Phloem Transport and Sugar Allocation
Explain pressure-driven translocation from source to sink and use phloem logic to interpret growth, pruning, girdling, and organ competition.
Educational reference · evidence, sources, and limits shown below
Explain pressure-driven translocation from source to sink and use phloem logic to interpret growth, pruning, girdling, and organ competition.
Terms to know
- Phloem
- Living vascular tissue transporting sugars, amino acids, ions, RNAs, hormones, and signals.
- Sieve element
- Conducting phloem cell joined into sieve tubes and supported metabolically by companion cells.
- Loading
- Entry of transported solutes into source-region phloem.
- Unloading
- Exit of transported solutes from phloem into a sink region.
Core science
Photosynthetic leaves export much of their fixed carbon as sucrose. Loading raises solute concentration in sieve elements, water enters osmotically from nearby xylem, and pressure increases. At sinks, unloading and metabolism reduce solute concentration and pressure. The resulting pressure gradient drives bulk flow through sieve tubes.
Phloem transport is not simply downward. A mature leaf can export to a growing apex, root, branch, or inflorescence according to vascular connections and sink demand. Different source-sink pathways can operate simultaneously, and a developing leaf can shift from importer to exporter as it matures.
Living sieve elements are vulnerable to crushing, heat, pathogens, and girdling. Wounding triggers sealing responses that limit leakage but also restrict transport. Carbohydrate accumulation above a damaged region and starvation below it may appear later than the injury.
Why this matters in cultivation
- Pruning removes both sinks and future source leaves. The result depends on timing, remaining leaf area, vascular connections, plant reserves, and the ability of other sinks to use redirected carbon.
- Do not assume foliar sugar products replace source-leaf function. Entry, concentration, microbial growth, osmotic injury, and metabolism must be demonstrated.
Measure and record
Source tissue
Leaf age, area, light exposure, health, carbohydrate status if measured, and export stage.
Sink tissue
Root, apex, branch, flower, seed, wound, or storage tissue; stage and growth rate.
Vascular event
Pruning, topping, bending, girdling, lesion, tie pressure, or pathogen observation.
Allocation proxy
Organ dry mass, soluble sugar/starch, node growth, flower growth, and root response.
Timing
Event date, photoperiod stage, response lag, recovery, and final harvest outcome.
Common misconceptions
Correction: Movement follows pressure gradients between sources and sinks in multiple directions.
Correction: Remaining sinks compete through vascular connections and developmental regulation.
Correction: Phloem also carries nitrogen compounds, ions, hormones, RNAs, and defense signals.
Evidence limits
Allocation cannot be proven from appearance alone. Isotopic tracing, repeated organ biomass, metabolite analysis, or transcript data provide stronger evidence than one harvest weight.
Related encyclopedia topics
- THC-ENC-010–014, THC-ENC-071–079, THC-ENC-181–200, and training/reproduction 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
- 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
- THC – Teaching Healthy Cultivation. Cannabis Plant Science Source Packet v1. May 2026. Project source packet.
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.