Linkages in Plant Physiology

The transport System in Plants

An overview of phloem
translocation
 The Phloem and its Function
• The function of the phloem tissue is
to transport food nutrients such
as glucose and amino acids from the leaves
and to all other cells of the plant, this is called
translocation.
 The phloem
• Phloem, like xylem, is a complex conducting
tissue of vascular plants.
• Its main function is the long distance transport
of sugars and other photosynthates from the
source (mature leaves), or reserves
(germinating seedlings) towards the sinks,e.g.
roots, developing reproductive structures
(flowers, fruits and seeds), meristems and
young leaves
 The phloem
• Phloem almost always accompanies xylem,
and their elements run parallel to each other
in vascular bundles.
• They must remain alive in order to conduct.
• Phloem consists of several types of cells,
including conducting and parenchyma cells,
phloem (or bast) fibers, sclereids and, in some
plants, secretory ducts and laticifers
 The phloem
• The phloem collects photo-assimilates in
green leaves, distributes them in the plant and
supplies the heterotrophic plant organs (e.g.
fruits, buds and roots).
• Phloem structure is specialized for loading,
long-distance transport and unloading of
assimilates.
 The phloem
• The difference in turgor pressure that is
generated by osmotically active assimilates
within this living conduit is the physical force
that drives long-distance transport through
sieve elements.
 The phloem
• Signalling molecules accessing the phloem are
swept along with assimilates and trigger
important growth processes such as
flowering.
 The phloem
• Plant species have evolved a variety of
strategies to generate and maintain turgor
pressure between source tissues, where
assimilates are synthesized or released, and
sink tissues, where assimilates are utilized or
stored.
 The phloem
• In the phloem, tubular cells assemble into so-
called sieve tubes which form a continuous
micro-fluids network.
• In this network, the products of
photosynthesis are distributed throughout the
plant body from sources (mature leaves) to
sinks (young leaves, roots, fruits etc.).
 The Xylem Vessel and its Function
• Xylem consists of the inner heartwood (the
dead part) and the outer portion called the
sapwood (the mostly living part.)
• The actual transport of the water and
minerals from the roots (sap) is carried out by
the outer portion of the sapwood (nearest the
bark).
• The largest percentage of the sap is
transported in the first few growth rings.
 The Xylem
• The xylem vessel is specialized
to transport water and dissolved minerals
from the root up to all the other parts of the
plant, and also to helps supporting the stem and
strengthening it.
The Xylem Vessel
 Now!!!!
• State the differences between the Xylem and
the Phloem

• USE 3 mins to do this.

 What is this slide depicting
• Xylem imports water and minerals while Phloem transports
water and food. Xylem exists as non-living tissue at
maturity, but phloem is living cells.

Xylem: Hard wall cells transport water and mineral nutrients

Phloem: Relatively soft -walled cells transport organic

nutrients
 What is this slide depicting
• "Hardness and softness" is a function of the
amount of lignification and extractive content
of the individual cell walls not there location
in the tree.
Dia. Of Phloem and Xylem
What is this slide Depicting?
The Cross section of a Mature Stem
An overview of phloem translocation
• The flow of sap and the distribution of foods
in plants has presented one of the most
demanding and perplexing problems with
which botanists have been faced for the past
one hundred years.
An overview of phloem translocation
• The problem has been tackled by many plant
physiologists during the 19th and 20th
centuries-- amongst these:
• Hales
• Hartig
• von Mohl et. al
An overview of phloem translocation
• During the past fifty years or so great strides
were made in the development of the ideas
and theories on phloem translocation.
An overview of phloem translocation
• Some of these theories are based on:
1.simple mass flow mechanism
2.more elaborate mechanism
3.combination of mechanisms
 Mass Flow or Pressure Flow
Hypothesis
• Put forward by Munch (1927, 1930)
• Organic substances move from a region of high
Osmotic pressure to region of low osmotic
pressure due development of a gradient of
turgor pressure.
• The sieve tube element is adapted to mass flow
of solutes, due to permeable vacoules.
• High osmotic concentration develops in these
tubes
• The tubes absorbs water from the
surrounding xylem and develop a high turgor
pressure.
• This causes the flow of organic solution
towards the area of low turgor pressure .
• A low turgor pressure is maintained in the sink
region by converting soluble molecule into
insoluble form.
An overview of phloem translocation
• Firstly, a study of the anatomy and
ultrastructure of the phloem tissue in
Gymnosperm, and Angiosperm material was
looked at.
An overview of phloem translocation
• Great progress has been made in the
betterment of the embedding techniques:
1.Cell preservation
2.less unacceptable plasmolysis of the difficult
to fix phloem tissues
An overview of phloem translocation
• Secondly, Many studies have been performed
using aphids and other sap-suckers, in an
effort to determine the rate at which
substances move in an otherwise undisturbed
conduit.
• This experiments in the 1959 is one
experiment most accepted by scientist.
An overview of phloem translocation
• To test the pressure flow hypothesis,
researchers used aphids that feed on phloem
sap.
• Aphids could be and was used as direct
pipeline to the phloem.
• An aphid probes with a hypodermic-
like mouthpart called a stylet that penetrates
a sieve-tube member.
An overview of phloem translocation
• As sieve-tube pressure force-feeds aphids,
they can be severed from their stylets, which
serve as taps exuding sap for hours.
• Researchers measured the flow and sugar
concentration of sap from stylets at different
points between a source and sink.
 An overview of phloem translocation
EXPERIMENT

25 m

Sieve-
tube
member Sieve-
Tube
member

Sap
droplet
Sap droplet
Stylet

Aphid feeding Stylet in sieve-tube Severed stylet

member exuding sap

RESULTS
The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration.

CONCLUSIONThe results of such experiments support the pressure flow hypothesis.

Figure 36.19
An overview of phloem translocation
• Velocities have been calculated
• The normal inorganic and organic composition
of the phloem has been determined for a
great many species using aphids and severed
aphid mouthparts.
• Many experiments have been conducted to
determine the direction and velocity of the
flow
An overview of phloem translocation
• A number of eminent scientists, and their
students, have thus contributed greatly in this
area, notably Weatherly, Peel, Eschrich and
Evert.
An overview of phloem translocation
• It was Eschrich for example, who first
postulated that substances could move in
opposite directions within the same sieve
tube, or within the same file of sieve tubes
An overview of phloem translocation
• This was achieved by application of 14C-Urea
acropetally to some feeding aphids, and by
the simultaneous application of fluorescein
below the feeding aphids.
An overview of phloem translocation
• Thirdly, and possibly least successful, have
been experiments which have been based
upon tracer studies.

• However these were least successful

 Why least successful ?
• Because early studies relied on water-based
photographic emulsions, which were placed in
the microautoradiographs of sections of the
stem, leaf or root material which had been fed
a radiotracer.
 Later Research
• Since then, much effort has been ex- pended
in the elucidation (clarification) of their
structure and their mode of function in the
plant.
• There is possibly no other tissue in plants,
which has caused such heated debate,
discussion, even open animosity between the
various supporters of the various theories,
which have been presented over this time, for
the mechanism of phloem translocation
 The Mechanism of phloem
transport
• When the movement of minerals and water
via xylem is driven mostly by negative
pressure and movement via phloem is driven
by hydrostatic pressure. This process is called
as translocation and accompanied by a
process known as phloem loading and
unloading.
It is important to distinguish between

1. Phloem loading mechanisms

2. Phloem transport mechanisms
3. Phloem unloading mechanisms
 The loading process
Two approaches
1.a passive pathway
2.an active (accumulating) step.
• In the first instance, there may be no energy
or thermodynamic demands placed upon the
system.
• In the second, ATP & NADPH would be needed
directly to drive co-transport across
membranes.
 The loading process
• Accumulation of sugars in the source phloem
is called phloem loading and depends either
on sucrose transporters in the plasma
membrane of sieve-element companion cell
complexes, or enzyme activity in the
companion cells of the source phloem
 Loading Process
• Mass Flow Mechanism
• Symplastic – Loading and movement of
sugars through the plasmadesmata
• Apoplastic – loading and movement of sugars
from the apoplast (Extra-Cellular cell wall
spaces) across the plasma membrane and into
the cell.
 The transport process
• Phloem transport can be viewed as an entirely
passive process, that makes no demands upon
the energy cycles of the plant other than
energy required for the maintenance of plant
membranes.
 The transport process
• The mechanism of phloem transport is based
on a high sugar concentration in the sieve
element-companion cell complexes of source
leaves, which osmotically builds up a high
turgor pressure.
 The Transport Process
• Solutes (sucrose and other solutes) move
across the membrane symplastic through
sieve tubes (from the mesophyll via minor leaf
veins) to sites where it will be utilized for plant
growth and development.
 The Transport Process
• Solute concentration is maintained high at the
source end of the system as sugars and other
solutes move into the sieve tubes, there the
concentration are low at the sink end as
solutes move out.
 The Transport Process
• Lowered concentration of solutes at the sink end
allows water to move out in response to the
pressure transmitted from the source end (or in
response to even high concentration of solutes in
apoplast at the sink).

• Growth of storage cell at the sink will cause

absorption of water from the apoplast cell wall
lowering its water potential and facilitating exit
of water from the sieve tubes there.
 The transport process
• Phloem transport is utilized for signal
transport, including:
 rapid electro-potential waves
 mobile non-coding ribonucleic acids (RNAs)
 mobile transcription factors that are
responsible for wound responses and
developmental switches in the target tissue
 The transport process
• If transport is passive then an entirely bulk
flow system, driven by concentration
gradients established and maintained
between the source and the sink.
• Transport would thus be along or down
a concentration gradient.
 The transport process
• If transport is passive, then metabolic
inhibitors would and should have no effect
upon the process.
 The transport process
• Alternatively one could argue that phloem
transport is an active process, and one
requiring energy (physiological or
thermodynamic) in order to drive and
maintain it.
• In light of this it could be viewed that ATP
NADPH or H+ K+ ion exchange as the driving
force.
 The unloading process
• Essentially, this must be the opposite of the
loading process, as conversion of the soluble
carbohydrate into a less osmotically-active
form will be necessary in order to set up the
driving force of the unloading process.
 The unloading process
• The processes that are involved could be:
1.entirely symplastic
2.mixed symplastic-apoplasmic
3.or apo-plasmic at the terminal sinks (such as
in parenchyma surrounding and beyond
terminal proto-phloem in shoot and root
apices).
 What about mechanisms
• These can be placed in one of two categories.
1. Those in which osmotic potential is the
driving force
2. Those where energy transformations are
necessary
Source Sink Relationship
 Studying Solute Transport in the
Phloem
• Phloem is difficult to study in plants because:

• (1) the transport cells/tissue in plants are

small (microscopic) in comparison to the
transport structures in animals
 Studying Solute Transport in the
Phloem
(2) there is a very rapid response of the phloem
to wounding (contents under pressure)
(3) transport in plants is intracellular (vs.
extracellular in animals)
(4) the transport cells are alive
 Solute Transport in the Phloem
• Phloem is the primary transport tissue for
photosynthates (photoassimilates, or simply
stated - organic materials).
Radiotracer studies in which leaves are
briefly exposed to 14C-labeled carbon dioxide
show that radioactive photosynthates are
localized in the phloem.
• Aphids Don't Suck
 Solute Transport in the Phloem
• Kennedy & Mittler (1953) first noted that
aphids could be used as a direct pipeline to
the phloem.
• Phloem-feeding aphids stick their hollow,
syringe-like stylet directly into phloem cells.
 Solute Transport in the Phloem
• the phloem contents are forced into the aphid
(thus the phloem is under pressure) and the
excess oozes out the anus (honeydew).
 Solute Transport in the Phloem
• Thus, aphid studies demonstrate that the
phloem is under pressure.
• Further, the honeydew can be collected and
we can identify its composition.
• Better yet, after anaesthetizing the aphid with
CO2 the body is severed from the stylet
leaving a miniature spile tapped directly into
the phloem
 Phloem Content
• Analysis - early studies to determine the
content of the phloem involved cutting into
the plant and analyzing the contents of the
sap that was recovered.
• The problem is that you couldn't be sure that
your sample wasn't contaminated by xylem
exudates or other materials
 Phloem is rich in
• 1. Carbohydrates - make up 16-25% of sap. The
major organic transport materials are sucrose,
stachyose (sucrose-gal), raffinose (stachyose-gal).
• Stachyose is a tetrasaccharide consisting of two
α-D-galactose units, one α-D-glucose unit, and
one β-D-fructose unit sequentially linked as
galgalglcfru.
• Stachyose is naturally found in numerous
vegetables and plants. C24H42O21
 Phloem Content
• These are excellent choices for transport
materials for two reasons:
• (a) they are non-reducing sugars (the hydroxyl
group, the number one carbon, is tied up)
which means that they are less reactive and
more chemically stable;
 Phloem Content
• (b) the linkage between sucrose and fructose
is a "high-energy" linkage similar to that of
ATP.
• sucrose is a good transport form that provides
a high energy, yet stable packet of energy
 Phloem Content
• Amines/amides (0.04-4%) such as:
• Asparagine
• Glutamine
• Aspartic acid
• ureides like ureas
 Phloem Content
• Citrulline
• allantoin
• allantoic acid. These compounds serve to
transport "nitrogen";
 Phloem Content
3. ATP
 Hormones
 sugar alcohols like sorbitol (apple, pear,
prune)
 mannitol (mangrove, olive)
 an assortment of other organic materials

 4. Inorganic substances including magnesium

and potassium
 Direction of phloem transport
• These experiments showed the accumulation
of material above the girdle, and that
carbohydrates were not translocated below
the girdle.
• Thus, plants transport substances in the
phloem downward toward to the roots
 Direction of phloem transport
• (2) Sophisticated girdling experiments, using
tracers like 32P, 13C, and 14C demonstrate that
substances in the phloem are transported
downward towards the roots OR upwards
toward the shoot meristem
 Rate of phloem transport
• Aphid experiments once again provide an
answer...translocation rates average about 30
cm hour-1 or even faster.
 Rate of phloem transport
• The phloem is under pressure
Studies with aphids showed that the sap was
"pushed" out of the plant suggesting the
phloem is under pressure.
• More recent studies with sophisticated
pressure probes have shown a pressure
gradient from source to sink.
 Mechanism for phloem transport
• (1) speed of transport.
• The process is much faster than simple
diffusion.
 Mechanism for phloem transport
• (2) bidirectional flow - recall that substances
can be transported down or up in the phloem;
and
• (3) pressures in the phloem

• Pressure flow (or Bulk Flow).

This is the best model that fits the data.
Mass Flow Mechanism
Phloem Transport
Pressure Flow Mechanism
 Summary
• Phloem – complex conducting tissue of
flowering plants
- Some theories that explain movement of
substances in the phloem
- Progress in the observation of the ultra-
structure of phloem tissues
- 3 processes in phloem transport
 Summary
• 2 approaches in loading
• Utilization of phloem transport
- Active
- Passive
- Inhibitors
Mechanisms involved in loading
Solute transport
Work done with aphids
 Video
• http://www.youtube.com/watch?feature=pla
yer_detailpage&v=MxwI63rQubU

• http://www.youtube.com/watch?feature=pla
yer_detailpage&v=MxwI63rQubU
The end

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