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37
Plant Growth and Development
Black-eyed Susan. This plant
(Rudbeckia hirta), which grows
to 0.9 m (3 ft), produces flowers
in response to the shortening nights of spring and early
summer.
T
he ultimate control of plant growth and development, which
includes all the changes that take place during the entire
life of an individual, is genetic. If the genes required for development of a particular trait, such as the shape of a leaf, the color
Dwight R. Kuhn
of a flower, or the type of root system, are not present, that characteristic does not develop. When a particular gene is present, its
expression—that is, how it exhibits itself as an observable feature
of an organism—is determined by several factors, including signals from other genes and from the environment. The location of
K EY C ONCE P TS
a cell in the young plant body also has a profound effect on gene
expression during development. Chemical signals from adjacent
Growth and development are influenced by a plant’s internal conditions and signals from its external environment.
cells help a cell “perceive” its location within the plant body. Each
A plant may respond to an external stimulus, such as light,
gravity, or touch, by a directional growth response, or
tropism. Study of tropisms has helped biologists elucidate
links between the external environment and internal signals
such as hormones.
becomes.
Hormones are chemical signals responsible for coordinating
and regulating many aspects of plant development.
plant tissues and that act as chemical signals between cells. Envi-
Although there are many plant hormones, five have been
well-characterized: auxins, gibberellins, cytokinins, ethylene, and abscisic acid.
cipitation and temperature, exert an important influence on gene
Plants have different receptors that detect various colors
of light. Phytochrome detects red and far-red light, which
affect several aspects of development, including the timing
of flowering.
cell’s spatial environment helps determine what that cell ultimately
Growth and development, including a plant’s responses to various changes in its environment, are controlled by plant hormones,
organic compounds that are present in very low concentrations in
ronmental cues, such as changing day length and variations in preexpression and hormone production, as they do on all aspects of
plant growth and development.
For example, the initiation of sexual reproduction is often under environmental control, particularly in temperate latitudes, and
plants switch to reproductive growth after receiving appropriate
signals from the environment. Many flowering plants are sensitive
to changes in the relative amounts of daylight and darkness that
accompany the changing seasons, and these plants flower in re-
789
sponse to those changes (see photograph on page 789). Other
vironment and use this information to help regulate normal growth
plants have temperature requirements that induce sexual repro-
and development. We consider all of these aspects of growth and
duction. Thus, plants continually perceive information from the en-
development in this chapter.
■
TROPISMS
Learning Objective
Plants exhibit movements in response to environmental stimuli
such as light, gravity, and touch. A plant may respond to such
an external stimulus by directional growth — that is, the direction of growth depends on the direction of the stimulus. Such a
directional growth response, called a tropism, results in a change
in the position of a plant part. Tropisms are irreversible and may
be positive or negative, depending on whether the plant grows
toward the stimulus (a positive tropism) or away from it (a negative tropism). Tropisms are under hormonal control, which is
discussed later in the chapter.
Phototropism is the directional growth of a plant caused
by light (❚ Fig. 37-1). Most growing shoot tips exhibit positive
phototropism by bending (growing) toward light, something you
may have observed if you place houseplants near a sunny window. This growth response increases the likelihood that stems
and leaves receive adequate light for photosynthesis. The bending
response of phototropism is triggered by blue light with wavelengths less than 500 nm. (You may recall from Chapter 33 that
blue light also induces stomata to open.)
For a plant or any organism to have a biological response to
light, it must contain a light-sensitive substance, called a photoreceptor, to absorb the light. The photoreceptor that absorbs blue
light and triggers the phototropic response and other blue-light
responses (such as stomatal opening) is a family of yellow pigments called phototropins. Phototropins are light-activated kinases, enzymes that transfer phosphate groups. There is evidence
that phototropins become phosphorylated — that is, a phosphate
On day 3, turned on
its side
Runk /Schoenberger/Grant Heilman
Describe phototropism, gravitropism, and thigmotropism.
1
Figure 37-1
Animated
Phototropism
Stems of corn (Zea mays) seedlings grow in the direction of light and
therefore exhibit positive phototropism. The bending is caused by
greater elongation on the shaded side of a stem than on the lighted side.
group is added — in response to blue light. Thus, phosphorylation is an early step in the blue light – signaling pathway.
Growth in response to the direction of gravity is called gravitropism. Most stem tips exhibit negative gravitropism by growing away from Earth’s center, whereas most root tips exhibit positive gravitropism (❚ Fig. 37-2). The root cap is the site of gravity
perception in roots; when the root cap is removed, the root con-
One hour later
Figure 37-2
Animated
Gravitropism
(a) A corn (Zea mays) seedling was turned on its side 3 days after germinating. In 1 hour the root and shoot tips
had curved. (b) In 24 hours (seedling on the right), the new root growth was downward (positive gravitropism)
and the new shoot growth was upward (negative gravitropism). A control seedling germinated at the same time
is on the left.
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Chapter 37
Dennis Drenner
(a)
(b)
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tinues to grow, but it loses any ability to perceive gravity. Special
cells in the root cap possess starch-containing amyloplasts that
collect toward the bottom of the cells in response to gravity, and
these amyloplasts may initiate at least some of the gravitropic response. If the root is put in a different position, as when a potted
plant is laid on its side, the amyloplasts tumble to a new position,
always settling in the direction of gravity. The gravitropic response
(bending) occurs shortly thereafter and involves the hormone
auxin (discussed later in the chapter). Despite the movement
of amyloplasts in response to gravity, researchers question their
role in gravitropism. A mutant Arabidopsis plant that lacks amyloplasts in its root cap cells still responds gravitropically when
placed on its side, indicating that roots do not necessarily need
amyloplasts to respond to gravity. Ongoing research may clarify
how roots perceive gravity.
Thigmotropism is growth in response to a mechanical stimulus, such as contact with a solid object. The twining or curling
growth of tendrils or stems, which helps attach a climbing plant
such as a vine to some type of support, is an example of thigmotropism (see Fig. 33-13b).
Review
❚
❚
What is phototropism? How does blue light trigger the phototropic response?
How do phototropism and gravitropism differ?
PLANT HORMONES
AND DEVELOPMENT
3
5
List several ways that each of these hormones affects plant
growth and development: auxins, gibberellins, cytokinins,
ethylene, and abscisic acid.
Summarize the activities of these plant hormones and hormone-like signaling molecules: brassinosteroids, jasmonates,
salicylic acid, systemin, and oligosaccharins.
A plant hormone is an organic compound that acts as a chemical
signal eliciting a variety of responses that regulate growth and development. The study of plant hormones is challenging because
hormones are effective in extremely small concentrations (less
than 106 mol / L). In addition, the effects of different plant hormones overlap, and it is difficult to determine which hormone,
if any, is the primary cause of a particular response. Plant hormones may also stimulate a response at one concentration and
inhibit that same response at a different concentration.
Plant hormones are different from animal hormones in several ways. For example, most plant hormones are not produced
in one part of the plant and transported over long distances to another part, where they cause a particular response; instead, the effects of plant hormones often occur close to where hormones are
produced. Also, plant hormones are generally small molecules,
not large, complex molecules like many animal hormones.
For many years, biologists studied five major classes of plant
hormones: auxins, gibberellins, cytokinins, ethylene, and abscisic
acid. More recently, researchers have uncovered compelling
evidence for a variety of signaling molecules, such as brassinosteroids, jasmonates, salicylic acid, systemin, and oligosaccharins. (❚ Table 37-1 summarizes plant hormones and hormonelike signaling molecules discussed in this chapter.)
Plant hormones act by signal transduction
Learning Objectives
2
4
Describe a general mechanism of action for plant hormones,
using auxin as your example.
Describe early auxin experiments involving phototropism.
Researchers have used molecular genetic techniques to better understand the biology of plant hormones. Arabidopsis mutants are
particularly useful. For example, different mutants have defects
TABLE 37-1
Plant Hormones and Signaling Molecules
Hormone
Site of Production
Principal Actions
Auxins (e.g., IAA)
Shoot apical meristem, young leaves, seeds
Stem elongation, apical dominance, root initiation, fruit development
Gibberellins
(e.g., GA3 )
Young leaves and shoot apical meristems,
embryo in seed
Seed germination, stem elongation, flowering, fruit development
Cytokinins
(e.g., zeatin)
Roots
Cell division, delay of leaf senescence, inhibition of apical dominance,
flower development, embryo development, seed germination
Ethylene
Stem nodes, ripening fruit, damaged or
senescing tissue
Fruit ripening, responses to environmental stressors, seed germination,
maintenance of apical hook on seedlings, root initiation, senescence and
abscission in leaves and flowers
Abscisic acid
Almost all cells that contain plastids (leaves,
stems, roots)
Seed dormancy, responses to water stress
Brassinosteroids
(e.g., brassinolide)
Shoots (leaves and flower buds), seeds, fruits
Light-mediated gene expression, cell division, cell elongation, seed
germination, vascular development
Jasmonates
(e.g., jasmonic acid)
Leaves? Probably many tissues
Initiation of defenses against predators or disease organisms
Salicylic acid
Wound (site of infection)
Resistance to disease organisms
Systemin
Wound (site of herbivore or pathogen attack)
Initiation of defenses against predators (herbivores) or disease organisms
Oligosaccharins
Unknown
May function in normal cell growth and development; defense responses
to disease organisms
Plant Growth and Development
791
Key Point
Many plant hormones activate genes through a signal cascade involving a series
of enzymes.
Plasma membrane
Cell wall
Ubiquitin
2
Ubiquitin is
tagged to proteins
that inhibit certain
genes.
4
Previously repressed
genes are activated
and expressed.
Nucleus
Transcription
Auxin
1
Auxin binds to
TIR1 receptor.
3
Protein
Proteins are
targeted for
destruction.
DNA
Nuclear envelope
Cytoplasm
Figure 37-3
General mechanism of action for the hormone auxin
The numbered steps are explained in the text.
in hormone synthesis, hormone transport, signal reception, or
signal transduction. In signal transduction, a receptor converts
an extracellular signal into an intracellular signal that causes
some change in the cell. Such mutants enable plant biologists to
identify and clone genes involved in these aspects of hormone
biology. Studying the mutant phenotypes helps plant biologists
establish connections between the mutant genes and specific
physiological activities involved in growth and development.
Using this research, biologists are elucidating the general
mechanism of plant hormone action. It appears that many plant
hormones bind to enzyme-linked receptors located in the plasma
membrane; the hormone binds to the receptor, where it triggers
an enzymatic reaction of some sort.
Let us consider a specific example that involves the plant hormone auxin, which is known to cause rapid changes in gene ex1 , the receptor for auxin
pression (❚ Fig. 37-3). As shown in step ●
is located in the plasma membrane and has a three-dimensional
shape that binds to the auxin molecule. The binding of auxin to
its receptor catalyzes the attachment of the molecule ubiquitin to
2 ). Whenever
repressor proteins that inhibit certain genes (step ●
ubiquitin is attached to protein molecules, the cell targets those
3 ). As a result, the genes that
molecules for destruction (step ●
were repressed by those repressor proteins are now activated, re4 ).
sulting in changes in cell growth and development (step ●
The identification of the receptor for auxin was reported in
2005 by two groups working independently. This receptor, called
transport inhibitor response 1 (TIR1) protein, is an F-box protein.
The F-box is a short sequence of amino acids found in molecules
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Chapter 37
that catalyze the addition of ubiquitin tags to proteins targeted
for destruction. Interestingly, all animals, plants, and fungi examined to date use ubiquitin to target proteins for destruction,
but bacteria do not. This suggests that the use of ubiquitin tags
evolved early in eukaryote evolution and was conserved as the
various groups of eukaryotes diversified. Plants have about 700
F-box proteins, but not much is known about most of them. Future research may identify other plant hormones and signaling
molecules that use F-box proteins as receptors.
Auxins promote cell elongation
Charles Darwin, the British naturalist best known for developing
the theory of natural selection to explain evolution, also provided
the first evidence for the existence of auxins. The experiments
that Darwin and his son Francis performed in the 1870s involved
positive phototropism, the directional growth of plants toward
light. The plants they used were newly germinated canary grass
seedlings. As in all grasses, the first part of a canary grass seedling
to emerge from the soil is the coleoptile, a protective sheath that
encircles the stem. When coleoptiles are exposed to light from
only one direction, they bend toward the light. The bending occurs below the tip of the coleoptile.
The Darwins tried to influence this bending in several ways
(❚ Fig. 37-4). For example, they covered the tip of the coleoptile as soon as it emerged from the soil. When they covered that
part of the coleoptile above where the bend would be expected
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Dennis Drenner
Dennis Drenner
of the sun reset the biological clock so that
the cycle repeats every 24 hours. What happens if a plant’s circadian clock is mutated
so that it is not resynchronized by the day –
night cycle? In 2005, biologists at the University of Cambridge in England reported
the identification of mutants of the model
plant Arabidopsis in which the circadian
clock is not synchronized to match the external day –night cycle. The mutant plants
were found to contain less chlorophyll, fix
less carbon by photosynthesis, grow more
slowly, and have less of a competitive advantage than Arabidopsis plants with a normal circadian clock.
For many plants, two photoreceptors — the red light – absorbing phytochrome and the blue /ultraviolet-A light –
absorbing cryptochrome— are implicated
in resetting the biological clock. Certain
(b) Leaf position at midnight.
(a) Leaf position at noon in a bean
amino acid sequences of the protein por(Phaseolus vulgaris) seedling.
tion of phytochrome are homologous with
amino acid sequences of clock proteins in
Figure 37-19 Sleep movements
fruit flies, fungi, mammals, and bacteria;
this molecular evidence strongly supports
the circadian-clock role of phytochrome.
Circadian rhythms in plants affect such biological events as
The evidence for cryptochrome as a clock protein is also congene expression, the timing of photoperiodism (that is, of seavincing. First discovered in plants, cryptochrome counterparts
sonal reproduction), the rate of photosynthesis, and the opening
are found in the fruit fly and mouse biological-clock proteins.
and closing of stomata. Sleep movements observed in the comPossibly both photoreceptors are involved in resetting the biomon bean and other plants are another example of a circadian
logical clock in plants; researchers have evidence that phytorhythm (❚ Fig. 37-19). During the day, bean leaves are horizonchrome and cryptochrome sometimes interact to regulate similar
tal, possibly for optimal light absorption, but at night the leaves
responses.
fold down or up, a movement that orients them perpendicular to
their daytime position. The biological significance of sleep moveReview
ments is unknown at this time.
❚ What is phytochrome? What are two roles of phytochrome?
When constant environmental conditions are maintained,
❚ What are the steps in phytochrome signal transduction?
circadian rhythms such as sleep movements repeat every 20 to
❚ In what process is cryptochrome involved?
30 hours, at least for several days. In nature, the rising and setting
S UM M A RY WI T H KE Y TE RM S
Learning Objectives
1
Describe phototropism, gravitropism, and thigmotropism
(page 790).
❚ Tropisms are directional growth responses. Phototropism
is growth in response to the direction of light. Gravitropism is growth in response to the influence of gravity.
Thigmotropism is growth in response to contact with a
solid object.
Learn more about phototropism and gravitropism by clicking on the figures in ThomsonNOW.
2
804
Describe a general mechanism of action for plant hormones,
using auxin as your example (page 791).
❚ Plants produce and respond to hormones, organic
compounds that act as highly specific chemical signals
Chapter 37
to elicit a variety of responses that regulate growth and
development.
❚ Many plant hormones bind to enzyme-linked receptors
located in the plasma membrane; the hormone binds to
the receptor, triggering an enzymatic reaction of some
sort. The receptor for auxin, which is located in the plasma
membrane, binds to auxin. This catalyzes the attachment of ubiquitin to repressor proteins, targeting those
molecules for destruction. When genes repressed by those
proteins are then activated, changes in cell growth and
development result.
3 Describe early auxin experiments involving phototropism
(page 791).
❚ Darwin and his son performed phototropism experiments
in the 1870s on grass seedlings. When they covered the
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coleoptile tip, the plant did not bend. When they removed
the coleoptile tip, bending did not occur. When the bottom of the coleoptile was shielded from the light, the coleoptile bent. The Darwins concluded that some substance
is transmitted from the upper to the lower part that causes
the plant to bend.
❚ In the 1920s, Frits Went isolated the phototropic hormone
from oat coleoptiles by removing the coleoptile tips and
placing them on agar blocks. Normal growth resumed
when he put one of these agar blocks squarely on a decapitated coleoptile; the substance had diffused from the
coleoptile tip into the agar and, later, from the agar into
the decapitated coleoptile. Went named this substance
auxin.
See the auxin experiments in action by clicking
on the figure in ThomsonNOW.
4
List several ways that each of these hormones affects plant
growth and development: auxins, gibberellins, cytokinins,
ethylene, and abscisic acid (page 791).
❚ Auxin is involved in cell elongation; tropisms; apical
dominance, the inhibition of axillary buds by the apical
meristem; and fruit development. Auxin also stimulates
root development on stem cuttings.
❚ Salicylic acid triggers systemic acquired resistance that
helps defend plants against pathogens and insect pests.
❚ Systemin, a polypeptide with hormonal properties,
stimulates a natural defense mechanism in which the plant
produces molecules that disrupt insect digestion.
❚ Oligosaccharins, short branched chains of sugar molecules, inhibit flowering and stimulate vegetative growth.
6 Explain how varying amounts of light and darkness induce
flowering (page 800).
❚ Photoperiodism is any response of plants to the duration
and timing of light and dark. Flowering is a photoperiodic
response in many plants. Short-day plants detect the
lengthening nights of late summer or fall and flower at
that time. Long-day plants detect the shortening nights
of spring and early summer and flower at that time.
Intermediate-day plants flower when exposed to days
and nights of intermediate length.
Learn more about photoperiodism by clicking
on the figure in ThomsonNOW.
7
❚ Gibberellins are involved in stem elongation, flowering,
and germination.
❚ Cytokinins promote cell division and differentiation; delay senescence, the natural aging process; and interact
with auxin and ethylene in apical dominance. Cytokinins
induce cell division in tissue culture, a technique in which
cells are isolated from plants and grown in a nutrient
medium.
❚ Ethylene plays a role in ripening fruits; apical dominance;
leaf abscission; wound response; thigmomorphogenesis,
a developmental response to mechanical stressors such as
wind; and senescence.
❚ Abscisic acid is an environmental stress hormone involved
in stomatal closure caused by water stress and in seed
dormancy, a temporary state of reduced physiological
activity.
5
Summarize the activities of these plant hormones and
hormone-like signaling molecules: brassinosteroids, jasmonates, salicylic acid, systemin, and oligosaccharins
(page 791).
❚ Plant steroids known as brassinosteroids are involved in
several aspects of plant growth and development, such as
cell division, cell elongation, light-induced differentiation,
seed germination, and vascular development.
❚ Jasmonates affect several plant processes, such as pollen
development, root growth, fruit ripening, and senescence.
They are also produced in response to the presence of
insect pests and disease-causing organisms.
Describe the role of phytochrome in flowering, including
a brief discussion of phytochrome signal transduction
(page 800).
❚ The photoreceptor in photoperiodism is phytochrome,
a family of about five blue-green pigments. Each type of
phytochrome has two forms, Pr and Pfr, named by the
wavelength of light they absorb. Pfr is the active form,
triggering or inhibiting physiological responses such as
flowering, shade avoidance, and a light requirement for
germination.
❚ The pathway in phytochrome signal transduction begins
when inactive phytochrome in the cytoplasm absorbs
red light and is converted into the active form, Pfr, which
moves into the nucleus. There, phytochrome binds to
the transcription factor PIF3 (phytochrome-interacting
factor) and activates or represses the transcription of lightresponsive genes.
Learn more about phytochrome by clicking on
the figure in ThomsonNOW.
Define circadian rhythm, and give an example (page 800).
❚ A circadian rhythm is a regular period in an organism’s
growth or activities that approximates the 24-hour day and
is reset by the rising and setting of the sun. Two examples
of circadian rhythms are the opening and closing of stomata and sleep movements.
9 Distinguish between phytochrome and cryptochrome
(page 800).
❚ Both phytochrome and cryptochrome are photoreceptors
that sometimes interact to regulate similar responses, such
as resetting the biological clock. Phytochrome strongly
absorbs red light, whereas cryptochrome absorbs blue and
ultraviolet-A light.
8
T E ST Y OU R UN D E RS TA ND ING
1. In the signal transduction process for the hormone auxin,
the molecule ubiquitin (a) absorbs blue light (b) becomes
phosphorylated (c) tags certain proteins for destruction
(d) interacts antagonistically with gibberellins (e) binds to a
receptor in the plant cell’s plasma membrane
2. When you prune shrubs to make them “bushier”— that is, to
prevent apical dominance —you are affecting the distribution
and action of which plant hormone? (a) auxin (b) jasmonic
acid (c) systemin (d) ethylene (e) abscisic acid
Plant Growth and Development
805
3. A synthetic
known as 2,4-D is used as a
selective herbicide. (a) auxin (b) gibberellin (c) cytokinin
(d) ethylene (e) abscisic acid
12. Pfr, the active state of
, forms when red
light is absorbed. (a) photosystem I (b) phytochrome (c) farred light (d) phototropin (e) cryptochrome
4. Research on a fungal disease of rice provided the first clues
about the plant hormone (a) auxin (b) gibberellin (c) cytokinin (d) ethylene (e) abscisic acid
13. Which of the following represents the correct order in the
phytochrome signal transduction pathway?
5. This plant hormone interacts with auxin during the formation
of plant organs in tissue culture. (a) florigen (b) gibberellin
(c) cytokinin (d) ethylene (e) abscisic acid
6. The plant hormone
delays senescence,
whereas the plant hormone
promotes
senescence. (a) cytokinin; auxin (b) auxin; cytokinin (c) cytokinin; ethylene (d) abscisic acid; ethylene (e) gibberellin;
auxin
7. The stress hormone that helps plants respond to drought
is (a) auxin (b) gibberellin (c) cytokinin (d) ethylene
(e) abscisic acid
8. This hormone promotes seed dormancy. (a) auxin (b) gibberellin (c) cytokinin (d) ethylene (e) abscisic acid
9. Which signaling molecule triggers the release of volatile substances that attract parasitic wasps to plant-eating caterpillars?
(a) brassinosteroid (b) jasmonic acid (c) systemin (d) salicylic
acid (e) oligosaccharin
10. The three classes of photoreceptors that enable plants to
sense the presence and duration of light are (a) phytochrome,
cryptochrome, and circadian rhythms (b) chlorophyll, photosynthesis, and photoperiodism (c) phytochrome, photoperiodism, and chlorophyll (d) phytochrome, cryptochrome,
and phototropin (e) phototropin, photosynthesis, and
photoperiodism
11. A plant’s response to the relative amounts of daylight and
darkness is (a) apical dominance (b) bolting (c) gravitropism
(d) photoperiodism (e) phototropism
1. red light 2. light-responsive gene is switched on (or off )
3. movement of Pfr to nucleus 4. conversion of Pr to Pfr
5. formation of PFr –PIF3 complex that is bound to promoter
region
(a) 1, 3, 5, 4, 2 (b) 1, 5, 3, 2, 4 (c) 1, 2, 3, 4, 5 (d) 1, 4, 3, 2, 5
(e) 1, 4, 3, 5, 2
14. Which of the following statements about phytochrome is
incorrect? (a) phytochrome is the main photoreceptor for
photoperiodism (b) phytochrome is a family of about five
blue-green pigment proteins, each coded by a different gene
(c) most of our knowledge of phytochrome function is based
on mutant corn (Zea mays) plants (d) each member of the
phytochrome family exists in two forms: Pr and Pfr (e) phytochrome helps plants sense the presence of nearby plants with
which they must compete for light
15. Which photoreceptor(s) is /are implicated in resetting the
biological clock? (a) phytochrome only (b) cryptochrome
only (c) phototropin only (d) cryptochrome and gibberellin
(e) phytochrome and cryptochrome
16. The orientation of the growth of a plant according to the
, whereas the
direction of light is called
twining of tendrils is an example of
.
(a) thigmomorphogenesis; gravitropism (b) photoperiodism; thigmomorphogenesis (c) phototropism; gravitropism
(d) phototropism; thigmotropism (e) photoperiodism;
thigmotropism
C R I TI C AL TH I N KI N G
1. Predict whether flowering in a short-day plant with a minimum critical night length of 14 hours would be expected
to occur in the following situations. Explain each answer.
(a) The plant is exposed to 15 hours of daylight and 9 hours
of uninterrupted darkness. (b) The plant is exposed to
9 hours of daylight and 15 hours of darkness. (c) The plant
is exposed to 9 hours of daylight and 15 hours of darkness,
with a 10-minute exposure to red light in the middle of the
night.
2. If you transplanted the short-day plant discussed in question
1 to the tropics, would it flower? Explain your answer.
3. Evolution Link. What adaptive advantages are conferred on a
plant whose stems are positively phototropic and whose roots
are positively gravitropic?
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Chapter 37
4. Evolution Link. Explain why a plant with a mutation in one
of its phytochrome genes that affects flowering time would be
at an evolutionary disadvantage.
5. Analyzing Data. Examine Figure 37-14a. When the day
length is 10 hours and the night length is 14 hours, what
percent of the plants flower? When the day length is 16 hours
and the night length is 8 hours, what percent of the plants
flower? What is the critical night length for this plant?
Additional questions are available in
ThomsonNOW at www.thomsonedu.com/
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