Chapter 6 Lecture Notes---I’d copy to word and print 2 pages to 1 paper….

Land colonization

• Prompted greater shoot growth to reach and compete for sunlight

• Prompted development of a deeper root system

• SEPARATES PHOTOSYNTHESIZING REGIONS FROM AREAS WHERE SUGARS ARE USED

• REQUIRES A DRIVING FORCE FOR THIS LONG-DISTANCE TRANSPORT

Phloem transport

• A highly specialized process for redistributing:

– Photosynthesis products

– Other organic compounds (metabolites, hormones)

– some mineral nutrients

• Redistributed from

– SOURCE SINK

Phloem transport: Sources and sinks

• Source:

– Any exporting region that produces photosynthate above and beyond that of its own needs (leaf)

• Sink:

– any non-photosynthetic organ or an organ that does not produce enough photosynthate to meets its own needs (root)

Source-sink pathways follow patterns

• Although the overall pattern of transport can be stated as source to sink

• Not all sources supply all sinks in a plant

• Certain sources preferentially supply specific sinks

• In the case of herbaceous plant, such as Sugar-beet, the following occurs:

Source-sink pathways follow patterns

• Proximity: – of source to sink is a significant factor.

– Upper nature leaves usually provide photosynthesis products to growing shoot tip and young, immature leaves

– Lower leaves supply predominantly the root system

– Intermediate leaves export in both directions

Source-sink pathways follow patterns

• Development: – Importance of various sinks may shift during plant development

– Roots and shoots major sinks during vegetative growth

– But fruits become dominant sinks during reproductive development

Source-sink pathways follow patterns

• Vascular connections: –Source leaves preferentially supply sinks with direct vascular connections

– A given leaf is connected via vascular system to leaves above and below it on the stem

• Modifications of translocation pathways: - Interference with a translocation pathway by mechanical wounding (or pruning)

– vascular interconnections can provide alternate pathways for phloem transport

Phloem Structure

• In the phloem, sucrose, other organic compounds, and some mineral ions move through tubes formed by chains of cells, sieve-tube members.

– These are alive at functional maturity, although they lack the nucleus, ribosomes, and a distinct vacuole.

– The end walls, the sieve plates, have pores that presumably facilitate the flow of fluid between cells.

– A nonconducting nucleated companion cell, connected to the sieve-tube member, may assist the sieve-tube cell.

Phloem Structure

•SIEVE ELEMENTS ARE THE PRINCIPAL CELLULAR CONSTITUENTS OF THE PHLOEM

•The distinguishing feature of phloem tissue is the conducting cell called the sieve element.

• Also known as a sieve tube, the sieve element is an elongated rank of individual cells, called sieve-tube members, arranged end-to-end (Fig. 6.8).

•Unlike xylem tracheary elements, phloem sieve elements lack rigid walls and contain living protoplasts when mature and functional.

•The protoplasts of contiguous sieve elements are interconnected through specialized sieve areas in adjacent walls.

•Where the pores of the sieve area are relatively large and are found grouped in a specific area, they are known as sieve plates (Fig. 6.9).

Types of sieve elements

•Sieve plates are typically found in the end walls of sieve-tube members and provide a high degree of protoplasmic continuity between consecutive sieve-tube members.

•Additional pores are found in sieve areas located in lateral walls.

•These are generally smaller and are not, as a rule, grouped in distinct areas.

• These sieve areas nonetheless provide cytoplasmic continuity through the lateral walls of adjacent sieve elements.

•As noted earlier, mature sieve elements contain active cytoplasm.

•However, as the sieve element matures, it undergoes a series of progressive changes that result in the breakdown and loss of the nucleus, the vacuolar membrane (or tonoplast), ribosomes, the Golgi apparatus (or dictyosomes), as well as microtubules and filaments.

•At maturity, the cells retain the plasmalemma, and endoplasmic reticulum (although it is somewhat modified), and mitochondria.

•In addition to sieve elements, phloem tissue also contains a variety of parenchyma cells.

•Some of these cells are intimately associated with the sieve-tube members and for this reason are called companion cells.

•Companion cells contain a full complement of cytoplasm and cellular organelles.

•A companion cell is derived from the same mother cell as its associated sieve-tube member and shares numerous cytoplasmic connections with it.

•The interdependence of the sieve tube member and companion cell is reflected in their lifetimes-the companion cell remains alive only so long as the sieve-tube member continues to function.

• When the sieve-tube member dies, its associated companion cell also dies.

• Companion cells are believed to provide metabolic support for the sieve-tube member and, perhaps, are involved in the transport of sucrose or other sugars into the sieve tube.

•The rest of the phloem parenchyma cells are not always readily distinguishable from companion cells, even at the ultrastructural level.

•The single exception is found in the minor leaf veins of some plants, typically of herbaceous dicotyledonous plants.

•Here certain phloem parenchyma cells develop extensive ingrowths of the cell wall.

•The result is a significant increase in the surface area of the plasma membrane.

•These cells are called transfer cells.

• The precise role of transfer cells is not understood but, as the name implies, they are thought to be involved in collecting and passing on photoassimilates produced in nearby mesophyll cells.

Types of companion cells

• Ordinary Companion cells:

– Chloroplasts with well developed thylakoids, smooth inner cell wall, relatively few plasmodesmata.

• Connected only to it’s own sieve plate

• Transfer cells:

– Well developed thylakoids

– Have fingerlike cell wall ingrowths –increase surface area of plasma membrane for better solute transfer.

• Both of these types are specialized for taking up solutes from apoplast or cell wall space

Types of companion cells

• Intermediary cells:

– Appear well suited for taking up solutes via cytoplasmic connections

– Have many plasmodesmata connects to surrounding cells

• Most characteristic feature

– Contain many small vacuoles

– Lack starch grains in chloroplast

– Poorly developed thylakoids

• Function in symplastic transport of sugars from mesophyll cells to sieve elements where no apoplast pathway exists

Phloem transport requires
specialized, living cells

• Sieve tubes elements join to form continuous tube

• Pores in sieve plate between sieve tube elements are open channels for transport

• Each sieve tube element is associated with one or more companion cells.

– Many plasmodesmata penetrate walls between sieve tube elements and companion cells

– Close relationship, have a ready exchange of solutes between the two cells

Phloem transport requires
specialized, living cells

• Companion cells:

– Role in transport of photosynthesis products from producing cells in mature leaves to sieve plates of the small vein of the leaf

– Synthesis of the various proteins used in the phloem

– Contain many, many mitochondria for cellular respiration to provide the cellular energy required for active transport

– There ate three types

• Ordinary companion cells

• Transfer cells

• Intermediary cells

Protective mechanisms in phloem

• Sieve elements are under high internal turgor pressure

– When damaged the release of pressure causes the contents of sieve elements to surge towards the damage site

• Plant could lose too much of the hard worked for sugars if not fixed

• Damaged is caused by

– Insects feeding on manufactured sugars

– Wind damage, temperature (hot and cold)

– Pollution causing a change in light wavelength

The mechanism of phloem transport

The Pressure-Flow Model

•Originally proposed by E. Munch in 1930, the pressure-flow hypothesis remains the simplest model and continues to earn widespread support among plant physiologists.

• The pressure-flow mechanism is based on the mass transfer of solute from source to sink along a hydrostatic (turgor) pressure gradient (Fig. 6.10).

• Translocation of solute in the phloem is closely linked to the flow of water in the transpiration stream and a continuous recirculation of water in the plant.

The Pressure-Flow Model

Translocation is thought to move at 1 meter per hour

– Diffusion too slow for this speed

• The flow is driven by an osmotically generated pressure gradient between the source and the sink.

• Source

– Sugars (red dots) is actively loaded into the sieve element-companion cell complex

• Called phloem loading

• Sink

– Sugars are unloaded

• Called phloem unloading

The Pressure
-Flow Model

• yw = ys + yp + yg

• In source tissue, energy driven phloem loading leads to a buildup of sugars

– Makes low (-ve) solute potential

– Causes a steep drop in water potential

– In response to this new water potential gradient, water enters sieve elements from xylem

• Thus phlem turgor pressure increases

• In sink tissue, phloem unloading leads to lower sugar conc.

– Makes a higher (+ve) solute potential

– Water potential increases

– Water leaves phloem and enters sink sieve elements and xylem

• Thus phloem turgor pressure decreases

The Pressure-Flow Model

• So, the translocation pathway has cross walls

– Allow water to move from xylem to phloem and back again

– If absent- pressure difference from source to sink would quickly equilibrate

• Water is moving in the phloem by Bulk Flow

– No membranes are crossed from one sieve tube to another

– Solutes are moving at the same rate as the water

• Water movement is driven by pressure gradient and NOT water potential gradient

•Assimilate translocation begins with the loading of sugars into sieve elements at the source.

•Typically, loading would occur in the minor veins of a leaf, close to a photosynthetic mesophyll or bundle-sheath cell.

•The increased solute concentration in the sieve element lowers its water potential and, consequently, is accompanied by the osmotic uptake of water from the nearby xylem.

•This establishes a higher turgor or hydrostatic pressure in the sieve element at the source end.

•At the same time, sugar is unloaded at the sink end-a root or stem storage cell, for example.

• The hydrostatic pressure at the sink end is lowered as water leaves the sieve elements and returns to the xylem.

•So long as assimilates continue to be loaded at the source and unloaded at the sink, this pressure differential will be maintained, water will continue to move in at the source and out at the sink, and assimilate will be carried passively along.

•According to the pressure-flow hypothesis, solute translocation in the phloem is fundamentally a passive process; that is, translocation requires no direct input of metabolic energy to make it function.

•Yet for years it has been observed that translocation of assimilates was sensitive to metabolic inhibitors, temperature, and other conditions, suggesting that metabolic energy was required.

•But the effects of low temperature and metabolic inhibitors are either transient or cause disruption of the P-protein and plug the sieve plates.

•Energy requirements for translocation within the sieve elements are therefore minimal and compatible with the passive character of the pressure-flow hypothesis.

•

•Another question that is frequently raised in discussions of the pressure-flow hypothesis is that of bidirectional transport.

•The translocation of assimilates simultaneously in opposite directions would at first seem incompatible with the pressure-flow hypothesis, but it does occur.

•

•Bidirectional transport makes sense.

•At any one time, plants will likely have more than one sink being served by the same source-roots for metabolism and storage and developing apical meristems or flowers, for example.

• The pressure flow model explains why phloem sap always flows from sugar source to sugar sink, regardless of their locations in the plant.

• Researchers have devised several experiments to test this model, including an innovative experiment that exploits natural phloem probes: aphids that feed on phloem sap.

•PHOTOASSIMILATES ARE TRANSLOCATED OVER LONG DISTANCES

•An analysis of phloem exudate provides more direct evidence in support of the conclusion that photoassimilates are translocated through the phloem.

• Unfortunately, phloem tissue does not lend itself to analysis as easily as xylem tissue does.

•This is because the translocating elements in the phloem are, unlike xylem vessels and tracheids, living cells when functional.

These cells contain a dense, metabolically active cytoplasm and, because of an inherent sealing action of its cytoplasm, do not exude their contents as readily as do xylem vessels.

•Moreover, phloem contains numerous parenchyma cells that, while not directly involved in the transport process, do provide contaminating cytoplasm.

•Cutting the stems of some herbaceous plants will produce an exudate of largely phloem origin, but in some plants, the exudate may quickly gel on contact with oxygen, making collection and subsequent analysis difficult.

In spite of these difficulties, however, numerous investigators have successfully completed analyses of phloem exudates obtained by making incisions into the phloem tissue, assisted in part by the development of modern analytical techniques applicable to very small samples.

One intriguing solution to the problem of obtaining the contents of sieve tubes uncontaminated by other cells was provided by insect physiologists studying the nutrition of aphids.

•Aphids are one of several groups of small insects that feed on plants by inserting a long mouthpart (the stylus) directly into individual sieve tubes.

•When feeding aphids are anaesthetized with a stream of carbon dioxide and the stylus carefully severed with a razor blade, phloem sap continues to exude from the cut stylus for several days.

•The aphid technique works well for a number of herbaceous plants and some woody shrubs, but it is restricted to those plants on which the aphids naturally feed.

•The principal advantage of this technique is that the severed aphid stylet delivers an uncontaminated sieve tube sap.

•Although the volumes delivered are relatively low, this technique has proven extremely useful in studies of phloem transport.

•The continued exudation, incidentally, demonstrates that phloem sap is under pressure, an important observation with respect to the proposed mechanism for phloem transport to be discussed later.

• The closer the aphid’s stylet is to a sugar source, the faster the sap will flow out and the greater its sugar concentration.

•Another line of evidence involves the use of radioactive tracers, predominantly 14C and usually fed to a leaf.

• A typical example is the translocation of photoassimilate in petioles of sugarbeet (Beta vulgaris) leaves.

•In these experiments, attached leaves were allowed to photosynthesize in a closed chamber containing a radioactive carbon source (14C02).

• After 10 minutes, the radiolabeled photoassimilate being transported out of

the leaf was immobilized by freezing the petiole in liquid nitrogen.

Cross sections of the frozen petiole were prepared and placed in contact with X-ray film.

•The resulting image on the X-ray film indicated that the radioactive photoassimilate being translocated out of the leaf was localized exclusively in the phloem (Fig. 6.6).

• Similar experiments have been conducted on a variety of herbaceous and woody plants and with other radioactive nuclides, such as phosphorous and sulphur, with the same conclusion-the translocation of photoassimilates and other organic compounds over long distances occurs through the phloem tissue.

•There are exceptions to this rule, such as when stored sugars are mobilized in the spring of the year and translocated through the xylem to the developing buds.

• Translocation - Distribution of nutrients, especially carbohydrates, through the phloem – “source and sink”

– Osmosis plays an important role

– Phloem loading - Sucrose is actively loaded into phloem tubes

– Water moves in and carries sucrose along passively

– No energy required for this process - but

– Loading and unloading sucrose in phloem tubes does require energy

– Companion cells of phloem provide the ATP

Phloem Transport is Bidirectional

• Translocation - distribution of carbohydrates manufactured in leaves to rest of the plant

• Energy requirements for phloem transport

– mass-flow hypotheses

• Dissolved carbohydrates flow from a source and are released at a sink.

• . Translocation in the Phloem

• Pathways of Translocation

– Sugar is translocated in phloem sieve elements

– Mature sieve elements are living cells specialized for translocation

– Large pores in cell walls are the prominent feature of sieve elements

– Damaged sieve elements are sealed off

– Companion cells aid the highly specialized sieve elements

• Patterns of Translocation: Source to Sink

– Source-to-sink pathways follow anatomic and developmental patterns

• Materials Translocated in the Phloem

– Phloem sap can be collected and analyzed

– Sugars are translocated in nonreducing form

• Phloem Loading

– Phloem loading can occur from the apoplast or symplast

– Sucrose uptake in the apoplastic pathway requires metabolic energy

– Phloem loading in the apoplastic pathway involves a sucrose–H+ symporter

– Phloem loading is symplastic in plants with intermediary cells

– The polymer-trapping model explains symplastic loading

– The type of phloem loading is correlated with several factors

• Phloem Unloading and Sink-to-Source Transition

– Phloem unloading and short-distance transport can occur via symplastic or apoplastic pathways

– Transport into sink tissues requires metabolic energy

– The transition of a leaf from sink to source is gradual

• PHLOEM LOADING CAN OCCUR SYMPLASTICALLY OR APOPLASTICALLY

•The path traversed by assimilate from the site of photosynthesis to the sieve element is not long.

•Most mesophyll cells are within a few tenths of a mm, at most three or four cells' distance from a minor vein ending where loading of assimilate into the sieve element companion cell complex (se-cc) actually occurs., it is generally agreed that sucrose moves from the mesophyll cells to the phloem, probably phloem parenchyma cells, principally by diffusion through the plasmodesmata (i.e., the symplasm).

•At this point, the pathway becomes less certain and the subject of some debate.

•From the phloem parenchyma there are two possible routes into the se-cc complex (Fig. 6.12).

•Sucrose may continue through the symplasm-that is, through plasmodesmata-directly into the se-cc complex.

•This route is known as the symplastic pathway.

• Alternatively, the sugar may be transported across the mesophyll cell membrane and released into the cell wall solution (i.e. the apoplasm).

•From there it would be taken up across the membrane of the se-cc complex where it enters the long-distance transport stream.

•This route is known as the apoplastic pathway.

•UNLOADING MAY OCCUR SYMPLASTICALLY OR APOPLASTICALLY

•Once assimilate has reached its target sink, it must be unloaded from the se-cc complex into the cells of the sink tissue.

•In principle, the problem is similar to loading; only the direction varies.

•In detail there are some significant differences. As with phloem loading, phloem unloading may occur via symplastic or apoplastic routes (see Fig. 6.14).

•The symplastic route (pathway 1) has been described predominantly in young, developing leaves and root tips.

•Sucrose flows, via interconnecting plasmodesmata, down a concentration gradient from the se-cc complex to sites of metabolism in the sink.

• The gradient and, consequently, flow into the sink cell is maintained by hydrolyzing the sucrose to glucose and fructose.

There are two possible apoplastic routes, shown as pathways 2 and 3 in Figure 6.14.

Phloem Loading:
Where do the solutes come from?

• Triose phosphate – formed from photosynthesis during the day is moved from chloroplast to cytosol

• At night, this compound, together with glucose from stored starch, is converted to sucrose

– Both these steps occur in a mesophyll cell

• Sucrose then moves from the mesophyll cell via the smallest veins in the leaf to near the sieve elements

– Known as short distance pathway – only moves two or three cells

Phloem Loading:
Where do the solutes come from?

• In a process called sieve element loading, sugars are transported into the sieve elements and companion cells

• Sugars become more concentrated in sieve elements and companion cells than in mesophyll cells

• Once in the sieve element /companion cell complex sugars are transported away from the source tissue – called export

– Translocation to the sink tissue is called long distance transport

Phloem Loading:
Where do the solutes come from?

• Movement is via either apoplast or symplast

• Via apoplastic pathway requires

• Active transport against it’s chemical potential gradient

• Involves a sucrose-H+ symporter

– The energy dissipated by protons moving back into the cell is coupled to the uptake of sucrose

Symplastic phloem loading

• Depends on plant species

– Dependant on species that transport sugars other than sucrose

• Requires the presence of open plasmodesmata between different cells in the pathway

• Dependant on plant species with intermediary companion cells

Symplastic phloem loading

• Sucrose, synthesized in mesophyll, diffuses into intermediary cells

• Here Raffinose is synthesized. Due to larger size, can NOT diffuse back into the mesophyll

• Raffinose and sucrose are able to diffuse into sieve element

Phloem unloading

• Three steps

• (1) Sieve element unloading:

– Transported sugars leave the sieve elements of sink tissue

• (2) Short distance transport:

– After sieve element unloading, sugars transported to cells in the sink by means of a short distance pathway

• (3) storage and metabolism:

– Sugars are stored or metabolized in sink cells

Phloem unloading

• Also can occur by symplastic or apoplatic pathways

• Varies greatly from growing vegetative organs (root tips and young leaves) to storage tissue (roots and stems) to reproductive organs

• Symplastic:

• Appears to be a completely symplastic pathway in young dicot leaves

• Again, moves through open plasmodesmata

Phloem unloading

• Apoplastic: three types

• (1) [B] One step, transport from the sieve element-companion cell complex to successive sink cells, occurs in the apoplast.

• Once sugars are taken back into the symplast of adjoining cells transport is symplastic

Phloem unloading

• Apoplastic: three types

• (2) [A] involves an apoplastic step close to the sieve element companion cell.

• (3) [B] involves an apoplastic step father from the sieve element companion cell

• Both involve movement through the plant cell wall

Summary

• Pathway of translocation:

– Sugars and other organic materials are conducted throughout the plant in the phloem by means of sieve elements

• Sieve elements display a variety of structural adaptations that make the well suited for transport

• Patterns of translocation:

– Materials are translocated in the phloem from sources (usually mature leaves) to sinks (roots, immature leaves)

• Photosynthate Distribution: Allocation and Partitioning

– Allocation includes storage, utilization, and transport

– Various sinks partition transport sugars

– Source leaves regulate allocation

– Sink tissues compete for available translocated photosynthate

– Sink strength depends on sink size and activity

– The source adjusts over the long term to changes in the source-to-sink ratio

• The Transport of Signaling Molecules

– Turgor pressure and chemical signals coordinate source and sink activities

– Signal molecules in phloem regulate growth and development

•DIRECTION OF TRANSLOCATION IS DETERMINED BY SOURCE SINK RELATIONSHIPS

•Identification of an organ or tissue as a source or sink depends on the direction of its net assimilate transport.

•An organ or tissue that produces more assimilate than it requires for its own metabolism and growth is a source.

•A source is thus a net exporter or producer of photoassimilate; that is, it exports more assimilate than it imports.

•Mature leaves and other actively photosynthesizing tissues are the predominant sources in most plants.

• A sink, on the other hand, is a net importer or consumer of photoassimilate.

•Roots, stem tissues, and developing fruits are examples of organs and tissues that normally function as sinks.

•The underlying principle of phloem translocation is that photoassimilates are translocated from a source to a sink.

•Sink organs may respire the photoassimilate, use it to build cytoplasm and cellular structure, or place it into storage as starch or other carbohydrate.

•Any organ, at one time or another in its development, will function as a sink and may undergo a conversion from sink to source.

•Leaves are an excellent example.

•In its early stages of development a leaf will function as a sink, drawing photoassimilates from older leaves to support its active metabolism and rapid enlargement.

•However, as a leaf approaches maximum size and its growth rate slows, its own metabolic demands diminish and it will gradually switch over to a net exporter.

• The mature leaf then serves as a source of photoassimilate for sinks elsewhere in the plant.

•The conversion of a leaf from sink to source is a gradual process, paralleling the progressive maturation of leaf tissue.

•STARCH AND SUCROSE ARE BIOSYNTHESIZED IN TWO DIFFERENT COMPARTMENTS

•Many plants, such as soybean, spinach, and tobacco, store excess photoassimilate as starch in the chloroplast, while others, such as wheat, barley, and oats, accumulate little starch but temporarily hold large amounts of sucrose in the vacuole.

•The starch and sucrose will later be mobilized to support respiration and other metabolic needs at night or during periods of limited photosynthetic output.

•Sucrose exported from the leaf cell to nonphotosynthetic tissues may be metabolized immediately, stored temporarily as sucrose in the vacuoles, or converted to starch for longer-term storage in the chloroplasts.

•SUCROSE IS BIOSYNTHESIZED IN THE CYTOSOL

•Sucrose is a soluble disaccharide containing a glucose and a fructose residue.

• It is one of the more abundant natural products that not only plays a vital role in plant life but is also a leading commercial commodity.

•Sucrose may function as a storage product as it does in sugarbeets or sugarcane, where it is stored in the vacuoles of specialized storage cells.

•Alternatively, sucrose may be translocated to other, nonphotosynthetic tissues in the plant for direct metabolic use or for conversion to starch.

•Sucrose is by far the most common form of sugar found in the translocation stream.

STARCH AND SUCROSE BIOSYNTHESIS ARE COMPETITIVE PROCESSES

While the enzyme sucrose phosphate synthase (SPS) determines the maximum capacity for sucrose synthesis, it appears that cytosolic fructose- 1,6-bisphosphate phosphatase (FBPase) plays the more important role in balancing the allocation of carbon between sucrose and starch synthesis.

The highly exergonic reaction (fructose1,6-bisphosphate -* fructose-6-phosphate + P) occupies a strategic site in the sucrose synthetic pathway-it is the first irreversible reaction in the conversion of triose-P to sucrose.

Consequently the flow of carbon into sucrose can easily be controlled by regulating the activity of FBPase-similar to regulating the flow of water by opening or closing a valve.

Exactly what is transported in phloem?

WHAT IS THE COMPOSITION OF THE PHOTOASSIMILATE TRANSLOCATED BY THE

PHLOEM?

The chemical composition of phloem exudate is highly variable.

It depends on the species, age, and physiological condition of the tissue sampled.

Even for a particular sample under uniform conditions, there may be wide variations in the concentrations of particular components between subsequent samples.

For example, an analysis of phloem exudate from stems of actively growing castor bean (Ricinus communis) (Table 6.1) shows that the exudate contains sugars, protein, amino acids, the organic acid malate, and a variety of inorganic anions and cations.

The predominant amino acids are glutamic acid and aspartic acid, which are common forms for the translocation of assimilated nitrogen (Chap. 8).

Although not shown in Table 6.1, some plant hormones (auxin, cytokinin, and gibberellin) were also detected, but at very low concentrations.

Of course, many of the components identified in phloem exudate do not necessarily represent translocated photoassimilate.

Protein found in phloem exudates includes a wide variety of enzymes as well as one predominant protein (called P-protein) that is unique to the translocating cells. We will return to a discussion of P-protein later in this chapter.

The principal constituent of phloem exudate in most species is sugar.

In castor bean it is sucrose, which comprises approximately 80 percent of the dry matter (Table 6.1).

This suggests that sucrose is the predominant form of translocatable photoassimilate.

This suggestion has been amply confirmed by labeling experiments.

In the example of translocation in sugarbeet petioles described earlier, more than 90 percent of the radioactivity, following 10 minutes of labeling with ¹⁴CO₂, was recovered as sucrose.

A survey of over 500 species representing approximately 100 dicotyledonous families confirms that sucrose is almost universal as the dominant sugar in the phloem stream.

Sugars that are not generally in phloem

• Carbohydrates transported in phloem are all nonreducing sugars.

– This is because they are less reactive

• Reducing sugars, such as Glucose, Mannose and Fructose contain an exposed aldehyde or ketone group

– Too chemically reactive to be transported in the phloem

Sugars that are in phloem (polymers)

• The most common transported sugar is sucrose.

– Made up from glucose & Fructose

• This is a reducing sugar

– The ketone or aldehyde group is combined with a similar group on another sugar

– Or the ketone or aldehyde group is reduced to an alcohol

• D-Mannitol

• Most of the other mobile sugars transported contain Sucrose bound to varying numbers of Galactose units

Other compounds

• Water!!!!!!!!!

• Nitrogen is found in the phloem mainly in:

– amino acids (Glutamic acid)

– Amides (Glutamine)

• Proteins

• Downstream, at the sink end of the sieve tube, phloem unloads its sucrose.

– The mechanism of phloem unloading is highly variable and depends on plant species and type of organ.

– Regardless of mechanism, because the concentration of free sugar in the sink is lower than in the phloem, sugar molecules diffuse from the phloem into the sink tissues.

– Water follows by osmosis.

DISTRIBUTION OF PHOTOASSIMILATES BETWEEN COMPETING SINKS IS DETERMINED BY SINK STRENGTH

The distribution of assimilate between sinks is referred to as partitioning.

In a vegetative plant, the principal sinks are the meristem and developing leaves at the shoot apex, roots, and nonphotosynthetic stem tissues.

With the onset of reproductive growth, the development of flowers, fruits, and seeds creates additional sinks.

In general, sinks are competitive and the photoassimilate is partitioned to all active sinks.

If the number of sinks is reduced, a correspondingly higher proportion of the photoassimilate is directed to each of the remaining sinks.

This is the basis for the common practice of pruning fruit trees to ensure a smaller number of fruit per tree (see Chap. 15).

Partitioning the assimilate among a smaller number of fruit encourages the development of larger, more marketable fruit.

Partitioning of assimilate between competing sinks depends primarily on three factors: the nature of vascular connections between source and sinks, the proximity of the sink to the source, and sink strength.

Translocation is clearly facilitated by direct vascular connections between the source leaf and the sink.

2. Pressure flow is the mechanism of translocation in angiosperms

• Phloem sap flows from source to sink at rates as great as 1 m/hr, faster than can be accounted for by either diffusion or cytoplasmic streaming.

– Phloem sap moves by bulk flow driven by pressure.

– Higher levels of sugar at the source lowers the water potential and causes water to flow into the tube.

– Removal of sugar at the sink increases the water potential and causes water to flow out of the tube.

– The difference in hydrostatic pressure drives phloem sap from the source to the sink

(1) Loading of sugar into the sieve tube at the source reduces the water potential inside the sieve-tube members and causes the uptake of water.

(2) This absorption of water generates hydrostatic pressure that forces the sap to flow along the tube.

(3) The pressure gradient is reinforced by unloading of sugar and loss of water from the tube at the sink.

(4) For leaf-to-root translocation, xylem recycles water from sink to source.

• The closer the aphid’s stylet is to a sugar source, the faster the sap will flow out and the greater its sugar concentration.

• In our study of how sugar moves in plants, we have seen examples of plant transport on three levels.

– At the cellular level across membranes, sucrose accumulates in phloem cells by active transport.

– At the short-distance level within organs, sucrose migrates from mesophyll to phloem via the symplast and apoplast.

– At the long-distance level between organs, bulk flow within sieve tubes transports phloem sap from sugar sources to sugar sinks.

• Interestingly, the transport of sugar from the leaf, not photosynthesis, limits plant yields.

•PHLOEM EXUDATE CONTAINS A SIGNIFICANT AMOUNT OF PROTEIN

•Sieve elements characteristically accumulate protein in such large quantities that it can be observed under the light microscope.

•When this protein was first observed by early anatomists, it was assumed to be a carbohydrate and was called slime.

•The accumulations observed in the region of the sieve plates were called slime plugs.

• It was not until the 1960s that cytochemical tests revealed the proteinaceous nature of this material and the name P-protein (for phloem protein) was introduced.

•As the sieve elements mature, the P-protein bodies continue to enlarge.

•At the time the nucleus, vacuole, and other cellular organelles disappear, the P-protein bodies disperse in the cytoplasm.

•In some species, such as maple (Acer rubrum), the P-protein takes the form of a loose network of filaments, ranging from 2 to 20 nm in width.

•In others, such as tobacco (Nicotiana sps.), the filaments appear tubular in cross section.

•In yet others, such as some leguminous plants, P-protein takes the form of crystalline inclusions.

•P-protein has been the subject of considerable attention over the years because of its prominence in sieve elements and its propensity to plug the pores in the sieve plates.

•P-protein has been implicated in various ways in the transport function of sieve elements.

•According to some theories, P-protein is considered an active participant in the transport process.

•It is now generally accepted that, in intact, functioning sieve elements, P-protein is located principally along the inner wall of the sieve element and does not plug the sieve plate.

•The formation of plugs in the sieve plates occurs only when the sieve element is injured.

•This occurs because the sieve element is normally under positive hydrostatic pressure, as evidenced by the continued flow of exudate from aphid stylets.

• When the pressure is released through injury to the sieve element, the contents, including P-protein, surge toward the site of injury.

•This results in the accumulation of P-protein, possibly assisted by its gelling properties, as "slime" plugs on the side of the sieve plate away from the pressure release.

Protective mechanisms in phloem

• P proteins:

– Occurs in many forms (tubular, fibrillar, chrystaline – depends on plant species and age of cell)

– Seal off damaged sieve elements by plugging up the sieve plate pores

– Short term solution

• Callose:

– Long term solution

– This is a b-(1,3)-glucan, made in functioning sieve elements by their plasma membranes and seals off damaged sieve elements

• In many plants

– Phloem loading requires active transport

• Proton pumping and cotransport of sucrose and H⁺

– Enable the cells to accumulate sucrose

PHLOEM TRANSLOCATION OCCURS BY MASS TRANSFER

What is the mechanism for assimilate translocation over long distances through the phloem?

A number of factors must be taken into account... 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-') 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 )

Some of the newly fixed carbon or photoassimilate in a source leaf is retained within the leaf, and the rest is distributed to various nonphotosynthetic tissues and organs.

This raises several interesting questions.

What, for example, determines how much carbon is retained and in what form?

What determines how much is exported and to where?

What determines how much assimilate, for example, is exported to the roots of a wheat or corn plant and how much is translocated to fill the developing grain