1) Phloem transports photosynthates like sucrose from source to sink organs. Source organs like leaves produce more photosynthates than needed, while sink organs like roots and fruits import photosynthates. 2) The rate of phloem transport depends on source and sink strength, which are functions of organ size and activity. Strong sources and sinks maximize productivity. 3) Sucrose is the primary solute loaded into and transported via the phloem. Pressure flow hypothesis explains the mass flow of sap through sieve tube elements and companion cells from source to sink.

1. Source Sink relationship – Phloem transport
Lecture 15

2. Source and sink
Source - Producers
Any exporting region that produces
photosynthates above and beyond that of its own
needs

3. Any non-photosynthetic organ or an organ
that does not produce enough photosynthates
to meet its own needs
Sink – Utiliser or Storage unit
Roots, tubers, developing fruits, and immature leaves

6. Under normal conditions, productivity of the crop
depends on
Source size : Leaf area & No. of leaves
Source activity : Photosynthetic efficiency of leaf
Source strength: Source size x source activity
(Two components)
1. Source strength
2. Sink strength

7. Sink size: No. of flowers, grains/panicle, fruits/branch,
tubers/plant
Sink activity: Duration of filling of sink & Rate of filling sink
(Seed filling duration and seed filing rate)
Sink strength: Two components
Sink strength = Sink size x Sink activity

8. Imbalance in Source & Sink
Low productivity of crops: Due to Source & Sink limitations
Source limitation
Inadequate source - production of less photo assimilates
Eg. Plants with low LAI (Pulses & Oil seed crops)
Sink limitation
Sufficient sink might not be available
Eg. Cereal crops with insufficient spikelet's in panicles

9. DRY MATTER DISTRIBUTION or PARTITIONING
Photosynthates
Flowers
Stem or shoot
Root
Harvest Index or partitioning efficiency (PE)
Economic Yield / Biological Yield x 100

10. Which part is involved in
photosynthates translocation?
Phloem

11. Translocation of solutes - Phloem Transport
Phloem
1. Food conducting Tissue – Complex in nature
2. Primary form of food for transport - Sucrose
3. Kinds of cells – Sieve Tubes & Companion cells
4. Living cells

12. Sieve tubes formed by cells that are joined
end to end
End walls have perforations – Sieve Plate
Cytoplasm of the sieve tube cells is
continuous through the perforations of the
end walls
This helps in the transport of materials
Companion cells are smaller cells
associated with the sieve tubes
It is in contact with the sieve tube cell
through pores in the wall
Sieve Tube

13. Sieve Tube
Sieve Tube
Element
Sieve Tube
Element
Sieve Plate
Companion Cell
Phloem
Plasmodesmata

14. What is transported in phloem?
Sugars - Sucrose

16. Form of Carbohydrate translocated in Phloem
Primary form – Sucrose
In some species
Raffinose (Trisaccharide)
Stachyose (Tetrasaccharide)
Verbascose (Pentasaccharide)
Sorbitol

17. Mechanism of Phloem Translocation
Passive – Long Distance Active – Short Distance
Physical Process Physiological Process
Munch Hypothesis

18. Solute Movement
Water
Movement
A B
Pressure Flow Theory – Mass Flow Hypothesis
Munch Hypothesis

19. Phloem Loading and Unloading
Phloem Loading
Transport of carbohydrate from mesophyll cells to phloem
Phloem Unloading
Transport of carbohydrate from phloem sieve tube to sink

20. Source (Leaf)
Mesophyll cell
Photosynthesis
Sugar production
OP Increases
Water Influx
Turgor Pressure
Pushes sap to
Phloem of Petiole
Stem
Sink (Grain / Fruit)

21. Water
Water
Source
Sink
Loading
Unloading

22. 1. Source produces organic molecules
2. Glucose is photosynthetic product
3.Glucose converted to sucrose for transport
4. Companion cell actively loads the sucrose
5. Water follows from xylem by osmosis
6. Sap volume and pressure increased to
give Mass flow
7. Unload the organic molecules by the
companion cell
8. Sucrose stored as the insoluble and
unreactive starch
9. Water that is released is picked up by the
xylem
10. water recycles as part of transpiration to
re supply the sucrose loading

24. Materials translocated in the Phloem
1. Sugars
2. Amino acids
3. Organic acids
4. Protein
5. Inorganic ions – K, Cl, P,& Mg
6. Hormones – Auxins, Gibberellins, Cytokinin & ABA

25. Manipulation of Source & Sink
By using nutrients and PGR’s, the source and sink
size and activity may be manipulated
Nitrogen – Increases source size and activity
Phosphorus – Increases source and sink size and activity
Potassium – Increases sink size and activity
Boron – Sink size and activity
Plant Growth Regulators
Nutrients
Plant Growth Promoters – Auxin, Gibberellin & Cytokinin – ↑Source & sink
Plant Growth Retardants – Ethylene & ABA – Reduces source size
Plant Growth Inhibitors – CCC, TIBA, Mepiquat chloride – ↑Sink activity

26. Thanks lot