Download Lecture 4 plant structure growth and differentiation

Lecture 4
Plant Structure, Growth and
differentiation
Objectives
• Define plant nutrition
• Outline how nitrogen is taken up by plants
• Discuss the mechanism of leaf senescence
and abscission.
• Explain the concept of fruit and seed
dispersal.
• Describe process of seed germination
including seed dormacy.
1. What is meant by “plant nutrition”
Uptake from the soil of mineral
elements
“Plant nutrition” specifically
does not refer to photosynthesis.
In this lecture the uptake of
nutrients from the soil directly
by roots
In the next lecture mutualistic
relationships between plants and
fungi and microrganisms
5. Nitrogen and the effects of soil organic matter
on plant nutrition
Nitrogen is the element most required by plants, in terms of weight.
It is not a product of weathering of soil particles.
There are two sources: fixation of atmospheric nitrogen by
bacteria
decomposition of organic matter,
usually decaying plant material.
N-fixing bacteria
Most uptake from the soil is in the form of nitrate
Fig. 32.13
Organic material is important in agricultural soils both as a
source of nitrogen and because it can increase water holding
capacity, e.g. biosolids application effects
A characteristic of non-agricultural
soils is accumulation of organic
material and acidification of the soil.
Such soils typically develop a very
distinct stratification, with organic
mater at the top.
Spodic soil
The organic layers in such soils can
have a considerable total quantity of
nitrogen but little may be available
due to the high acidity, and sometimes
lack of oxygen, in the organic layer.
6. Leaf senescence and withdrawal of
nutrients to the plant
Senescence
is a
term for the collective
process leading to the
death of a plant or plant
part, like a leaf. Leaf
senescence is a part of
the process by which a
plant goes into dormancy
and is induced by a
change in day length.
Changing leaf color
As daylength decreases, the plants ability
to synthesize chlorophyll becomes
reduced.
Yellow and orange carotinoids and
xanthophylls, always present within the
leaf, begin to show.
Water and nutrients are drawn into the
stems and from the leaves.
Senescing cells also produce other
chemicals, particularly anthocyanins,
responsible for red and purple colors.
Some species, particularly oaks, contain
high quantities of tannins in the leaves
which are responsible for brown colors.
Nutrient retention during senescence
In deciduous tree species some 60 – 70% of N, 60 – 70% of P,
30% of K, 25% of Mg, and 15% of Ca are withdrawn from
leaves prior to them being shed. Storage is in the bark and
elements are re-mobilized in spring
A decline in photosynthesis with aging can be prevented by the
decapitation of the plant above the leaf in question. This implies a
regulatory action by the growing point. Results from primary leaves
of bean (Das, 1968).
Leaf Abscission
The final stage in leaf senescence is abscission ("cutting off")
Abscission is controlled by a special layer of cells at the
base of the petiole, the abscission layer.
This layer releases ethylene gas that stimulates production
of cellulase. This in turn breaks down cells walls so that
eventually the leaf is held on to the plant only by xylem
fibers. Wind eventually weakens these and leaf falls
Another special layer of cells adjacent to the abscission layer
produces cells impregnated with suberin. These form a
protective layer, which is seen as the leaf scar
Tyloses, as well as gums are formed inside the vessels and
plug them up before abscission occurs
Vascular tissue
Leaf
Abscission layer
Developing leaf scar
Axilliary
bud
Stem
KEY CONCEPT
Seeds disperse and begin to grow when conditions are
favorable.
Types of Fleshy
Fruits
Drupe
(Peach)
Berry
(Tomato)
Pome
(Apple)
Aggregate
(Strawberry)
Pepo
(Cucumber)
Multiple
(Pineapple)
Fruit Dispersal
• The form of the fruit gives clues about its
dispersal.
• Small, dry fruits with “wings” or
“parachutes” may be wind-dispersed.
Fleshy fruits are often animal dispersed.
Explosive fruits can fling seeds away.
Floating fruits may be water dispersed.
How are these Fruits
Dispersed?
Dandelion
Coconut
Maple
Cocklebur
Jewelweed
A Thinking Question?
• Why must fruits be dispersed away
from the parent plant?
Seeds
Seed Dormancy
• As seeds mature, they enter a period of
dormancy.
– Suspended growth and development.
– Lowered metabolic rate.
– Resistance to adverse environmental
conditions.
Seed Dormancy
• Conditions that break dormancy vary among plant
species.
– Some germinate as soon as the environment is
suitable (adequate moisture and temperature).
• Conditions that break dormancy vary among plant
species.
– Others have additional requirements:
• Initial drying (seeds within fleshy fruits)
• Exposure to prolonged cold (seeds of temperate and
arctic plants).
• Disruption of seed coat .
Seed Dormancy
• Seeds can remain dormant in the soil for long
periods of time. Dormancy ensures that seeds
only germinate when conditions are optimal.
• When we weed or cultivate a bare patch of
soil, the weeds that sprout up immediately
usually come from the “seed bank” already in
the soil.
Breaking Dormancy
• Seeds require moisture and the right
temperature to germinate.
• In addition, some seeds germinate only
after certain environmental signals:
• Drying
• Temperature (period of cold or heat)
• Disruption of the seed coat
? Thinking Question ?
• What could be the advantage of waiting for
each of these signals to germinate?
• Long period of cold
• High heat of a forest fire
• Drying out
• Disruption of the seed coat.
Monocot
Germination
• Monocot seeds
have one
cotyledon.
– Absorbs and
transfers most
nutrients
during seed
germination.
Dicot Germination
• Dicot seeds have two
cotyledons
– Absorb nutrients from
endosperm during
seed development and
store for future use.
– Transfer stored
nutrients to other parts
of embryo during seed
germination.
Seed Germination
• Emergence of the embryonic root is followed by
emergence of the embryonic shoot.
• Emerging roots and shoots must push through the soil
without abrading the apical meristems at their tips.
• Germination (resumption of growth) occurs when the
seed absorbs enough water to rupture the seed coat.
• Once photosynthesis begins, the plant is called a
seedling.
Cotyledons
• Food stored in the seed (as endosperm or within
cotyledons) provides the energy for sprouting.
• Cotyledons of dicots with
hypocotyl hooks are carried out of
the soil.
–
–
–
–
–
Remain below ground.
Digest endosperm and transfer nutrients to seedling.
Become photosynthetic.
Transfer stored nutrients and synthesized sugars to the seedling.
Wither and fall off when reserves have been consumed.
Coevolution
• 43.6 What Are some Adaptations for Pollination
and Seed Dispersal?
– Coevolution Matches Plants and Pollinators
– Fruits Help Disperse Seeds
Coevolution
• Some plants and pollinators have coevolved.
– Each acts as an agent of natural selection on the other.
• Animal-pollinated flowers must:
– Attract useful pollinators able to locate flowers and
extract pollen or nectar.
– Frustrate undesirable visitors who might eat nectar or
pollen without fertilizing the flower.
Coevolution
• Animal-pollinated flowers are grouped into
three categories based on how they attract
pollinators.
– Food
– Sex deceptions
– A nursery
Food
• Many animals forage
on flowers,
distributing pollen
from flower to flower
in the process.
• Examples: beetles,
bees, moths,
butterflies,
hummingbirds.
Food
• Bees are attracted by sweet odors and bright
colors.
• Bees do not see the same range of colors that
humans do.
• Bee-pollinated flowers are typically white, blue,
yellow, or orange, with markings that reflect UV
light pointing toward the center.
• Bee-pollinated flowers have structural adaptations to
ensure pollen transfer.
• When a bee visits a young plant, the stamens emerge
from a crevice between petals, brushing pollen onto her
back as her weight deflects the petals downward.
• In older flowers, the sticky stigma of the carpel
protrudes from the crevice, becoming pollinated by
pollen-coated, nectar-foraging bees.
Food
• Other adaptations for drawing
pollinators:
– Nectar-containing tubes to
accommodate the long tongues
of moths and butterflies
– Tubular flower shape to match
the long bills and tongues of
hummingbirds
– Flowers that heat up to
broadcast their scents
• Example: skunk cabbage
Sexual Deception
• Some plants capitalize on the
mating drive and behaviors of male
wasps.
• Example: some orchid flowers
mimic female wasps or bees in
scent and shape.
– The males attempt to copulate,
becoming covered with pollen
in the process.
– Pollen is transferred as they
repeat the process with other
orchids.
Nurseries
• Some insects fertilize a
flower and then lay their
eggs in the flower’s ovary.
• Example: yuccas and
yucca moths.
– A female moth collects
pollen from one flower and
transfers it to another,
smearing it over the stigma
and laying her eggs inside
the ovary.
– The developing seeds
provide nutrition for the
developing caterpillars.
Fruits Disperse Seeds
• Mechanisms to disperse seeds:
- Explosive fruits to eject seeds.
-
Light fruits with large wind-catching surfaces.
• Floating fruits for water dispersal.
• Clingy or tasty fruits that allow animal dispersal.
In Summary. . . .
• Plant Reproduction Structure and Functions
• Fruit and Seed Structure and Adaptations
• Coevolution
THE
END