Chapter 39 Reading Quiz
1.
2.
3.
4.
5.
What are the three parts to a signaltransduction pathway?
What is the general term for chemicals
that coordinate the parts of an organism?
Which hormone promotes the ripening of
fruits?
What effect do gibberellins have on
seeds?
What is “apoptosis”?
1. Explain how a hormone may cause its
effect on plant growth and development.
• Hormone  chemical signals that
coordinate the parts of an organism
• It is a compound that is produced by one
part of the body and then transported to
other parts
• Here it triggers responses in the target
cells and tissues 
2. Describe phototropism.
• Phototropism  the
growth of a shoot
toward or away from
light
- positive  towards
light
- negative  away
from light 
3. Describe what the hormone auxin does and
effects that it has on plants.
• Auxin  describes any chemical substance
that promotes the elongation of coleoptiles
(the sheath that encloses the plant shoot)
• Have multiple functions in both monocots &
dicots
• Stimulates stem elongation, root growth,
differentiation, and branching
• Development of fruit, apical dominance,
phototropism & gravitropism 
4. Describe what cytokinins do to a plant.
• They are synthesized in the roots and
transported to the other organs
• Affect root growth & differentiation
• Stimulates cell division and growth
• Stimulates germination and can delay
senescence 
5. Explain how the ratio of cytokinin to auxin
affects cell division and cell differentiation.
• If more cytokinin than auxin  shoot buds
develop from the callus
• If more auxin than cytokinin  roots form
• Cytokinin alone in a culture will have no
effect 
6. Define apical dominance and describe the
check-and-balance control of lateral
branching by auxins and cytokinins.
• Apical dominance  the ability of the terminal bud
to suppress the development of axillary buds
• The auxins & gibberellins are antagonistic in this
case
• Auxin transported down the shoot from the
terminal bud restrains axillary buds from growing,
causing the shoot to lengthen
• Cytokinins entering the shoot system from the
roots counter the action of auxin by signaling
axillary buds to begin growing
- auxin cannot suppress the growth of these buds
once it has begun 
7. List several factors besides auxin from
the terminal bud that may control apical
dominance.
• As you’ve seen, cytokinins also play a role in
apical dominance through stimulating the
lateral branches to begin, and overpowering
auxin
• This ends the apical dominance that the
terminal bud once had 
8. Describe gibberellins and their effects on
plants.
• Gibberellins  form in the meristems of
apical buds and roots, young leaves, and the
plant embryo
• Promote seed & bud germination
• Causes stem elongation and leaf growth
• Stimulates flowering and development of
fruit
• Affects root growth & differentiation 
9. Describe how stem elongation and fruit
growth depend upon a synergism between
auxin and gibberellins.
• Stem elongation 
- auxins stimulate stem elongation
- gibberellins also stimulate stem
elongation
• Fruit growth 
- dual control by auxins & gibberellins
- in some plants, both must be present for
fruit to set 
10. Explain the probable mechanism by which
gibberellins trigger seed germination.
• After water is imbibed, the release of
gibberellins from the embryo signals the
seeds to break dormancy and germinate
• Gibberellins support the growth of
seedlings by stimulating the synthesis of
digestive enzymes to mobilize food supply

11. Describe how abscisic acid (ABA) helps
prepare a plant for winter.
• It is advantageous for plants to be
dormant at times
• ABA is produced in leaves, stems, roots,
seeds
• ABA inhibits growth, closes stomata during
water stress, and counteracts the breaking
of dormancy
• Suspends primary & secondary growth 
12. Explain the antagonistic relationship between
ABA and gibberellins.
• In seed growth, the seed will germinate
when ABA is overcome by its inactivation
or removal or by the increased activity of
gibberellins
• The ratio of gibberellins to ABA
determines whether the seed will remain
dormant or germinate 
13. Give an example of how ABA can act as a
stress hormone.
• It helps plants cope with adverse
conditions
• When a plant begins to wilt, ABA
accumulates in the leaves and causes
stomata to close, preventing further water
loss 
14. Describe the role of ethylene in plant
senescence, fruit ripening and leaf
abscission.
• Ethylene gas  a gaseous hormone
• Senescence  the progression of
irreversible change that eventually leads to
death (ex: autumn leaves, withering petals)
• Fruit ripening  degradation of cell walls,
decrease chlorophyll content, dropping
• Leaf abscission  the loss of leaves every
autumn (keeps deciduous trees from
desiccating during winter 
15. Discuss how the study of mutant varieties
of plants has heightened our understanding
of plant hormones.
• Mutant varieties that don’t develop
correctly tell us about plant hormone
function
• Ex: one tomato variety has fruit that never
ripens (it is missing the ethylene receptor)
• By studying these mutations scientists are
beginning to understand the signaltransduction pathways that link hormonal
signals to cellular responses 
16. List two environmental stimuli for leaf
abscission.
1. Cooler temperatures
2. Less water 
19. Explain how light causes a phototropic
response.
• The cells on the darker side of the plant
stem elongate faster than the cells on the
bright side, curving the stem toward the
light source 
18. Define circadian rhythm and explain what
happens when an organism is artificially
maintained in a constant environment.
• Circadian rhythm  a physiological cycle with a
frequency of about 24 hours
• Rhythms could be internal, or responses to an
environmental cycle (rotation of earth)
• Rhythms persist even if organism is sheltered
from environmental cues
• Deviation from 24-hour clock can be 21 – 27 hours
when sheltered
• All research thus far points
to an internal clock 
19. List some common factors that entrain
biological clocks.
• Day length
• Synthesis of a protein that regulates it’s
own production through feedback control
• It’s possible that all eukaryotes are prone
to jet lag, the way we are when the clock
on the wall doesn’t match our internal clock

20. Define photoperiodism.
• Photoperiodism  a physiological response
to photoperiod
• Seasonal events  seed germination,
flowering, onset and breaking of bud
dormancy 
21. Distinguish among short-day plants, longday plants, and day-neutral plants; give
common examples of each; and explain how
they depend upon critical night length.
• Short-day  requires a light period shorter than
a critical length to flower
ex: poinsettias (flower in late summer, fall, or
winter)
• Long-day  flower only when the light period is
longer than a certain number of hours
ex: spinach, lettuce, irises (flower in late spring,
early summer)
• Day-neutral  unaffected by day-length; flower
at a certain stage of maturity
ex: tomatoes, rice, dandelions 
22. Explain how the interconversion of
phytochrome can act as a switching
mechanism to help plants detect sunlight and
trigger many plant responses to light.
• How does a plant measure a photoperiod?
- pigments called phytochromes
• Red light (660 nm) is the best at
interrupting dark periods & can induce a
long-day plant to flower even if it shouldn’t

23. Using photoperiodism as an example,
explain how an integrated control system can
regulate a plant process such as flowering.
• Photoperiodism is a series of complicated
systems working together
• Phytochromes detect the amount & quality
of light to determine if flowering will
happen
• Plant hormones may also play a large role,
as plants can communicate the flowerinducing substance 
24. Describe how plants apparently tell up
from down, and explain why roots display
positive gravitropism and shoots exhibit
negative gravitropism.
• Gravitropism functions as soon as a seed
germinates
• Statoliths (specialized plastids containing
dense starch grains) are located in certain
cells of the root cap
• The aggregation of these statoliths at the
low point may cause a redistribution of
calcium, triggering growth 
25. Distinguish between thigmotropism and
thigmomorphogenesis.
• Thigmotropism  the directional growth in
response to touch
ex: vine coiling response
• Thigmomorphogenesis  a developmental response
to mechanical perturbation, usually results from an
increased production of ethylene to chronic
mechanical stimulation
ex: stunted trees in windy places 
26. Describe how motor organs within pulvini
can cause rapid leaf movements and sleep
movements.
• Pulvini  specialized motor organs located at the
joints of leaves
• Results from a rapid loss of turgor by cells within
these pulvini
• The motor cells become flaccid after stimulation
because they lose potassium, which causes water
to leave the cells
 Sleep movements are powered by daily changes in
the turgor pressure of the pulvini
 The potassium gain & loss governs water gain &
loss, drooping or raising the leaves 
27. Provide a plausible explanation for how a
stimulus that causes rapid leaf movement can
be transmitted through the plant.
1.
From the point of stimulation, the
message that produces this response
travels wavelike through plant 1 cm/s
2. Chemical messengers probably have a role
in this transmission
3. An electrical impulse can also be
detected (action potential) 
28. Explain the molecular basis of resistance
to nonvirulent pathogens.
• Pathogens & plants have coevolved to compromise
• Specific resistance to a disease is based on genefor-gene recognition
• Requires a precise match-up between an allele in
the plant and the allele in the pathogen
• This renders the pathogen “nonvirulent” and it
doesn’t destroy the plant or itself in the process

29. Describe the local and systemic response
to virulent pathogens.
• Localized  the cells attacked release
molecular signals that sets the rest into
action and forms a barricade of sorts to
slow pathogen
• Systemic  the same response in localized
can produce a signal molecule that triggers
generalized defense responses in the
organs distant from the original site of
infection  The End!
18. Define tropism and list three stimuli that
induce tropisms and a consequent change of
body shape.
•
Tropism  growth responses that result
in curvatures of whole plant organs
toward or away from stimuli
1. Light
2. Gravity
3. Touch 
17. Describe the components of a signaltransduction pathway.
1.
Reception  hormone binds to the
specific hormone receptor on cell surface
2. Signal transduction  information is
taken into the cell
3. Induction  the activation of the desired
cellular responses 