UNIT 2.3.2

Source-Sink Relationships

How plants allocate resources for growth and yield

🎯 After this unit, you will be able to:

Define sources and sinks in plant transport
Explain how sink strength determines assimilate partitioning
Describe how source-sink relationships change during development
Apply source-sink concepts to improve crop yield

⚖️ The Source-Sink Concept

In plant physiology, sources are organs that produce or release more photosynthates than they need, while sinks are organs that import photosynthates for growth, storage, or respiration .

Key concept: Plants constantly balance source supply with sink demand. Understanding this balance is essential for improving crop yield—we want to maximize sugar production AND ensure it gets to harvestable parts (fruits, seeds, tubers) .

🌿 [Diagram: Source-sink overview showing leaves (sources) and fruits/roots (sinks) with phloem connections — to be inserted]

🔋 What Are Sources and Sinks?

🌿 Sources

Net exporters of photosynthates

Mature leaves (primary sources)
Green stems and pods
Storage organs during mobilization (e.g., germinating seeds, sprouting tubers)

Source strength = rate of production × ability to export

🥔 Sinks

Net importers of photosynthates

Roots and tubers
Developing fruits and seeds
Meristems and young leaves
Flowers and reproductive structures

Sink strength = sink size × sink activity

🔄 Did you know? The same organ can be a sink at one developmental stage and a source at another. For example, a young leaf imports sugars (sink), but once mature, it exports sugars (source). A potato tuber is a strong sink during bulking, but becomes a source when it sprouts .

📊 Sink Strength: What Makes a Strong Sink?

Sink strength determines how much assimilate a sink can attract. It has two components :

Sink Strength = Sink Size × Sink Activity

Sink size: Physical size of the sink organ (number of cells, volume)
Sink activity: Metabolic activity—rate of sugar import, conversion to storage products (starch, proteins, oils), and respiration

Sinks compete for assimilates. Stronger sinks get more sugars. Factors that increase sink strength include :

Factor	Effect on sink strength	Example
Cell division rate	Increases demand for building materials	Young fruits with many dividing cells
Storage capacity	More space to store sugars/starch	Tubers, grains, fleshy fruits
Metabolic activity	Rapid conversion of imported sugars to starch, proteins, or oils maintains concentration gradient	Oilseeds (high metabolic rate), grains filling
Hormonal signals	Hormones like auxin, cytokinins can activate sinks	Fruit set and development
Phloem unloading efficiency	Ease of sugar movement from phloem into sink cells	Some genotypes have more plasmodesmatal connections

🔄 Source-Sink Dynamics

Sources Can Be Limited

If source activity is reduced (e.g., by defoliation, disease, low light), sinks compete for limited assimilates. This can lead to :

Smaller fruits and seeds
Fruit drop (abscission of weaker sinks)
Reduced yield

Sinks Can Be Limited

If sink demand is low (e.g., few fruits set), sugars accumulate in leaves, which can feedback-inhibit photosynthesis . This is why fruit trees with a light crop often have lush growth but smaller fruit the next year .

🍎 Alternate Bearing in Apple Trees

Many fruit trees exhibit alternate bearing (biennial bearing)—a heavy crop one year followed by a light crop the next. This is a classic source-sink phenomenon :

In the "on" year, heavy fruit load (strong sinks) depletes carbohydrate reserves and inhibits flower bud formation for next year
In the "off" year, few fruits allow reserves to rebuild, leading to abundant flower buds

Management strategies include fruit thinning in the "on" year to balance source-sink relationships and reduce biennial bearing .

📈 [Graph: Relationship between source supply, sink demand, and photosynthesis rate — to be inserted]

🚪 Phloem Unloading: How Sugars Enter Sinks

Sugars must exit the phloem to reach sink cells. Three main pathways exist :

1. Symplastic unloading

Moves through plasmodesmata from sieve elements to sink cells. Common in growing tissues like root tips and young leaves .

2. Apoplastic unloading

Sugars are exported into the cell wall space (apoplast), then taken up by sink cells via transporters. Common in developing seeds and fruits where maternal and filial tissues are separated .

3. Transition (mixed) unloading

Both pathways operate, often with developmental changes .

Seed Development: A Special Case

In developing seeds, the maternal seed coat and the filial embryo/endosperm are not connected by plasmodesmata. Sugars must be unloaded into the apoplast and then taken up by the embryo . This creates an opportunity for regulation—the mother plant can control how much sugar reaches each seed .

🌾 Did you know? In wheat, the number of endosperm cells that divide early in grain development determines potential grain size. Later, starch filling rate determines final weight. Both are influenced by sink activity and assimilate supply .

🥇 Sink Priorities: Which Sinks Get Sugars First?

Sinks compete for assimilates, and there is a general hierarchy :

Roots and shoots (maintenance respiration) — highest priority, essential for survival
Meristems (growth) — next priority
Developing leaves (becoming sources)
Reproductive structures (flowers, fruits, seeds)
Storage organs (tubers, bulbs, grain filling) — lowest priority, filled after other needs met

This hierarchy explains why stress during grain filling (drought, defoliation) reduces yield—the plant prioritizes survival over reproduction .

🌽 Maize Kernel Abortion

In maize, drought stress during pollination and early grain fill can cause kernel abortion—especially at the tip of the ear. The basal kernels (closer to the cob attachment) are stronger sinks and outcompete tip kernels for limited assimilates. This is why tip kernels often fail to fill under stress .

🧑‍🌾 Manipulating Source-Sink for Higher Yield

1. Increasing Source Strength

Optimize leaf area index (LAI) — ensure enough leaves to capture light
Extend leaf duration — delay senescence (stay-green varieties)
CO₂ enrichment — increases photosynthesis rate
Fertilization — especially nitrogen, to maintain photosynthetic capacity

2. Increasing Sink Strength

Fruit thinning — reduces number of sinks but increases strength of remaining sinks (larger fruit)
Breeding for larger sinks — more cells per fruit, larger endosperm capacity
Improving phloem unloading — enhance transporter activity or plasmodesmatal connections
Hormonal treatments — cytokinins can increase sink activity

3. Balancing Source and Sink

The ideal is a balance where sink demand matches source supply. Too few sinks (low demand) feedback-inhibits photosynthesis. Too many sinks (excessive demand) leads to competition and smaller individual organs .

Scenario	Outcome	Example
Source-limited	Sinks compete; yield reduced; fruit drop	Defoliation, low light, drought
Sink-limited	Sugars accumulate; photosynthesis downregulated	Poor fruit set, alternate bearing "off" year
Balanced	Optimal yield and quality	Well-managed orchards, optimal plant populations

🍎 Practical Applications in Horticulture

Fruit Thinning

Thinning excess fruits early in development reduces sink number but increases sink strength of remaining fruits. Benefits include :

Larger fruit size
Better fruit quality (sugar content, color)
Prevents alternate bearing
Improves return bloom next year

Pruning and Canopy Management

Pruning affects source-sink balance by :

Removing sinks (branches, shoots)
Improving light penetration to sources (leaves)
Balancing vegetative and reproductive growth

Rootstock Selection

Rootstocks influence sink strength of roots and scion vigor. Dwarfing rootstocks (like M9 for apple) reduce root sink strength, directing more assimilates to fruits .

🍇 Grape Berry Sink Strength

In grapes, the onset of ripening (véraison) is marked by a dramatic shift in sink activity. Berries switch from importing sugars for cell division to accumulating sugars for flavor development. Understanding this helps growers time irrigation and nutrient applications for optimal fruit quality .

🇪🇹 Ethiopian Applications

Enset (False Banana)

Enset is a unique crop where the pseudostem and corm are harvested for starch. Understanding source-sink relationships helps optimize :

Timing of transplanting
Leaf removal practices (traditional harvesting of leaves for fiber may affect source strength)
Optimal harvest time when corm sinks are maximally filled

Coffee

Coffee bean development is a classic sink process. The endosperm (bean) imports sugars from the parent plant. Shade management in coffee plantations affects source strength (photosynthesis) and ultimately bean yield and quality .

Maize in Ethiopia

In the maize-growing regions of Ethiopia, understanding kernel abortion under drought stress (source-limited) could help farmers manage risk through plant density adjustments and supplemental irrigation at critical times .

📌 Unit Summary

Sources (mature leaves) produce and export sugars; sinks (fruits, roots, meristems) import and use them
Sink strength = size × activity — determines how much assimilate a sink attracts
Sinks compete for assimilates; stronger sinks get more
Phloem unloading can be symplastic (through plasmodesmata) or apoplastic (across membranes)
Source-sink balance affects yield: source-limited → competition; sink-limited → feedback inhibition
Management strategies: fruit thinning, pruning, rootstock selection, optimizing LAI

Reflection question: A mango grower in Ethiopia notices that some trees produce many small, low-quality fruits while others produce fewer but larger, sweeter fruits. Using source-sink concepts, explain what might be happening and suggest management practices to improve fruit quality.

📌 Key terms introduced

Source Sink Sink strength Sink activity Source strength Phloem unloading Symplastic unloading Apoplastic unloading Assimilate partitioning Alternate bearing Kernel abortion

✅ Check your understanding

Define "source" and "sink" in plant physiology. Give two examples of each.
What two factors determine sink strength? How could a breeder increase sink strength in a crop?
Explain why drought during grain filling can cause kernel abortion in maize.
How does fruit thinning improve the size and quality of remaining fruits?
A greenhouse tomato crop has lush leaves but small fruits. Is this likely source-limited or sink-limited? What management change would you recommend?

Discuss your answers in the course forum.

Plant Biochemistry for Horticulture · HORT 202 · Dilla University · Last updated March 2026