8. Trans Location of Photo Assimilates

8/4/2019 8. Trans Location of Photo Assimilates

1/35

Translocation of photoassimilates

By

Fredeslinda C. Evangelista, PhD.


2/35

Photoassimilate- product ofphotosynthesis

Translocation- long distancetransport of photoassimilates

Translocation occurs in thephloem


3/35

Evidences for phloemtranslocation of photoassimilates

By girdling

Analysis of phloem exudateusing the aphid technique

Use of radioactive tracersusually on the leaf


4/35


5/35

Sievetubemember

stylet Phloem sapdroplet

Honeydewdroplet


6/35


7/35

The composition of phloem

exudate Principal constituent is sugar i.e. sucrose

Why sucrose?

It is nonreducing so less likely to reactwith other substances along the way.

It may also contain minerals, amino acids,proteins and hormones.


8/35


9/35


10/35


11/35


12/35

Direction of long distance

translocation in the phloem From sourceto sink

Source

net exporter or producer of

photoassimilate

a plant organ in which sugar is beingproduced by either photosynthesisor the breakdown of starch.

Examplemature leaf


13/35

From sourceto sink

Sink

net importer or consumer of

photoassimilate

Or a storer of sugar

Example- growing fruit

Direction of long distance

translocation in the phloem


14/35

Partitioning of assimilate

among sinks Sinks are competitive and are partitioned to

all active sinks

Partitioning depends on 3 factors

Nature of the vascular connectionsbetween source and sinks

Proximity of the sink to the source

Sink strength


15/35

sinks-photoassimilate will move preferentiallytoward the sink leaf above and in line (same

rank) with the source leaf


16/35

Sink strength- is a measure of the capacity of a sink toaccumulate metabolites


17/35

Mechanism of translocationin the phloem

By pressure flow/mass flow

Mass transfer of solute from source to sinkalong a hydrostatic (turgor) pressure gradient

Translocation is fundamentally a passive

process Transport is bidirectional


18/35


19/35

Pressure flow is the mechanismof translocation in angiosperms

Phloem sap flows from source to sink atrates as great as 1 m/hr, faster than can be

accounted for by either diffusion orcytoplasmic streaming.

Higher levels of sugar at the source

lowers the water potential and causeswater to flow into the tube.


20/35

Removal of sugar at the sinkincreases the water potential and

causes water to flow out of the tube.

The difference in hydrostatic pressuredrives phloem sap from the source tothe sink



21/35

(1) Loading of sugar into the sievetube at the source reduces the

water potential inside the sieve-tube members and causes theuptake of water.

(2) This absorption of watergenerates hydrostatic pressure thatforces the sap to flow along the

tube.



22/35

Pressure flow is the mechanism

of translocation in angiosperms

(3) The pressure gradient isreinforced by unloading of sugarand loss of water from the tube atthe sink.

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


23/35

Phloem loading

Initial transfer ofassimilate frommesophyllcells intosieve elements at thesource end

Movement of

sucrose frommesophyll tophloem(parenchyma) is by

the symplast


24/35

From the phloem parenchyma, thereare 2 possible routes:

May continue to companion cell-sieve-tube member complex via thesymplast.

Sucrose reaches sieve-tube membersby a combination of symplastic andapoplastic pathways.

Phloem loading


25/35


26/35


27/35

Evidence indicates that a sucrose-H+

cotransport mediates sucrose transport

between the apoplast and symplast Proton pumps generate an H+ gradient,which drives sucrose across the

membrane via a cotransport protein thatcouples sucrose transport with thediffusion of H+ back into the cell.

Phloem loading


28/35


29/35

symplastic pathway

Sucrose-H+ cotransport exists as a

mechanism for retrieving sucrose thatleaked from photosynthetic cells

To prevent unneccessary loss of sugarfrom the transport stream

Phloem loading


30/35


31/35

Cc-sc complex symplastically connectedwith adjacent cells- exhibit symplastic

loading Cc-sc complex symplastically isolated

exhibit apoplastic loading

Phloem loading


32/35

Phloem unloading

Final transfer of assimilate of the sieveelements into target cells

Mechanisms

By symplastic pathway

Flow maintained by hydrolyzingsucrose into glucose and fructose

By apoplastic pathway


33/35


34/35

Mechanisms

By apoplastic pathway

Passive unloading into apoplast(e.g. maize seeds)

Active unloading into apoplast (e.g.legume seeds)

Phloem unloading


35/35

Documents

8. Trans Location of Photo Assimilates