Transcript Plant Growth Regulators 23.02.16 File

Plant Growth
Regulators
Chemical Messengers
Hormones

In plants, many behavioural patterns and
functions are controlled by hormones.

These “chemical messages” influence many
patterns of plant development.

Plant hormones – are a natural substance
(produced by the plant) that acts to control
plant activities - chemical messengers.
Hormones

Are produced in one part of a plant and
then transported to other parts, where they
initiate a response.

They are stored in regions where the
stimulus are and then released for transport
through either phloem or mesophyll when
the appropriate stimulus occurs.
Growth Regulators
Plant growth regulators – include
plant hormones (natural &
synthetic), but also include nonnutrient chemicals not found
naturally in plants that when
applied to plants, influence their
growth and development.
Growth Regulators
5 recognised groups of natural plant
hormones and growth regulators.
1. Auxins
2. Gibberellins
3. Cytokinins
4. Ethylene
5. Abscisic acid
1. Auxins
Found in leaves and stems.
 Causes cell enlargement or elongation –
located in meristems and shoot tips
(terminal & lateral buds).
 Auxins move mainly from apex (top)
down.
 Lengthening of the internodes and
influence the developing embryos in
the seed.

When auxin is added to a stem, the stem
will bend away from the auxin. It
elongates faster on the extra auxin side.
Auxins
Apical dominance – high levels of auxin
in the stem just above lateral buds
block their growth (blockage of growth
of lateral buds by presence of terminal
buds).
 If shoot tip is removed. The auxin level
behind the lateral buds is reduced and
the lateral buds begin to grow (the
auxin which formed the blockage to
keep lateral buds small is reduced so
they can grow).

Auxins

Photo (light) and geotropism (gravity) –
involved in tropism responses – positive
responses

Flower initiation and development

Root initiation and development – used on
cuttings to help stimulate root growth
Examples of Auxins
Plant Growth Regulators

Indobutyric acid (IBA) – synthetic e.g.
Seradix

2,4-dichlorophenoxyacetic acid (2-4D)
- synthetic
Hormone

Indoleactic acid (IAA) -naturally
occurring.
Commercial uses of synthetic
Auxin

Adventitious root initiation

Weed control

Increasing fruit set

Preventing pre-harvest fruit drop

Spray thinning

Inhibition of stem sprouting

Micro-propagation.
2. Gibberellins

Gibberellic Acid (GA)

Have a regulatory function

Are produced in the shoot apex
primarily in the leaf primordial (leaf
bud) and root system

Stimulates stem growth dramatically
Gibberellins

Stimulates cell division, cell elongation
(or both) and controls enzyme
secretions.

E.g. dwarf cultivars can be treated with
GA and grow to normal heights –
indicates dwarf species lack normal
levels of GA.

Involved in overcoming dormancy in
seeds and buds.

GA translocates easily in the plant (able
to move freely) in both directions –
because produced in not only shoot apex
but also in the root structure.
Gibberellins
Used commercially in:

Increasing fruit size of seedless grapes

Stimulating seed germination & seedling
growth

Promoting male flowers in cucumbers for
seed production.

Overcoming cold requirements in some
seed. Application of GA foregoes the cold
requirements (some seed require to be
frozen or placed in the refrigerator for a
period of time before they will germinate).
3. Cytokinins

Promotes cell division

Found in all tissues with considerable
cell division.
 E.g.
embryos (seeds) and germinating
seeds, young developing fruits

Roots supply cytokinins upward to the
shoots.

Interact with auxins to influence
differentiation of tissues (may be used
to stimulate bud formation).
Cytokinins

As roots begin to grow actively in the
spring, they produce large amounts of
cytokinins that are transported to the
shoot, where they cause the dormant
buds to become active and expand.

Tissue culture uses cytokinins to induce
shoot development.

Cytokinins may slow or prevent leaf
senescence (leaf ageing or leaf fall).
4. Ethylene
Gaseous hormone.
 Produced in the actively growing
meristems of the plant, in ripening or
ageing fruits, in ageing or dying flowers,
in germinating seeds and in certain plant
tissues as a response to bending,
wounding or bruising.
 Ethylene as a gas, diffuses readily
throughout the plant.

Ethylene

May promote leaf senescing and
abscission (leaf fall).

Increases female flowers in cucumbers
(economically - will increase fruit
production).

De-greening of oranges, lemons and
grapefruit – ethylene gas breaks down
chlorophyll and lets colours show
through.
5. Abscisic Acid (ABA)

Widespread in plant body – moves readily
through plant.

ABA appears to be synthesized (made) by the
leaves.

Interacts with other hormones in the plant,
counteracting the growth – promoting the
effects of auxins & gibberellins.
Abscisic Acid
 Involved
with leaf and fruit abscission
(fall), onset of dormancy in seeds and
onset of dormancy (rest period) in
perennial flowers and shrubs.
 ABA
is effective in inducing closure of
stomata in leaves, indicating a role in
the stress physiology in plants.
 E.g.
increases in ABA following water,
heat and high salinity stress to the
plant.
Shoot and Root Development
The balanced growth of a plant is the result of varying auxin and
cytokinins ratios within the plant.

Auxins are produced in actively growing aerial buds in
significant concentrations.

Cytokinins are found throughout the plant.

High auxin / cytokinins ratios result in lateral bud break and
subsequent shoot development.

These ratios can be used to explain observed seasonal growth
patterns:

In spring, low auxin / cytokinins ratios result in vigorous shoot
development from dormant buds.

Actively growing terminal buds increase the auxin / cytokinins
ratio, resulting in increased root growth. Increased root growth
lowers the auxin / cytokinins ratio, inhibiting further shoot
growth.
Fruit Set – Fertilisation.
Successful fertilisation requires;
1.
Pollen compatibility – successful germination of the
pollen tube.
2.
Correct environmental conditions – this affects the rate
of pollen tube growth to the ovule.

Zygote development leads to the formation of the fruit
and usually seeds within the fruit.

Plant hormones are involved with this fruit set.

When the zygote fails to develop and no seed forms,
the immature fruit usually drops.

With some fruits e.g. grapes and citrus, the fruit does
not fall, even through the seed fails to develop.
Fruit Set – Growth regulators

In the fruits of some species, auxin application can replace
the stimulation of the developing zygote; e.g. the use of
Fullset in tomatoes. Auxin sprays are also used to increase
the production of green peppers, egg plants and figs.

After initial fruit set, natural cytokinins ratios can be
disrupted by the application of NAA (a synthetic auxin) to
cause fruit thinning. Other compounds can also be used as
chemical thinning agents e.g. the insecticide Carbaryl is
used to thin pip fruit and the fungicide Saprol can have a
similar effect on stone fruit.

Fruit thinning is a management technique to regulate fruit
size. Research suggests that the ratio of leaves to
developing fruit is important, and should be in the range of
20/40 leaves per fruit.
Growth inhibition
Use of some growth hormones to restrict plant growth, e.g.

Delay of flowering in male kiwifruit

Dwarfing of ornamentals e.g. chrysanthemums and
poinsettias.

Inhibiting elongation in punnet grown annual plants e.g.
tomatoes.

Inhibit sprouting of stored potatoes.

To maintain the quality of harvested onions.

The prevention of lodging in cereal crops. Lodging is the
result of a weak stem and a full seed head. The seed
head bends over and reduces the yield when harvesting.
Questions
1.
2.
Link up the plant growth substances with the plant growth
processes it controls.
Auxin
Fruit ripening
Gibberellin
Growth towards the light
Cytokinin
Cell division
Abscisic acid
Breaking seed dormancy
Ethene
Direction of root growth
Auxin has a different effect on roots, shoots and _______ buds.
Low concentrations of auxin _______ roots, while higher
concentrations _______ roots. The apical tip produces high
concentrations of ______ which inhibits the ______ buds and
stops them from growing. If you want a busy plant, you can
remove the apical _______ and the lateral buds will start to
grow.
3.
Each plant ______ to an environmental ______ is controlled
by a ______ that is produced where the stimulus is ______,
then transported to where it has its ______.
4.
Auxins are produced at the tip of _______ and control the
response to _______. In root tips auxins control the growth
response to _______.
5.
_______ break seed and winter dormancy; promote
germination and cell expansion.
6.
Cytokinins promote cell _______.
7.
Abscisic acid maintains _______ in seeds and ______ plants
over winter.
8.
_______ promotes _______ ripening and loss of leaves from
deciduous trees.