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Hormones help control growth patterns. Some hormones mainly stimulate growth (auxins, gibberellins,
Like animals, plants use a reception-transductionresponse pathway when they respond to a stimulus. Tropisms are growth responses toward or away from unidirectional stimuli. Positive phototropism of stems is
growth toward light. Negative gravitropism of stems is
growth away from the direction of gravity. Thigmotropism occurs when a plant makes contact with an object. Nastic movements are not directional.
Plants exhibit circadian rhythms, such as the sleep
movements of prayer plants and the closing of stomata.
A biological clock most likely controls these circadian
rhythms.
S -r L./ onr
and cytokinins), and some mainly inhibit growth (abscisic acid and ethylene).
Photoperiodism refers to the physiological plant
responses to changes in the length of day or night. Flowering in plants is a photoperiodic event, leading to the classification of plants as short-day, long-day, or day-neutral.
The response of the pigment phytochrome is believed to
be involved in the flowering event.
E.)CE.FtISES
Study the text section by section as you answer the questions that follow.
•
•
•
•
Plants use a reception-transduction-response pathway when they respond to a stimulus.
Nastic movements do not involve growth and are not dependent on the direction of the stimulus.
Tropisms are growth responses in plants toward or away from unidirectional stimuli such as light or gravity.
Plants sometimes exhibit circadian rhythms (e.g., closing of stomata) that recur every 24 hours.
1. Identify the type of response described, and add the word positive or negative only if appropriate.
a.
A plant tendril encircles an object.
b.
A root grows down into the soil, in the same direction as gravity.
c.
When sensitive hairs are touched, the leaves of a Venus's flytrap snap shut.
d.
Leaves track the sun, and the stem bends toward the light.
e.
Stomata open and close every 24 hours.
Label the following diagram to explain the actions of a hormone. Then answer the questions.
.._
.. ,.
b.
8.
4r
.
.-,
..
' C.
..
..
.
,..
reactions
I:
hormone
receptor
----1( cellular
activities
----.
.
'0
I '-,.
223
a. What happens during reception? _
b. What happens during transduction?
c. What happens during celliilar activities?
•
•
•
•
•
Each class of plant hormones can be associated with specific responses to environmental stimuli.
Auxins bring about a response to both light and gravity.
The most obvious effect of gibberellins is elongation between nodes.
In tissue culture, the proportion of cytokinins to auxins affects differentiation and development.
Among other effects, abscisic acid helps regulate the closing of stomata, and ethylene causes fruits to ripen.
Auxins (PP. 482
-
483)
3. Indicate whether these statements concerning oat seedling experiments are true (T) or false (F). Rewrite any
false statements to make them true.
a._ Oat seedlings with tips removed still bend toward the light. Rewrite:
b.
When an agar block containing auxin is placed on one side of a tipless coleoptile, the shoot curves
toward that side. Rewrite:
c.
Auxin normally moves to the shady side, and thereafter the stem bends toward the light. Rewrite:
4. Study the following diagram.
Golgi apparatus
cell wall
materials
second messenger
and DNA-binding
protein
plasma —
membrane
cell wall
Explain what is happening at each numbered event.
224
Gibberelli7s (p. 484)
5. What effect do gibberellins have on the following?
a. stems
b. dormancy of seeds and buds
c. dwarf plants
6. Study the following diagram, and then answer the questions that follow concerning how gibberellin stimulates
growth of a plant embryo.
•
gibberellic acid
Ca24- channel
•
receptor
Ca 2 ' (second
messenger)
DNA-binding
protein
cytoplas
DNA
nucleus
V --Nf-N-,j--mRNA
c
oc
c
c cc
amylase
t
plasma
membrane
cell wall
a. What happens after gibberellin attaches to a plasma membrane receptor?
b. How does calcium (Ca2+) enter a cell?
c. What does the Ca2÷—DNA-binding protein complex do?
d. How does this help the embryo grow?
Cytakinins
(P.
486)
7. Indicate whether these statements about tissue culture experiments that demonstrate plant hormone interaction
are true (T) or false (F).
a.
Cellular enzymes affect the activity of plant hormones.
Oligosaccharins function in animals but not in plants.
b.
c.
The acidity of the culture medium affects cell differentiation.
The ratio of auxin to cytokinin affects cell differentiation.
d.
Abscisic Acicl
(p. 487)
8. What effect does abscisic acid have on the following?
a. seed and bud dormancy
b. a bud in the fall
c. stomata
225
9. Study the following diagram and list the events that occur after abscisic acid (ABA) is received at a guard cell
plasma membrane.
a.
inside
4
!)
-.:—'. :
-.
outside
H20
-. ...,
„ ,. :
..,.
,
.
b.
.
,-'...-.
'
0 •
.
•
----
ABA
C.
•
b. Guard cell plasma
membrane
-.-..
a. Open stoma
c. Closed stoma
Ethylene (p. 487)
10. What effect does ethylene have on the following?
a. ripening of fruit
b. abscission
1,41,101:.WeilifiglEMNIPIPI4T18-4
- 8 9)
Min
• Plant responses controlled by the length of day or night (photoperiod) involve the pigment phytochrome.
11. Study the following diagram. For questions a f, choose either shorter or longer as the answer. For g and h,
choose either night or day as the answer
-
Long-day
plant
°XI
Short-day
plant
Critical
length
a.
In a, a long-day plant flowers when the night is a-
than a critical length.
In a, a short-day plant will not flower when the night is h.
than a critical length.
In b, a long-day plant will not flower when the night is c-
than a critical length.
In b, a short-day plant will flower when the night is d-
than a critical length.
In c, a long-day plant will flower if a flash of light interrupts a night that is ecritical length. A short-day plant will not flower if a flash of light interrupts a night that
is f.
the g.
226
than a critical length. The conclusion is that the length of
, not the h-
,
controls flowering.
than a
12. a. Label the three arrows in the following diagram with these terms:
far-red light (shade and evenings)
red light (daytime)
metabolic conversion at night
d. Place the following statements to one side of P r or Pfr as appropriate:
Stem elongation is inhibited.
Seed germination is promoted.
Pfr
e. The conclusion is that
Review key terms by completing this crossword puzzle, using the following alphabetized list of terms:
ABA
abscission
auxin
cytokinin
ethylene
gibberellin
gravitropism
hormone
photoperiodism
phototropism
phytochrome
Across
2 Chemical messenger produced in one part of the
body that controls the activity of other parts
3 Plant hormone regulating growth, particularly cell
elongation; most often indoleacetic acid (IAA)
5 Relative lengths of daylight and darkness that affect the physiology and behavior of an organism
6 Plant hormone that causes stomata to close and that
initiates and maintains dormancy
7 Photoreversible plant pigment whose active form
seems to be involved in regulating transcription of
certain genes
Down
1 Directional growth of plants in response to the
Earth's gravity
3 Dropping of leaves, fruits, or flowers from a plant
4 Plant hormone producing increased stem growth; also
involved in flowering and seed germination
5 Directional growth of plants in response to light
8 Plant hormone that promotes cell division; often
works in combination with auxin during organ development in plant embryos
9 Plant hormone that causes ripening of fruit and is
also involved in abscission
227