35.1. Plant
Responses
A. Organisms Respond to Stimuli
1. One defining characteristicoflife is an ability to respond to
stimuli.
2. Adaptive organisms respond to
environmental stimuli because it leads to longevity and survival.
3. Animals have nerves and
muscles; plants respond by growth patterns.
B. Tropisms
1. A tropism is plant growth toward or away from a directional
stimulus.
a. Directional
is important; the stimulus comes from only one direction instead of many.
b. Growth
toward a stimulus is a positive tropism; growth away from a
stimulus is a negative tropism.
c. By
differential growth, one side elongates faster; result is curving toward or
away from a stimulus.
2. Three well-known tropisms are
named for the stimulus that causes the response.
a. Phototropism
is growth of plants in response to light; stems show positive phototropism.
b. Gravitropism
is response to earth's gravity; roots demonstrate positive gravitropism
and stems demonstrate negative gravitropism.
c. Thigmotropism
is unequal growth due to touch (e.g., coiling of tendrils around a pole).
C. Phototropism
1. Early researchers, including Charles Darwin and his son, observed plants
curve toward light.
2. Phototropism occurs because cells
on shady side of stems elongate.
3. It is believed that a yellow
pigment related to riboflavin acts as a photoreceptor for light.
a. Following
reception, plant hormone auxin migrates from bright side to shady side of a
stem.
b. How
reception of stimulus couples to production of auxin is not yet known.
4. Auxin is also involved in
gravitropism, apical dominance, and root and seed development.
D. Gravitropism
1. An upright plant placed on its side displays negative gravitropism;
it grows upward opposite gravity.
2. Charles Darwin and his son were
first to show that roots display positive gravitropism.
a. If root
cap is removed, roots no longer respond to gravity.
b. Later
researchers showed root cap cells contain statoliths, starch
grains within amyloplasts.
c. Due to
gravity, amyloplasts settle to lowest part of cell. (Fig. 38.2c)
3. The hormone auxin
underlies both positive and negative gravitropisms.
a. The two
tissues respond differently to auxin, which moves to lower side of both stems
and root.
b. Auxin
inhibits growth of root cells; cells of upper surface elongate and root curves
downward.
c. Auxin
stimulates growth of stem cells; cells of lower surface elongate and stem
curves upward.
E. Thigmotropism
1. Unequal growth due to contact with solid objects is thigmotropism.
2. Coiling of morning glory or pea
tendrils around posts, etc. is a common example.
3. Cells in contact with an object
grow less while those on opposite side elongate.
4. This process is quite rapid;
tendrils have been observed to encircle an object in ten minutes.
5. A couple of minutes of stroking
can bring about a response that lasts for several days.
6. Response can be delayed; tendrils
touched in the dark will respond when illuminated.
a. ATP
rather than light can cause the response; need for light is a need for ATP.
b. Hormones
auxin and ethylene are involved; they induce curvature of tendrils in absence
of touch.
7. Thigmomorphogenesis
is a touch response involving the whole plant.
a. Entire
plant responds to presence of wind or rain.
b. A plant
growing in a windy location has a shorter, thicker trunk.
c. Even
rubbing of a plant inhibits cellular elongation and produces a shorter,
sturdier plant.
F. Nastic Movements
1. In contrast to tropisms, nastic movements are independent of
the direction of stimulus.
2. Seismonastic movements
result from touch, shaking, or thermal stimulation.
3. This response takes only a second
or two, is due to a loss of turgor pressure within cells.
4. When a Mimosa pudica
leaf is touched, leaflets fold because petiole droops. (Fig. 38.4)
5. A pulvinus is a
thickening at base of such leaflets where turgor pressure can rapidly drop.
6. Mechanisms are potassium ions
that move out of the cell; water follows by osmosis.
7. A single stimulus such as a hot
needle can cause leaves to respond.
8. Venus's-flytrap
a. This
plant has three sensitive hairs at base of the trap.
b. When
touched by an insect, an impulse-type stimulus triggers the trap to close.
c. Turgor
pressure in leaf cells propel the trap.
G. Sleep Movements
1. Sleep movements are nastic responses to daily changes in light
level.
2. Movement is due to changes in
turgor pressure of motor cells in a pulvinus.
3. Some plant movements correspond
to environmental changes in light, temperature, etc.
4. Circadian rhythm is
a biological rhythm with a 24-hour cycle.
5. Biological clock is
an internal mechanism maintaining biological rhythms in absence of stimuli.
6. Biological clocks are
synchronized by external stimuli to twenty-four-hour rhythms.
7. Photoperiod is more reliable an
indicator of seasonal changes than temperature change.
8. Stomates and flowers usually open
in morning, close at night; some plants secrete nectar at same time of day.
35.2. Plant
Hormones
A. For plants to respond to stimuli, activities of plant cells and
structures have to be coordinated.
1. Almost all plant communication is done by hormones.
2. Hormones are low
concentration chemical messengers active in another part of an organism.
3. A responses is influenced by
several hormones; requires a specific ratio of two or more hormones.
4. Hormones are synthesized in one
part of a plant; they travel in phloem after plant receives appropriate
stimulus.
5. Each naturally-occurring hormone
has a specific chemical structure.
6. Other chemicals that differ only
slightly from natural hormones also affect the growth of plants.
7. Plant growth regulators
are hormone imitators plus naturally-occurring plant growth hormones.
B. Auxin
1. Indoleacetic acid (IAA) is most common naturally-occurring
auxin.
a. It is
produced in shoot apical meristem and found in young leaves, flowers, and
fruits.
b.
Apically-produced IAA prevents growth of axillary buds; this provides for apical
dominance.
c. When a
terminal bud is removed, nearest buds begin to grow and the plant branches.
d.
Application of a weak solution of auxin causes roots to develop from ends of
cuttings.
e. Auxin
production by seeds promotes growth of fruit.
f. As long
as auxin is concentrated in leaves and fruits rather than stem, they do not
fall off.
2. Auxin-controlled cell elongation
is involved in gravitropism and phototropism.
a. When
gravity is perceived, auxin moves to lower surface of roots and stems.
b. Darwin
discovered with oat seedlings, phototropism would not occur if tip of a
seedling is cut off
or covered by a cap; they concluded cause of curvature moved from coleoptile
tip to rest of shoot.
3. Frits W. Went experimented with
coleoptiles in 1926. (Fig. 38.7)
a. He cut
off tips and placed them on agar.
b. Agar
block was placed to one side; coleoptile would curve away from that side
regardless of light.
c. He
deduced a chemical caused curved growth and named it auxin after Greek word for
"to grow."
C. How Auxins Work
1. In a plant exposed to unidirectional light, auxin moves from bright side to
shady side of a stem.
2. Auxin binds to receptors and
activates the ATP-driven proton (H+) pump.
3. As hydrogen ions are pumped out of
the cell, cell wall becomes acidic, breaking hydrogen bonds.
4. Cellulose fibrils are weakened
and activated enzymes further degrade the cell wall.
5. Electrochemical gradient
established causes uptake of solutes; water follows by osmosis.
6. Turgid cell presses against cell
wall, stretching it so that elongation occurs.
7. Auxin-mediated elongation occurs
in younger cells; older cells may lack auxin receptors.
D. Gibberellins
1. Gibberellins are a group of 70 plant hormones that chemically
differ.
2. GA3 is the most
common of natural gibberellins.
3. Gibberellins are
growth promoters that elongate cells.
4. Gibberellins were discovered in
1926 by Ewiti Kurosawa, a Japanese scientist investigating a fungal
disease of
rice plants called "foolish seedling disease."
a. His
fungus-infected plants produced an excess chemical gibberellin, named after the
fungus.
b. By 1956,
gibberellic acid was finally isolated from a flowering plant rather than
fungus.
5. Mode of action (Fig. 38.11)
a. Hormone
GA3 binds to a receptor; a second messenger (Ca2+) inside cell combines with
the protein calmodulin.
b. Ca2+
-calmodulin complex activates a gene codinng for the enzyme amylase.
c. Amylase
acts on starch to release sugars used as a source of energy by the growing
embryo.
E. Cytokinins
1. Cytokinins are a class of plant hormones that promote cell
division.
2. Cytokinins are
derivatives of the purine base adenine.
3. A natural cytokinin zeatin
is found in corn kernels; kinetin is a synthetic cytokinin.
4. Researchers discovered cytokinins
in work on growing plant tissues in culture. (Fig. 38.12)
5. Oligosaccharins,
chemical fragments released from cell wall, also direct differentiation.
6. Researchers hypothesize that
auxin and cytokinins are part of a reception-transduction-response
pathway that
activates enzymes that release these fragments from cell wall.
F. Senescence
1. Aging processes are senescence; large molecules break down and
transported elsewhere.
2. Cytokinins prevent senescence of
leaves; they also initiate development of leaf growth.
3. Cytokinins initiate growth of
lateral buds despite apical dominance.
G. Inhibitory Hormones
1. Abscisic acid (ABA) is called "stress hormone";
it maintains seed and bud dormancy and causes
closure of
stomates.
2. Dormancy occurs
when a plant organ readies itself for adverse conditions by stopping growth.
a. ABA moves
from leaves to vegetative buds in fall; thereafter these buds are converted to
winter buds
which are covered by thick, hardened scales.
b. Reduction
in ABA and increase in gibberellins break seed and bud dormancy; seeds
germinate and
buds send forth leaves.
3. Abscisic acid brings about
closing of stomates when a plant is under water stress.
a. By some
unknown mechanism, ABA causes K+ ions to leave guard cells.
b. As a
result, guard cells lose water and stomates close.
4. Although external application of
ABA promotes abscission, it is not believed to function in this process;
the hormone
ethylene is considered to have this natural function.
H. Ethylene
1. It was an early practice to prepare citrus fruit for market by storage in a
room with a kerosene stove.
2. Later work revealed incomplete
combustion of kerosene produced ethylene which ripens fruit.
3. Ethylene is a
gaseous plant hormone; it ripens fruit by increasing activity of enzymes that
soften fruit.
4. Ethylene stimulates production of
cellulase enzyme that hydrolyzes cellulose in plant cell walls.
5. A barrel of ripening apples can
induce ripening of a bunch of bananas some distance away.
6. Ethylene releases from site of a
physical wound; therefore one rotten apple spoils the whole bunch.
7. The presence of ethylene in air
inhibits growth of plants in general.
8. Ethylene is present in auto
exhaust and in homes heated with natural gas.
9. Inhibition of plant growth occurs
in low concentrations (one part ethylene per 10 million parts of air).
10. Ethylene is involved in abscission,
the dropping of leaves, fruits, or flowers.
a. Lower
levels of auxin in these areas (compared to stem) probably initiate abscission.
b. Once
abscission begins, ethylene stimulates production of enzymes such as cellulase
that cause leaf,
fruit, or flower drop.
35.3.
Photoperiodism
A. Many physiological changes in plants (e.g. seed germination, the breaking
of bud dormancy, flowering,
and the onset of senescence) are
related to a seasonal change in day length.
1. Photoperiodism is a physiological response to relative lengths
of daylight and darkness.
2. Research by U.S. Department of
Agriculture in controlled greenhouses revealed this mechanism.
B. Plants can be divided into three groups, based on photoperiodism.
1. Short-day Plants
a. These
flower when day length was shorter than a critical length.
b. Examples
include cockle-bur, poinsettia, and chrysanthemum.
c. In
effect, they require a period of darkness that is longer than a critical length
to flower.
2. Long-day Plants
a. These
flower when the day length is longer than a critical length.
b. Examples
include wheat, barley, clover, and spinach.
c. In
effect, they require a period of darkness that is shorter than a critical
length to flower.
3. Day-neutral Plants
a. These are
plants for which flowering is not dependent on day length.
b. Examples
include tomato and cucumber.
C. A long-day and a short-day plant can have same critical length.
1. These plants differ in specific sequence of day lengths as a season
progresses.
2. Spinach is a long-day plant that
flowers in summer when day length increases to 14 hours.
3. Ragweed is a short-day plant that
flowers in fall when day length shortens to 14 hours or less.
4. In 1938, K. C. Hammer and J.
Bonner experimented with artificial lengths of dark and light periods.
a.
Cocklebur, a short-day plant, flowers as long as the dark period lasts over 8.5
hours.
b. If dark
period is interrupted by a flash, it does not flower; darkness amidst day cycle
has no effect.
c. Long-day
plants require a dark period shorter than a critical length regardless of
length of light period.
d.
Therefore, length of the dark period controls flowering, not length of the
light period.
D. Phytochrome and Flowering
1. U.S.D.A. scientists discovered phytochrome, a blue-green leaf
pigment that exists in two forms:
2. Pr(phytochrome red)
absorbs red light (wavelength of 660 ;nm); it is converted to Pfr.
3. Pfr is phytochrome
far-red and absorbs far-red light (wavelength of 730 ;nm); it is
converted to Pr.
4. During a 24-hour period, there is
a shift in ratio of these two pigments.
a. Direct
sunlight contains more red than far-red light; Pfr is present in plant
leaves during the day.
b. Shade and
sunsets have more far-red than red light; Pfr is converted to Pr
as night approaches.
c. There is
a slow metabolic replacement of Pfr by Pr during night.
5. Phytochrome
conversion may be a first step in reception-transduction-response pathway
resulting in flowering.
E. Other Functions of Phytochrome
1. The Pr ----> Pfr conversion cycle controls other growth
functions in plants.
2. In addition to being involved in
flowering, Pfr promotes seed germination and stem branching.
3. Following germination, presence
of Pr dominates; stem elongates and grows toward sunlight while
leaves
remain small.
4. Once a plant is exposed to
sunlight and Pr is converted to Pfr, the plant begins to grow
normally-leaves
expand and
the stem branches.
5. Pfr form of phytochrome
triggers activation of one or more regulatory proteins in cytosol.
6. These proteins migrate to nucleus
and bind to "light-stimulated" genes coding for proteins found in
chloroplasts.