Source–Sink Dynamics

What source–sink balance is

Source–sink dynamics describe the balance between: - Sources: tissues that produce carbohydrates (mainly mature leaves) - Sinks: tissues that consume carbohydrates (roots, fruits, shoots, storage organs)

Growth and yield depend on how effectively sugars move from source to sink.

Common sources

Mature, healthy leaves
High-light canopy regions

Source strength depends on: - Light availability - Leaf area - Nutrient status - Temperature

Common sinks

Growing roots
Young leaves
Flowers and fruits
Storage tissues

Sink strength depends on: - Developmental stage - Temperature - Hormonal signalling - Water availability

Why source–sink balance matters

Yield does not fail because photosynthesis stops — it fails when: - Sink demand exceeds source supply - Transport is disrupted - Respiration consumes carbohydrates faster than they are produced

This often happens before visible stress appears.

Environmental influences

Source–sink balance is strongly affected by: - Radiation sum - Night temperature (respiration losses) - VPD (transpiration and transport) - Root-zone oxygen

Warm nights are particularly damaging because they increase respiration without increasing photosynthesis.

Sink limitation vs source limitation

Source-limited systems

Low light
Small leaf area
Nutrient stress

Symptoms: - Poor growth - Low sugar levels - Reduced yield potential

Sink-limited systems

Excessive vegetative growth
Limited fruit load
Poor partitioning

Symptoms: - High leaf sugars - Soft growth - Delayed maturation

Agronomic implications

Yield is limited by the weakest link in the source–sink chain
Increasing fertiliser cannot overcome low radiation
Reducing stress improves partitioning more than increasing feed

Key concept

Plants fail quietly when energy allocation breaks down, not when inputs run out.

Encyclopaedia