a simple explanation of Translocation of Solutes through phloem and Mass flow hypothesis

2. SUBMITTED BY
ANANDHALAKSHMI A. L
BSc Botany 2016-19
GOVERNMENT COLLEGE KOTTAYAM

3.  Long distance transport of materials through xylem and
phloem.
 Xylem translocation involves the transport of absorbed
water and minerals from roots to leaves.
 Phloem transport involves the transport of the organic
food from green parts to non-green parts for consumption
or storage.
 Translocation of photosynthates, translocation of
photoassimilates or translocation of organic solutes.
 It is polar and occurs from source to sink.

8. Downward
translocation
 Occurs from leaves to
stem and roots
 The food synthesised in
leaves are transported to
sinks in the stems and
roots.
Upward
translocation
 Occurs from leaves to
developing buds,
flowers and fruits for
consumption and
storage.
 In germinating seeds,
stored food is
transported upward
from cotyledons or
endosperm to the
growing apex
 Transport of organic
food during the
appearance of new
foliage
 During the renewal of
growth on deciduous
trees in spring.
Radial(lateral)
translocation
 Occurs from the cells
of pith to those of cortex
and epidermis
 It is common during the
development of fruits and
seeds

9.  Occurs near stem apices or at the
growing points of stem nodes.
 The food received from mature leaves is
transported upward to growing points
and downward to storage regions or
consumption regions.

11. Active
Physiological process
Passive
Physical process

12.  Phloem loading
 Phloem unloading

13.  The initial transfer of photosynthate from leaf mesophyll to
the seive elements of minor veins.
 It is the transfer of sugars from mesophyll cells to phloem
cells.
 It occurs against the concentration gradient. Hence it
involves energy expenditure
 Thus, its an active process
 Selective process that occurs at the source
 Amino acids, reducing sugars and organic ions are most
usually taken up.
 Phloem loading
symplastic
apoplastic

14.  Seive tubes are connected to adjacent cells
through plamodesmata.
 Primitive and less efficient mechanism
 Plants with symplastic loading have low relative
growth rate.
 Van Bel and Gamalet (1992) postulated that the
mode of phloem loading depends on the degree of
plasmodesmatal connectivity i.e, connectivity,
symplastic translocation.

15.  Takesplace in the absence of
plasmodesmata connections.
 Acts against environmental adversaries,
such as drought and low temperature.
 Plants with apoplastic phloem loading have
relatively higher growth rate and better
adaptations to environmental stresses.

16.  Reverse of phloem loading
 Transfer of solutes from sieve tubes to
the sink
 Occurs in a concentration gradient
 Can be symplastic or apoplastic

17. Short-distance translocation
 Protoplasmic streaming
 Contractile protein hypothesis
 Diffusion hypothesis
 Activated diffusion theory
Long-distance translocation
 Munch’s mass flow hypothesis

18. Protoplasmic
streaming
hypothesis
•de Vries in 1885
•Later on accepted by Cany (1952) and Thaine (1962)
•Organic solutes move inside the sieve tubes along with protoplasmic streaming and they
diffuse from one tube to the next through cytoplasmic strands in the sieve plate
•Solutes are carried downward and upward
Contractile
protein
hypothesis
• Proposed by Fenson and Williams in 1974
• A network of interconnecting microfibrils in the sieve tube elements bind the
solute molecules and move them by their contraction
Diffusion
hypothesis
• The translocation of solute occurs by the simple physical process of
diffusion from the place of manufacture (high concentration) to place of
consumption (low concentration)
• Not convincible because the rate of translocation in phloem is much higher
than that could be achieved by simple diffusion
• The translocation rate is also affected when the supply of oxygen is stopped
or when the tissue is poisoned.

19. Activated
diffusion theory
• Proposed by Mason and Maskell (1936)
• Theory states that acceleration of the diffusion of
food materials in the phloem is activated by cellular
metabolism.
Electro-osmotic
theory
• Proposed by Fensom (1957) and Spanner (1958)
• States that an electrical potential across the sieve
plate is responsible for faster rate of liquid flow
• The sugar molecules tightly link to K+ ions in the
sieve tube sap and are carried along with the
movement of the K+ ions in the sieve tube sap
• The circulation of K+ can maintain electric
potential across the sieve plate

20.  Proposed by Munch (1926)
 Osmotic mechanism controls downward movement of organic
substances in phloem.
 During photosynthesis the cell sap concentration of mesophyll cells
at the top is maintained high. As a result osmotic pressure
increases and causes absorption of water. These two factors
combine to produce a high turgor pressure in mesophyll cells.
 The mesophyll cells remain interconnected through small pores
known as plasmodesmata. These connections occur from cell wall
of mesophyll to sieve tubes.
 Through these connections some of the solution is forced down to
sieve tubes due to torgur pressure.
 The loss of solutes from mesophyll cells would then be
compensated by newly formed photosynthetic products and
movement of water from xylem
 The movement is always from a region of higher to lower turgor
pressure.
 A turgor pressure gradient is established between source and sink.

22. Solute
concentr
ation high
• As Carbohydrates
are continuously
synthesized
Osmotic
potential
increases
• As a result they
absorb water from
neighboring cells
Turgor
pressure
increases
• Allows some of the
cell contents to pass
into the sieve tubes
• Phloem loading at
the source
Mesophyll cells are
interconnected
through
plasmodesmata
Through these some
amount of solution is
forced down to sieve
tubes under the influence
of turgor pressure
Newly formed
photosynthetic
products
Compensation
of solutes
Mesophyll cells

23. At the sink end like roots and storage
organs sugar is unloaded
Cells either consume it or convert it
into insoluble storage products. This
is phloem unloading
Creates an increase in osmotic
potential and a decrease in turgor
pressure
In sieve elements solute
concentration increases due to
phloem loading
As a result water potential lowers
and an osmotic uptake of water
from the nearby xylem occurs
Results in a higher turgor or
hydrostatic pressure at the source
end.
A turgor pressure gradient is
established and a mass flow of
solutes takes place through this
gradient
Phloem translocation is an active
process involving no direct input
of metabolic energy (it has been
now proved that requires minimal
energy requirement)

24.  Widely accepted
 It is supported by the fact that phloem sap,
containing high sugar content, exudates from
the cut end of a stem.
 Maskell and Zimmermann have revealed that
in phloem there exists a positive
concentration gradient for the mass
movement of solutes from higher to lower
concentration

25.  Explains about unidirectional flow whereas in plants its usually
bidirectional
 Does not account for an active role of cytoplasm and strongly
holds that phloem translocation is absolutely a physical process.
 Swanson proposed and demonstrated that phloem loading and
phloem translocation are active process. Munch’s hypothesis holds
that phloem translocation is a passive physical process
 It is generally held that sieve pores remain plugged with dense
protoplasm preventing the mass flow of solute molecules
 According to this hypothesis high turgor pressure is needed to
overcome the resistance at the cross wall plasmodesmata which is
not possible.

26.  Define
 Source
 Sink
 Downward translocation
 Upward translocation
 Radial translocation
 Bidirectional translocation
 Explain mechanism of translocation
 Write a short note on theories about translocation
 Explain Munch’s mass flow or pressure flow hypothesis
 What are the objections against pressure flow hypothesis?
Reference : Plant Physiology by S. N Pandey and
B K Sinha,
Plant Physiology and