Botany Basics
The science of plants — how they are built, how they function, how they grow
Chapter 9: Plant Hormones and Growth Responses
Why this matters: Plants respond to their environment constantly — they grow toward light, roots grow toward gravity, vines wrap around supports, plants flower when days get longer. Understanding hormones explains why pinching a tip makes a plant bushy, why ethylene ripens fruit, why rooting powder works, and why plants behave the way they do when you manage them.
9.1 What Are Plant Hormones?
Plant hormones (phytohormones) are chemical signals produced in small amounts that regulate growth, development, and responses to the environment. Unlike animal hormones, plant hormones often work at the site of production AND travel to other parts of the plant.
THE FIVE CLASSICAL PLANT HORMONES:
1. AUXIN (IAA)
Growth, cell elongation, apical dominance,
root formation, gravitropism, phototropism
2. GIBBERELLINS (GA)
Stem elongation, seed germination,
flowering, fruit development
3. CYTOKININS (CK)
Cell division, shoot initiation,
delay of aging (senescence)
4. ABSCISIC ACID (ABA)
Stress response, stomatal closure,
seed dormancy
5. ETHYLENE (C₂H₄)
Fruit ripening, leaf/fruit drop,
responses to wounding and stress
(NOTE: a gas, not dissolved in water)9.2 Auxin — The Master Growth Regulator
Auxin (primarily indole-3-acetic acid, IAA) is produced primarily at shoot tips and young leaves.
AUXIN KEY EFFECTS
From: 9.2 Auxin — The Master Growth Regulator
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig01.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig01.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig01.png" alt="AUXIN KEY EFFECTS"></figure>
Phototropism — Growing Toward Light
PHOTOTROPISM
From: 9.2 Auxin — The Master Growth Regulator
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig02.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig02.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig02.png" alt="PHOTOTROPISM"></figure>
Apical Dominance
APICAL DOMINANCE
From: 9.2 Auxin — The Master Growth Regulator
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig03.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig03.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig03.png" alt="APICAL DOMINANCE"></figure>
Rooting Hormone
AUXIN and ROOT FORMATION
From: 9.2 Auxin — The Master Growth Regulator
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig04.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig04.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig04.png" alt="AUXIN and ROOT FORMATION"></figure>
9.3 Gibberellins — Stretch and Grow
Gibberellins (GAs) primarily stimulate cell elongation and cell division.
GIBBERELLIN EFFECTS
From: 9.3 Gibberellins — Stretch and Grow
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig05.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig05.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig05.png" alt="GIBBERELLIN EFFECTS"></figure>
9.4 Cytokinins — Divide and Stay Young
Cytokinins are produced primarily in ROOT TIPS and transported upward. They promote cell division (cytokinesis = cell division).
CYTOKININ EFFECTS
From: 9.4 Cytokinins — Divide and Stay Young
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig06.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig06.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig06.png" alt="CYTOKININ EFFECTS"></figure>
9.5 Abscisic Acid — The Stress Responder
ABA is produced when plants are under stress — primarily drought. It acts fast.
ABA EFFECTS
From: 9.5 Abscisic Acid — The Stress Responder
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig07.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig07.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig07.png" alt="ABA EFFECTS"></figure>
9.6 Ethylene — The Ripening Gas
Ethylene is unusual — it is a gas at room temperature. It diffuses through plant tissue and into the air around the plant.
ETHYLENE EFFECTS
From: 9.6 Ethylene — The Ripening Gas
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig08.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig08.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig08.png" alt="ETHYLENE EFFECTS"></figure>
9.7 Plant Tropisms — Directed Growth Responses
Tropisms are growth responses directed toward or away from a stimulus.
PHOTOTROPISM — response to light
From: 9.7 Plant Tropisms — Directed Growth Responses
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig09.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig09.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig09.png" alt="PHOTOTROPISM — response to light"></figure>
9.8 Photoperiodism — Day Length and Flowering
Many plants use day length (or more accurately, night length) to time flowering with the right season.
PHOTOPERIODISM
From: 9.8 Photoperiodism — Day Length and Flowering
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig10.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig10.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig10.png" alt="PHOTOPERIODISM"></figure>
Chapter Summary
FIVE HORMONES
From: Chapter Summary
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig11.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig11.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig11.png" alt="FIVE HORMONES"></figure>
1. Auxin promotes cell elongation in stems.
2. Apical dominance is caused by high auxin concentrations suppressing lateral buds.
3. Gibberellins are primarily responsible for seed dormancy.
4. Cytokinins promote cell division.
5. Abscisic acid (ABA) is called the "stress hormone" because it helps plants respond to water stress.
6. Ethylene promotes fruit ripening.
7. Thigmotropism is the growth response to gravity.
8. Phototropism is the growth response to light.
9. ABA causes stomata to open during drought.
10. Cytokinins delay senescence (aging) in leaves.
11. Gibberellins can stimulate seed germination by breaking dormancy.
12. Ethylene is a gas, not a liquid or solid hormone.
13. Long-day plants flower when the night length is longer than a critical minimum.
14. Short-day plants flower when nights are longer than a critical length.
15. Day-neutral plants flower regardless of day length.
16. The photoreceptor that measures day length for flowering is called phytochrome.
17. Gravitropism causes roots to grow downward and shoots to grow upward.
18. Auxin moves from shoot tip toward the shaded side, causing the lit side to elongate more.
19. Ethylene from ripe fruit can trigger ripening in nearby unripe fruit.
20. Wounding a plant can trigger ethylene production.
1. What is auxin and what are its primary functions?
2. How does auxin cause phototropism?
3. What are gibberellins and when would a grower use them?
4. What is the role of ABA in drought response?
5. How do ethylene and ripening interact?
6. What is thigmomorphogenesis and why does it matter for plants?
7. What is the difference between short-day and long-day plants?
8. What is phytochrome and how does it measure day length?
9. How do cytokinins interact with auxin to determine plant form?
10. Name a practical application of each of the five main plant hormones.
1. The plant hormone primarily responsible for cell elongation in shoots is .
2. The suppression of lateral bud growth by the shoot tip is called dominance.
3. Plant hormones that stimulate cell division are called .
4. The "stress hormone" that causes stomatal closure during drought is .
5. The gas hormone that promotes fruit ripening is .
6. Growth response to gravity is called .
7. Growth response to touch or physical contact is called .
8. The photoreceptor protein that detects day/night length is .
9. Plants that flower when nights exceed a critical minimum length are called plants.
Complete in the field. Check each off as you go.
1. Apical Dominance Demonstration: Grow two identical bean or tomato seedlings. Pinch the growing tip of one when 4 inches tall. Observe weekly for 4 weeks. Measure branching in each.
2. Phototropism Setup: Grow seedlings in a box with a single light hole on one side. Observe and record the angle of bending over 5-7 days. Try rotating the plant and observe correction.
3. Ethylene Ripening Test: Place an unripe banana or avocado in a bag alone, and another with a ripe apple or ripe banana. Check ripening rate daily. Explain the difference.
4. Gravitropism Observation: Germinate seeds on a damp paper towel. After roots emerge, rotate 90°. Observe over 48-72 hours. How does the root change direction?
5. Day Length and Flowering: Research which of the following are short-day, long-day, or day-neutral: poinsettias, chrysanthemums, spinach, strawberries, tomatoes. Explain why florists put poinsettias in darkness in fall.
Practice Exercises
- You put a ripening banana in a bag with a hard avocado. Two days later the avocado is ripe. What happened chemically?
- Why does pinching the tip of a basil plant make it grow bushier?
- Spinach bolts and goes bitter in your summer garden. What hormone and environmental signal caused this, and when should you plant it instead?
- You stratify apple seeds in the refrigerator for 12 weeks. What hormone are you degrading, and what is the physiological change that allows germination?
- A vine tendril is curling around a fence post. What tropism is this and what is the hormone mechanism?
- You buy a tomato at the grocery store and it is red but tasteless. It was picked green and treated with ethylene gas. What flavor compounds are missing and why?
Next Chapter → Plant Classification — Families, Genera, Species
Connections to Other Topics
→ C01 Ch04: Stems — apical dominance and branching
From: Connections to Other Topics
Image file: ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig12.png
Save image as ../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig12.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c01-botany-basics/ch09/c01-botany-basics_ch09_plant_hormones_fig12.png" alt="→ C01 Ch04: Stems — apical dominance and branching"></figure>
Next Chapter → Plant Classification