Plant hormones (also known as plant growth regulators (PGRs) and phytohormones) are chemicals that regulate a plant's growth. Plant hormones on the other hand, are not like animal hormones, they are often not transported to other parts of the plant and production is not limited to specific locations. Plants lack tissues or organs specifically for the production of hormones; unlike animals, plants lack glands that produce and secrete hormones to be moved around the body. Plant hormones shape the plant, effecting seed growth, time of flowering, the sex of flowers, its longevity, senescence of leaves and fruits, they affect which tissues grow up and which grow downward, leaf formation and stem growth, fruit development and ripening, and even plant death. Hormones are vital to plant growth and lacking them plants would be mostly a mass of undifferentiated cells.
Plant Growth Regulators
•In plants, many behavioral patterns and
functions are controlled by hormones.
These are “chemical messengers”
influencing many patterns of plant
oPlant hormones – a natural substance
(produced by plant) that acts to
control plant activities. Chemical
•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
stimulus are and then released for
transport through either phloem or
mesophyll when the appropriate
•Plant growth regulators – include plant
hormones (natural & synthetic), but also
include non-nutrient chemicals not found
naturally in plants that when applied to
plants, influence their growth and
•5 recognized groups of natural plant
hormones and growth regulators.
o5. Abscisic acid
•Influence plant growth – found in leaves
and stems – growth regulators and
•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
What happens when auxin is added to a stem.
The stem will bend away from the auxin. It
elongates faster on the extra auxin side.
•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
•Photo (light) and geotropism (gravity) –
involved in tropism responses – positive
•Flower initiation and development
•Root initiation and development
(rootone) – used on cuttings to help
stimulate root growth
•Have a regulatory function
•Are produced in the shoot apex primarily in
the leaf primordial (leaf bud) and root
•Stimulates stem growth dramatically
•Stimulates cell division, cell elongation
(or both) and controls enzyme
secretions. Ex: 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
•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.
•Used commercially in:
oIncreasing fruit size of seedless
oStimulating seed germination &
•Promoting male flowers in cucumbers
for seed production.
•Overcoming cold requirements – for
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).
•Promotes cell division
•Found in all tissues with considerable cell
oEx: embryos (seeds) and germinating
seeds, young developing fruits
•Roots supply cytokinins upward to the
•Interact with auxins to influence
differentiation of tissues (may be used
to stimulate bud formation).
•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 cultures use cytokinins to induce
•Cytokinins may slow or prevent leaf
senescence (leaf ageing or leaf fall).
•Produced in the actively growing
meristems of the plant, in senescing
ripening or ageing fruits, in senescing
(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.
•May promote leaf senescing and
abscission (leaf fall).
•Increases female flowers in cucumbers
(economically - will increase fruit
•Degreening of oranges, lemons and
grapefruit – ethylene gas breaks down
chlorophyll and lets colors show through.
•Abscisic Acid (ABA)
oWidespread in plant body – moves
readily through plant
oABA appears to be synthesized (made)
by the leaves.
oInteracts with other hormones in the
plant, counteracting the growth –
promoting the effects of auxins &
•Involved with leaf and fruit abscission (fall),
onset of dormancy in seeds and onset of
dormancy (rest period) in perennial flowers
•ABA is effective in inducing closure of
stomata in leaves, indicating a role in the
stress physiology in plants. (ex: increases
in ABA following water, heat and high
salinity stress to the plant)