2. 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.
3. SOURCE AND
SINK
SOURCE:
Any transporting organ capable of mobilizing
organic compounds or producing photosynthate in
excess of its own needs, e.g., mature leaf,
SINK:
Non photosynthetic organs and organs that do not
produce enough photoassimilate to meet their own
requirements, e.g., roots, tubers, developing fruits,
immature leaves.
Translocation of sucrose can be in any direction
depending on source and sink location and strength.
5. SOURCE STRENGTH :
Source strength = Source size x source activity
It is determined by the rate of photosynthesis and the efficiency of photosynthesis
(C3 &C4) Cycle
High plant density
It is influenced by the leaf characteristics such as leaf area, number of mesophyll
cells, number of Sucrose-H+ Symporters and the ability of the sieve elements to
carry the photosynthates.
6. SINK STRENGTH
Sink strength = Sink size x Sink
activity
It depends up on the potential capacity of
the sink to accumulate the photosynthates
It is also significantly influenced by the
distance between the source and sink
7. TRANSLOCATION
Translocation of sucrose can be in any direction –
depending on source and sink location and strength.
The flow of water in plants is almost always from
roots to leaves.
Examples:
Beta maritime (wild beet) root is a sink during the
first growing season.
In the second season the root becomes a source,
sugars are mobilized and used to produce a new
shoot.
• In contrast, in cultivated sugar beets roots are sinks
during all phases of development.
9. MECHANICS OF SOURCE SINK
• Both the source and the sink can grow individually, or there may be a ratio
between them that allows for growth. Three alternative scenarios can be visualised
in crop plants:
1) The source might have a larger physical area than the sink;
2) The sink could have a larger size than the source; and
3) Both could be in dynamic equilibrium.
10. Condition 1: If the source exceeds the sink
• Excess Photosynthates:
The plant produces more photosynthates
(carbohydrates, primarily) than the sink can
accommodate. If the sink is small (e.g., not
enough reproductive organs or not enough growth
in fruits/grains), some of the excess sugars may be
trapped in the source tissues (such as leaves or
stems)
.
• Poor Leaf and Plant Health:
The excess photosynthates that are not effectively
moved to the sink may accumulate in the leaves or
stem, which can lead to leaf distortions (e.g.,
twisting or discoloration). This can also result in a
decline in the photosynthetic rate (Pr) of the
leaves, reducing the plant’s overall productivity
11. • Environmental and Nutrient
Impact:
• External factors like soil-water conditions,
weather, and nutrient availability can influence
the balance between source and sink. For
instance:
• Low nitrogen (N) levels can cause cereals like
rice and wheat to develop abundant leaves but
weak panicles (flower clusters), with ineffective
tillers (secondary stems that produce grains).
• Nutrient deficiencies (such as phosphorus (P),
zinc (Zn), manganese (Mn)) or toxicities (like
iron (Fe) and aluminum (Al)) can also lead to a
situation where the plant develops sufficient
leaves but lacks the proper sink development to
convert the excess photosynthates into grain.
12. Condition 2: If the sink exceeds the source
• In crops like fruit trees (mango, guava,
citrus), vegetables (cucurbits, tomato), and
pulses (pea, gram, cotton), the sink size
(e.g., flower buds, fruit set, pods, bolls)
exceeds the source (photosynthetic
capacity).
• Early Decline of Sinks:
The sinks tend to decline early due to a
lack of assimilates (photosynthates) needed to
support their growth.
• Potential Yield Increase:
If fruit set can be raised by just 1-2% in crops
like mango, guava, and citrus, crop yields
can increase by 50-100%.
• Breeding for Larger Sink Size:
Breeding has been used to increase the sink
size in cereals and grain crops, but even with
an adequate sink size, grain production
remains low.
13. • Leaf Area Index (LAI) and Grain Output:
In crops like rice, increasing the Leaf Area
Index (LAI) does not lead to higher grain
output and instead approaches a plateau
(Venkateswarlu, 1976).
• Effect of Increased LAI:
As LAI increases, the distance between
spikelets and grains widens, limiting the
potential for grain production.
• Leaf Area vs. Functional Efficiency:
Sometimes, the actual leaf area may exceed
the required amount, but the functional
efficiencies of the leaves are much lower,
reducing productivity.
14. Condition 3: An equilibrium maintained
between source and sink
• At balanced nutrient levels , the source and sink work in a harmony the plant
produces enough photosynthates to meet the needs of the developing reproductive
organs. As a result, the energy produced by the leaves is efficiently directed to the
sink, leading to better crop yields and improved productivity
• This equilibrium can vary based on environmental conditions, such as soil fertility,
and plant genotype. Different plant varieties may show differing efficiencies in how
well the source and sink are balanced.
15. SOURCE AND SINK CONSTRAINTS
Yield constraints or limitations may be due to source limitation or sink limitation or both.
SOURCE CONSTRAINTS
Reduced Leaf Area Index (LAI):
A drop in LAI limits leaf area development,
reducing the source capacity.
Reduced Net Assimilation Rate (NAR):
NAR decreases due to factors like defoliation
from severe weather, pests, or diseases, limiting
photosynthesis.
Reduced Light Intensity:
Reduced light reaching the plant canopy can also
diminish photosynthetic activity, weakening the
source.
• SINK CONSTRAINTS
Floret Sterility:
Floret sterility can significantly reduce sink capacity
(e.g., fewer grains or fruits).
Nutrient Supply Limitations:.Lack of essential
nutrients can hinder the development of sinks.
Environmental Stressors
Factors like canopy temperature, water logging, and
salinity can impair sink function.
Hormonal and Climatic Factors
Hormonal imbalances and unfavorable weather
conditions (e.g., heat stress) can impede sink
development and function.
16. How to enhance yield by Source-Sink relationship ?
• Improving photosynthetic C assimilation
• Nitrogen-use efficiency
• Rubisco efficiency
• Water use efficiency Transgenic interventions, recently demonstrated that
reduction in stomatal aperture or density can improve WUE without
reducing C assimilation or Crop yield
• Leaf anatomy (C3, C4, and CAM)
• Species specific variations in photosynthetic capacities and modes of C
fixation
17. Regulating Feed back inhibition of Photosynthesis
Enhancing the constitutive non-rectifying AKT2 Variant
Compartmentation and control of Apoplastic sucrose Ameleoration feed back
inhibition) (
SWEET proteins Sucrose leakage in Sieve elements (leaked out sucrose is utilised
by sieve element maintainance) reducing sucrose leakage
low oxygen supply is likely to limit ATP production and, hence, energization of
phloem loading
• Enhancing Photosynthesis
18. Genes involved in the regulation of source sink dynamics
• Sucrose Transporters (SUTS):
•
• These genes encode proteins that responsible
for transporting source to sink sucrose are from
• They are regulated by hormones cytokinins and
auxins
• Starch Synthase (SS):
• These genes encode enzymes that are
involved in the synthesis of starch in sink
tissues
• They are regulated by various factors
including light intensity and hormonal
signals
They include SS1, SS2, SS3 and SS4
Invertases
They are involved in the breakdown of
sucrose
They include IV1, IV2 and IV3
They are regulated by hormones and
developmental cues
Cellulose synthase (CESA)
CESA1, CESA3 and CESA6
Transcription factors
Several TFs are involved in regulating gene
expression related to source-sink dynamics.
They include BZIP11, WRKY75 and NAC029
19. MANIPULATION OF SOURCE SINK
1.Defoliation and Leaf Senescence:
• Defoliation, or the removal of leaves, impacts the source of
assimilates. The leaves are the primary sites of photosynthesis,
and their removal reduces the plant’s capacity to produce the
sugars and other metabolites required for growth.
• Conversely, delaying leaf senescence (the process of leaf aging
and breakdown) can extend the period during which the plant
continues to photosynthesize.
• The retention of green leaf area is important because it allows for
continued assimilation of carbon into sugars that are essential for
seed and fruit development.
20. 2.Decapitation:
• Decapitation, or the removal of the apical bud, has
a significant effect on plant growth.
• This technique can maintain chloroplast integrity,
reduce chlorophyll loss, and alter sink strength,
thus positively impacting growth and yield.
• For example, In soybean, decapitation at specific
growth stages (such as 35 days after sowing, when
the main stem has four nodes) combined with the
application of growth regulators, has resulted in
improved pod set, increased numbers of pods per
plant, and higher seed yield.
21. 3.Employment of growth retardant:
• The use of growth retardants, like mepiquat
chloride (MC) and paclobutrazol, is an effective
strategy to manipulate plant growth and optimize
the source-sink relationship.
• By controlling excessive vegetative growth and
enhancing canopy structure, these chemicals help
to extend the duration of photosynthesis, delay leaf
senescence, and ensure better allocation of
assimilates toward developing seeds or fruits.
22. 4.Light Quantity and Source-Sink Balance:
Light intensity directly influences photosynthesis, which is
the primary source of energy for plant growth.
• Lowering light intensity can reduce the rate of
photosynthesis, which in turn decreases the plant’s ability to
produce assimilates (such as sugars) required by the plant’s
sink tissues (fruits, seeds, etc.).
•
Under reduced light intensity, the source-sink balance is
affected as the plant’s ability to supply enough energy for
growth and reproduction is compromised. This might limit
the growth of sinks (like developing fruits) or cause a shift in
the allocation of assimilates to maintain the plant’s survival.
23. 5.Abiotic Stress and source sink manipulation :
• Plants under abiotic stress (e.g., salinity, alkalinity)
face challenges in maintaining an efficient source-
sink relationship. Stress often reduces photosynthetic
efficiency and can impair the plant’s ability to
produce sufficient assimilates for sink growth.
•
By modifing the light spectrum, plants under stress
(such as salinity or alkalinity) can achieve a more
favorable source-sink balance, which promotes better
growth and yield despite challenging conditions.
24. 6.Reducing inflorescence and flower production
Reducing inflorescence and flower production through source-sink
manipulation can have significant effects on plant yield, depending on the
balance between the source (photosynthesizing leaves) and the sink (developing
flowers, seeds, or fruits).
• The results from various studies indicate that the strength of the source (leaf
photosynthesis) plays a more prominent role in limiting yield during
flowering and reproductive stages than sink capacity.
• By adjusting sink demand (e.g., through deflowering, spike removal, or
selective inflorescence removal), plants can better allocate resources to their
remaining reproductive organs, leading to improved seed or fruit
development and overall yield.
25. CONCLUSION
Sources and sinks affects both growth and yield. The growth may be controlled by
both source and sink strengths.
Such control involve physically manipulating the plant or its environment: for
example, source activity may be altered by elevated CO2, or shading, while sink
activity may be altered by sink removal or sink chilling
However, modern genetic approaches may now be used to alter source and sink
activity and enhance growth and production
• Also application of nutrients at right time, right amounts and right place is
necessary
