4.2 Hormones

Question 1

Growth inhibitors, meaning and example.

Answer 1

Meaning: Regulation substances which retard processes such as root and stem elongation, seed germination and bud opening.
They act as antagonists to plant hormones such as auxin, gibberellin and cytokinins.
They stop plant growth and promote production of flowers.

Example ABA - promotes abscission of leaves, initiates bud dormancy, forms tubers on potatoes.

Question 2

Hormone mimics.

Meaning and example.

Answer 2

Meaning: Synthetic hormones mimic the activities of naturally - occurring substances - they may modify plant growth and development.

Example: 2, 4- D - selective weed killer, kills broad leaves but not grasses, plant grows rapidly, leaves & stems curl then die. Mimics IAA

Question 3

Growth retardants.

Meaning and example.

Answer 3

Meaning : Slows growth, ‘holds’ plants in greenhouse at required stage of development, plants need to be compact with short inter-nodes.

Example Dazide/Daminozide, suppresses gibberellin (which elongates stems) in order to produce more compact plants.

Question 4

Advantages of synthetic PGRs.

Answer 4

Can be manufactured in quantity.
Can be used for cuttings to promote root growth.
Can increase number of flowers.
Can extend life of cut flowers.
On turf it can control vegetative growth > reduce need for mowing.
Can eliminate unwanted suckers.
Can reduce pollen

Question 5

interaction of PGRs:

Seed dormancy

Answer 5

Dormancy in seeds is regulated by balance between abscisic acid and gibberellins.
Ethylene regulates balance by controlling abscisic acid.
Ethylene - over-sees balance.
Gibberellins - bring seed to dormancy.
Abscisic acid puts seed in dormancy.

Question 6

Interaction of PGRs

Apical dominance

Answer 6

Cytokinins form lateral buds.
Auxin causes apical dominance.

Apical but removed > cytokinins released > outward growth from lateral bud. Auxin is decreased .

Giberellin promotes outward growth.

Question 7

Interaction of PGRs

Micropropagation.

Answer 7

A high auxin to cytokinins ration generally favors root formations.

A high cytokinins to auxin ratio generally favors shoot formation.

Balanced auxin: cytokinin ratio favours callus development.

Giberellins - used to determine plant height and fruit set.

Question 8

Interaction of PGRs

Cell division and differentiation.

Answer 8

Cytokinins - promotes cell division at meristem of new shoots.
Cytokinins are regulated by auxin.
When ratio of cytokinins to auxin is high > increased stem and leaf growth.
When ration of auxin to cytokinins is high > root growth increases.
Auxin stimulates cell division in cambium and xylem and phloem differentiation.
Ethylene - promotes lateral growth in roots, with auxin it works as a growth inhibitor.
Gibberellins - with ethylene is involved in root differentiation

Question 9

Micropropagation

Answer 9

Micropropagation.

Single plant cell, small leaf portion or meristematic shoot apices used to produce whole plant.
Plant is a form of clone.
Piece of plant placed in agar with nutrients and growth regulators.
Environment must be sterile.
Grown in controlled conditions (light and temp.)
Concentration of auxin: cytokinin controls the differentiation of plant tissues.

Question 10

PGR interaction and micropropagation

Answer 10

GAs antagonize the impact of abisic acid (ABA).
They make or break dormancy.
Auxin affect apical dominance.
Cytokines promote business in a plant

Question 11

Difference between synthetic and endogenous PGRs

Answer 11

Endogenous PGRs come from within plant.

Synthetic PGRs applied to plant

Question 12

Ethelylene (Ethene) C2 H4

Answer 12

Gas
Produced in all cells especially ripe fruit and young shoot apices.
Transported as a precursor to ACC.

Effects
Senescence
Ripening
Abscission

Question 13

Abscisic Acid

ABA

Answer 13

Properties.
Transported up and down in xylem and phloem.
Found in all major plant organs.
ABA levels fluctuate, continuously broken down and remade.
Produced in leaves, stems, fruits and seeds.

Effects:
Stomata closure.
Dormancy in seeds.
Decreased growth in buds of deciduous trees in winter.

Question 14

Gibberellins (GAs)

Answer 14

Properties:
Non polar movement.
Present in young plant growth.

Effect:
Cell division and elongation - stem elongation.
Seed germination promotion.
Release of dormancy in seeds and buds.
Rôle in inducing flowering following vernalisation as biennials.

Question 15

Cytokinins eg. Zeatin

Answer 15

Properties:
Produced in meristems.
Cytokinesis means process of cell division.
Non polar movement.
Present where cell division is occurring.

Effects:
Regulation of cell division.
Interacts with auxin in control of apical dominance and lateral branching.
Retards senescence if leaves and other plant tissue.

Question 16

Endogenous and Synthetic Growth Regulators.

Answer 16

Chemical controls within plant.

Known as plant growth regulators (PGRs)

May be ENDOGENOUS -naturally produced or SYNTHETIC -artificially produced.

Control growth and other physiological functions.

Growth promoters - Auxins, Cytokinins, Gibberellins
Growth inhibitors - Ethylene, Abscisic Acid (ABA)

Ethylene can be promoter and inhibitor..

Question 17

Auxin eg. IAA or PAA

Answer 17

Properties:
Moves up and down plant by polar movement.
Specific sensitivity where cells are actively dividing - tips of roots and shoots.
Produced in stem tip, bud and young leaves.
Transported to roots from sites of synthesis.

Effects.
Cell elongation, maintains apical dominance, rooting is promoted and formation of lateral roots, halting of Auxin is linked to leaf abscission., stimulate cell division in cambium.