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1. reception – signal molecule lands on
receptor
2. Transduction – relay molecules called
second messengers
3. Response – activation of cellular
response

Etiolation – morphological adaptations
for growing in darkness
› Energy is spent elongating stems

De-etiolation “greening” – shoot reaches
sunlight – phytochrome (photoreceptor)
› Elongation slows, leaves expand, roots
elongate, shoot produces chlorophyll

Hormones – chemical signals that
coordinate the different parts of an
organism
› In plants, response is governed by interaction
of two or more hormones
› First plant hormone discovered was Auxin
(IAA)
› Cells have receptors for hormones which
then activate processes in cell such as:
 Transcription
 Activate enzymes
 Membrane transport
Types of Hormones
Auxin (IAA)
Cytokinins
Gibberellins
Brassinosteroids
Abscisic acid
Ethylene

Tropism – any growth response that
results in curvatures of whole plant
organs toward or away from stimuli
› Phototropism
 Positive phototropism – growth toward light
 Negative phototropism – growth away from
light

Used opaque covers on tip and collar at
bottom to test phototropism – light hitting
tip caused phototropism

Cut coleoptile & put it back on with
gelatin or mica separating it – a
substance moved through the gelatin
that caused bending
Extracted auxin from the gelatin
 In the dark, placed gelatin with auxin off
center and the coleoptile bent away
from the side with the gelatin

For monocots, auxin causes
phototropism by accumulating on the
dark side of the shoot and causing cell
elongation
 Not the case for dicots

Produced in shoot tips
 Causes cell elongation in low conc but
inhibits elongation in higher conc.
 Stimulates lateral and adventitious root
formation
 Synthetic auxin in high doses kills dicots
 Developing seeds give off auxin which
promotes fruit growth
 Interplay with cytokinin causes apical
dominance

Interplay between auxin & cytokinin
 How do these two
hormones produce the
Christmas tree look?

Produced in embryos, roots, fruits –
moves from root upward in xylem
 Stimulates cell division in conjunction
with auxin
 Balance b/w 2 causes differentiation

› Apical dominance
 Cytokinin stimulates lateral buds & growth
 Auxin inhibits lateral buds & growth

Antiaging effect – cut pieces of leaves
stay green by inhibiting senescence
(aging)
Stem elongation
 Fruit gets larger

› Ex - Thompson seedless grapes – grapes get
larger (with auxin) and internodes elongate
to make room for bigger grapes
Help seeds break dormancy (stimulate
amylase release) & mobilize food
 Contributes to flower bolting

Slows growth
 Promotes seed dormancy – inhibit
germination & help w/ dormancy
 Drought tolerance

› Causes stomata to close
› Warns leaves of H2O shortage
Produced in response to stress & high
levels of auxin
 Triple response to mechanic stress –
allows growing shoot to avoid obstacle
 Programmed cell death (apoptosis)

› Occurs in xylem vessels, cells in flowers after
pollination, leaves in autumn
Leaf abscission
 Fruit ripening

Produced in seeds, fruit, shoots, leaves,
and floral buds
 Inhibit root growth; slow leaf abscission;
promote xylem differentiation

The effect of light on plant morphology
 Light also allows plants to measure the
passage of days and seasons
 Photoreceptors

› Blue light receptors control: phototropism,
stomatal opening,
› Red light receptors (phytochrome) controls:
seed germination in lettuce, stimulates
branching & inhibits vertical growth, sets
circadian rhythms to 24 hrs

Flowering
› Short day plants require a short enough day
and long enough night
 Tobacco, mums, poinsettias, some soybeans –
late summer or fall
› Long day plants require a long enough day
and short enough night
 Spinach, radish, lettuce, iris, cereal grains – late
spring or early summer
› Day neutral flower when they are old
enough
 Tomatoes, dandelions, rice
› Some plants must have cold treatment
before photoperiod will induce it to flower

Gravitropism – response to gravity
› Roots display positive gravitropism
› Shoots exhibit negative gravitropism
› Auxin plays rolls in gravitropism
› Statoliths – specialized starch plastids settle
to lower portions of cells

Thigmotropism – directional growth in
response to touch
› Action potentials cause response
› Ex – Mimosa pudica and Venus fly trap
›
›
Mimosa pudica video
Venus fly trap video

Drought
Problem – loss of turgor,
dehydration

Flooding
Problem – O2 deprivation

Salt
Problem – hypertonic environment
– roots lose turgur pressure

Heat
Problem - denaturing proteins

Cold
Problem – lose fluidity of membrane