Plants create their own food through the process of photosynthesis, making them autotrophs. Additionally, the process' end result is referred to as a photosynthate or photo-assimilate. In plants, the phloem is a conducting tissue that carries photosynthate (food) to every part of the plant. While storage or the point of use is referred to as the Sink, the source of production or manufacturing is referred to as the Source. The source and sink connection notion is explained in the slides. The mechanisms cover these and other crucial aspects of the topic.

1. SOURCE : SINK
RELATIONSHIP 2010
Course title: Plant growth and Development
C. code: AGR-502
Presented By: Alhaji Alusine Kebe
19-Arid-4491
6th semester
Agronomy
2022
Assigned by : Dr. Imran Mahmood

2.  Introduction
 Definition of Source and Sink
 Photo-assimilate or Photosynthate
 Translocation of Photosynthate
 Factors affecting the Translocation of
photo-assimilates from source to sink
 Source as Sink or Sink as a Source
 Significant of Sink and source Relationship
 Vote of Thank

3. Introduction
Plants and some other microbes are autotrophs, i.e., they prepare
their own food.
Therefore the process of self food preparation in plant is known
as photosynthesis.
And the product is called photosynthate or photo-assimilate
The source of production or manufacturing is called Source while
storage or point of utilization is called Sink
 However, this food needs to be transported to different parts of
the plant.
Phloem is a conducting tissue in plants that transports food to all
the parts of a plant.
The movement of food from the source to the sink is known as
phloem translocation.

5. Source
The term source are referred to as the photosynthetically active parts
of a plant.
Sources include any exporting organs, typically mature leaves, that
are capable of producing photosynthate in excess of their own needs.
SINK
Sinks include any nonphotosynthetic organs of the plant and organs
that do not produce enough photosynthetic products to support their
own growth or storage needs. Roots, tubers, developing fruits, and
immature leaves, which must import carbohydrate for normal
development, are all examples of sink tissues.

6. However, a source is
not always a source,
and a sink is not
always a sink.
For example, leaves
that act as sources
when the plant is fully
grown are known to
act as sinks when the
plant is growing.
Source is not always a source, and a sink is not always a sink.

7. Source in Plants Sink in Plants
Description
The photosynthetically active parts of the plants
are referred to as the sources.
The areas of active growth and the areas of
storage are referred to as the sinks.
Loading/Unloading
The phloem elements are loaded at these sites. The phloem elements are unloaded at these
sites.
Nutrients
The source in the plant is responsible for
synthesising the sugars required for plant
growth.
The sinks in the plants use the sugars for
immediate use and store the rest for future
metabolic needs.
Example
The leaves act as a source in fully grown plants. Seeds, fruits, flowers, roots and storage organs
act as sinks in fully grown plants.

8. Photo-assimilate or Photosynthate
 The term photosynthate refers to products of
photosynthesis.
 Sap (photosynthate) in the phloem is not
translocated exclusively in either an upward or a
downward direction, and translocation in the
phloem is not defined with respect to gravity.
Rather, sap is translocated from areas of supply,
called sources, to areas of metabolism or storage,
called sinks.

9. Translocation
 Long-distance transport of solutes from one tissue system to
another is referred to as translocation.
 Translocation of photo-assimilates occur in phloem which can be
functionally characterized into three different zones along the
source-to-sink pathway.
 At the sources, these are often referred as collection phloem,
while at the sinks, these are termed as release phloem.
 The connecting pathways of the two are known as transport
phloem.
 Supply of photoassimilates from all sources does not reach all
sinks.

11. • Phloem loading (transfer of photosynthates from the mesophyll cells
of the leaf to the phloem sieve tube elements) and phloem unloading
(transfer of photosynthates from phloem sieve tube elements to the
cells of a sink) can be rate limiting and can affect translocation.
• During phloem loading the mesophyll cells are typically at a lower
osmotic potential (higher water potential) than the sieve tube
elements, thus the phloem loading requires an energy input to move
sugars into an area of higher concentration.
• Phloem loading generates increased osmotic potential in the sieve
tube elements, supplying the driving force for mass flow of
assimilates.
• It consists of movement of sugars from simplast (mesophyll cells)
into apoplast (cell walls) and then into symplast (phloem cells).

13.  Proximity of source to sink
 Developmental stage
 Vascular connections
 Modification in translocation pathways
 Sink strength
 Source strength
 Moisture stress
 Mineral nutrition
 Temperature
 Other climatic condition
 Light intensity

14. 1. Proximity of source to sink
• The source provides its photosynthates to sink located closest to
it. Leaves present on the lower portion of stem predominantly
supply underground parts of plants, whereas leaves present in the
middle portion of the stem supply in both upward and downward
directions.
• Partitioning of assimilates to the sink is generally higher when
sinks are closest to the source.
• Upper leaves usually export to the shoot apex, lower leaves to the
roots and middle leaves to both.
For Example, the upper expending leaves of soybean will import
more assimilates from the second leaf below them, which is on the
other side of the stem.

15. 2.Developmental stage
• Root and shoot apices are usually the major sinks during vegetative growth,
whereas developing fruits become the major sinks during reproductive
phase.
• At the time of senescence, mature leaves serve as sink.
• Thus, there is change in the source and sink status of the growing organs
during plant development.
• In Wheat with the during grain formation stage, the leaves start
transporting the dry matter to the grains and grains act as sink.
• Initially the translocation starts at a slow speed but after some time the
transportation rate becomes very high.
• In later growth phases rate of translocation becomes reduced. Dry matter
translocation also depends upon the demand.
• More the dry matter lost through respiration by grains higher will be the
translocation rate

16. 3. Vascular connections
• Sinks which have direct vascular connections with
source are preferred.
• The accumulation of dry matter into sink stops at
physiological maturity stage as the vascular connection
to the seeds / grains from the source is broken by
formation of abscission layer. The stage is marked by
maximum seed dry weight.
• During grain filling stage the dry matter in leaves show
a decline followed by a corresponding increase in dry
matter of grains.

17. 4. Modification in translocation pathways
Wounding interferes with the translocation pathway and
leads to the alteration of translocation patterns in
relation to proximity and vascular connections.
5. Sink strength
• Ability of a sink to store or to metabolize sugar
imports determines its capacity to compete for sugars
exported by various source tissues.
• Removal of a sink results in increased translocation
of sugars to other available and competing sinks.

18. 6. Source strength
 Source Size x Source activity
 Differences in CO2 fixation, C3 vs C4 CO2 fixation (Rubisco &
PEP Case)
 Leaf characters – size, thickness, mesophyll size, compaction,
vascular bundle
 Carrying capacity of sieve element (temp., H2O, nutrients,
hormone)
 Potential capacity of the sink to accumulate assimilates
 Young leaves act as stronger sinks in comparison to roots when
supply from a source is compromised.
Sink strength = sink size × sink activity

19.  Another type of source is a storage organ during the exporting
phase of its development.
 For example, the storage root of the biennial wild sugar beet (Beta
maritima) is a sink during the growing season of the first year, when
it accumulates sugars received from the source leaves.
 During the second growing season the same root becomes a source;
the sugars are remobilized and utilized to produce a new shoot,
which ultimately becomes reproductive.
 Thus, roots of the cultivated sugar beet (Beta vulgaris) can
increase in dry mass during both the first and the second
growing seasons, so the leaves serve as sources during both
flowering and fruiting stages.

20. • The leaves formed in the initial stages also works as sink as
they withdraw the nutrition from other parts of plants, but
very soon they start producing the dry matter or
photosynthates through photosynthesis and become source.
• Flag leaf in cereals provides more dry matter to the sink as
compared to other parts. It has the advantage of being
located near to the sink. It also receives more amount of solar
radiation and subsequently produce more dry matter.
• In wheat the grain yield is determined by either supply of
assimilates from source during grain filling or by the number
and capacity of kernels to be filled by the source.

21. Significant of Sink source Relation
• Source-Sink Relationships in Crop Plants and
Their Influence on Yield Development and
Nutritional Quality.
• For seed crops, yield is the cumulative result
of both source and sink strength for
photoassimilates and nutrients over the course
of seed development