Transcript photoperiodism

9.3 Growth in Plants
Chapter 9: Plant Biology
Root Cross
Section
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Pith – usually soft (sometimes solid) – storage
Procambium – cell layer that produces the vascular tissues
Periclycle – layer that may produce the roots
Endodermis – layer that surrounds the vascular bundle
Parenchyma – the bulk of the roots; storage; involved in conducting
water from the soil in to the interior tissue
Epidermis – outer layer
Stem Cross Section
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Epidermis – provides protection; pores for gas exchange
Xylem – in woody plants also proved support
Cambium – separates the xylem and phloem area of rapidly
dividing cells that differentiate into xylem or phloem
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Plants show growth throughout their lives
(indeterminate growth), however, they do die
Death occurs based on the plant’s life cycle
Some plants are annuals – they complete
their life cycle in one year
Biennials – take 2 years to complete their life
cycle
Perennials – live many years- usually die
because of infections or other environmental
factors
Meristems
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Growth in plants in confined to regions known
as meristems
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Meristem – the undifferentiated tissues in
plants (like stem cells!)
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https://www.youtube.com/watch?v=h9kCPO7oMf8
Apical Meristem
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Found at the tips of roots and stems, and the
tips of buds and shoots
Produces primary growth (i.e. longer stem or
roots)
Apical Meristems
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Root Meristem -responsible for the growth of
the root
Shoot meristem is responsible for
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Growth (lengthening) of stem
Cells that will produce leaves and flowers
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With each division, one cell remains in the
meristem
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The other increases in size and differentiates
as it is pushed away from the meristem
region
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Each apical meristem can give rise to
additional meristems:
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Protoderm  epidermis
Procambium  vascular tissue
Ground meristem  pith
(cambium)
(epidermis)
(pith)
Lateral Meristems
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Found in the cambium
Allow for secondary growth (i.e. for the plant
to get thicker)
Auxin
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Auxins are hormones that have a broad
range of function including initiating root
growth, development of fruit, and regulating
leaf development
IAA (indole-3-acetic acid) is the most
abundant auxin
IAA is produced in the apical meristem and is
transported down the stem to stimulate
growth
At high concentrations it can inhibit growth
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Axillary buds are shoots
that form at a node (the
junction of the stem and
the base of a leaf).
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Auxin produced at the
shoot apex inhibits
growth at the nodes. This
is known as apical
dominance.
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As the plant grows the shoot
apex is further away from
the node
 This means less auxin
means the node can now
grow
Now the axillary bud can
produce a stem or a fruit
Cytokinins are hormones
produced in the roots that
also promote axillary bud
development
Plant Tropisms
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A tropism is a growth in response to a
stimulus
Phototropism: the growth in response of light
Gravitropism: growth in response to a
gravitational force
Phototropism and Auxin
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Plants require light for photosynthesis; thus
seedlings must grow toward sunlight
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Auxins cause the positive phototropism of
plant shoots and seedlings (so long as the
plant cells have auxin receptors)
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Auxin increases the flexibility of plant cell
walls enabling cell elongation on the side of
the shoot necessary to cause growth towards
the light.
Auxin is actually concentrated on the side of
the stem away from the light source
The elongation of the cells on the side away
from the light allows for curvature toward the
light source.
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Root auxin inhibits shoot elongation
Micropropagation of Plants
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This is an in vitro procedure to clone plants
Tissues from the shoot apex of a plant with
desirable features is sterilized and cut into
pieces known as explants
The explants are grown in a nutrient agar that
also contains growth hormones.
Once the roots and shoots are developed,
the clones plant can be transferred to soil
Micropropagation
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Useful for propagating virus-free plants, since
meristematic tissues are likely virus-free.
Can be used to clone endangered plants
Some plants, like orchids, are difficult to
germinate the traditional way.
Micropropagation is high successful.
The plantlets can be stored in liquid nitrogen cryopreservation
9.4 Reproduction in
Angiospermophytes
Dicotyledonous Flower Parts
FLOWER PART
FUNCTION
Sepals
Protect the developing flower while in the bud
and at night when the buds close.
Petals
Modified leaves; often colourful to attract
pollinators
Stamen
The “male” reproductive structure; made of
anther and filament
Anther
Produces and releases pollen
Filament
Stalk of stamen that holds up anther
Carpel
The “female” reproductive structure; made of
ovary, style, and stigma
Pistil
Can refer to a single carpel or a group of fused
carpels
Stigma
Sticky top of carpel which pollen lands on
Style
Supports and holds up the stigma; gives the
stigma exposure to pollen
ovary
Base of carpel in which the female sex cells
develop; if fertilization occurs, it will turn into
a protective fruit
Ovules
Found in the ovary; contain female sex cells,
eggs
Pollen
Contain male sex cells (sperm)
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Flowers occur in various colours, shapes and
types – reflective of their pollinator
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Complete flowers – contain all four basic
flower parts (sepals, petals, stamen, and
carpel)
Incomplete flowers – lack at least one of
these parts
Staminate flowers – have only stamens
Carpellate flowers – have only carpets
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Pollination
the process in which pollen (which contains the
male sex cells –sperm) is placed on the
female stigma.
 Can occur via a variety of vectors
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Wind
water
Insects
Birds
Bats
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Angiosperms and their pollinators have
coevolved (supported by fossil evidence)
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The flowers colours, patterns, odours, shapes
and even the time of day it blooms are
designed to attract a specific pollinator
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Often, the flower provides the gift of food to
the pollinator in exchange for the pollinator
unintentionally transporting pollen to the
stigma. A mutualistic relationship!
Examples
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Red flowers – pollinated by birds
Yellow and orange flowers – bees
Heavily scented flowers – nocturnal animals
Inconspicuous, odourless flowers – wind
As the humming bird eats nectar at the base
of the flower, it picks up pollen which it will
transfer to the stigma of the next flower it visits
These orchids look
similar to a particular
wasp species. A wasp
will come to the
orchid and a attempt
to mate with it. In the
process it will pick up
pollen and transfer it
to the next flower it
attempts to mate with.
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Self Pollination – when pollen from the anther
of a plant falls on its own stigma
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A form of inbreeding – thus less genetic variation
Cross Pollination – pollen lands on the stigma
of a different plant.
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Increases variation and offspring with different
fitness
Fertilization
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When the male and female sex cells unite to
form a diploid zygote.
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The female sex cells are in the ovules. The
sperm from the pollen that has attached itself
to the stigma must make its way to the ovules
in the ovary.
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Pollen attaches to stigma and begins to
grow a pollen tube through the style
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Within the growing pollen tube is the
nucleus that will produce the sperm.
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The pollen tube completes growing by
entering an opening at the bottom of the
ovary
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The sperm moves from the tube to combine
with the egg of ovule to form a zygote.
The Seed and Seed Dispersal
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Once the zygote is formed, it develops with
the surrounding tissue into the seed
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As the seed is developing, the ovary around
the ovule mature into a fruit
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Seed dispersal can be aided by water, wind,
animals
SEED
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Is the means by which an embryo can be
dispersed into to distant locations.
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It is a protective structure for the embryo
Seed Part
Function
testa
Tough, protective outer coat
cotyledons
Seed leaves that function as nutrient storage
structures
microphyle
Scar of the opening where the pollen tube
entered the ovule
Embryo root
and embryo
shoot
Become the new plant when germination
occurs
Pre-Germination
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Once seeds are formed, a maturation
process follows.
The seed dehydrates until the water content
of the seeds is about 10 -15% of its weight.
At this point, the seed goes into a dormant
period where there is low metabolism and no
growth or development.
Duration is variable for different types of seed
It is an adaptation to environmental
conditions
FUN FACT
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In 1995, a team of biologists
found some seeds in a dried-up
lakebed. The seeds were from a type of lotus
plant. After germinating some of the seeds,
the biologists found them to be nearly 1300
years old!!! (They used radiometric dating to
determine this age.)
Fun Fact
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In 2005, a 2000 year old Judean Date palm
(found in the ruins of Herod the Great’s
palace)was germinated
Germination Conditions
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If conditions become favourable, the seed will
germinated.
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GERMINATION – is the development of the
seed into a functional plant.
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There are several conditions that must be
fulfilled for a seed to germinate.
WATER
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Required to rehydrate the dried seed tissues
Makes the seed swell
As a result the seed coat will crack and
hydrolytic enzymes are activated- they will
start to catabolize large molecules (storage
polysaccharides such as starch is converted
into maltose) for cellular respiration.
OXYGEN
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Required for the break down of those sugars
in cellular respiration
TEMPERATURE
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Appropriate temperature is required, that is
varied among plants depending on their
natural environment.
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Ex. Period of low temperature followed by high temps –
ensures that the seed does not geminate until the winter
has passed.
Temperature is important for enzyme activity
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Many plants have specific conditions other
than these that must be met in order to
germinate.
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Ex:Lodgepole Pine
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The emerging seedling is fragile and will be
exposed to hard weather, parasites,
predators, and other hazards.
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Many seeds will not produce a functional
plant because of these threats
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To compensate, plants produce a large
number of seeds
Metabolic Processes during
Germination of a Starchy Seed
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Seed absorbs water (which leads to many
metabolic changes)
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Gibberellin is released after the uptake of
water
Gibberellin – plant growth hormone
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Gibberellin triggers the release of the
enzyme amylase
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Amylase causes the hydrolysis of the starch
into maltose
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Maltose is hydrolyzed into glucose which
can be used for cellular respiration or
converted into cellulose to build cell walls for
new cells
6.
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Stored proteins and lipids will also be
hydrolyzed to make proteins/enzymes and
phospholipids and energy metabolism.
Germination uses the food stored in
cotyledons to grown until it reaches light
when it starts to photosynthesize
Control of Flowering
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Light important factor for growth and
development
Plants are able to detect the presence of
light, its direction, wavelength, intensity
PHOTOPERIODISM – the plant’s response
to light involving the relative lengths of day
and night.
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*To ensure continued existence in an area, a
plant must flower when pollinators are
available and when necessary resources are
plentiful
PLANT TYPE
FLOWERING AND LIGHT
EXAMPLES
LONG-DAY
PLANTS
Bloom when days are longest
and nights the shortest
(midsummer)
Radishes, spinach,
lettuce
- Require Pfr
SHORT-DAY
PLANTS
Bloom in spring, late summer,
and autumn when days are
shorter
Poinsettias,
chrysanthemums, asters
- Inhibited by Pfr
DAY-NEUTRAL
PLANTS
Flower without regard to day
length
Roses, dandelions,
tomatoes
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It is actually the length of night that controls
the flowering process.
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The control is brought about by a special
blue-green pigment called phytochrome.
Phytochrome
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Phytochrome is a photoreceptor and a
pigment
It absorbs light
There are 2 forms of phytochrome
P – absorbs red light
P – absorbs far-red light/darkness
r
fr
Far-Red Light
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Wavelengths between 700-800nm
At the far end of the visible light spectrum
(Between red light and infrared light)
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During the day, when there is light, red light
(wavelength of 660nm) is present
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Pr absorbs red light and is rapidly converted
into Pfr
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At the end of the day, after many hours of
light, plants will have most of their
phytochrome in the form of Pfr
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During the night, when there isn’t light
(therefore no red light), Pfr is slowly converted
back into Pr
By morning, most of the phytochrome will be
Pr again
If there is even a flash of light interrupting the
darkness during the night, it will disrupt the
process of Pfr turning into Pr
Pr
Pfr
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Pfr
Pr
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Long-day plants, require Pfr to flower
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Long day = short night!
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At the end of a short night, there will still be lots
of Pfr remaining.
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The remaining Pfr at the end of a short night
stimulates the plant to flower.
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In short-day plants, the Pfr acts as an
inhibitor for flowering.
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So after a short night, the remaining Pfr
will prevent the plant from flowering.
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If it was a long night, all the phytochrome
will be in the form of Pr (there will be no
Pfr) so flowering CAN occur.