Transcript Plants

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Chapter 25 and 26
25.1 Plant Nutrition and Soil
• Essential Inorganic Nutrients
– About 95% of a plant’s dry weight is carbon, hydrogen, and
oxygen
– Primary nutrients are carbon dioxide and water
– Mineral – inorganic substance usually containing two or
more elements
• A nutrient is essential if
– It has an identifiable role,
– Another nutrient cannot substitute for it, and
– If deficiency of the nutrient causes a plant to die
• Macronutrients – C, H, O, P, K, S, Ca, Mg
• Micronutrients – Fe, B, Mn, Cu, Zn, Cl, Mo
2
Overview of Plant Nutrition
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CO2
O2
H2O
O2
H2O
CO2
minerals
3
Plant Nutrition and Soil
• Soil erosion occurs when water or wind
carry soil away to a new location
• Worldwide, erosion removes about 25
billion tons of topsoil annually
– Deforestation (removal of trees)
– Desertification (increase in deserts due to
overgrazing and over farming marginal lands)
4
25.2 Water and Mineral Uptake
• Water and minerals enter the roots of flowering
plants through the same pathways
– Between porous cell walls, then forced into
endodermal cells by the Casparian strip
– Through root hairs, through cells across the cortex and
endodermis via cytoplasmic strands within
plasmodesmata
• Water enters root cells when their osmotic
pressure is lower than that of the soil
• Minerals are actively taken up by plant cells and
are transported in the xylem along with water
5
Water and Mineral Uptake
• Adaptations of roots for mineral uptake
• Important Symbiotic Relationships
– Rhizobium bacteria live in root nodules
• Bacteria fix atmospheric nitrogen
• Host plant provides the bacteria with carbohydrates
– Mycorrhizal association between fungi and plant roots
• Fungus increases the surface area for water and mineral
uptake and break down organic matter
• Root provides the fungus with sugars and amino acids
• Parasitic plants
• Carnivorous plants
6
Root Nodules
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
root
nodule
bacteria
Portion of infected cell
(Top): © Dwight Kuhn; (Circle): © E.H. Newcomb & S.R. Tardon/Biological Photo Service
7
Mycorrhizae
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Mycorrhizae present
Mycorrhizae not present
mycorrhizae
(Top): © B. Runk/S. Schoenberger/Grant Heilman Photography; (Circle): © Dana Richter/Visuals Unlimited
8
Other Ways to Acquire Nutrition
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dodder
(brown)
bulbs release
digestive enzymes
a. Dodder, Cuscuta sp.
Sundew leaf
enfolds prey
sticky
hairs
narrow
leaf form
b. Cape sundew , Drosera capensis
a: © Kevin Schafer/Corbis; b(Plant): © Barry Rice/Visuals Unlimited; b(Leaf): © Dr. Jeremy Burgess/Photo Researchers, Inc.
9
25.3 Transport Mechanisms in
Plants
• Vascular tissues transport water and nutrients
– Xylem transports water and minerals
• Two types of conducting cells
– Tracheids
– Vessel elements
• Water flows passively from an area of higher water potential
to an area of lower water potential
– Phloem transports organic materials
• Conducting cells are sieve-tube members
– Have companion cells to provide proteins
– End walls are sieve plates
– Plasmodesmata extend through sieve plates
10
Plant Transport System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Leaf
xylem
phloem
intercellular
spaces
stoma
O2 CO
2
H2O
O2 CO
2
H2O
sugar
H2O
xylem
H2O
sugar
Stem
phloem
Root
H2O
11
xylem
phloem
Transport Mechanisms in Plants
• Potential energy is stored energy
• Water potential is the energy of water.
– Water moves from a region of higher potential
to a region of lower potential
• In terms of cells, two factors usually
determine water potential:
– Water pressure across a membrane
– Solute concentration across a membrane
12
The Concept of Water Potential
• Pressure potential is the effect that pressure has
on water potential.
– Water moves across a membrane from the area of
higher pressure to the area of lower pressure.
– The higher the water pressure, the higher the water
potential.
• Osmotic potential takes into account the
presence of solutes
– Water tends to move from the area of lower solute
concentration to the area of higher solute
concentration.
– The lower the concentration of solutes (osmotic
potential), the higher the water potential.
13
Water Potential and Turgor
Pressure
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
central vacuole
Wilted
central vacuole
cell wall
Turgid
cell wall
H2O
enters
the cell
higher
Extracellular fluid:
water potential
pressure potential
osmotic potential
Equal water
potential inside and
outside the cell
lower
Inside the cell:
water potential
pressure potential
osmotic potential
a. Plant cells need water.
Pressure potential
increases until
the cell is turgid
b. Plant cells are turgid.
© The McGraw Hill Companies, Inc./Ken Cavanagh, photographer
14
Transport Mechanisms in Plants
• Water Transport
– Xylem vessels form an open pipeline
• The vessel elements are separated by perforated
plates
• Water moves into and out of tracheids through pits
– Water entering roots creates a positive
pressure (root pressure)
• Pushes xylem sap upward
– May be responsible for guttation
» Water forced out vein endings along edges
of leaves
15
Guttation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Ed Reschke/Peter Arnold, Inc.
16
Transport Mechanisms in Plants
• Cohesion-tension model of xylem
transport suggests a passive xylem
transport
– Cohesion is the tendency of water molecules
to cling together
– Adhesion is the ability of the polar water
molecules to interact with molecules of vessel
walls
– A continuous water column moves passively
upward due to transpiration
17
Transport Mechanisms in Plants
• Leaves
– Transpiration causes water loss through stomata
– Water molecules that evaporate are replaced by water
molecules from leaf veins
– Due to cohesion, transpiration exerts a pulling force
(tension) drawing water through the xylem to the leaf
cells
– Waxy cuticle prevents water loss when stomata are
closed
• Stem
– Tension in xylem pulls the water column upward
• Roots
– Water enters xylem passively by osmosis and is pulled
upward due to tension in xylem
18
Transport Mechanisms in Plants
• Opening and Closing of Stomata:
– Each stoma in leaf epidermis is bordered by
guard cells
• Increased turgor pressure in guard cells opens
stoma
• Active transport of K+ into guard cells causes water
to enter by osmosis and stomata to open
• Opening and closing of stomata is regulated by
light
19
Opening and Closing of Stomata
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Open Stoma
H2 O
H2O
vacuole
K+
guard cell
H+
stoma
K+ enters guard cells, and water follows.
a.
25 µm
Closed stoma
H2O
H2O
K+
K+ exits guard cells, and water follows.
b.
25 µm
a: © Jeremy Burgess/SPL/Photo Researchers, Inc.; b: © Jeremy Burgess/SPL/Photo Researchers, Inc.
20
Chapter 26
26.1 Plant Hormones
• Flowering plants respond to environmental
stimuli
– Stimuli include light, gravity, carbon dioxide
levels, pathogen infection, drought, and touch
– Response to stimuli leads to the survival of the
species.
• The responses can be:
– Short term
• Stomata open and close in response to light levels.
– Long term
• The response to gravity causes downward growth of the
root and the upward growth of the stem.
22
Plant Hormones
• Response of plants to environmental stimuli
involves signal transduction
– The binding of a molecular “signal” that initiates
and amplifies a response.
– Signal transduction involves the following:
– Receptors – proteins activated by a specific signal
– Transduction pathway – a series of relay proteins
or enzymes that amplify and transform the signal
to one understood by the machinery of the cell
– Cellular response – the result of the transduction
pathway
23
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Plant Hormones
• Hormones
– Enable plant cells to communicate
– Are synthesized in one part of the plant
– Travel within phloem or from cell to cell in
response to the appropriate stimulus
25
Signal Transduction in Plants
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
defense
hormones
hormone-binding site
3
blue light
signal
2
Receptor: Molecule in
the plasma membrane,
cytoplasm, or nucleus
that receives signal and
becomes activated.
Response: Most often
a change in gene expression
or a cellular process affects
plant growth and development.
Transduction pathway: A series
of relay proteins that amplify and
convert the original signal into one
that affects cellular machinery .
activated
phototropin
auxin
relay
proteins
Defense
responses
auxin carrier
Responses
include bending
of stem
1
activated
auxin receptor
Cytoplasm
Gene
expression
changes
Nucleus
Responses
include growth
of roots
26
Plant Hormones
• Auxins
– Produced in shoot apical meristem
– Found in young leaves, flowers, and fruits
• Effects of auxin on growth and development:
– Apically produced auxin prevents the growth of
axillary buds
• Apical dominance
– Promotes growth of roots and fruit
– Prevents loss of leaves and fruit
– Promotes positive phototropism of stems
27
Auxin and Phototropism
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1. Coleoptile tip is
intact.
2. Coleoptile tip is
removed.
3. Tips are placed on
agar, and auxin
diffuses into the agar.
4. Agar block is placed
to one side of the
coleoptile.
5. Curvature occurs
beneath the block.
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Plant Hormones
• How Auxins Cause Stems to Bend
• When a stem is exposed to unidirectional
light, auxin moves to the shady sides
• Auxin binds to plasma membrane receptors;
the complex leads to the activation of a proton
pump
• Activated proton pumps H+ out of cell
– Cell wall loosens
– Turgor pressure increases due to the entry of
water
– Cell enlarges
29
Expansion of the Cell Wall
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
cellulose fiber
in cell wall
auxin
nucleus
H+
H+
enzyme
(inactive)
H+
H+
chloroplast
1
Cytoplasm
H+
H+
H+
active
enzyme
H2O
H+
2
turgor
turgor
3
30
Plant Hormones
• Gibberellins are growth-promoting
hormones
– Gibberellins cause stem elongation
– There are about 70 gibberellins
• Each differ slightly chemically
• The most common is gibberellic acid
31
Gibberellins Cause Stem
Elongation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
b.
a: © Robert E. Lyons/Visuals Unlimited; b: © Sylvan Whittwer/Visuals Unlimited
32
Plant Hormones
• The cytokinins are a class of hormones that
promote cell division
– found in dividing tissues of roots, in seeds, and in
fruits
– have been used to prolong the life of flower cuttings
as well as vegetables in storage
– Auxin and cytokinins interact
– prevent senescence (aging process)
33
Interaction of Hormones
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
b.
c.
d.
Courtesy Alan Darvill and Stefan Eberhard, Complex Carbohydrate Research Center, University of Georgia
34
Plant Hormones
• Abscisic acid (ABA) is produced by any
“green tissue” (i.e., tissue containing
chloroplasts)
– sometimes called the stress hormone
• initiates and maintains seed and bud dormancy
• brings about the closure of stomata
35
Dormancy and Winter Buds
36
Dormancy and Germination
37
Abscisic Acid Promotes Closure
of Stomata
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
inside
outside
H2O
K+
K+
K+
Ca2+
ABA
Open stoma
Guard cell plasma
membrane
Closed stoma
38
Plant Hormones
• Ethylene (H2C = CH2) is a gas formed from the
amino acid methionine.
• Effects of ethylene
– Abscission
• Ethylene stimulates certain enzymes, such as cellulase,
which helps cause leaf, fruit, or flower drop
– Ripening of fruits
• Increases the activity of enzymes, such as cellulase, that
soften fruits
• It also promotes the activity of enzymes that produce the
flavor and smell of ripened fruits.
– Axillary bud inhibition
– Suppression of stem and root elongation
39
Ethylene and Abscission
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
No abscission
© Kingsley Stern
Abscission
40
Ethylene and Fruit Ripening
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
gene for ethylene
biosynthesis enzyme
ripe tomatoes
harvested
DNA
transcription
mRNA
translation
functional
enzyme for
ethylene
biosynthesis
ethylene synthesis (in plant)
green tomatoes
harvested
no ethylene
synthesis
41
Arabidopsis Is a Model
Organism
• Arabidopsis thaliana
– A small flowering plant related to cabbage and
mustard plants
– Has no commercial value
– It has become a model organism for the study of
plant molecular genetics, including signal
transduction.
• It is small, so many hundreds of plants can be
grown in a small amount of space.
• Generation time is short – 5-6 weeks until maturity.
• It normally self-pollinates, but it can easily be crosspollinated.
• The number of base pairs in its DNA is relatively small.
42
Overall Appearance of Arabidopsis thaliana
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Arabidopsis thaliana
Courtesy Elliot Meyerowitz/California Institute of Technology
43
26.2 Plant Responses
• Tropism
– Plant growth toward or away from a
unidirectional stimulus
• Positive tropism is growth toward the stimulus
• Negative tropism is growth away from the stimulus
– Gravitropism - movement in response to
gravity
– Phototropism - movement in response to
light
– Thigmotropism - movement in response to
touch
44
Plant Responses
• Gravitropism
– When a plant is placed on its side, the stem
grows upward, opposite of the pull of gravity
– Stems with root caps grow downward
• Response depends on sensors called statoliths
– Auxin may be responsible for:
• gravitropism of roots and shoots
45
Gravitropism
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a.
gravity
b.
c.
25 mm
a: © Kingsley Stern; b: Courtesy Malcolm Wilkins, Glascow University; c: © BioPhot
46
Plant Responses
• Phototropism
– Positive phototropism of stems
• Occurs because cells on the shady side of the
stem elongate due to the presence of auxin
• A pigment absorbing blue light initiates
phototropism
47
Phototropin
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1
cytoplasm
blue light
2
3
blue light
phot
blue light
phot
ADP
transduction
pathway
phot
P
plasma
membrane
AT P
ATP
48
Plant Responses
• Thigmotropism
– Unusual growth due to contact with solid
objects
• Coiling of tendrils
– Thigmomorphogenesis occurs when the
entire plant responds to the presence of
environmental stimuli
• Wind
• Rain
49
Plant Responses
• Nastic movements:
– Do not involve growth and
– Are not dependent on the stimulus direction
• Turgor movements result from touch, shaking, or
thermal stimulation
– Mimosa pudica
– Venus flytrap
• Sleep movements:
– Occur daily in response to light and dark changes
– Circadian rhythm
50
Turgor Movement
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
pulvinus
Before
vascular tissue
cell retaining
turgor
cell losing
turgor
After
© John Kaprielian/Photo Researchers, Inc.6
51
Plant Responses
• Circadian rhythms:
– Biological rhythms with a 24-hour cycle
– Tend to be persistent
• Rhythm is maintained in the absence of
environmental stimuli
• Caused by a biological clock
52
Sleep Movements and Circadian Rhythms
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Prayer plant (morning)
a.
Prayer plant (night)
Morning glory (morning)
b.
Morning glory (night)
Circadian Rhythm
flowers
open
flowers
close
Period
(about 24 hours)
0
12
24
36
48
Time (hours)
c.
(Top, both): © Tom McHugh/Photo Researchers, Inc.(Bottom left): © BIOS A. Thais/Peter Arnold, Inc.; (Bottom right): © BIOS Pierre Huguet/Peter Arnold, Inc.
53
Plant Responses
• Photoperiodism:
– Any physiological response prompted by
changes in day or night length
– influences flowering in some plants
– requires participation of a biological clock and
a plant photoreceptor called phytochrome
54
Plant Responses
• Phytochrome is a blue-green leaf pigment
that alternately exists in two forms
– Phytochrome red (Pr) is inactive
– Phytochrome far-red (Pfr) is active
• Conversion of forms allows a plant to
detect photoperiod changes
• Also promotes seed germination and
inhibits stem elongation
55
Plant Responses
• Flowering
– Flowering plants can be divided into three
groups based on their flowering status.
• Short-day plants flower when the day length is
shorter than a critical length
• Long-day plants flower when the day length is
longer than a critical length
• Day-neutral plants are not dependent on day
length for flowering
– Some plants may require a specific sequence
of day lengths in order to flower
56
Plant Responses
• Responses to the biotic environment:
• Plants are always under attack by herbivores and
parasites.
• Physical and Chemical Defenses
– Cuticle-covered epidermis and bark
– Secondary metabolites
• Tannins
• Alkaloids
• Cyanogenic glycosides
– Wound responses – proteinase inhibitors and systemin
– Hypersensitive response (HR) – initiates wound response
and seals wounded area
– Indirect defenses – prevent egg laying in insects
– Mutualistic relationships with animals - acacia tree and
acacia ant
57