Source Leaves and Sink Tissues
Classify changing source and sink roles and explain how sink strength, vascular connection, organ stage, and genotype influence allocation.
Educational reference · evidence, sources, and limits shown below
Classify changing source and sink roles and explain how sink strength, vascular connection, organ stage, and genotype influence allocation.
Terms to know
- Source
- Organ or tissue exporting more assimilate than it imports over a defined interval.
- Sink
- Organ or tissue importing assimilate for growth, storage, maintenance, or metabolism.
- Sink strength
- Combined ability of a sink to attract and use imported resources, often described as size multiplied by activity.
- Remobilization
- Transfer of stored or structural nutrients from one tissue to another.
Core science
Source and sink are functional states, not permanent organ names. Mature illuminated leaves commonly export sucrose. Young leaves, roots, meristems, wounded tissues, developing flowers, and seeds commonly import. As a leaf expands and its photosynthesis exceeds its own demand, it transitions toward source behavior. During senescence, nutrients can be remobilized even while photosynthetic capacity declines.
Sink strength depends on organ size, metabolic activity, developmental program, vascular connection, hormones, temperature, nutrient status, and genotype. A large sink that cannot unload or metabolize imported carbon may accumulate sugars and feed back on source photosynthesis. Additional source leaf area does not guarantee greater flower or total biomass when transport, sink demand, or feedback limits carbon use.
A controlled comparison of two drug-type chemovars found major differences in flower biomass, nutrient allocation, leaf starch, source activity, and sink-related gene expression. The study supports genotype-specific source-sink behavior; it does not establish one nutrient or carbohydrate recipe for all chemovars.
Why this matters in cultivation
- Protect productive source leaves unless a defined canopy, disease, or airflow objective justifies removal. Judge defoliation by remaining functional leaf area and subsequent organ growth, not by light penetration alone.
- When flower growth stalls, evaluate source capacity, roots, transport, stage, temperature, light, nutrition, and genotype before adding carbohydrate or bloom products.
Measure and record
Organ classification
Source, transitional, sink, senescing, or uncertain; basis and date.
Source capacity
Leaf area, light exposure, assimilation or chlorophyll proxy, age, and health.
Sink activity
Organ number, size, growth interval, developmental stage, and dry mass.
Allocation
Root, stem, leaf, flower, and discarded dry mass with a defined harvest standard.
Genotype/context
Clone/seed line, environment, nutrition, training, plant density, and replicate.
Common misconceptions
Correction: Young, shaded, damaged, or senescing leaves may import or contribute little net carbon.
Correction: Position, stage, light, vascular connection, genotype, and local environment differ.
Correction: Uptake route, metabolism, osmotic effects, and microbial risk must be demonstrated.
Evidence limits
Source-sink status changes over hours and development. Organ dry mass and sugar pools are indirect; flux measurements or isotopic tracing are needed for direct allocation evidence.
Related encyclopedia topics
- THC-ENC-065, THC-ENC-067, THC-ENC-070, THC-ENC-072, THC-ENC-079–080, and training/flowering lessons.
Source notes
- Taiz L, Moller IM, Murphy A, and Zeiger E. Plant Physiology and Development. 7th ed. Oxford University Press, 2022. Publisher record
- Jost R et al. Sink Strength, Nutrient Allocation, Cannabinoid Yield, and Associated Transcript Profiles Vary in Two Drug-Type Cannabis Chemovars. Journal of Experimental Botany. 2025;76:152-174. Open source
- Danziger N and Bernstein N. Too Dense or Not Too Dense: Higher Planting Density Reduces Cannabinoid Uniformity but Increases Yield per Area in Drug-Type Medical Cannabis. Frontiers in Plant Science. 2022;13:713481. Open source
- Alden MJ and Faust JE. Cultivation Strategies to Modify Biomass Partitioning and Improve Yield in Controlled-Environment Production of High-Cannabidiol Cannabis. HortScience. 2024;59:1511-1519. Open source
This lesson summarizes the source material and its evidence limits for education. Use direct measurement, controlled comparison, and the cited sources when conditions differ or a decision carries meaningful risk.