The document discusses the mechanisms and theories of phloem transport in plants, highlighting the role of sieve elements and companion cells in the transport of solutes. It evaluates various theories, including electro-osmosis theory, protoplasmic streaming, and the widely accepted pressure-flow hypothesis, while addressing objections to these theories and factors affecting transport rates such as temperature, inhibitors, and nutrient deficiencies. Additionally, it explains the processes of phloem loading and unloading of sugars from source to sink, emphasizing the energy requirements and pathways involved in these translocations.

1. TRANSLOCATION OF SOLUTE IN
PLANTS
CONTENTS
1. MECHANISM OF PHLOEM TRANSPORT
2. P-PROTEINS
3. ELECTRO-OSMOSIS THEORY
4. MUNCH’S MASS FLOW THEORY
5. FACTORS AFFECTING PHLOEM TRANSPORT
DR. AMIT KUMAR
ASSISTANT PROFESSOR
UNIVERSITY OF LUCKNOW
LUCKNOW-226007.

2. Mechanism of Phloem Transport:
The mechanism of long-distance transport through the sieve
tube is soundly based on the internal organization of sieve tubes,
without which it remains speculative.
Phloem tissue is composed of the sieve elements, companion
cells or albuminous cells, phloem parenchyma and phloem fibres. Of
them, the sieve elements and companion cells are important for
transport.
The sieve elements are anucleated, elongated living
cells, through which transport actually takes place. They are connected
end to end with porous sieve plates in between, forming long cellular
channels called sieve tubes.
The companion cells have dense cytoplasm with small vacuoles.
Mitochondria, dictoysomes and endoplasmic reticulum are abundant.
The nucleus is well-defined.

3. The sieve tube ultrastructure shows continuous smooth
endoplasmic reticulum. Mitochondria in the sieve tubes are capable of
carrying out cellular respiration. In mature sieve elements plastids are
present with rudimentary internal membrane system. Microfilament
bundles have been reported in mature sieve elements.
There are several kinds of fibrilar proteins having diameter of 7-
24 nm and the molecular weights vary from 14,000 to 150,000. These
proteins are referred to as P-proteins (phloem proteins). The pores of
the sieve plates are blocked with these P-proteins. The occlusion of
pores does not favour the pressure flow hypothesis. Many theories,
however, suggest that P-proteins play some kind of active role in
pumping solution through the pores. Whether the sieve plate pores are
open or occluded by P-protein is still a question.
Electron microscopic studies are post-vital observations.
Observation of living functioning sieve tubes is exceedingly difficult
because of their fragility. With these uncertainties of the internal
structure actual explanation of the transport mechanism is still lacking.

4. Various theories to explain the transport of photosynthates in the
phloem which are as follows:
(i) Electro-Osmosis THEORY:
This mechanism suggests that an electric potential is maintained
across the sieve plate. The electric potential exerts a force on the solution
around the filamentous material fixed in the pores, thus causing flow through
the plats.
Electric potentiality is maintained in the form of a continuous
circulation of ions through the sieve pores and back through companion cells
or even through walls of the sieve tubes (Spanner, 1958) (Fig. 6. 11).
So, according to this mechanism the sieve plates are the origins of the
force for movement and not an obstruction. It is believed that K+
ions are
moved through the pores and again circulated back to the same side of the
plate by an ATP-driven potassium ion pump present in the membrane.
Potassium ions have been found in adequate concentration in sieve tubes.
The fixed negative charges on the proteinaceous plug were assumed to
be balanced by mobile potassium ions, which would be pulled by an electric
potential difference across the sieve plate, in turn pulling along water and other
solutes.

5. The main objection to this theory is that it does not show
transport of ions of both positive and negative charges and polarized
potentials across the sieve plates have not been found. Further, the
efficiency of water movement (the number of water molecules moved per
ion) have been found to be higher than that observed during electro-
osmosis in non-living system.

6. (ii) Protoplasmic Streaming:
De Vries in 1885 suggested that protoplasmic streaming was
responsible for the transport of sugar through the phloem.
According to him protoplasm was circulated around the periphery of
the sieve elements. Thus, like a conveyer belt or two-way escalator it facilitated
bidirectional movement of trans-locates through the same sieve tube.
In the 1960s, Thaine observed intercellular strands of protoplasm
moving through the sieve pores from one sieve element to the next of the entire
length of a sieve tube. He suggested that peristaltic pump and counter-current
were responsible for the movement of translocates.
This cytoplasmic pumping in trans-cellular strands can account for the
bidirectional translocation through different strands within a single sieve tube.
But the mechanism is incapable of explaining the speeds and SPECIFIC MASS
TRANSFER/SMTs in the higher ranges, and the membrane bound strands have
not been found.
Dempsey et al. (1975), however, found strands of appropriate size but
not surrounded by membranes. In addition the maximum streaming rates
observed in plant cells are less than 1 mm min-1
for slower than the rates
typical of phloem transport (often 1 cm min-1
or max).

7. (iii) Contractile Protein Variants:
Fensom and Peel reported the presence of fibrilar proteins called
P-proteins which oscillated in a manner resembling moving flagella. They
also found particles attached to the micro fibrils moving in a bouncing
motion resembling Brownian movement, but several times more rapid.
They claimed that these P-proteins played some kind of active role
in pumping solution through the pores. Various such proposals have been
made suggesting the generation of movement by the contractility (actin
like activity) of P-protein filaments.
No positive actin reactions, such as binding with heavy
meromyosin, have been detected. Sabins and Hart showed that the P-
proteins are highly variable in their composition and are not contractile in
nature.
iv) Pressure-Driven Flow:
Pressure-flow or mass-flow hypothesis is the most widely accepted
hypothesis at present, though there are a number of reservations. The
hypothesis was proposed in its elemental form by a German scientist, E.
Munch, in 1926.

8. The hypothesis rests on the assumption that a turgor-pressure gradient
exists between the source and the sink.
Trans-locates are carried passively in response to the pressure gradient
caused by osmotic diffusion of water into the sieve elements at the source end
and out of the sieve elements at the sink end. This type of translocation is
called osmotically generated pressure flow (OGPF).
The hypothesis is simple and based upon a model that can be made in
laboratory (Fig. 6.12). Two osmometers A and B, permeable only to water, are
connected to each other with a tube.
Osmometer A contains solution that is more concentrated than its
surrounding solution and osmometer B contains a solution less concentrated
than that in A, but still higher than its surrounding medium. Both the
surrounding solutions have open channels.
Since osmometer A contains more solutes, it will develop a higher
turgor pressure which is transmitted throughout the system through the open
channel, causing a passive mass-flow of water and solutes from A to B.
Water comes out of B influenced by the pressure and is re-circulated
through the open channel. If solutes can be added into A and removed from B
continuously, the flow will continue.

10. 1. According to Munch, the living plant contains a comparable system (Fig.
6.13). The sieve elements near mesophyll cells are analogous to A.
2. The sieve elements in this region are continuously loaded with sugars by the
mesophyll cells and the concentration is kept high. In the sink end the sugar
concentration in the sieve elements is always kept low as sugars become
osmotically inactive through metabolism or are utilized in growth, stored as
starch, or converted to fats.
3. The connecting channel between source and sink is the phloem and the
surrounding dilute solutions are those of the apoplast and that in the xylem.
So, according to Munch’s hypothesis the flow through the sieve tubes is
passive, although there is evidence of involvement of metabolism in bulk flow.
4. Sieve plate pores are open channels as they favour the mechanism.
Knoblauch and Van Bel, using a confocal laser scanning microscope, have
been able to visualize the transport of sugars along with a green phloem –
mobile fluorescent dye in the living sieve elements.
The sieve plates showed staining within the pores that were lined with
plasma membrane and that the pores were open and not occluded. It should be
kept in mind that pores in sieve areas and sieve plates are modified
plasmodesmata.

11. 5. Simultaneous bidirectional transport in a single sieve tube has not been
detected. Transport in both directions has been detected in sieve elements of
different vascular bundles in stems. The unidirectional transport through a single
sieve element supports the Munch’s model.
(v) Objections to Pressure-Flow Hypothesis:
The hypothesis suggests that substances should move in the same
direction and at the same velocity. It has been found that 14
C sugars moved most
rapidly, 32
P-phosphates moved more slowly, and 3
H,0 moved slowest of all. It is
explained that water is exchanged rapidly along the pathway.
It goes out through the sieve-tube membrane into the surrounding
tissues and again diffuses back into the sieve tube. While sucrose and phosphate
do not move as readily through the membrane, they might move much faster than
the water molecules.
There are a number of situations in which sieve tubes appear to carry two
substances in opposite directions simultaneously. Trip and Gorham clearly
demonstrated the presence of 14
C assimilates and 3
H glucose that moved from
opposite directions in a single sieve tube. In minor veins of leaves, movement
appears to go either way or both ways. Many workers, however, suggest that
bidirectional movement occurs in separate phloem ducts, a possibility under the
pressure-flow system.

12. Again, the sieve plates themselves offer a considerable resistance to
passive bulk-flow as postulated in the Munch’s model.
The phloem ultra-structure suggests that the pores are partially or
completely blocked with P-proteins. So, though these objections remain a
rudimentary barrier to the universal acceptance of the pressure-flow hypothesis, it
may well turn out that pressure-flow is the most probable mechanism of phloem
translocation.
Factors Affecting Phloem Transport:
Phloem transport is affected by several important factors which are as
follows:
(i) Temperature:
Temperature plays an important role in translocation. There is an
optimum range of temperature for maximum translocation rate. Hewitt and Curtis
observed that the optimum range of temperature for translocation in bean plants
was 20°C – 30°C.
Translocation has also been found to be irreversibly inactivated by
temperatures above 50°C. Similarly, too low temperatures affect translocation
rate. Low temperatures inhibit active phloem transport by preventing the
involvement of metabolic energy.
There are two types of plants according to low-temperature sensitivity.

13. For some plants such as cucumber and tomato the inhibition temperature
is around 10°C and the inhibition persists for a longer period. They are chilling-
sensitive plants. Another group of plants such as sugar beet, potato, etc., are
chilling-insensitive. In these plants low temperature has a transient effect.
They can recover translocation speed and SMT after 60 to 90 min. even
when the local cooling of an organ is maintained at 0°C. Low temperature
increases viscosity of the phloem sap which reduces the speed and alters
membrane structures which disorganizes the contents and causes plugging of the
sieve pores. In chilling-insensitive plants probably the membrane remains
unaltered.
(ii) Inhibitors:
Certain metabolic inhibitors such as cyanide and dinitrophenol have been
shown to inhibit carbohydrate translocation, supporting the use of respiratory
energy in helping movement. Cyanide applied locally to phloem prevents
translocation through the treated zone. The inhibitors do not reach phloem in
intact plants and so to apply it the vascular bundle is exposed surgically. Whether
the inhibitor has its effect on the transport phenomenon or on the loading and
unloading phenomena is difficult to assess. In both the cases translocation is
inhibited. However, translocation rate is regulated more by the metabolism of the
source and sink cells than by the metabolism of the conducting cells themselves.

14. (iii) Potassium and Boron Deficiency:
Potassium is abundantly present in phloem sap. Potassium deficiency
affects the growth of fruits and storage organs.
Potassium circulation around the sieve plate increases translocation of
sugar in sieve tubes. Circulation of potassium establishes a potential difference
across the sieve plates which actually favours sugar translocation. But there is no
general acceptance of this explanation. It is now believed that K+
ions are
involved in loading in the minor veins in leaves.
Boron is also essential for sugar transport. There is no sufficient
explanation for the function of boron in sugar transport. However, Gauch and
Dugger, suggested that boron complexes with sucrose to form sugar-borate
complex which might pass through the negatively charged membranes more
readily than neutral sugar molecules.
(iv) Hormones:
In the actively growing regions growth-promoting phytohormones are
present in high amounts. The actively growing regions act as stronger sinks and
thus attract most of the nutrients from the source regions. Growth hormones
stimulate growth in these regions.
So, it is suggested that growth hormones have a strong influence on phloem
translocation. But this effect is indirect. It is very difficult to distinguish hormonal-

15. --effects on translocation from hormonal effects on the metabolism of sink tissue
for the attraction of trans-locates.
It can be said that phloem translocation is at least partially under the
control of phytohormones such as the cytokinins, indole-3-acetic acid (IAA), and
gibberellic acid (GA).
Of course, in intact plants there is no evidence that the endogenous
levels of hormones in the various tissues bear any relation to phloem
translocation. They affect assimilate partitioning by controlling sink growth, leaf
senescence, and other developmental processes.
Phloem Loading and Unloading in Plants–
Translocation of organic solutes such as sucrose (i.e., photosynthetic)
takes place through sieve tube elements of phloem from supply end (or source) to
consumption end (or sink). But, before this translocation of sugars could proceed,
the soluble sugars must be transferred from mesophyll cells to sieve tube
elements of the respective leaves.
This transfer of sugars (photosynthetic) from mesophyll cells to sieve
tube elements in the leaf is called as phloem loading. On the other hand, the
transfer of sugars (photosynthetic) from sieve tube elements to the receiver cells
of consumption end (i.e., sink organs) is called as phloem unloading. Both are
energy requiring processes.

16. Phloem Loading: As a result of photosynthesis, the sugars such as sucrose
produced in mesophyll cells move to the sieve tubes of smallest veins of the leaf
either directly or through only 2-3 cells depending upon the leaf anatomy.
Consequently, the concentration of sugars increases in sieve tubes in comparison
to the surrounding mesophyll cells.
The movement of sugars from mesophyll cells to sieve tubes of phloem
may occur either through symplast (i.e., cell to cell through plasmodesmata,
remaining in the cytoplasm) or the sugars may enter the apoplast (i.e., cell walls
outside the protoplasts) at some point en route to phloem sieve tubes.
In the latter case, the sugars are actively loaded from apoplast to sieve
tubes by an energy driven transport located in the plasma membrane of these
cells. The mechanism of phloem loading in such case has been called as sucrose-
H+
symport or cotransport mechanism.
According to this mechanism (Fig. 15.5 ) protons (H+
) are pumped out
through the plasma membrane using the energy from ATP and an ATPase carrier
en­
zyme, so that concentration of H+
becomes higher outside (in the apoplast)
than inside the cell. Spontaneous tendency toward equilibrium causes protons to
diffuse back into the cytoplasm through plasma membrane coupled with transport
of sucrose from apoplast to cyto­
plasm through sucrose -H+
symporter located in
the plasma membrane.

18. The mechanism of the transfer of sugars (sucrose) from
mesophyll cells to apoplast is however, not known.
Phloem loading is specific and selective for transport sugars.
Both symplastic and apoplastic pathways of phloem loading are used in
plants but in different species. In some species however, phloem loading
may occur through both the path­
ways in the same sieve tube element or
in different sieve tube elements of the same vein or in sieve tubes in
veins of different sizes.
Experimental findings have revealed certain patterns in
apoplastic and symplastic loading of sugars in phloem (Table 15.1),
which appears to be related with the type of sugar transported to phloem,
type of companion cells (ordinary, transfer or intermediary) and number
of plasmodesmata (few or abundant ) connecting the sieve tubes
(including the com­
panion cells) to surrounding cells in smaller veins.
To some extent, phloem loading is also correlated with the family
of plant, its habit (trees, shrubs, vines or herbs) and climate such as
temperate, tropical or arid climate.

19. Phloem Unloading:
It occurs in the consumption end or sinks organs (such as
developing roots, tubers, reproductive structures etc.)
Sugars move from sieve tubes to receiver cells in the sink in­
volving
following steps:
(i) Sieve element unloading:
In this process, sugars (imported from the source) leave sieve
elements of sink tissues.

20. (ii) Short distance transport:
The sugars are now transported to cells in sink by a short distance
pathway which has also been called as post-sieve element transport.
(iii) Storage and metabolism:
Finally, sugars are stored or metabolized in the cells of the sink.
As with the phloem loading process, sucrose unloading also occurs
through symplast via plasmodesmata or through apoplast at some point en route
to sink cells.
Phloem unloading is typically symplastic in growing and respiring sinks
such as meristems roots, and young leaves etc. in which sucrose can be rapidly
metabolized. (Young leaves act as sink until their photosynthetic machinery is
fully developed, at which point they become sources).
Usually, in storage organs such as fruits (grape, orange etc.), roots (sugar
beet) and stems (sugarcane), sucrose unloading is known to occur through
apoplast.
However, according to Oparka (1986), phloem unloading in potato tubers
from sieve elements to cortical cells is a symplastic passive process. Because,
there are wide varieties of sinks in plants which differ in structure and function,
no one scheme of phloem unloading is available.