Growth Responses and Regulation of Growth
CHAPTER 11
GROWTH & DEVELOPMENT
Growth- the increase in size of a plant
 Development- the gradual changes over the life
of the plant
 Both are controlled by

 Genetics
 Hormones
(ex. Auxins, gibberellin…)
 Environment (ex. Tropism)
WHAT ARE HORMONES?
Organic compounds that act as chemical
signals between cells
 Regulate growth & development

5 MAJOR CLASSES OF PLANT HORMONES
5.
Auxin
Gibberellin
Cytokinin
Ethylene
Abscisic Acid

Basically, these are just signaling molecules
1.
2.
3.
4.
AUXIN

Hormone involved in
 Stem
elongation
 Apical
dominance (inhibition of lateral growth)
 Encourages
 Helps
root formation on cuttings
plant grow towards light
 Targets
cells that are not reached by light (in shade)
 Responsible
for Phototropism
Animation
AUXIN AND PHOTOTROPISM

Coleoptile = the
first leaf of a
monocot seedling
AUXIN AND ROOT DEVELOPMENT
Control
(roots placed in water)
LOW AUXIN
CONCENTRATION
HIGH AUXIN
CONCENTRATION
GIBBERELLIN

Hormone involved in
 Stem
elongation
 Flowering
 Seed
germination
EFFECTS OF GIBBERELIN
CYTOKININ

Hormone involved in
 REQUIRED
 Delay
FOR CELL DIVISION!!!
of senescence (aging)
 Interacts
with auxin in the control of apical
dominance (encouraging primary growth)
HORMONES AND TISSUE CULTURE
CYTOKININ AND
SENESCENCE
ETHYLENE

A gaseous plant hormone involved in
 Leaf
abscissions (the normal falling off of
leaves/fruits/flowers)
 Ripens
fruits
 Weakening
 Leaf
cell walls during autumn
senescence (aging)
ETHYLENE AND FRUIT RIPENING
ABSCISIC ACID

Plant Hormone involved in
 Dormancy
(temporary state of arrest/growth does
not occur)
 Response
 Ex.
to stress
Drought – triggers closing of stomata
PHOTOPERIODISM

The response of a plant to the relative lengths of
daylight and darkness (such as flowering)

Short-day plants (long night)
 Flower
when the night length is = or > ~ 12 hours
 Late Summer or fall

Intermediate-day plants
 Do
not flower when day is either too long or too short
 Spring or Fall

Long-day plants (short night)
 Flower
when night length is = or < 12 hours
 Late spring or summer

Day-neutral plants
 photoperiod
does not affect flowering
TEMPERATURE AND REPRODUCTION

Vernalization
 The
low-temp requirement for flowering in some
plant species
 Some plants need to be exposed to low temp for
several weeks for flowering to occur after seeds
germinate and grow
 Example of plant with low T requirement:
 Winter
Wheat:
planted in fall and germinates.
 Seedlings exposed to cold winter
 Flower after resuming growth in spring

TEMPERATURE REQUIREMENTS
TROPISM
A tropism is a directional growth response
 Permanent change in position
 3 types of tropism

 Phototropism
- Stimulus  photo = “light”
 Gravitropism – Stimulus  grav = “weight”
 Thigmotropism – Stimulus  thigmo = “touch”
TROPISMS

PHOTOTROPISM
 Directional

GRAVITROPISM
 Plant

growth of a plant caused by light
growth in response to direction of gravity
THIGMOTROPISM
 Growth
in response to contact with a solid object