Plant Physiology & Development
Source–Sink Carbon Allocation
Leaves that export photosynthate act as sources, while actively growing, storage, root, and reproductive tissues act as sinks. Source–sink strength changes with plant stage, tissue age, environment, and stress, so carbon allocation is dynamic rather than fixed.
Key concepts
- Mature photosynthetically active leaves are major carbohydrate sources
- Expanding leaves, roots, meristems, flowers, and developing seeds can act as sinks
- Sink strength depends on tissue demand and capacity to import and use assimilates
- Stress can reduce source supply, sink demand, transport capacity, or several at once
- Training and pruning change canopy structure and may alter the balance among competing sinks
What to measure or observe
- Track which leaves are fully expanded versus still developing
- Record growth rate of competing shoots and reproductive sites
- Compare canopy light distribution with lower-canopy senescence
- Note whether growth slows after root, water, temperature, or light stress
- Use repeated observations rather than inferring carbon allocation from one plant photograph
Common mistakes
- Assuming every large leaf is always a net source under all conditions
- Treating pruning as a guaranteed way to redirect a fixed amount of energy
- Ignoring root demand when interpreting shoot growth
- Assuming larger sink number always increases total production
- Using source–sink language as a substitute for measurements of light, water, and plant development
Visuals this lesson still needs
- Whole-plant source–sink flow diagram
- Leaf age and source transition sequence
- Vegetative versus reproductive sink map
- Pruning and competing-sink comparison