Plant Repsonses & Hormones
Nancy G. Morris
Volunteer State Community College
Campbell, 5th edition, Chapter 39
Plant Hormones
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Hormone = Gr. “to excite”
1) active in small amounts
2) produced in one part of plant &
transported to another for action
3) action is specific for that site
Each has a Multiplicity of Effects
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Depending on site of action
Developmental stage of plant
Concentration of hormone
Auxins
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stimulate growth but too much
inhibits growth
functions:
1) root initiation, stem elongation
2) retard abscission (loss) of leaves &
fruits (Figure 39.10)
3) stimulates cell differentiation
4) apical dominance (Figure 39.6)
Apical dominance
Gibberellins
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discovered due to a fungus
Giberella which causes Japanese
“foolish” rice seedling disease
Figure 39.7
induces flowering
stimulates growth by increasing
cell size & numbers (Figure 39.8)
“Foolish seedling disease” in rice
39.11 Effect of Gibberellin
Dwarf pea plant treated with gibberellin
Cytokinins
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1) induces cell division (cytokinesis)
2) affects root growth &
differentiation
3) stimulates germination
4) delays senescence (aging); the
progression of irreversible change
that eventually leads to death
Abscisic Acid
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1) stimulates abscission
2) converts vegetative buds
(active) to dormant buds
3) inhibits growth
39.16 Abscission
Ethylene
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1) promotes fruit ripening
2) stimulates production of
cellulase
Photochromes
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Plant pigment
Important in processes where
light is a critical factor, such
as, flowering in long day short
night plants
Figure 39.16
Plant Movements due to growth
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Phototrophism – movement toward
(positive) or movement away from
(negative) light
 Geotrophism – movement toward
(positive) or away from (negative) center
of the earth
 Thigmotrophism – directional growth
responding to contact (tendrils)
Photoperiodic control of Flowering
Plant Movements not due to Growth
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Thigmonastic response – touch
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Photonastic response – shamrock
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stimulus, e.g. mimosa leaves
Sleep response – caused by change
in osmotic pressure
Sleep movements of a bean plant:
Movement of materials
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Path from roots:
1) epidermis of root & root hair cells
2) roots absorb water, minerals, gases
3) stems conduct
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Xylem & phloem are vertical conductors
Parenchyma of cortex, pith, rays are lateral
conductors
Movement
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Root pressure –osmotic pressure &
pressure created by active absorption &
secretion by root cells; drives materials
up xylem
Transpiration – evaporation of water
through leaf stomata; cohesion of water
molecules exerts a pull on columns of
water in xylem
Guttation – release of water droplets at
leaf tips; occurs when too much water is
absorbed by plant & when humidity is high
Translocation – movement of material
from one location to another
Guttation
Fighting Back….
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Response to herbivory
(animals eating plants):
1) physical barriers – thorns & spines
2) chemical defenses – distasteful or toxic
compounds
3) chemical signaling – salivary enzyme from
herbivore triggers a pathway that produces a
volatile attractant that recruits a parasitoid
wasp. The wasp lays eggs within the caterpillar,
and the larvae upon hatching destroy the
caterpillar by consuming its tissues. (Figure 39.22)