SOURCE-SINK relationship.pptx

PLANT NUTRITION AND CROP
PRODUCTIVITY
SAID H. MARZOUK
SOURCE - SINK RELATIONSHIPS AND
LIMITATIONS ON PLANT GROWTH RATE
AND YIELD
binmarzouk@gmail.com

Source – sink
Source : regions of photoassimilate
productions
Sink: regions of photoassimilate
storage
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.

• The flow of water in plants is almost
always from roots to leaves.
• Translocation of sucrose can be in
any direction – depending on
source and sink location and
strength.
• Examples:
• Beta maritima (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

• The continued growth of any plant depends primarily on
photosynthetic activity in the leaves and the transport of
organic compounds from the leaves to heterotrophic cells.
• The source supplies assimilates (C-compounds) to the
sink.
• The sink accepts and consumes the assimilates for its own
growth or accumulates them for a certain period.

• Translocation
• Translocation is the movement of organic solutes e.g.
SUCROSE, from a source to a sink through the phloem
by means of mass flow.
• The sucrose transporter gene in Rice is OsSUT1

• Phloem loading is the process of
loading carbon into the phloem for transport to
different 'sinks' in a plant.
• Sinks include metabolism, growth, storage, and
other processes or organs that need
carbon solutes to persist

• Plants take up a range of resources from the environment
in order to sustain growth, including water, carbon dioxide
and mineral nutrients.
• Plant organs responsible for taking up a particular resource
from the environment are known as the ‘source’ for that
resource.
• Whilst organs utilizing the resource for growth,
metabolism or storage are the ‘sink’ for that resource.
• Source and sink organs must be in balance in order for
plants to function effectively in their environment, and this
balance depends upon a coordinated relationship between
sources and sinks.

Source and Sink Organs for
Carbon
and Nitrogen
In the simplest case, consider a
plant composed of two organs
only: leaf and root.
Leaves carry out photosynthesis
and are net sources of carbon,
whilst roots are net sinks for
carbon.
In contrast, roots are net sources
for nitrogen which they take up
from the soil, whilst leaves are
net sinks for this element

• Since both leaves and roots require both carbon and
nitrogen, the net role played by each organ represents the
overall balance of resource transfer.
• All plant cells use carbon for respiration, growth and the
building of essential metabolites, and nitrogen for building
proteins, enzymes and genetic material.
• Thus, mature photosynthetic leaves have a relatively small
carbon sink activity and a relatively large carbon source
activity, meaning that they are net sources for carbon.
• Roots have a small nitrogen sink activity and a greater
nitrogen source activity making them net sources for
nitrogen.

• CO2 from the atmosphere is fixed by plants in the process
of photosynthesis, to generate triose phosphate which is
converted to sucrose, which is a readily available store of
carbon, or starch, for longer-term storage.
• The amount of starch stored in leaves varies between
different species; many temperate grass species also use
the polymer fructan for long-term carbon storage (Paul and
Foyer, 2009).
• Structural components – such as cellulose, which is a
major component of plant cell walls – are also important
repositories for fixed carbon.
• Besides sucrose, some plant species transport
oligosaccharides and sugar alcohols.

• Nitrogen is reduced and then used to create amino
acids, the building blocks of proteins, a process
which occurs in both root and shoot.
• Once assimilated into amino acids, nitrogen can
be stored as protein for long-term storage, or
amino acids for readily available nitrogen to be
combined with carbon fixed in photosynthesis.
• In tropical legumes and some other species,
nitrogen is stored as ureides (Rao et al., 2017).

• The balance between carbon- and nitrogen-containing
metabolites is an important indicator of source–sink status.
• For example, the ratio of free amino acids to sucrose
expresses the relative availability of nitrogen and carbon
• With a high ratio indicating an excess of available nitrogen
and a low ratio indicating an excess of available carbon.
• This balance is attuned to enable plants to optimize their
growth and development.
• For example, the rate of photosynthesis is correlated with
the rate of nitrogen assimilation, whilst increased
availability of nitrogen leads to a reduction of starch
synthesis, making carbon available for assimilation into
amino acids. binmarzouk@gmail.com

HOW TO DIFFERENTIATE SOURCE AND SINK
• There are 3 criteria to differentiate source from sink
1. Morphology : Seeds, fruits, roots, tubers, are generally
considered as sinks. Mature leaf is considered as source.
2.Direction of transport: Source is a plant part which
exports the photosynthates sink imports the assimilates.
• Contrast: Leaf at young stage is a sink till it gets 75% of
its total expansion it will become source.
• A fully expanded mature leaf is a source. A senescing leaf
is also treated as a source because the stored food material
from it will be exported back to growing points.

• Measures of sink
• Sinks are measured in terms of carbohydrates, oils, and
proteins etc.
• largely based on the biological energy units they produce.
• In general sinks are measured in terms of number
(coconut), (in this case weight is not important but number
is important)
• Volume (timber), the stem biomass
• Weight (agricultural crops). Eg rice

Strength of source and sink
• The strength of a sink or source depends upon the
size of the organ and the rate at which it is taking up
or utilizing a particular resource:
• Source strength = source size × source activity (1)
• Sink strength = sink size × sink activity (2)
• Where strength is net transport, size refers to the
biomass of the organ (g) and activity is the specific
resource uptake rate. based on Geiger and Shieh
(1993). binmarzouk@gmail.com

Measures of source:
Source Strength = source Size * Source Activity
= Leaf area/plant * Average photosynthetic rate per
unit area.
• Eg: Leaf Area Index*Net Assimilation Rate
• The size means how much assimilates are
available for export.

• The source size and activity is highly dynamic or
sensitive to , nutritional and climatic factors.
• Eg. N deficiency significantly decrease the
photosynthetic CO2 assimilation capacity of
leaves. (Havlin et al., 2005).
• Potassium (K) is essential nutrients for all plants
and is involved in many vital plant functions such
as plant enzyme activation, energy capture from
photosynthesis, N uptake and protein synthesis

• Starch is a primary insoluble carbohydrate produced by
higher plants and consists of amylose and amylopectin
as a major fraction.
• The enzymes responsible for synthesis of starch, starch
synthetase, is activated by potassium.
• Under inadequate potassium levels, the level of starch
declines, while soluble carbohydrates and nitrogen-
based compounds accumulate (Mae ,1997).

• Other nutrients like Mg, Mn and Co also play major role
in photosynthesis, their by acting upon oxidation
reduction in PSII and PSI
• Other factors like light intensity, CO2 concentration,
temperature and moisture also play a major role in
photosynthesis and so affects source strength.

Types of assimilates are:
1.Current assimilates: They are translocated and
linked to photosynthesis
Eg. Sugars produced through carbon fixation- these
assimilate contribute for maximum phytomass.
• These are carried out by expanded leaves.

2. Accumulated assimilate: Mainly as stored
food material to serve as some for seedling
growth.
• These accumulated assimilates are from plant
parts.
• Eg. Endosperm of germinating seeds,
cotyledons, Sugarcane sets.

Sink strength
• Sink Strength = Sink size * sink Activity
• Sink size: It is the maximum space available for the
accumulation of photosynthetic products.
• In grain crops it is expressed as number and size of grains.
• Eg: no. of panicles/plant *avg. no. of spikelets per
panicle*specific grain weight, sink activity refer to
photoassimilate use and storage. Eg tubers, seeds
If rate of conversion of sucrose to starch is more
↓
Translocation gradient will be more.
↓
Higher will be the rate of sink

• Eg. The major part of the starch in rice grains at
harvest is the photosynthetic product of the leaves,
which is translocated from the leaves directly to the
growing grains after flowering.
• The ripening of rice grains can be considered a
process of accumulation of contents in a container.
• The grain yield may be limited by either the size of
the container or the amount of its contents .
• If the container is small the yield cannot be high. On
the other hand, even if the container large, the yield
cannot be high if the contents are limited.

Limitations of source and sink
• Yield constraints or limitations may be due to source
limitation or sink limitation or both.
Source size limitation:
Source size is limiting due to defoliation (decrease in LAI)
caused by certain environmental factors or due to diseases
or sucking insects
i.e. any factor that limits the leaf area development affects
the source size.
• Similarly source activity refers to NAR which is limited
due to
1. Certain environmental factors like light intensity. If light
intensity is not sufficient then source activity in decreased.

2. Mutual shading of leaves within the crop.
Sink Limitation: It is due to
A. Floret sterility ( cereals)
B. Insect damage
C. Flower drop

Possible reasons for flower drops:
1. deficiency of P synthesis.
2. deficiency of N availability.
3. Reduced light intensity in plant canopies.
4. Canopy temperature and Water logging
5. Shading and salinity
6. Hormonal factors

Relationship between source-sink and growth
• Growth is controlled by contiguous physiological and
developmental mechanisms, but initially depends upon
ecological adaptations and evolutionary history.
• Plants with different growth strategies succeed in different
ecosystems, and in different niches within those
ecosystems.
• Proximate causes of growth rate variation include both
external and internal factors (Körner, 1991).

• Externally, plants are affected by a number of abiotic and
biotic factors including nutrient and light levels,
temperature, competition, and all of which influence the
supply and demand for essential resources, and plants
must ensure that growth rates are adjusted accordingly
(Bloom et al., 1985).
• Internally, plant growth is constrained by molecular,
physiological, and developmental processes.
• These internal processes can all be understood within the
framework of source–sink interactions.

• Since plants are sessile can influence external factors to
a limited degree, the internal factors are well controlled.
• As a consequence, it is these internal interactions of
source and sink activity that must be responsible for the
large fundamental variation in relative growth rate
among species under common environmental
conditions.
• The primary focus for many of these is boosting
photosynthetic carbon acquisition (source activity), yet
sink activity is also believed to limit grain development
in many major crops (Acreche and Slafer, 2009).

• Sink activity is maintained through breeding and genetic
techniques to provide crops which is more productive
(Reynolds et al., 2012).

Improve Harvest index (HI)
• Increase biomass production
• Synchronized development of reproductive organ
• Reproductively determinate
• Reduced growth of non harvestable organ
• Reduced leaf growth at reproductive stage
• Decline in duration of Vegetative growth and increased
duration of Reproductive growth.

Summary
• 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

REFERENCES
• Gray, S. B., Dermody, O., Klein, S. P., Locke, A. M.,Mcgrath, J. M., Paul, R.
E., et al. (2016). Intensifying drought eliminates the expected benefits of
elevated carbon dioxide for soybean. Nat. Plants 2:16132. doi:
10.1038/nplants.2016.132.
• Paul, M. J., and Foyer, C. H. (2001). Sink regulation of photosynthesis. J.
Exp. Bot. 52, 1383–1400. doi: 10.1093/jexbot/52.360.1383.
• Rao, I. M., Beebe, S. E., Polania, J., Grajales, M., Cajiao, C., Ricaurte, J., et
al. (2017). Evidence for genotypic differences among elite lines of
common bean in the ability to remobilize photosynthate to increase
yield under drought. J. Agric. Sci. 155, 857–875. doi:
10.1017/S0021859616000915
• Hav_e M, Marmagne A, Chardon F, Masclaux-Daubresse C. 2016.
Nitrogen remobilisation during leaf senescence: lessons from
Arabidopsis to crops. Journal of Experimental Botany 68: 2513–2529.
• Forde BG, Cutler SR, Zaman N, Krysan PJ. 2013. Glutamate signalling via
a MEKK1 kinase-dependent pathway induces changes in Arabidopsis
root architecture. Plant Journal 75: 1–10.

• THANK YOU

SOURCE-SINK relationship.pptx

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