Transcript Chapter 10: Plant Reproduction, Growth, and Development

Chapter 10: Plant
Reproduction, Growth, and
Development
10-1
Sexual Reproduction in
Flowering Plants
Sexual reproduction requires gametes,
often as egg and sperm.
In flowering plants, the structures that
produce the egg and sperm are located
within the flower.
Flowers have special structures to
enable fertilization of egg by sperm.
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Structure of Flowers
The reproductive portions of a flower are
the stamen, consisting of a stalk-like
filament bearing an anther, and the
pistil, made up of stigma, style, and
ovary.
The ovary contains one or more ovules.
Sepals enclose a whorl of petals that are
usually colored to attract pollinators.
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Flower structure
10-4
Alternation of Generations
Plant life cycles includes two alternating
generations.
The sporophyte (2N) produces haploid
spores by meiosis.
A spore develops into a haploid
gametophyte that produces gametes.
The sporophyte of flowering plants is
dominant and produces two types of
spores, microspores and megaspores.
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Alternation of generations in a
flowering plant
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Microspores within the anther mature
into pollen grains that are the
microgametophyte generation.
A megaspore within an ovule develops
into the megagametophyte generation
that produces an egg.
During pollination pollen grains move by
wind or animal carrier from the anther
to the stigma where a pollen grain
produces a pollen tube.
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Sperm travel down the pollen tube; one
sperm unites with the egg and
becomes a zygote that develops into an
embryo.
The other sperm unites with the polar
nuclei within the megagametophyte.
This produces triploid (3n) endosperm
that nourishes the embryo.
These two fusions are known as double
fertilization.
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Life cycle of a flowering plant
10-9
The ovule wall hardens and becomes the
seed coat.
The seed consists of the sporophyte
embryo, stored food, and a seed coat.
The ovary may develop into a fruit.
10-10
Growth and Development in
Plants
Plant growth and development involve
cell division, cellular elongation, and
differentiation of cells into tissues and
organs.
Development is a programmed series of
stages from a simpler to a more
complex form.
Cellular differentiation is specialization
during development.
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Development of the Dicot
Embryo
After double fertilization, the singlecelled zygote lies beneath the
endosperm nucleus.
The dicot embryo has two cotyledons or
seed leaves that take up and store
nutrients from the endosperm.
The monocot embryo has only one
cotyledon that rarely stores food.
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The epicotyl is the portion above the
cotyledons and becomes shoot; the
hypocotyl is the portion below the
cotyledons and will become stem.
The hypocotyl ends in the radicle that will
grow into root tissue.
The embryo plus stored food is now
contained within a seed.
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Development of a dicot embryo
10-14
Development of Seeds and Fruits
In flowering plants, seeds are enclosed
within a fruit that usually develops from
the ovary.
The ovary wall becomes the pericarp.
Fleshy fruits have a fleshy pericarp; dry
fruits have a dry pericarp.
An aggregate fruit such as blackberry is
derived from many ovaries on the
flower.
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Fruit diversity
10-16
Dispersal of Seeds
Seeds are modified to be distributed far
from the parent plant.
Many disperse by wind or animal carrier
and some float by ocean currents.
Plants have various means, such as
hooks, spines, or wings, to ensure that
dispersal occurs.
Birds and animals eat fruits and defecate
seeds away from the parent plant.
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Germination of Seeds
Germination of seeds occurs if there is
sufficient water, warmth, and oxygen to
sustain growth.
Dormancy may be required before
germination, and some seeds require
periods of cold or minimal moisture.
In many fruits, germination does not
occur until seeds are removed and
washed; mechanical action may also be
required.
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Dicot Versus Monocot
Development
In dicots, the cotyledons supply nutrients
to the developing plant.
The epicotyl bears young leaves called a
plumule.
The hypocotyl becomes the stem and the
radicle becomes the root.
The dicot shoot is hook-shaped to
protect the delicate plumule.
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Common garden bean, a dicot
10-20
Germination in darkness causes
etiolation, which involves elongation
and lack of color.
Monocots have only one cotyledon and
the endosperm, rather than the
cotyledon, is food-storage tissue.
The coleoptile and coleorhiza are
protective sheaths around the monocot
plumule and radicle.
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Corn, a monocot
10-22
Asexual Reproduction in
Flowering Plants
Non-differentiated meristem tissue allows a
plant to reproduce by asexual vegetative
propagation.
Strawberry stolons, potato tuber eyes,
sweet potato roots, tree suckers, African
violet cuttings, and grapevine cuttings
are common methods of asexual
propagation.
In horticulture, identical offspring
produced by vegetative cuttings are
clones.
10-23
Propagation of Plants in Tissue
Culture
Plant cells are totipotent, having all the
genetic potential to become mature
specialized plants.
Using tissue culture, whole plants can be
grown from protoplasts, naked plant
cells produced by enzymes that digest
cell walls.
Cell suspension culture permits
production of chemicals from single
cells derived from leaf, stem, or root
tissue.
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Tissue culture trees
10-25
Genetic Engineering of Plants
Various techniques introduce foreign
DNA into protoplasts that are
propagated in tissue culture.
Adult plants are generated from these
cells and could produce insecticideresistant plants or plants that can grow
in nutrient-limited soil.
Plants can also be engineered to treat
human diseases.
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Control of Plant Growth and
Development
Since each plant cell is totipotent,
hormones have a role in determining
cellular differentiation.
Plant Hormones
There are five common groups of plant
hormones: auxins, gibberellins,
cytokinins, abscisic acid, and ethylene.
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Natural and synthetic hormones are also
called plant growth regulators.
Plant hormones elicit a physiological
response in target cells by binding to a
receptor protein in the plasma
membrane.
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Mode of action of auxin, a plant
hormone
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Stimulatory Hormones
Auxins include indoleacetic acid (IAA).
Apically produced auxin prevents the
growth of axillary buds (a phenomenon
called apical dominance), causes roots
to develop, and prevents fruit drop.
Auxin’s ability to cause cell elongation is
involved in a plant’s response to light
and gravity.
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Cytokinins, such as zeatin and kinetin,
promote cell division and help to
regulate the plant cell cycle.
Cytokinins interact with auxin to affect
differentiation, producing roots or
shoots or flowers depending on the
ratio of these hormones.
Cytokinin also prevents senescence,
death of plant parts due to aging.
Gibberellins promote growth of stems
and can break the dormancy of seeds.
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Effects of plant hormones
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Inhibitory Hormones
Abscisic acid (ABA), the stress hormone,
initiates and maintains seed and bud
dormancy and closes stomata in the
fall.
Ethylene ripens fruit by increasing
enzyme activity, particularly cellulase
that hydrolyzes cellulose.
Ethylene in the air inhibits growth of
plants in general, and along with low
levels of auxin and gibberellin in a leaf,
initiates abscission.
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Effects of ethylene
10-34
Plant Responses to
Environmental Stimuli
Plant growth and development are
influenced by environmental stimuli
such as light, day length, gravity, and
touch.
Environmental signals determine the
seasonality of growth, reproduction,
and dormancy.
The plant’s ability to respond to the
environment enhances its survival.
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Plant Tropisms
Positive phototropism (stems bend
toward the light) is due to the migration
of auxin from the bright side to the
shady side of a stem.
After auxin arrives, the cells on that side
elongate and the stem bends toward
the light.
In negative gravitropism, stems curve
away from gravity due to auxin on the
lower side of the stem.
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Positive phototropism
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Negative gravitropism
10-38
Flowering
Photoperiod is the ratio light to darkness
in a 24-hour cycle.
Short-day (long night) plants flower when
the days get shorter than a critical
length and long-day (short night) plants
flower when the days get longer than a
critical length.
Day-neutral plants do not depend on day
length for flowering.
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Effect of day/night length on
flowering
10-40
Phytochrome and Plant
Flowering
If flowering is dependent on length of day
and night, plants must have some way to
detect photoperiod.
Phytochrome is a plant pigment believed
to be involved in regulating the response
of plants to day length.
Phytochrome exists in two forms, Pfr
(active form) that is converted to Pr
(inactive form) as night approaches;
phytochrome conversion may signal day
length.
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Other Functions of Phytochrome
The Pr → Pfr cycle also signals seeds
when sunlight is present and
conditions are good for germination,
and it causes stems to etiolate.
The Pfr binds to regulatory proteins in the
cytoplasm and the complex migrates to
the nucleus where it binds to specific
genes.
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Phytochrome control of a growth
pattern
10-43
Transport in the Mature Plant
Water and Mineral Transport in Xylem
A plant uses active transport to
concentrate minerals in root cells and
xylem.
Thereafter minerals are transported within
water, in tracheids and vessel elements
that form a pipeline to the leaves.
Water entering root cells forms a positive
root pressure.
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Conducting cells of xylem
10-45
Cohesion-Tension Model
The cohesion-tension model of xylem
transport explains how water is
transported to great heights against
gravity.
Polar water molecules are cohesive and
adhere to the walls of the xylem vessel
and fill the water pipeline.
Transpiration, evaporation of water from
leaves, creates a negative pressure that
pulls the water column upward.
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Cohesion-tension theory of
xylem transport
10-47
Mineral Transport
In addition to the carbon, hydrogen and
oxygen obtained from water and
carbon dioxide, a plant needs mineral
elements.
A plant uses active transport to take in
minerals beyond what is provided by
the flow of water.
Plants are important for concentrating
minerals that are used by consumers
including humans.
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Adaptations of Roots for Mineral
Uptake
Bacteria in the root nodules of legumes
are symbionts that convert the nitrogen
in the atmosphere to NH4+.
Fungi have a symbiotic relationship with
plant roots, called mycorrhizae, that
increase water and mineral uptake and
improve nutrient transfer.
Plants without mycorrhizae grow in
limited environments.
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Root nodules
10-50
Mycorrhizae
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Some plants have poorly developed or
no roots and have minerals and water
supplied by other means.
Epiphytes (air plants) do not grow in soil
and therefore must use roots to extract
moisture from air and catch rain and
minerals in leaves.
Parasitic plants send out root-like
haustoria that tap into the xylem and
phloem of the host stem.
A few carnivorous plants supplement
their diet by capturing insects.
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Opening and Closing of Stomata
Guard cells on either side of a stoma
regulate its opening and closing.
When water enters guard cells, the stoma
opens; when water leaves, it closes.
When a plant is photosynthesizing, a
pump actively transports hydrogen
ions (H+) out of guard cells.
Then potassium (K+) ions followed by
water enter guard cells and the stoma
opens.
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When the pump is not working, K+
followed by water exits guard cells and
the stoma closes.
The blue-light component of sunlight
appears to be absorbed by a pigment
that leads to activation of the H+ pump.
Abscisic acid also causes stomata to
close.
Stomata continue to open and close
when kept in the dark, indicating an
internal biological clock.
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Opening and closing of stomata
10-55
Organic Nutrient Transport in
Phloem
Green parts of plants produce sucrose
that is transported in phloem.
Conducting cells are sieve-tube
elements, accompanied by a
companion cell and strands of
cytoplasm called plasmodesmata.
Transport of organic materials within
phloem is termed translocation.
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Sieve-tube elements of phloem
10-57
Pressure-Flow Model of Phloem
Transport
The pressure-flow model of phloem
transport explains how organic
nutrients are transported in a plant.
During the growing season, the leaves
are a source of sugar.
Using energy from ATP, sugar is actively
transported into sieve-tube elements
and water follows passively.
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Buildup of water creates a pressure and
starts a flow of phloem sap toward a
sink where sugar is actively
transported out of sieve-tube elements.
Because the direction that phloem sap
flows is from the source to the sink,
this explains transport to newly formed
leaves or to fruits.
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Pressure-flow theory of phloem
transport
10-60
Chapter Summary
In a plant life cycle, pollen grains carry
sperm from flower to flower.
Seeds within fruits are the products of
sexual reproduction in flowering plants.
Seeds must be dispersed and must
germinate to complete the life cycle.
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Plants reproduce asexually; this ability
can be commercially utilized to mass
produce identical plants, sometimes
after genetic engineering.
Plants respond to outside stimuli by
changing their growth patterns.
Plant hormones regulate plant growth
patterns.
Some plant responses are controlled by
the length of daylight (photoperiod).
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Transpiration pulls water and minerals
from the roots to the leaves in xylem
according to the cohesion-tension
model.
Stomates are open for evaporation to
occur.
The pressure-flow model of phloem
transport states that sugar is actively
transported into phloem at a source,
and water follows by osmosis.
The resulting increase in pressure
creates a flow, which moves water and
sucrose to a sink.
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