Plant Hormones – Detailed Study Notes

Defined in the transcript as “plant growth substances” that cause a reaction in the organ where they are produced.
Five principal groups mentioned (common name / Afrikaans alternate in brackets):
• Auxins (Ouksiene)
• Gibberellins (Gibberelliene)
• Cytokinins – promotes cytokinesis (Sitokiniene)
• Ethylene – involved in germination & fruit ripening (Etileen)
• Abscisic acid – stress & dormancy signal (Absissiensuur)
Each hormone group elicits a specific developmental or stress-related response, together coordinating overall plant growth, tropic movements, reproduction, and survival.

Concentrated in meristematic regions immediately behind the tips of stems and roots.
Act as the key coordinator of plant growth; their effects are often most visible as tropisms (growth movements relative to external stimuli).
Primary functions highlighted:
• Tropical responses (phototropism, geotropism, etc.)
• Cell elongation ‑ stimulates loosening of cell walls, enabling expansion.
• Apical dominance – suppresses growth of lateral (axillary) buds below an active shoot tip.
• Promotes development of certain fruits (e.g.", parthenocarpic fruit set in agriculture).
Biochemical note: Most common naturally occurring auxin is indole-3-acetic acid \text{(IAA)}.

High auxin concentration near the terminal bud inhibits lateral buds.
When the growing tip is removed (decapitation / pruning) or when auxin diffuses farther from the tip, the inhibitory effect is lost:
• Dormant axillary buds become active ⇒ plant becomes bushier.
Agricultural / horticultural relevance:
• Selective pruning manipulates plant architecture, fruiting potential, and light interception by intentionally removing apical buds.

Positive phototropism: stems bend toward light; negative: away.
Mechanism outlined:
• Light striking one side of stem triggers relocation of auxins to the shaded side.
• \uparrow Auxin on dark side ⇒ \uparrow cell elongation on that side.
• Differential growth rate causes curvature of stem toward the light source.
Biological significance: maximizes photosynthetic efficiency by orienting leaves toward optimal illumination.

Unequal auxin distribution also underlies gravity-sensing responses.
Roots (positive geotropism):
• When a root is horizontal, auxin accumulates on the lower side due to gravity.
• In roots, high auxin concentration inhibits elongation, so cells on the lower side grow more slowly.
• Upper side grows faster ⇒ root curves downward.
Shoots (negative geotropism):
• Same lateral auxin shift, but in shoots a high auxin level stimulates elongation.
• Lower (auxin-rich) side of shoot elongates more ⇒ stem curves upward.
Ensures correct orientation: roots anchor and seek water/nutrients; shoots seek light.

Principal role: elongation of internodes, making the plant taller.
Stimulate seed germination by:
• Activating the gene that encodes the enzyme amylase.
• Amylase converts stored starch \rightarrow maltose, supplying soluble sugars to the embryo.
Agricultural notes: exogenous gibberellins break dormancy in some seeds, increase fruit size in grapes, and induce bolting in rosette plants.

Categorized as an inhibitory or stress hormone.
Functions described:
• Promotes seed & bud dormancy, safeguarding against unfavorable conditions (cold, drought).
• Short-term water stress: triggers stomatal closure, reducing transpirational water loss.
• Long-term cold hardening: induces genes that bolster tolerance to freezing (e.g., antifreeze proteins, osmoprotectants).
ABA therefore balances the growth-promoting effects of auxins, gibberellins, and cytokinins, maintaining homeostasis.

External defensive structures deter herbivores/pathogens:
• Thorns, spines, trichomes, thick cuticles.

Rely on secondary metabolites – organic compounds not central to primary growth & development but vital for defense.
Examples supplied:
• Nitrogen compounds (alkaloids, cyanogenic glycosides) – bitter or toxic.
• Terpenoids – aromatic, may disrupt insect molting or attract predators of herbivores.
• Phenolics – lignin, tannins; reinforce cell walls, reduce digestibility.
Some compounds make cell walls indigestible to bacteria, fungi, or grazing animals.

Horticulture and agriculture routinely harness plant hormones:
• Synthetic auxins for rooting cuttings, weed control.
• Gibberellins to enlarge fruit clusters, break dormancy.
• ABA analogs explored to enhance drought resistance.
Tropism knowledge informs greenhouse lighting placement, optimizing yield and energy use.
Defense metabolites inspire pharmaceuticals, natural pesticides, and biomaterials.

Auxin effect often observed at concentrations of 10^{-5} to 10^{-6} M in laboratory assays.
ABA-induced stomatal closure can cut transpiration by up to 50\% within minutes under wilting conditions (contextual data supplementing the transcript theme).
In germinating barley, gibberellin-activated amylase can elevate maltose concentration from \approx 0 to 20\,\text{mg}\,\text{g}^{-1} endosperm within 24 h (illustrative of the “activates the amylase gene” point).

[ ] Know the five hormone groups & alternate names.
[ ] Understand auxin distribution patterns for phototropism vs.0geotropism.
[ ] Explain apical dominance & practical pruning outcomes.
[ ] Describe gibberellin role in stem elongation & seed germination (amylase activation).
[ ] List ABA functions in dormancy, stomatal regulation, cold hardening.
[ ] Differentiate mechanical from chemical plant defenses with examples.
[ ] Be able to relate hormone knowledge to agricultural/horticultural practices.