1. “Hormone” was first used to describe substances in animals:
“a substance produced in a gland that circulates in the blood and has an effect far away from the site of production”
In plants used to mean a compound that acts at low concentrations to affect growth and development. Produced in one part of the plant and utilized in another part.

2. Five plant hormones known by the mid 1960s, new compounds called plant growth regulators.
The five classical hormones
Auxins
Cytokinins
Gibberellins
Ethylene
Abscisic acid
Other plant growth regulators
Brassinosteroids
Salicylic acid
Jasmonic acid
Systemin
Florigen

3. Hormones Class Endogenous Hormone Growth Regulators
auxin indoleacetic acid IBA, NAA, 2,4-D, others
cytokinin zeatin, zeatin riboside kinetin, BA, 2iP, TDZ
gibberellin GAx GA3, GA4+7
abscisic acid abscisic acid (ABA)
ethylene ethylene Ethephon, Ethrel

5. Frits Went’s experiments

10. Auxin (indoleacetic acid)
Produced in apical and root meristems, young leaves, seeds in developing fruits
cell elongation and expansion
suppression of lateral bud growth
initiation of adventitious roots
stimulation of abscission (young fruits) or delay of abscission
hormone implicated in tropisms (photo-, gravi-, thigmo-)

11. Auxin Indole acetic acid and related molecules Photo-and gravitropism
The shoot hormone, made in the shoot apex
Travels down the stem
Polar Auxin Transport

13. Auxin promotes rooting

14. Auxin also: Promotes apical dominance Prevents leaf abscission
Enhances fruit growth
Auxin from the developing seeds results in fruit growth

16. Auxin-like growth regulators
indolebutyric acid (IBA)
2, 4 dichlorphenoxyacetic acid (2,4D)
2, 4, 5 trichlorophenoxyacetic acid (2, 4, 5 T)
picloram

17. Napalm, Agent Orange, dioxin

18. Herbicide 2,4-D – 2,4 dichlorophenoxy acetic acid
Causes a plant to grow itself to death
More readily absorbed by broad-leaved plants
Most often the “weed” of ‘Weed and Feed’ lawn fertilizers

20. Commercial uses - auxins
stimulation of adventitious rooting
2,4-D as a herbicide for dicots
sprout prevention in pruned trees
fruit thinning or fruit holding depending on stage
of development

21. Cytokinin Cytokinin – the root hormone Discovered by Folke Skoog
The last unknown compound needed to get plant cells to grow undifferentiated
Cytokinins delay and even reverse senescence
Release buds from apical dominance

22. Cytokinin (zeatin, ZR, IPA)
cell division factor
stimulates adventitious bud formation
delays senescence
promotes some stages of root development

23. Cytokinins Cell division and cytokinesis - Moves in xylem sap
- Stimulates RNA and protein synthesis
- Works in conjunction with auxin b.
Apical dominance
- Cytokinin and auxin are antagonistic- auxin from terminal bud causes shoot to lengthen
- Cytokinins from roots stimulate axillary bud
-Auxin stimulates lateral root formation cytokinins restrain it
- Anti-aging - slows leaf deterioration

24. Commercial uses - cytokinins
Applied as kinetin, benzyladenine, or zeatin conjugates
axillary bud growth in orchids, daylilies
antioxidant (browning preventer) in cut salads
mix with GAs as fruit size stimulator

26. Gibberellins
A large family of compounds (80) with a few biologically active members
Produced in roots and young leaves
Now known to be essential for stem elongation
Dwarf plant varieties often lack gibberellins

27. Gibberellins
Found as the toxin produced by some fungi that caused rice to grow too tall – “Foolish Disease”
Induces parthenocarpic fruit development
Delays senescence

28. Gibberellins are involved in bolting of rosette plants
Gibberellins are used to improve grapes
Gibberellins are involved in seed germination
gibberellins will induce genes to make enzymes that break down starch

29. Commercial uses - GA inhibitors
Cycocel, Bonsai, Sumagic
height control in flowering pot plants (lilies, orchids)
height control in bedding plants

30. Commercial uses - GA application
Apply as GA3, or GA4+7
increase flower size on certain ornamentals (eg, “gibbing” camellias)
increase berry separation and size in bunch grapes
overcome shallow dormancies in vegetative buds
stimulate seed germination

32. Ethylene The smallest hormone A gas
Important in seed germination, fruit ripening, epinasty, abscision of leaves
Sex expression in cucurbits

33. Ethylene Gaseous hormone produced in many plant tissues
autocatalytic (stimulates its own production)
volatile gas
production stimulated during ripening, flooding, stress, senescence, mechanical damage, infection
product of combustion of petrochemicals

35. Commercial uses - ethylene application
Applied as ethylene gas or Ethephon or Ethrel sprays
flower initiation (bromelliads, pineapples)
stimulation of ripening (bananas, tomatoes)
degreening of citrus
abscission induction prior to mechanical harvest (cherries)
increased color development in once-over harvested processor type tomatoes

36. Commercial uses - ethylene inhibition
Removed by chemical “scrubbing” or low atmosphere
long term storage of apples in CA storages
treatment of cut flowers with silver thiosulfate
long-keeper (delayed ripening) mutants (or rDNA) of tomato
hypobaric storage of many fruits, vegetables, flowers

38. Abscisic acid (ABA)
Found in stressed leaves, dormant seeds, dormant buds
stomatal closure
inhibits germination of some seeds
inhibits active growth of axillary buds

39. Abscisic acid Incorrectly named, not related to abscission
Important in water stress and other stresses
Causes stomatal closure
Prevents premature germination of seeds
Changes gene expression patterns