Plant Hormones - Volunteer State Community College

Plant Repsonses & Hormones
Nancy G. Morris
Volunteer State Community College
Campbell, 5th edition, Chapter 39
Plant Hormones
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
Depending on site of action
Developmental stage of plant
Concentration of hormone
stimulate growth but too much
inhibits growth
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
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
1) induces cell division (cytokinesis)
2) affects root growth &
3) stimulates germination
4) delays senescence (aging); the
progression of irreversible change
that eventually leads to death
Abscisic Acid
1) stimulates abscission
2) converts vegetative buds
(active) to dormant buds
3) inhibits growth
39.16 Abscission
1) promotes fruit ripening
2) stimulates production of
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
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
Thigmonastic response – touch
Photonastic response – shamrock
stimulus, e.g. mimosa leaves
Sleep response – caused by change
in osmotic pressure
Sleep movements of a bean plant:
Movement of materials
Path from roots:
1) epidermis of root & root hair cells
2) roots absorb water, minerals, gases
3) stems conduct
Xylem & phloem are vertical conductors
Parenchyma of cortex, pith, rays are lateral
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
Fighting Back….
Response to herbivory
(animals eating plants):
1) physical barriers – thorns & spines
2) chemical defenses – distasteful or toxic
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)