Phloem Transport in Plants | Botany

In plant growth and development, materials are moved from the source (where they enter the plant or are synthesized) to the sink (where they are utilized). Inter-organ translocation in the plant is primarily through the vascu­lar system, the xylem and phloem. Movement in the xylem tissue is essentially a one-way acropetal (upward) movement from the roots via the transpiration stream.

In contrast, substances in the phloem have bidirectional movement; movement may be acropetal or basipetal (downward). Assimilate produced in leaves moves to sinks, while substances absorbed by roots move upward. In both xylem and phloem there are lateral connections, plasmodesmata, which allow some lateral movement.

The bulk of translocated substances, other than water are the result of photosynthesis or remobilization of assimilates in storage. This is indicated by the fact that 90% of the total solids in the phloem consists of carbohydrates, mostly non-reducing sugars (sugars without an exposed aldehyde or ketone group, e.g., sucrose and raffinose), which occur in phloem sap at the rather high concentrations of 10 to 25%. The predominant sugar translocated in the phloem of most crop species is sucrose; in some species it is the only one.

The phloem sap also contains nitrogenous substances, especially amino acids, amides, and urides, at concentrations of 0.03 to 0.4%. Extremely low quanti­ties of many other compounds are also translocated in the phloem, including many growth regulators, nucleotides, some inorganic nutrients, and systemic pesticides. However, many compounds, such as reducing sugars, contact her­bicides, proteins, most polysaccharides, calcium, iron, and most micronutrients, do not normally move in phloem.

The most widely proposed translocation mechanism is the mass flow or pressure flow hypothesis originally suggested by Munch (1930), which postu­lates that assimilate moves in a mass flow along a hydrostatic pressure gra­dient.

The active (metabolic) loading and unloading of assimilate in the source and sink regions, respectively, are responsible for differences in osmotic poten­tial in the sieve tubes in these regions. At the source, where sugars are produced, the phloem increases in sugar concentration. This re­duces the water potential in sieve tubes, which causes water to move into sieve tubes from surrounding tissue.

This, in turn, increases the hydrostatic pres­sure, causing mass flow of water and assimilates to areas of less pressure. At sinks the sugar concentration is reduced by sink utilization.

This removes sugars from the sieve tubes, which increases the water potential, and water moves in from the sieve tubes, which reduces the hydrostatic pressure in the tubes and thus results in a hydrostatic pressure gradient from source to sink. A presentation of the pressure flow hypothesis has recently been presented by Milburn (1975).

Translocation Rates:

The rate at which a compound is moved in the phloem can be affected by the rate of acceptance by sinks (phloem unloading), the chemical nature of the compound as it affects movement in phloem tissue, and the rate at which the source is moving the compound into sieve tube elements (phloem loading).

One way of measuring the translocation rate of assimilate is to allow leaves to photosynthesize ¹⁴CO₂ and measure the rate of ¹⁴C movement from the leaf. Velocity of front molecules with ¹⁴C have been measured at over 500 cm. hr^-1. However, when the bulk of assimilate is measured, velocities usually range 30-150 cm. hr^-1.

For yield, velocity is less important than specific mass transfer (SMT), which the weight is of assimilate moved per cross-sectional area of phloem per unit of time. SMTs measured for several species have been surprisingly similar, ranging 3-5 g. cm^-1. hr^-1. These observa­tions suggest that the cross-sectional phloem area might limit the translocation rate.

Phloem size seems to develop according to the size of the source or sink it is serving. For example, the cross-sectional area of phloem within the peduncle of modern wheat is greater than that of wheat ancestors and is correlated to greater translocation rates. The fact that larger leaves have a proportionally larger cross-sectional phloem area than do smaller leaves is specific for leaves of the same species and gener­ally true for leaves among species.

Although the cross-sectional phloem area is fairly uniform among plants, there seems to be more phloem tissue than is needed for adequate transloca­tion. In experiments in which the cross-sectional phloem area of peduncles was reduced by incision, the grain growth rate was not reduced in either wheat or sorghum.

In addition, when the cross-sectional phloem area of wheat roots was reduced the specific mass transfer (based on cross-sectional phloem area) increased more than 10 times. Considering these results, it seems unlikely that the volume of phloem tissue limits the flow from source to sink in most crops.

Different translocation rates occur among species, especially between the plants exhibiting C₄-type and C₃-type photosynthesis. Leaves of C₄ species have higher CO₂ exchange rates, a larger ratio of cross-sectional phloem area to leaf area, and greater translocation rates. Leaves of C₄ species also export a larger percentage of their assimilation within a few hours than do C₃ species.

This improved export of assimilate by leaves of C₄ species may be due to their specialized anatomy, in which vascular sheath cells have chloroplasts (Kranz anatomy), or the result of a greater cross-sectional phloem area. However, there is evidence to indicate that improved export might be related more to higher CO₂ exchange rates than to leaf anatomy.

Phloem Loading and Unloading:

Phloem loading (transfer of photosynthate from the mesophyll cells of the leaf to the phloem sieve tube elements) and phloem unloading (transfer of photosynthate from phloem sieve tube elements to the cells of a sink) can be rate limiting and can affect translocation. During phloem loading the meso­phyll cells are typically at a lower osmotic potential (higher water potential) than the sieve tube elements; thus phloem loading requires an energy input to move sugars into an area of higher concentration.

Phloem loading generates the increased osmotic potential in the sieve tube elements, supplying the driv­ing force for mass flow of assimilate. It consists of movement of sugars from symplast (mesophyll cells) into apoplast (cell walls) and then into symplast (phloem cells). When sugars move into sieve elements, the movement may be aided by adjacent companion cells.

The greater rate of movement in C₄ species may be due to the vascular sheath cells, which surround the veins in the leaf and have chloroplasts. Under illumination, chloroplasts can help provide photosynthetic energy (adenosine triphosphate, or ATP) needed for loading. It has also been sug­gested that under high leaf sucrose levels the bundle sheath cells might have a higher osmotic potential than adjacent sieve tubes to facilitate loading through a sugar concentration gradient.

At the other end of the translocation process, phloem unloading can also limit the rate at which a sink receives assimilate. Studies on unloading are scarce, so description is difficult. Some studies have shown that unloading is similar to loading in that the sugars move from the phloem symplast to the apoplast and then are transferred to the symplast of sink cells.

However, there are indications that unloading may occur by a direct symplast transfer from phloem cells to sink cells. Current indications are that unloading occurs by different mechanisms in different tissues and may vary with the developmental status of the sink.