🌿 Source Sink Relationship
Source Sink Relationship.
This lesson provides exam-focused context on key concepts in crop physiology and connects the section topics for quick revision.
Source
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It is the regions of photoassimilates production
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Export photoassimilates
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Chlorophyllous tissues
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Leaves, stipules, fruit wall, young stem, pedicel, awns, peduncle, calyx, bract etc
Sink
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Regions of photoassimilates consumption
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Import photoassimilates
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Growing regions
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Storage organs – Fruit and Seed
Source strength
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Source Size x Source activity
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Differences in CO2 fixation (Rubisco & PEP Case)
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Leaf characters – size, thickness, mesophyll size, compaction, vascular bundle
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Carrying capacity of sieve element (temp., H2O, nutrients, hormone)
Sink strength
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Sink size x Sink activity
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Potential capacity of the sink to accumulate assimilates
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Competition among different sink
Source sink interaction
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Source sink equilibrium
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Small surplus source for stress
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High source size during sink differentiation
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Improve strength by activity
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Synchrony of sink organ development
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Increased HI is reached – increase DMA
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Reduce photorespiration in C3 plants
Evans (1983)
Reduced growth of non harvestable organ
Prolonged faster storage
Enhanced competition of storage organ
Enhanced competition of regulatory process
Reduced stem weight and height
Reduced root weight with adequate nutrient and H2O
Improved agronomic support (avoid biotic & abiotic stress)
Hormonal regulation
Developmental plasticity (small surplus source for stress)
Efficient system
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Quick export of photoassimilates to avoid end product inhibition
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Efficient root system
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More photosynthetic rate
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Optimum LAI (4 to 6)
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High photosynthetic rate & high DMA
Blackman’s law of limiting factor
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A process is controlled by several factors
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The phase of the process is limited by slowest factor
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Compensation mechanism working under canopy level
Dry matter accumulation (DMA)
G x E interaction; nutrients; CO2 fixation rate (path way); photorespiration; vascular
network; LAI & LAD; source-sink limiting condition; root-shoot balance
HI
Ye = Yb x h
/ HI = {Yield (Eco)/ Yield (Biol)} x 100
Improve Harvest index (HI)
Increase biomass production (DMA)
Synchronized development of reproductive organ
Reproductively determinate
High source strength at the time of sink differentiation
Reduced growth of non harvestable organ
Reduced leaf growth at reproductive stage with high LAD
Optimum LAI and early peak LAI
More prolonged and faster storage, enhanced competitiveness among of the storage organ High photosynthetic rate
Improved HI by increased size and number of sink organ
Decline in duration of Vegetative growth and increased duration of Reproductive
growth.
Limitations
Source: wheat, rice, pulses, oilseeds
Sink: bajra, ragi
Transport: sorghum, maize (green leaf at harvest; senescence of phloem
Parenchyma)
Sink limitation:
Late anthesis (Long Vegetative phase)
Indeterminate (Vegetative & Reproductive growth)
Vegetative growth at Reproductive phase
Less sink number and size
Hormonal imbalance
Any Stress
Multi-sink demand (nodules supply 25 – 75 % of N demand)
Source limitation:
Low canopy photosynthesis
Low optimum LAI
Slow peak LAI (lag vegetative growth)
Low LAD at filling
Early leaf senescence
Stress – nutrients, water
Plant Growth Regulators (PGRs)
ABA inhibit sucrose uptake in source (Loading)
Auxin promotes source uptake
Starch accumulation in chloroplast inhibit photosynthesis
ABA in leaves causes closer of stomata (Inhibit CO2 fixation)
Cytokinin delays senescence of source and sink
Cytokinin in sink increases photoassimilates import
Ethylene induces senescence process.
Summary Cheat Sheet
- Review each concept section above in sequence to connect definitions, processes, and applied crop-physiology outcomes.
- Focus on high-yield terms, pathways, and condition-dependent responses for exam-ready recall.
- Use the listed examples, comparisons, and cycles as rapid-revision anchors before practice questions.
References
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References
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