Plant Hormones: Auxin
Explain auxin synthesis, polar transport, concentration- and tissue-dependent responses, and its roles in apical dominance, tropisms, vascular development, and adventitious rooting.
Educational reference · evidence, sources, and limits shown below
Explain auxin synthesis, polar transport, concentration- and tissue-dependent responses, and its roles in apical dominance, tropisms, vascular development, and adventitious rooting.
Terms to know
- Auxin
- Class of growth regulators represented naturally by indole-3-acetic acid and acting through concentration, transport, and tissue competence.
- Polar auxin transport
- Directional cell-to-cell auxin movement created by asymmetrically positioned transport proteins.
- Apical dominance
- Suppression of axillary-bud outgrowth associated with the active shoot apex and interacting signals.
- Adventitious root
- Root initiated from non-root tissue such as a stem cutting.
Core science
Auxin is produced strongly in young shoot tissues and developing organs and is transported through regulated cellular and vascular routes. It influences cell expansion, vascular differentiation, tropic curvature, lateral-root development, apical dominance, fruit development, and wound responses. Response depends on concentration, tissue, developmental state, other hormones, and transport direction.
Apical dominance is a network, not an auxin faucet. Auxin moving from the shoot apex affects branching indirectly through interactions with cytokinins, strigolactones, sugar availability, and bud-local signals. Removing the apex changes those relationships and can release several buds, but branch number and vigor still depend on genotype, node maturity, roots, light, and resources.
At a cutting base, wounding, carbohydrate status, auxin, oxygen, moisture, temperature, and tissue age interact during adventitious-root formation. A cannabis cutting study found treatment effects of substrate and hormone application, with cultivar context. More rooting compound is not automatically better; excessive concentration or poor application can inhibit or injure tissue.
Why this matters in cultivation
- Record active ingredient, concentration, formulation, lot, exposure method, and untreated control when evaluating rooting hormones. Brand name alone is not reproducible.
- After topping, monitor specific buds and branch growth rather than assuming equal release. Training outcomes reflect the whole signaling and resource system.
Measure and record
Plant material
Genotype, mother ID, node, tissue age, cutting size, and mother condition.
Auxin treatment
Active ingredient, concentration, formulation, lot, exposure, date, and control.
Environment
Rooting medium, temperature, humidity/VPD, PPFD, oxygen risk, and sanitation.
Branch response
Topping position, bud break dates, branch number, length, angle, and symmetry.
Root response
Callus, days to root, root number/length, rooting percentage, survival, and abnormalities.
Common misconceptions
Correction: It participates in many developmental and transport processes.
Correction: Dose-response can be inhibitory or toxic and differs by tissue and genotype.
Correction: New shoots establish their own dominance and interact with resources and other signals.
Evidence limits
Commercial formulations and legal uses vary. Cannabis rooting results from one cultivar, substrate, or concentration should be validated with controls before becoming an SOP.
Related encyclopedia topics
- THC-ENC-013–014, THC-ENC-074–079, THC-ENC-201–220, and THC-ENC-221–240.
Source notes
- Taiz L, Moller IM, Murphy A, and Zeiger E. Plant Physiology and Development. 7th ed. Oxford University Press, 2022. Publisher record
- Campbell BJ et al. Evaluation of Substrate Composition and Exogenous Hormone Application on Rooting Success of Cannabis sativa Cuttings. PLOS ONE. 2021;16:e0249160. Open source
- THC – Teaching Healthy Cultivation. Cannabis Plant Science Source Packet v1. May 2026. Project source packet.
- THC – Teaching Healthy Cultivation. Cultivation SOP Source Materials Packet v1. May 2026. Project source packet.
- Mason MG et al. Sugar Demand, Not Auxin, Is the Initial Regulator of Apical Dominance. Proceedings of the National Academy of Sciences. 2014;111:6092-6097. Open source
- Bertheloot J et al. Sugar Availability Suppresses the Auxin-Induced Strigolactone Pathway to Promote Bud Outgrowth. New Phytologist. 2020;225:866-879. Publisher record
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.