Transporting Organic Solutes in Phloem:

Theodor Hastig, a German botanist discovered the new cell kind in back of trees. He called them sieve tube members and supposed that they were conduits for moving sugars from leaves to roots. To test this he reasoned that if sieve tubes of back were cells by which nutrients moved, then removing the ring of back from tree trunk must cause nutrients to collect above so called girdle. This is exactly what occurred.

Hastig started other experiments which eventually related translocation with sieve tubes. He prepared the series of shallow cuts into sap oozed from the incisions. Hastig accomplished that organic solutes moved in sieve tubes. More current studies with radioactive tracers have established Hastig's conclusion: sugars and other organic substances move approximately fully in sieve tubes of phloem.

Structure of Conducting Cells:

Sieve tube members are cylinders connected by sieve like areas known as sieve plates, each of which has several sieve pores. Sieve pores may occupy more than 50% of area of sieve plates. Sieve pores were originally thought to be clogged; but studies plates must that sieve pores are open in functional sieve tubes. Sieve tubes in many plants are short-lived; they generally function only during season in which they are created. In these plants, sieve tubes are finally replaced by cells denied from vascular cambium. During periods of dormancy, sieve pores turn out to be clogged with callose and turn into non-functional. When growth resumes, callose is hydrolyzed, and sieve tubes again transport sugars. Products of callose hydrolysis are utilized as substrates for renewal of growth. Callose and P-protein are situated along border of functioning sieve tube members. Callose is quickly synthesized when sieve tubes are wounded. Callose plays pores of wounded sieve tubes and prevents loss of assimilated nutrients through wound. Likewise, P-protein quickly clogs pores of wounded sieve tubes and minimizes loss of sugars.

How Substances Move in the Phloem:

Many models for phloem transport have been suggested. Validity of the models is judged by the ability to describe and precisely forecast phloem transport such a model should also describe rate of phloem transport.

Rates of Phloem Transport:

Solutes move unexpectedly fact in phloem peak rates of transport may exceed 2mh-1. As much as 20 litres of sugary sapcan be collected per day from several stems of sugar palm. At cellular level, sieve element empties and fills every two seconds.

Models for Phloem Transport:

Hypothesis 1:

Solutes move by phloem using diffusion:

Diffusion is very slow to account for phloem transport in plants. Let the 10% sucrose solution connected to pan of pure water by a 1- meter tube with cross-sectional area of 1cm2: Diffusion of only 1 mg of sucrose to pan of pure water would need almost 3 years. Thus, mechanisms more rapid than diffusion should be involved in solute transport in phloem.

Hypothesis 2:

Solutes move by Phloem using Cytoplasmic Streaming Cytoplasmic streaming is very slow to account for phloem transport. Furthermore, streaming actually doesn't happen in mature sieve tubes. Thus, cytoplasm streaming alone can't account for movement of solutes in phloem.

Hypothesis 3:

Solutes move by Phloem using Pressure Flow:

This model was suggested by German plant physiologist, Ernst Munch in 1926. Model expresses that tugor-pressure gradient drives unidirectional mass flow of solutes and water by sieve tubes of phloem. According to the model, solutes move by sieve tubes along pressure gradient in manner like movement of water by a garden hose.

Four requirements should be fulfilled for Munch's model to work:

1. There should be osmotic gradient between two osmometers.

2. Selectively permeable membranes should be present to set up pressure a gradient.

3. There should be open channel between two osmometers to let flow

4. Surrounding medium should have water potential which exceeds (that is less negative than that of most negative osmometers).

Pressure-flow model is attractive as it describes source-to-sink movement in plants. Sources are sites where sugars are prepared by photosynthesis or hydrolysis of starch, they have large amounts of sugar, and solute concentration is high. Sinks are sites where sugars are utilized or stored are in solute starch. They have less sugar than do sources. Their solute concentration is comparatively low. Roots, active meristems and growing fruits are examples of sinks.

1. Sucrose made at the source is loaded in sieve tube

2. This loading decreases water potential that causes water to enter sieve tubes by osmosis.

3. Influx of water in sieve tube creates the pressure gradient which carries sucrose to sink, where it is unloaded.

4. Removing sucrose at sink increases water potential there, causing water to movement of sieve tube at sink.

5. Sucrose exiting sieve tube at sink returns to xylem and is re-circulated.

Ability of pressure-flow model to account for and exactly forecast feature of phloem transport makes it most widely accepted model for phloem transport.

Loading and Unloading the Phloem:

Phloem Loading:

Let chlorenchymes cell (that is a source) in leaf and cortical cell (i.e. a sink) in the root. Many chlorenchymes cells are 2-4 layers from vein. Therefore, sugars made in chlorenchymes cells should be transported across many others chlorenchyma cells before they can be loaded in sieve tube. Movement of solutes between adjacent chlorenchyma cells is symplastic and is improved by the several plasmodesmas which link the cells. That is symplastic transport accounts for movement of solutes to chlorenchyma cells boarding vein. There are no symplastic links between chlorenchyma cells and companion and sieve cells. Sugars, thus, move through cell wall (i.e. apoplast) before being loaded in sieve tubes. Sugars in cell wall are loaded in sieve tubes by companion cells that several frequently have several plasmodesmata and comprise structures similar to transfer cells. Cross section of leaf vein from common grounded (Senecio eu1goris) illustrating sieve elements and transfer cells. Wall ingrowths of transfer cells make large surface areas which are utilized to improve loading and unloading of sieve elements. Elaborate invaginations of cells wall and cell membrane of transfer cells give the large surface area for transporting sugars from cell wall in sieve tube. Loading of sieve tubes form apoplast needs metabolic energy and is driven indirectly by protein gradient generated at expense of ATP. Sugars and other solutes are loaded selectively in sieve tubes. Only those solutes which are transported will be loaded. For instance, sucrose is always present in sieve tubes, that glucose is hardly ever present. If veins are bathed in solution of sucrose and glucose, only sucrose will be loaded in sieve tubes.

Phloem Unloading:

Unloading of solutes from sieve tube members happens symplastically or apoplasticatly. In vegetative sinks which are growing like roots and young leaves, unloading is generally symplastic. In other sinks, unloading is apoplastic. Mechanism underlying phloem unloading may differ in different species.

Influence of Environment on Phloem Transport:

Many environmental factors affect phloem transport. One of them is light, that promotes photosynthesis and thus increases production of sucrose. As the result, increased light intensity usually helps transport to roots. Likewise, darkness stimulates translocation from roots to shoots.

Mineral deficiencies are also significant in phloem transport that is strongly affected by nutritional status of a plant. For instance, phloem transport is slow in boron-deficient plants; transport increases dramatically when the plants are supplied with boron. Potassium deficiencies also reduce phloem transport, most probably due to dependence of phloem loading on k+ uptake.

Contents of Sieve Tubes:

Most abundant compound in sieve tube is water that is significant since as much as half of waters in several fruits are delivered in sieve tubes. More than 90% of solutes in sieve tubes are carbohydrates. In many plants, these carbohydrates move largely as completely as sucrose. Concentration of sucrose may be as high as 30%, thus giving phloem sap syrupy thickness. Though, not all plants transport only sucrose; few plant families also transport others sugars like stachyose, raffinose, and verbascose. These sugars are similar and comprise sucrose, attached to one or more D-galactose units. Similar to sucrose, they are all no reducing sugars, that are less reactive and less labile to enzymatic breakdown than are reducing sugars like glucose and fructose.

Few plants also transport sugar alcohols in the phloem. For instance, apple and cherry trees transport sosbitol, and mannitol moves in phloem of ash (frxinus). While Biblical manna which was miraculously supplied to Israelites came from heaven, commercial manna (source of mannitol) is dried phloem-exudate of manna ash (frxinus omus) and related plants. Sieve tubes also have ATP and nitrogen containing compounds like amino acids, particularly during senescence of leaves and flowers. Sieve tubes also transport hormones, viruses, alkaloids, and inorganic ions, particularly k+.

Exchange between Phloem and the Xylem:

Contents of xylem and phloem are in aqueous equilibrium; i.e., they contain similar water potential. For instance, water entering loaded sieve tubes comes from xylem and water leaving unloaded sieve tubes returns to xylem and is re-circulated. Movement of water between xylem and phloem is rarely accompanied by exchange of solutes of the tissues.

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