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Source sink relationship and different growth models
• TOPIC: CONCEPTS OF SOURCE AND
SINK –TRANSLOCATION OF
PHOTOSYNTHATES AND FACTORS
INFLUENCING TRANSPORT OF
SUCROSE AND CONCEPTS OF CROP
GROWTH MODELING- DIFFERENT
MODELS – ADVANTAGES AND
DISADVANTAGES OF CROP GROWTH
MODELS- MODELS FOR TESTING
PRESENTED BY : ZUBY GOHAR ANSARI
CONCEPTS OF SOURCE AND SINK –TRANSLOCATION
OF PHOTOSYNTHATES AND FACTORS INFLUENCING
TRANSPORT OF SUCROSE
• Introduction: Source is an organ which synthesis
the food material. In general we consider a mature
leaf as source. An organ which stores the
photosynthesis is called a sink. In plant 92.94% dry
matter comes from photosynthesis and 6 to 8%
comes from mineral matter. Photosynthates
generally moves in the form of sucrose from source
to sink. This source is used for growth and
development . Photosynthates stored as starch or
sent for growth purpose in the shape of sucrose,
depends upon the source – sink relationship.
HOW TO DIFFERENTIATE SOURCE AND
• There are 3 criteria to differentiate source from sink
• 1. Morphology : Seeds, fruits, roots, tubers, are
generally considered as sinks. Mature leaf is considered
• 2.Direction of transport: Source is a plant part which
exports the photosynthates sink imports the
• Contrast: Leaf at young stage is a sink till it gets 75% of
its total expansion it will not 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
• Seed: A germinating seed is generally
considered as source as it supplies food
material for radical elongation.
• However, a growing seed on a ear head is
treated as a sink.
• 3. Metabolism: Source is an organ which
capable of synthesizing photosynthates to
satisfy its demand as well as to supply the
extra food material to other growing points.
• Sink is an organism which utilizes these photo
assimilates for growth or stage.
TYPES OF SINK
• 1.Primary sink: Economically useful parts
• Seeds- rice , timber- forest crops, fruits-
mango , leaves-green, stem-sugarcane
• Primary sinks are those which are of prime
importance to man.
• 2.Secondary sink: Rhizomes, tubers, corns.
• In a plant tubers are produced as storage organs. This
plant might also produce seeds. These storage
organs are produced much earlier to the
reproductive organs(seeds ultimately) development.
• Sec. sinks are also useful for vegetative reproduction.
• 3.Alternate sinks: Fiber, lint or stem in perennial
plants are called as alternate sinks.
• Eg. Sun hemp, flax-fiber
• Dry weight of these alternate sinks develops after the
• In cotton lint is formed after the seed development.
• 4.Additional sinks: Rhizobium nodules, galls, parasitic
weeds, symbiosis or parasitism.
• 5.Metabolic sinks: Stem tips, root tips where
photosynthesis are used for active growth and
• Finally the classification of source or sink is highly
dynamic in nature in nature which changes with time,
plant age, organ age etc.
• Some organs act as both source & sink
• Green pods of beans, okra etc., which are capable of
photosynthesizing and also draws resources as sink
(to a greater extent they are sinks).
• 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), weight(agricultural crops).
• Weight is the most dependable measure of sink size
is expressed as
• SINK SIZE=SINK CAPACITY*SINK ACTIVITY
• Sink capacity: It is the max. space available for the
accumulation of photosynthetic products.
• In grain crops it is expressed as number and size of
• Eg: no. of panicles/plant *avg. no. of spikelets per
panicle*specific grain weight
• Sink Activity: It is the inherent capacity of sink to
create a translocation gradient for photosynthates
assimilated at source.
• Always the photosynthates translocated in the form
of sucrose from leaves to grains where it gets
converted to starch. The rate of conversion
influences the rate of translocation.
• If rate of conversion of sucrose to starch is more
• Translocation gradient will be more.
• Higher will be the rate of translocation
• Measures of source: source is generally
measured in the terms of Cm square/ plant i.e. on
area basis eg: LAI
• SOURCE STRENGTH=SOURCE SIZE * SOURCE
• =Leaf area/plant*Average photosynthetic rate per
• Eg: LAI*NAR
• The source size and activity is highly dynamic or
sensitive to cultural, nutritional and climatic factors.
• Source: Region which export the assimilates
• Eg. Expanded green leaves
• Two 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
• 2.Accumulated assimilate: Mainly as stored food
material to serve as some for seedling growths.
These accumulated assimilates are from plant parts.
• Eg. Endosperm of germinating seeds, cotyledons,
• SINK: Region which imports the assimilates Eg.
Growing points of roots, shoots, storage organs(
pods, rhizomes, tubers, seeds and fruits)
• There is a relationship between source & sink
• A. In cereals reproductive organs initially serve as
sink and later when seeds formed serve as source for
• B. This relationship was given by Mason and Mask
well (1928 working as cucurbitacae fruit serve as sink
while leaf serves as source.
• C. In Maize husk contributes enormously for grain
• D. Flag leaf will serve as source in cereals to about
50% for grain development. Rest of the organs like
glumes and other organs serve for the rest 50%.
• Metabolic sink: The utilization of assimilates
during respirations give rise to energy to plants and
in turn produce the growth.
• Carbohydrates metabolism: Utilized in
respiration- in turn give rise to energy-and finally
growth of the plants.
• Beevers (1969) done extensive work on metabolic
• Warren and Wilson(1967) mentioned that
• Source strength (capacity) formed by 2
• =source size* source activity
• The size means how much assimilates are
available for export. Activity means rate of
photosynthesis source size Eg. Expanded area
i.e. leaf area and no. of leaves
• Source activity : P.S. rate mg CO2 dm per
square per hour or NAR (ULR)
• Sink strength (capacity):Sink Size*Sink
SINK SIZE SINK ACTIVITIES
No. of Rep. structures
No. of growing points
Wt. of flowers
Shoot and root tips
Growth rate of sink i.e.
Growing points viz Shoot
Root apex or Rep.
Like flower buds and
• This relationship can be used for single plant or for a
crop depending upon the objective of study.
• Yield constraints or limitations may be due to
source limitation or sink limitation or both.
• Source size limitation: Suppose 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
• 2. Mutual shading of leaves with in the crop.
• 3. Intercropping due to different crops. Above three
considerably affects the source activity.
• Sink Limitation: It is due to
• A. Floret sterility ( cereals)
• B. Insect damage
• C. Flower drop due to many factors
• Possible reasons for flower drops:
• 1. Limitation of P.synthesis.
• 2. Limitation of N availability.
• 3.Reduced light intensity in plant canopies.
• 4. Canopy temperature
• 5. Hormonal factors
• 6. Gas exchange in canopies
• 7. Humidity in canopies
• 8. Soil & water factors
• 9. Water logging
• 10. Shading and salinity
• Sink activity is limited due to unfavorable environmental
conditions ( High temperature in wheat, low temperature
in monsoon season)
• Source Size Manipulation: These can be experimentally
accounted for source limitation . If it occurs during rep.
stage it will be crucial and affect yield. one can know its
effect by reducing source size by way of defoliation (
25%, 50%, 75%, 100% & 0%)at reproductive stage (50%
flowering ) as was done in sorghum.
• In Maize by cutting the leaf size to 25%, 50%, 75% of
leaf to adjust source size and see the effect on yields
components in cereals and in legumes( G.nut).
• Instances where hormonal treatment prevent flower
drop and increase in yield one may explain in such
genotypes that the extra source was utilized by the
increased sink size.
• The source activity can be manipulated by reducing
light intensity (shading of crops)with muslin cloth
with different layers to have different degree of
shading. The light intensity can be measured under
different degree of shades. Light intensity without
shade is taken as full light (Zero shade, 75% shade,
50% shade, & 25% shade).
• Sink manipulation: Major sink is reproductive
parts. It does not mean that it is the only sink but the
vegetative meristems (apical shoots) and root
meristems etc., whose sink is stronger in
requirement of , assimilates.
• Size of reproductive sink: Size of inflorescence
(cereals), No. of pods & capsules (dicots), weight of
tubers (potato) form the sink size. If the
inflorescence is reduced by half or one fourth or any
%, the sink size can be manipulated. In case of
number, reduced the number by removing flowers by
25%or any %. Then study the export of
Photosynthates to the sink, since the sink size is
reduced, the photosynthates may be diverted to
other plant parts.
• Seed size in cut ear head is big as compared to in
normal ear head. Sometimes the assimilates
accumulates in source itself causing leaf wt. to
increase. Once the process of accumulation of
assimilates takes place in leaves, starch content
increases and finally the P.S. rate goes down.
• Sink Activity : RGR of sink should have the capacity
to store materials transported to it under natural
• Temp stress-At extreme the RGR is less.
• Light- It is important for reproductive structures.
• Sink capacity can be manipulated by growth
retardants. Sink act as source. Reproductive
structures can be covered (shading) and light
intensity is reduced to reduce sink activity
• The translocation of assimilates mainly sucrose
transported through phloem sieve tubes to the
• Excess Source →Translocations →Sink (Increase in
• If excess assimilates does not increase the sink
size the reasons may be
• 1. Physical construction in the phloem vessel.
• 2.The phloem loading may be more at source side
due to fewer diameters at the delivery and
receiving point, less photosynthate reaches to
• 3.Chemical nature of metabolites itself.
MODELS OF SOURCE- SINK
• For easy demonstrations. Source is of 2 kinds
• A. Fixed Source (Accumulated assimilates )-
seed and tuber at the time of germination.
• B. Producing (Current assimilates) and
exporting the assimilation.
• We can study the source in seed, the rate &
amount of translocation to the root & shoot
(can be studied) in germinating seedlings
which is a simple growth model.
• Rooting of leaves: If leaves are cut and pit in
water, develops callus from petioles & produce
the roots. EX. Cowpea, Tapioca & groundnut.
• A mature leaf along with petioles taken & provide
light to leaves, these will produce photosynthesis
and translocate to petioles (units)sink. After
certain time sink is not in a position to take
assimilates and these assimilates are stored in
source itself. When this is put in humid condition
(in a beaker of water) the petiole at its base
produces a white callus tissue and then callus will
give rise to roots. This is the simple model to
study source and sink relationship.
DYNAMICS OF SOURCE AND SINK
• Relationship b/w sink activity & source activity
• A. Sink activity dictates the source activity.
• Case 1: In detached rooted leaves of dwarf bean the
photosynthetic rate is related to the activity of the
• In this system the photosynthetic rate will be very
less initially however when it starts developing the
root system rate of photosynthesis also starts picking
• Increase root demand for photosynthates → source
• Other argument: Development of roots → ↑
cytokinin concentration → ↑ source activity.
• More the demand higher the photosynthetic activity.
• Case 2: Nature of root stocks control the net
assimilation rate in spinach, sugar beet.
• Where sugar beet is used as root stock it improved
the rate of photosynthesis on spinach scion.
• Case 3: The assimilation rate of potato is controlled
by tuber activity at least during bulking.
• Conclusion: Assimilation in these plants is linked
with size & activity of their sinks.
• Case 4: In Rice,
• Genotypes with High sink & source ratio.( High H.I.)
• High photosynthetic rate per unit leaf area was
• Very low amount of carbohydrates are accumulate in
vegetative & structural organs.
• Genotypes with Low sink & source ratio.(Low H.I.)
• Low photosynthetic rate per unit leaf area was
• Vegetative & structural organs accumulate more
amounts of photosynthesis
• Case 5: In Wheat, Flag leaf gives more contribution
• Assimilation rate ↓ immediately after the removal of
• Case 6 : In tomato a compound leaf
opposite to trees ( Inflorescence)
provides photosynthates for the
development for the development of
fruits on a trees. If that trees is removed,
leaf translocated assimilates to another
trees which increases the yield 10-15% ,
but the total yield decreases.
• B. Increasing source capacity increases grain
• Additional nitrogen supply during flowering & pod
filling stage increases pod yield.(This is to increase
the source activity but not the source size)
• CO2 fertilization: Maintaining plant under 600 ppm
CO2 from flowering to harvesting increasing the
yield, pod set, pod no., seed no., seed set also
• Low temp. during grain filling pd. Increases seed wt.
& yield ( because of reduction in the dark
• Source – Conclusions: Source size may not be a
limiting factor, but source activity limits the yield.
• Increased availability of photosynthates from source
increases the yield.
• Source development & distribution in the canopy must
• Characters associated with high source sink
• In such varieties leaves remain green even at harvest.
• Seed No. & seed wt. per pod remain almost constant to
the first formed pod to last formed.
• Large No. of biomass is partitioned into vegetative parts (
stems, leaves etc.)
• Very low H.I.
• In pulses & oilseeds most of the varieties are of
determinate type both vegetative & reproductive growth
• Low source to high sink ratio:
• Leaves show very early senescence forced
maturity of the plant is seen.
• Pod wt., pod No., seed wt./pod decreases
from the first formed pod to last formed pod.
• Very low amount of biomass is partitioned to
• H.I. is very high.
• Most of the varieties of this type are of
• Traditional varieties in several crop plants
possess greater source than sink (they are
• During crop development leafy ness is
reduced and sinks size gradually increased.
• What happens if greater source capacity is
• It results in poor crop performance as
fertilization & any other cultural practices
results in a greater foliage & poor productivity.
• Sink No. though increased flower drop
• In few crop plants like vegetables (cucurbits, beans,
tomato) pulses (determinate cowpea, soybean) &
oilseeds ( bunchy type g.nut, sunflower) though the
sink in terms of more number is initiated they drop
prematurely due to lack of adequate supply of
photosynthates proper time.
• General recommendations: In crops growing
assured water supply and available nutrients.
• Large No. of flowers must be produced
synchronously in one week to 10 days. This is to
avoid the competition among sinks.
• Photosynthates should not be wasted in such
structures like peduncle & thalamus more than a
• Increase in duration of grain filling.
• Determinate growth of inflorescence terminal & few
auxiliary inflorescences are needed.
• Translocation of photosynthates to sink is important.
It is generally influenced by the sink characters such
• 1. Rate of utilization of carbohydrates by sink or rate
of conversion should be more.
• 2. No. of competing sinks with in a plant (more No.
of sinks more competition).
• 3. No. of competing sinks with in on
• 4. Distance of sink from the source.
• 5. Endogenous hormones concentration ( like
Auxin, GA3 & cytokinins).
• More the hormone production more will be
• One of the ways for increasing food production is
either genotype may be modified to suit the
prevailing environment or environment could be
modified and made conducive to the existing
• What is environment and what are the climatic
factors that limits yield.
• BILLINGS(1952) defined environment of a plant as a
external forces and substances affecting growth,
structure and reproductions.
• Envt. is dynamic, constantly changing from season to
season, day to day and hour to hour.
• The primary factors that determine the
productivity of crop plants are water, temp., soil
physical, and chemical factors. The interaction of
pests with envt. is a major factor is crop
• For increasing production from present cultivated
land by reduction of climatic stress conditions.
• For thorough understanding of crop, micro
climate interactions and interactions among soil,
crop, insects, weed and weather- a new
methodology called crop modeling was
introduced – a new technique to crop scientists.
CROP GROWTH MODEL (MONTEITH,
• It is defined as “ quantitative scheme to predict the
growth, development and yield of a crop under
specific set of genetic and environmental factors”.
• Reynolds and Acock (1985) divided crop stimulation
models into 2 groups.
• 1.Empirical model: in these models one or several
independent variables representing weather or soil
characteristics are related to dependent variable
representing crop response such as growth or yield .
• These models do not explain cause and
effects of relationship but provide most
practical approach for the assessment
and prediction of yield and are largely
used many countries like India. This
growth model indicates the relationship
b/w variables without reffering
underlying physical or biological
structures that exists b/w variables.
• 2. Mechanistic Models: These models explain
not only the relationship b/w the weather and
yield but also the mechanism of these models.
Explain relationship of influencing dependent
• In these models, processes like
photosynthesis, respiration, ET, nutrient
balance involved are calculated numerous
equations and integrated. Many models
contain a mixture of above two.
• NORMAN (1979) further divided the models
into 3 arbitrary categories.
1. Statistical Models : These models express the
relationship b/w the yield and yield components and the
weather parameters. In these types relationships are
measured in a system using statistical techniques. They
do not require detailed information about the plant
involved, but depends mainly on statistical techniques
such as correlations or regressions of plant and
environmental variable. Example: crop yield weather
• 2. Para materialization or crop weather analysis models:
this model proposed by Bair (1973). In these model, it was
assumed that the crop yields depend basically on 3 agro
meteorological variables solar energy temp., and soil
moisture or evapotranspiration. These 3 variables modify
each other during the life cycle of a crop and produce
positive or negative effect on final yield.
3.Analog- physical Models: use for detailed
plants- envt. formulations. Among them,
Dynamic stimulation models are
prominent. It predicts the change in crop
status with time as a function of
Ex: Soil water content of oat at certain
depths through out the season.
Changing number of bolls on a cotton
plants with advancement of season.
ADVANTAGES OF CROP GROWTH
• 1.To reduce site specific long term field experiments.
• 2. To interpret climatological records in terms of
production potential and limitations.
• 3.To evaluate risk associated with management
• 4.To communicate research results b/w locations.
• 5.To conceptualize multi disciplinary activities.
• 6.These models give pre-season and in-season
management discussions on cultural practices like
fertilization, irrigation and pesticides.
• 7.It is easy way for avoiding experimentation.
• 8. These models can assist policy makers by
predicting soil erosion, leaching of agricultural
chemicals, effects of climatic changes and large yields
• 9.Modeling relates to plant growth and development
from seedling to maturity.
• 10.The variability of growth and development is
understood by basic concepts explained on
• 11.The response of plants to their macro and micro
environment are actually quantified.
• 12.It also helps in knowing missing data to have a
complete picture of processes.
• 13.It will give new ideas leading to experimental
• 14.Models will indicate priorities for applied
• 15.It will help the manager in making suitable
• 16.It evaluates expected returns of soil and crop
• 17.It enhances understanding of biological and
physical system and their interactions.
• The information required for production of growth
• A. Aerial environment:
• 1.Mean day/night temperature
• 2.Total amount of visible radiation in each photoperiod.
• 3.Total net radiation in each photo and dark period.
• 4.Length of photoperiod.
• 5.Profile of visible radiation through crop land.
• 6.Profile of visible temperature through crop land.
• 7.Profile of visible water vapor content through crop land.
• 8.Profile of net radiation through land crop.
• 9.Daily wind velocity.
• B.Soil environment:
• 1.Amount of water in soil layers at the start of
• 2.Soil water content at wilting point and field capacity.
• 3.Amount of available N and Pin each soil layer.
• 4.Temperature profile through soil.
• 5.Rate of fixation and release of N and P in soil.
• C.Crop characteristics:
• 1.Light perception-PAR.
• 2.CO2 fixation.
• 3.Carbon partitioning.
• 4.Tissue expansion.
EXAMPLES OF GROWTH MODELS
• 1.Sorghum: SORGF model-used to evaluate crop-
• Simulates timing of development of plant, dry matter
partitioning and partitioning of dry matter into different
plant parts based upon the stage of development as
influenced by environmental conditions. Huda et al,
• 2.Pearlmillet: SORGF with slight modifications. Based
on dry matter accumulation, dry matter partitioning and
evapo transpiration models simulates final grain yield.
Both simulated and observed grain yield and TDM are in
• 3. Maize: Weaich et al(1996) at Australia for
simulating maize emergence using soil and climate
• 4. Cotton: Staggren boog et al(1994)-GOSS Y M
model- to assess Water use in cotton and also as a
tool for scheduling irrigations in SAR.
• 5. Soybean: GLYCIM simulation model-Acock et al
• 6. Groundnut: PNUT GRO-model, Singh et al 1992.
to predict pod yield in different environment as
determined by season, short days and moisture
• 7.Wheat: CERES Model- Havi et al 1993 climate
change and wheat yields in Punjab.
• Input data required for SORGF- a sorghum
Leaf number Total no. of leaves produced
Leaf area Max. area of each individual leaf
Planting data Sowing data, Plant population, Row width,
Row directions, Depth of sowing
Climatic data Max. temperature, Min. temperature, Solar
radiation, Rainfall (Daily from planting to
Soil data Soil water holding capacity, Initial available
water content, location data, latitude.