RHS Level 3: Plant taxonomy, structure, and function Q4

4. Understand the role of plant growth regulators in plant development and their relevance to horticulture.

4.1 Describe the properties and key effects of endogenous plant growth regulators (PGRs) and their interactions.

Describe the 5 major groups of endogenous PGRs to include the following properties and effects:


Examples: IBA, IAA

Properties: polar movement, root/shoot sensitivity, produced at shoot apex

Effects: cell elongation and enlargement, root development, apical dominance, fruit set and development.


Example: GA

Properties: non-polar movement; produced in young expanding organs.

Effects: promotes stem elongation; promotes seed germination and release from bud and seed dormancy; promotion of flowering and its link to vernalisation e.g. in biennials.


Example: zeatin

Properties: non-polar movement, produced in meristems.

Effects: promotion of cell division; shoot development; retardation of senescence.

Abscisic acid

Properties: non-polar movement; produced in leaves, stems, fruits and seeds; continuously broken down and remade.

Effects: stomatal closure; promotion of dormancy in seeds.

Ethene (ethylene)

Properties: gas, produced in all cells; transported as a precursor ACC.

Effects: senescence and ripening; abscission.

Describe the interactions of PGRs in: cell division and differentiation (micropropagation); apical dominance; seed and bud dormancy.

Cell division and differentiation:

Cytokinins – promote cell division at the meristems growing new shoots, cytokinins are regulated by auxins to determine how active they are. When the ratio of cytokinins to auxins is higher, stem and leaf growth is stimulated. When the ratio of cytokinins to auxins is relatively low, root growth is stimulated.

Auxins – stimulate cell division in cambium and differentiation between phloem and xylem

Ethylene – promotes lateral growth in the roots. Works as a growth inhibitor (ie suppresses cell division) in conjunction with auxins

Gibberellins – along with ethylene, are involved in root differentiation

In micropropagation:

A high auxin to cytokinin ratio generally favours root formation, whereas a high cytokinin to auxin ratio favours shoot formation. An intermediate ratio favours callus production. Giberellins are used to determine plant height and fruit set.

Apical dominance:

Cytokinins – form lateral buds

Auxins – cause apical dominance

After the apical bud is removed, cytokinin is released causing the outward growth of the lateral buds. Auxin is decreased, gibberellin is promoted and the bud continues its outward growth.

Seed dormancy:

Dormancy in seeds is regulated by the balance between abscisic acid and gibberellins. Ethylene regulates this balance by controlling the abscisic acid.

Ethylene – oversees the balance

Gibberellins – bring seeds out of dormancy

Abscisic acid – puts seeds into dormancy by suppressing cell growth.

4.2 Describe the use of synthetic PGRs in horticultural situations.

Distinguish between synthetic and endogenous PGRs. State the advantages of synthetic PGRs. State what is meant by and give examples of: hormone mimics, growth retardants and growth inhibitors.

Hormone mimics: mimic or partly mimic naturally occurring hormones in the plant.

Example: 2,4-D the herbicide mimics a hormone by making a plant grow rapidly, growing curled leaves and stems and then dying. This is mimicry of the plant hormone indoleacetic acid (IAA)

Example: Cycocel (CCC) is a synthetic growth retardant used for wheat

Growth retardants: these are synthetically created to suppress the hormones that stimulate growth.

Example: dazide (Daminozide ) suppresses gibberellin which elongates stems, in order to make more compact plants. paclobutrazol  suppresses the growth of fungal diseases.

Growth inhibitors note: I’m confused by this, because the terms growth inhibitor and growth retardant are used interchangeably. The only possible difference is that inhibitors stop growth altogether, whereas retardants merely slow it down – a matter of degree, but the below example is described as an inhibitor, but only slows growth, so I don’t know.

Example: Ethephon mimics ethylene, reduces plant growth and increases leaf density by encouraging new leaves to grow and slowing down the senescence of older leaves.

Difference between synthetic and endogenous PGRs: Endogenous PGRs come from inside the plant, synthetic are applied to the plant.

Advantages of synthetic PGRs

  • Can be manufactured in quantity
  • Can be used for cuttings to promote root growth
  • Can be used to increase the number of flower buds
  • Can improve the longevity and quality of cut flowers
  • In turf can control vegetative growth and reduce the need for mowing
  • Can reduce or eliminate unwanted suckers
  • Can reduce pollen – for hayfever sufferers

Describe TWO synthetic PGRs used in different horticultural situations from the following list (purpose, application method, timing and amounts):

Note: I’ve chosen Paclobutrazol and Trinexapac-ethyl to elaborate on, but there are other options on the syllabus.

Paclobutrazol –growth control in ornamental plant production;

Purpose: Inhibits gibberellin. Creates stouter plants by reducing internodal growth, increases root growth, brings about early fruit set, and increases seed set. Used on trees and tomatoes.

Application method: apply to soil or soak seed (foliar feed is ineffective)

Timing: Some growers have success using multiple sprench applications, applied, for example, every few weeks, where the rates are adjusted based on the size and vigor of the crop. Another approach is to apply a drench at a high rate once the crop reaches a desirable size. However, late drenches are usually not recommended because the growth-inhibiting effect can continue after plants are planted into the landscape. A late drench may be appropriate for crops that are meant to remain in their containers, such as hanging baskets and potted flowering plants.

For aggressive crops, early paclobutrazol applications are desirable once roots have reached the pot edges, typically seven to 10 days after transplant. Late applications of paclobutrazol, particularly when delivered as a spray, can delay flower development and reduce flower size. Therefore, early and proactive applications are strongly recommended, and late applications should generally be used as a last resort.

Amounts: A paclobutrazol spray at 5 to 10 ppm can be appropriate for bedding plants with moderate vigor, whereas at least twice that may be needed for aggressive crops, especially when grown during the late spring. On aggressive herbaceous perennials, typical spray rates are 60 to 90 ppm.

Trinexapac-ethyl –growth retardation in amenity grassland and managed turf; Ethene (ethylene) –sprout inhibition in potato storage.

Purpose: Controls growth and inhibits gibberellin. Used on grains and turf. Stunts growth so that plants put energy into reproduction. Reduces need for mowing in grass, keeps grain stems shorter for support.

Application method: Applied as a foliar spray, post emergence. It is translocated to the growing shoot.

Timing: Apply Trinexapac-ethyl 1 ME to actively growing turf. If turf is going into dormancy because of high or low temperatures or lack of moisture, apply a lower rate of Trinexapac-ethyl 1 ME. Repeat applications of Trinexapac-ethyl 1 ME may be made as soon as the turf resumes growth or more suppression is desired.

Amounts: Trinexapac-ethyl 1 ME can be applied at rates of 0.1 to 0.2 fl. oz. per 1,000 sq. ft. every 7 to 14 days to suppress basal rot anthracnose. Use the lower rate when seed heads are present.


4.3 Describe tropic plant movements.

Describe and explain the mechanism of: Phototropism and gravitropism (geotropism) in the root and shoot; thigmotropism (seismotropism).


Description: plant growth in response to light. It does this in a positive way in shoots and a negative way in roots.

Mechanism: cells on the side furthest from the light have a concentration of auxin, so they elongate

Gravitropism (AKA geotropism)

Description: turning or growth movement in response to gravity. It does this in a positive way in roots and a negative way in stems, so that roots grow down and stems grow up.

Mechanism: In roots the Auxin inhibits cell growth and thus the cells elongate and grow faster on the opposing side of the root. This causes the root to curve downward with the direction of gravity

Thigmotropism (AKA seismotropism)

Description: plant growth in response to touch or contact, occurs when plants grow round a surface eg climbing tendrils

Negative: of roots when they make contact they grow away.

Mechanism: the touched cells produce auxin and transport it to untouched cells on the other side of the stem, which then grow faster so growth bends.

4.4 Describe how flowering is controlled in plants.

State what is meant by the terms: photoperiodism, critical day length, day length categories (short day, long day, day neutral plants), ‘florigen’.

Photoperiodism: the physical/physiological reaction of organisms to the length of day or night.

Critical day length: is really critical night length or photoperiod, but was misnamed due to initial misunderstanding. It’s the length of daylight necessary to initiate flowering in long day plants, or inhibit flowering in short day plants.

Short day: a plant that requires a long period of darkness in order to flower (longer than 12 hours) eg Poinsettias (Euphorbia pulcherrima) and Chrysanthemum spp.

Long day: a plant that needs a short period of darkness in order to flower eg Aster spp., potato, Echinacea spp.

Day neutral: these form flowers regardless of day length eg tomatoes, corn

Florigen: A hypothesized, hormone-like molecule responsible for controlling and/or triggering flowering in plants. It was thought to be produced in leaves and to act in buds..

Name ONE plant example for EACH day length category. Describe the role of phytochrome in the photoperiodic response. State what is meant by the term vernalisation (cold treatment required for flowering) State the photoperiodic/vernalisation requirements (day length, temperature) for flowering in a NAMED horticultural crop: AYR Chrysanthemum, poinsettia (Euphorbia pulcherrima) OR strawberry (Fragaria x annanassa). NO PRODUCTION DETAILS REQUIRED

Phytochromes: are the photoreceptors that detect light. They regulate leaf growth, seed germination and chlorophyll production (all of which are affected by light availability). And they affect the timing of flowering by detecting how long night and day lengths are, which determines whether the photoperiodic conditions are right to form flowers.

Vernalisation: a plant’s ability to flower in spring by exposure to the prolonged cold of winter, or by artificial equivalent.

Euphorbia pulcherrima: in autumn it requires total uninterrupted darkness 13-14 hours per night for 8-10 weeks at a temperature no lower than 15°