Plant growth regulators (PGRs) are chemicals used to modify plant growth such as increasing branching, suppressing shoot growth, increasing return bloom, removing excess fruit, or altering fruit maturity. Numerous factors affect PGR performance including how well the chemical is absorbed by the plant, tree vigour and age, dose, timing, cultivar, and weather conditions before, during, and after application. Plant growth regulators can be grouped into five classes: compounds related to auxins, gibberellins and inhibitors of gibberellin biosynthesis, cytokinins, abscisic acid and compounds affecting the ethylene status. Products that block the biosynthesis of plant hormones are also available (Apogee, Retain) The five groups of plant growth regulators used in fruit crops include: Auxins: These are growth promoting substances that contribute to the elongation of shoots, but at high concentrations they can inhibit growth of lateral buds. In addition to being used as plant growth regulators, auxins can also be herbicides (2, 4-D etc.). In apple production napthaleneacetic acid (NAA) is a synthetic auxin that can be used to thin fruit and prevent fruit drop shortly before harvest. For more information on the use of products for thinning see Ontario.ca/apples and find Thinning of Tree Fruit. Gibberellins: Gibberellins (GA) promote cell elongation, shoot growth, and are involved in regulating dormancy. Promalin®/Perlan® (containing GA4+7 and 6-benzyladenine) have been used to improve fruit size and reduce russetting in apples. ProGibb 40SB and Falgro (containing GA3) are used to delay ripening, improve fruit firmness and extend the harvest period in sweet cherries. Gibberellins are used in tart cherries to manage flowering to avoid over production. Apogee or Kudos 27.5 WDG® (Prohexidione-calcium) inhibits the biosynthesis of gibberellins. Apogee or Kudos 27.5 WDG are used to modify the morphology of trees (apple and cherries) and to control runner production in strawberries. Cytokinins: Cytokinins promote cell division. Cytokinins are involved in branching and stimulating bud initiation. They are used as fruit thinners (Maxcel® or Cilis Plus® 6-BA) in apples. For more information on the use of these products for thinning see Ontario.ca/apples and find Thinning of Tree Fruit. Absicisic Acid: Absicsic acid controls the dormancy of buds and seeds, inhibits shoot growth and is involved in regulating water loss from plants. Ethylene: Ethylene promotes abscission of leaves and fruits, inhibits shoot elongation and inhibits lateral bud development. In apples and cherries, ethylene is involved in the transition of fruit from being physiologically mature to ripe. Ethephon (Ethrel®) is a synthetic compound that releases ethylene upon application. Retain interferes with ethylene biosynthesis and allows fruit to hang on trees longer and lengthens storage life.

1. Plant Growth Hormones
Amrinder Singh
Ph.D Scholar

2. Plant Growth and Development
Hormones diffuse throughout the plant to promote
growth and development.

3. What is a Plant Hormone?
A naturally-occurring compound which regulates change in
physiology, growth or development of the plant.

4. 5 Types of Plant Hormones
I. Auxin
II. Cytokinin
III. Gibberellin
IV. Abscisic Acid
V. Ethylene

5. Auxin
Role of
Hormone
Cell elongation (increase cell size)
Site of
Productio
n
Shoot Tips
Effect of
Hormone
Growth of plant in response to the
environment, production of roots.
Tropism: a plant’s response to environment
Phototropism- response to light
Geotropism-response to gravity
Thigmotropism-response to touch

6. Auxin Growth Effects
Stimulates Adventitious
Root Formation.
Adventitious roots grow
from stems or leaves rather
than from the original root
system of the plants.
This is especially useful when
cutting and transplanting plants.

7. Auxin Growth Effects
Thigmotropism-touch
Phototropism-light
Geotropism-gravity
Tropisms-How a
plant grows in
response to the
environment

8. Auxin
How does this hormone stimulate tropism?
Cell Elongation
Auxin travels away from the
sunlight and expands the cells

9. Cytokinin
Role of
Hormone
Cell division (increase number of cells)
Site of
Production
Root Tips
Effect of
Hormone
Mitosis of new cells;
Stimulates seed germination and new
shoot growth

10. AUXIN stimulates the
production of roots.
CYTOKININ stimulates
the production of shoots.

11. Gibberellin
Role of
Hormone
Internode Elongation (height)
Site of
Production
Root and Shoot Tips
Effect of
Hormone
Controls yearly cycles (flowering/bolting,
seeding and dormancy exiting)
Rapid growth of stems and seeds.

12. Internode Elongation
Internode: the part of the plant between two nodes or joints.
Node: the part of the stem where stems leaves and buds
emerge; point at which stems intersect and branch.

13. Gibberellin and Cycle Control
How gibberellin in a plant recognizes seasonal changes
Photoperiod: a plant’s recognition of daylight length
in a 24 hour period.
As daylight increases in the spring, the plant
recognizes a longer photoperiod. Gibberellin
triggers the plant to exit dormancy.

14. Why is it called Gibberellin?
Japanese rice farmers discovered a fungus
(Gibberella fujikuroi) that caused an increase in
this hormone.
From this, they learned that plants have a
hormone that causes shoot and seed growth.
They named this hormone, “Gibberellin”,
after the fungus.

15. Gibberellin Growth Effects
Increases
plant
height
Increases
fruit
size

16. Abscisic Acid
Role of
Hormone
Dormancy
Site of
Production
Chloroplasts
Effect of
Hormone
Enters dormancy: (leaves drop off trees,
seeds fall, the stomata close to
reduce water loss during drought stress)

17. Abscisic Acid
Abscisic Acid in a plant recognizes seasonal changes
Dormancy: a period of no growth
As daylight decreases in the
winter, the chloroplasts in the
plant recognize a shorter
photoperiod.
Abscisic Acid
triggers the plant
to enter dormancy.
The flowers, seeds
and leaves fall from
the trees

18. Ethylene
Role of
Hormone
Ripening and Death
Site of
Production
Ripening fruits, aging flowers,
germinating seeds and wounded tissues
Effect of
Hormone
Stimulates fruits to ripen, flowers to
enter senescence (to grow old and die)

19. What causes my fruits and veggies to go bad?
The answer is ETHYLENE!
Ethylene is the only plant hormone that exists in a gas
form.
It can be synthesized from anywhere in the plant. It can
even diffuse outside the origin plant and affect another
plant nearby.

20. Is Ethylene good or bad?
A gas that speeds up plant death?!!
How is this good news?!
Examples:
flowers or fruit
that are not
“ripe” need
ethylene to
reach their peak

21. If the amount of Auxin is greater than Cytokinins…
Auxin
Cytokinis
Shoots grow less rapidly,
while root growth is dramatically increased
How do these Hormones Work
Together?

22. If the amount of Cytokinins is greater than Auxin…
Cytokinin
Auxin
How do these Hormones Work
Together?
Shoot growth is dramatically increased,
while roots grow is less rapidly.

23. If the amount of Cytokinins is greater than Auxin…
Abscisic Acid
Gibberellin
Flowers are no longer blooming
and leaves begin to fall from the trees.
How do these Hormones Work
Together?

24. If the amount of Cytokinins is greater than Auxin…
Gibberellin
Abscisic Acid
Green leaves and flowers begin to regrow.
This usually occurs in the spring time.
How do these Hormones Work
Together?

25. In summary…
Auxin
• Produces Roots
• Cell elongation
• Acts through tropism responses
Cytokinin
• Produces new Shoots
• Mitosis of new cells
• Seed Germination
Gibberellin
• Promotes Internode elongation
• Controls yearly cycles
Abscisic Acid
• “Dormancy hormone”
• Causes leaves to drop and stomatas to close
Ethylene
• “Death Hormone”
• Speeds up ripening and senescence

26. Thank You