abundant mitochondria

Ordinary companion cells have chloroplasts with well-developed thylakoids and a cell wall with a smooth inner surface. Of most significance, relatively few plasmodesmata connect this type of companion cell to any of the surrounding cells except its own sieve element. As a result, the symplast of the sieve element and its companion cell is relatively, if not entirely, symplastically isolated from that of surrounding cells.

Transfer cells are similar to ordinary companion cells, except for the development of finger-like wall ingrowths, particularly on the cell walls that face away from the sieve element. These wall ingrowths greatly increase the surface area of the plasma membrane, thus increasing the potential for solute transfer across the membrane.

Because of the scarcity of cytoplasmic connections to surrounding cells and the wall ingrowths in transfer cells, the ordinary companion cell and the transfer cell are thought to be specialized for taking up solutes from the apoplast or cell wall space. Xylem parenchyma cells can also be modified as transfer cells, probably serving to retrieve and reroute solutes moving in the xylem, which is also part of the apoplast.

Though ordinary companion cells and transfer cells are relatively isolated symplastically from surrounding cells, there are some plasmodesmata in the walls of these cells. The function of these plasmodesmata is not known. The fact that they are present indicates that they must have a function, and an important one, since the cost of having them is high: They are the avenues by which viruses become systemic in the plant. They are, however, difficult to study because they are so inaccessible.

Intermediary cells appear well suited for taking up solutes via cytoplasmic connections. Intermediary cells have numerous plasmodesmata connecting them to surrounding cells, particularly to the bundle sheath cells. Although the presence of many plasmodesmata connections to surrounding cells is their most characteristic feature, intermediary cells are also distinctive in having numerous small vacuoles, as well as poorly developed thylakoids and a lack of starch grains in the chloroplasts.

Patterns of translocation: Source to sink

Sap- the fluid contents of plant cells found in the phloem, is not translocated exclusively in either an upward or a downward direction, and translocation in the phloem is not defined with respect to gravity. Rather, sap is translocated from areas of supply, called sources, to areas of metabolism or storage, called sinks.

Sources include any exporting organs, typically mature leaves, that are capable of producing photosynthate in excess of their own needs. The term photosynthate refers to products of photosynthesis. Another type of source is a storage organ during the exporting phase of its development. For example, the storage root of the biennial wild beet (Beta maritima) is a sink during the growing season of the first year, when it accumulates sugars received from the source leaves. During the second growing season the same root becomes a source; the sugars are remobilized and utilized to produce a new shoot, which ultimately becomes reproductive.

Sinks include any non-photosynthetic organs of the plant and organs that do not produce enough photosynthetic products to support their own growth or storage needs. Roots, tubers, developing fruits, and immature leaves, which must import carbohydrate for normal development, are all examples of sink tissues. Both girdling and labelling studies support the source-to-sink pattern of translocation in the phloem.

Source-to-sink pathways follow anatomic and developmental patterns

Although the overall pattern of transport in the phloem can be stated simply as source-to-sink movement, the specific pathways involved are often more complex. Not all sources supply all sinks on a plant; rather, certain sources preferentially supply specific sinks. In the case of herbaceous plants, such as sugar beet and soybean, the following generalizations can be made.

Proximity. The proximity of the source to the sink is a significant factor. The upper mature leaves on a plant usually provide photosynthates to the growing shoot tip and young, immature leaves; the lower leaves supply predominantly the root system. Intermediate leaves export in both directions, bypassing the intervening mature leaves.

Development. The importance of various sinks may shift during plant development. Whereas the root and shoot apices are usually the major sinks during vegetative growth, fruits generally become the dominant sinks during reproductive development, particularly for adjacent and other nearby leaves.

Vascular connections. Source leaves preferentially supply sinks with which they have direct vascular connections. In the shoot system, for example, a given leaf is generally connected via the vascular system to other leaves directly above or below it on the stem. Such a vertical row of leaves is called an orthostichy. The number of internodes between leaves on the same orthostichy varies with the species.

Modification of translocation pathways. Interference with a translocation pathway by wounding or pruning can alter the patterns established by proximity and vascular connections that have been outlined here. In the absence of direct connections between source and sink, vascular interconnections, called anastomoses (singular anastomosis), can provide an alternative pathway. In sugar beet, for example, removing source leaves from one side of the plant can bring about crosstransfer of photosynthates to young leaves (sink leaves) on the pruned side. Removal of the lower source leaves on a plant can force the upper source leaves to translocate materials to the roots, and removal of the upper source leaves can force lower source leaves to translocate materials to the upper parts of the plant.

The plasticity of the translocation pathway depends on the extent of the interconnections between vascular bundles and thus on the species and organs studied. In some species the leaves on a branch with no fruits cannot transport photosynthate to the fruits on an adjacent defoliated branch. But in other plants, such as soybean (Glycine max), photosynthate is transferred readily from a partly defruited side to a partly defoliated side.

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Materials translocated in the phloem:

The main materials translocated in the phloem are sucrose, amino acids, hormones, and some inorganic ions.

Water is the most abundant substance transported in the phloem. Dissolved in the water are the translocated solutes, mainly carbohydrates. Sucrose is the sugar most commonly transported in sieve elements. There is always some sucrose in sieve element sap, and it can reach concentrations of 0.3 to 0.9 M.

Nitrogen is found in the phloem largely in amino acids and amides, especially glutamate and aspartate and their respective amides, glutamine and asparagine. Reported levels of amino acids and organic acids vary widely, even for the same species, but they are usually low compared with carbohydrates.

Almost all the endogenous plant hormones, including auxin, gibberellins, cytokinins, and abscisic acid, have been found in sieve elements. The long-distance transport of hormones is thought to occur at least partly in the sieve elements. Nucleotide phosphates and proteins have also been found in phloem sap.

Proteins found in the phloem include filamentous Pproteins (which are involved in the sealing of wounded sieve elements), protein kinases (protein phosphorylation), thioredoxin (disulfide reduction), ubiquitin (protein turnover), chaperones (protein folding), and protease inhibitors (protection of phloem proteins from degradation and defense against phloem-feeding insects) (Schobert et al. 1995; Yoo et al. 2000).

Inorganic solutes that move in the phloem include potassium, magnesium, phosphate, and chloride. In contrast, nitrate, calcium, sulfur, and iron are relatively immobile in the phloem.

Rates of movement

The rate of movement of materials in the sieve elements can be expressed in two ways: as velocity, the linear distance travelled per unit time, or as mass transfer rate, the quantity of material passing through a given cross section of phloem or sieve elements per unit time. Mass transfer rates based on the cross-sectional area of the sieve elements are preferred because the sieve elements are the conducting cells of the phloem. Values for mass transfer rate range from 1 to 15 g h–1 cm–2 of sieve elements.

In early publications reporting on rates of transport in the phloem, the units of velocity were centimeters per hour (cm h–1), and the units of mass transfer were grams per hour per square centimeter (g h–1 cm–2) of phloem or sieve elements. The currently preferred units (SI units) are meters (m) or millimeters (mm) for length, seconds (s) for time, and kilograms (kg) for mass.


The mechanism of phloem translocation in angiosperms is best explained by the pressure-flow model, which accounts for most of the experimental and structural data currently available.

Any comprehensive theory must take into account a number of factors. These include:

  1. the structure of sieve elements, including the presence of active cytoplasm, P-protein, and resistances imposed by sieve plates;
  2. observed rapid rates of translocation (50 to 250 cm hr-1) over long distances;
  3. translocation in different directions at the same time;
  4. the initial transfer of assimilate from leaf mesophyll cells into sieve elements of the leaf minor veins (called phloem loading); and
  5. final transfer of assimilate out of the sieve elements into target cells (called phloem unloading).

In early research on phloem translocation, both active and passive mechanisms were considered. All theories, both active and passive, assume an energy requirement in both sources and sinks. In sources, energy is necessary to move photosynthate from producing cells into the sieve elements. This movement of photosynthate is called phloem loading. In sinks, energy is essential for some aspects of movement from sieve elements to sink cells, which store or metabolize the sugar. This movement of photosynthate from sieve elements to sink cells is called phloem unloading and will also be discussed later. The passive mechanisms of phloem transport further assume that energy is required in the sieve elements of the path between sources and sinks simply to maintain structures such as the cell plasma membrane and to recover sugars lost from the phloem by leakage. The pressure-flow model is an example of a passive mechanism. The active theories, on the other hand, postulate an additional expenditure of energy by path sieve elements in order to drive translocation itself (Zimmermann and Milburn 1975).

Diffusion is far too slow to account for the velocities of solute movement observed in the phloem. Translocation velocities average 1 m h–1; the rate of diffusion is 1 m per 32 years! The pressure-flow model, first proposed by Ernst Münch in 1930, states that a flow of solution in the sieve elements is driven by an osmotically generated pressure gradient between source and sink (∆Ψp). The pressure gradient is established as a consequence of phloem loading at the source and phloem unloading at the sink.

Ψw = Ψs + Ψp; that is, Ψp = Ψw – Ψs. In source tissues, energy-driven phloem loading leads to an accumulation of sugars in the sieve elements, generating a low (negative) solute potential (∆Ψs) and causing a steep drop in the water potential (∆Ψw). In response to the water potential gradient, water enters the sieve elements and causes the turgor pressure (Ψp) to increase.

At the receiving end of the translocation pathway, phloem unloading leads to a lower sugar concentration in the sieve elements, generating a higher (more positive) solute potential in the sieve elements of sink tissues. As the water potential of the phloem rises above that of the xylem, water tends to leave the phloem in response to the water potential gradient, causing a decrease in turgor pressure in the sieve elements of the sink.

Fig. 7.2. Pressure-flow model of translocation in the phloem. Possible values for Ψw, Ψp, and Ψs in the

xylem and phloem are illustrated. (After Nobel 1991.)


If no cross-walls were present in the translocation pathway—that is, if the entire pathway were a single membrane-enclosed compartment—the different pressures at the source and sink would rapidly equilibrate. The presence of sieve plates greatly increases the resistance along the pathway and results in the generation and maintenance of a substantial pressure gradient in the sieve elements between source and sink. The sieve element contents are physically pushed along the translocation pathway as a bulk flow, much like water flowing through a garden hose. Close inspection of the water potential values shows that water in the phloem is moving against a water potential gradient from source to sink.

The predictions of the pressure-flow model have been confirmed some important predictions emerge from the pressure-flow model:

  • The sieve plate pores must be unobstructed. If P-protein or other materials blocked the pores, the resistance to flow of the sieve element sap would be too great.
  • True bidirectional transport (i.e., simultaneous transport in both directions) in a single sieve element cannot occur.
  • Great expenditures of energy are not required in order to drive translocation in the tissues along the path, although energy is required to maintain the structure of the sieve elements and to reload any sugars lost to the apoplast by leakage. Therefore, treatments that restrict the supply of ATP in the path, such as low temperature, anoxia, and metabolic inhibitors, should not stop translocation.
  • The pressure-flow hypothesis demands the presence of a positive pressure gradient. Turgor pressure must be higher in sieve elements of sources than in sieve elements of sinks, and the pressure difference must be large enough to overcome the resistance of the pathway and to maintain flow at the observed velocities. The available evidence testing these predictions supports the pressure-flow hypothesis.

Phloem loading: from chloroplasts to sieve elements

Several transport steps are involved in the movement of photosynthates from the mesophyll chloroplasts to the sieve elements of mature leaves, which is called phloem loading (Oparka and van Bel 1992):

  1. Triose phosphate formed by photosynthesis during the day is transported from the chloroplast to the cytosol, where it is converted to sucrose. During the night, carbon from stored starch exits the chloroplast probably in the form of glucose and is converted to sucrose. (Other transport sugars are later synthesized from sucrose in some species.)
  2. Sucrose moves from the mesophyll cell to the vicinity of the sieve elements in the smallest veins of the leaf. This short-distance transport pathway usually covers a distance of only two or three cell diameters.
  3. In a process called sieve element loading, sugars are transported into the sieve elements and companion cells. In most of the species studied so far, sugars become more concentrated in the sieve elements and companion cells than in the mesophyll. Note that with respect to loading, the sieve elements and companion cells are often considered a functional unit, called the sieve element–companion cell complex. Once inside the sieve elements, sucrose and other solutes are translocated away from the source, a process known as export. Translocation through the vascular system to the sink is referred to as long-distance transport.

In many ways the events in sink tissues are simply the reverse of the events in sources. Transport into sink organs, such as developing roots, tubers, and reproductive structures, is termed import. The following steps are involved in the import of sugars into sink cells.

  1. Sieve element unloading. This is the process by which imported sugars leave the sieve elements of sink tissues.
  2. Short-distance transport. After sieve element unloading, the sugars are transported to cells in the sink by means of a short-distance transport pathway. This pathway has also been called post–sieve element transport.
  3. Storage and metabolism. In the final step, sugars are stored or metabolized in sink cells. These three transport steps together constitute phloem unloading, the movement of photosynthates from the sieve elements and their distribution to the sink cells that store or metabolize them (Oparka and van Bel 1992)

Fig. 7.3. Schematic diagram of pathways of phloem loading in source


Photosynthates allocation and partitioning

The photosynthetic rate determines the total amount of fixed carbon available to the leaf. However, the amount of fixed carbon available for translocation depends on subsequent metabolic events. The regulation of the diversion of fixed carbon into the various metabolic pathways is termed allocation.

The vascular bundles in a plant form a system of pipes that can direct the flow of photosynthates to various sinks: young leaves, stems, roots, fruits, or seeds. However, the vascular system is often highly interconnected, forming an open network that allows source leaves to communicate with multiple sinks. Under these conditions, what determines the volume of flow to any given sink? The differential distribution of photosynthates within the plant is termed partitioning.


Sink strength is a function of sink size and sink activity

Various experiments indicate that the ability of a sink to mobilize photosynthate toward itself, the sink strength, depends on two factors—sink size and sink activity—as follows:

Sink strength = sink size × sink activity Sink size is the total weight of the sink tissue, and sink activity is the rate of uptake of photosynthates per unit weight of sink tissue. Altering either the size or the activity of the sink results in changes in translocation patterns. For example, the ability of a pea pod to import carbon depends on the dry weight of that pod as a proportion of the total number of pods (Jeuffroy and Warembourg 1991).

Changes in sink activity can be complex because various activities in sink tissues can potentially limit the rate of uptake by the sink. These activities include unloading from the sieve elements, metabolism in the cell wall, uptake from the apoplast, and metabolic processes that use the photosynthate in either growth or storage.

Cooling a sink tissue inhibits activities that require metabolic energy and results in a decrease in the speed of transport toward the sink. In corn, a mutant that has a defective enzyme for starch synthesis in the kernels transports less material to the kernels than does its normal counterpart (Koch et al. 1982). In this mutant, a deficiency in photosynthate storage leads to an inhibition of transport. Sink activity and thus sink strength are also thought to be related to the presence and activity of the sucrose-splitting enzymes acid invertase and sucrose synthase because they catalyze the first step in sucrose utilization. Whether these enzymes control sink strength or are simply correlated with sink metabolism and growth is currently an active topic of research.






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