Transcript Plant Science notes

Plant Science
9.1 PLANT STRUCTURE AND GROWTH
9.2 TRANSPORT IN ANGIOSPERMOPHYTES
9.3 REPRODUCTION IN
ANGIOSPERMOPHYTES
Remember…
 Plant cell!
Plant Evolution
Plants originated from green algae that lived in ponds that
occasionally dried out.
Angiosperms
 Angiosperms have dominated the land for over 100
million years.
 Known as “flowering plants”
 There are about 250,000 known species of flowering
plants living today.
 Most of our food comes from flowering plants
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Roots, such as beets and carrots
Fruits of trees and vines, such as apples, nuts, berries, and
squashes
Fruits and seeds of legumes, such as peas and beans;
Grains, such as rice, wheat, and corn
Angiosperms
 Divided into two groups:
 Names refer to the first leaves that appear on the plant
embryo.
 Embryonic leaves are called seed leaves, or cotyledons:
Monocots (embryo has one seed leaf)
 Dicots (embryo has two seed leaves)

Angiosperms
 Monocots:
 Orchids, bamboos, palms, and lilies, as well as grains and
other grasses
 Leaves have parallel veins
 Stems have vascular tissues arranged in a complex array of
bundles.
 Flowers have petals and other parts in multiples of three.
 Roots form a fibrous system (a mat of threads) that spread out
below the soil surface.
 Make excellent ground cover that reduces erosion.
Angiosperms
Angiosperms
 Dicots:
 True dicots include most shrubs and trees (except for conifers),
as well as many food crops.
 Leaves have a multibranched network of veins
 Stems have vascular bundles arranged in a ring.
 Flower usually has petals and other parts in multiples of four
or five.
 Large, vertical root (called a taproot) goes deep into the soil

You can see this if you try to pull up a dandelion
Angiosperms
Plant Body
 Composed of organs with various tissues reflective of
their evolutionary history as land-dwelling
organisms.
 Must draw resources from two environments:
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Water and minerals from soil
CO2 and light from air
Plant Body
 Plant body is divided up to two main parts:
 Subterranean part root
 Aerial part shoot
Plant Body
 Root system:
 Anchors in the soil, absorbs and transports minerals and
water, and stores food.
 Monocots Fibrous root system consists of a mat of generally
thin roots spread out shallowly in the soil
 Dicots have one main vertical taproot with many small
secondary lateral roots growing outward.
 Both Monocots and Dicots have tiny projectsions called root
hairs:

Enormously increase the root surface area for absorption of water
and minerals.
Plant Body
 Shoot system:
 Made up of stems, leaves, and adaptations for reproduction
(flowers)
 Stems are parts of the plant that are generally above ground
and support the leaves and flowers. Composed of:
Nodes
 Points at which leaves are attached
 Internodes
 Portions of the stem between nodes


Leaves are the main photosynthetic organs in most plants
(green stems also perform photosynthesis)

Consist of a flattened blade and a stalk, or petiole, which joins the
leaf to a node of the stem.
Plant Body

Shoot system (continued):
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Two types of buds that are undeveloped shoots:
Terminal bud
 Found at the apex (tip) of the stem, has developing leaves and a
compact series of nodes and internodes
Axillary bud
 one of each of the angles formed by a leaf and the stem, are
usually dormant.
Plant Body
Apical dominance
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Results from the terminal bud producing hormones that
inhibit growth of the axillary buds.
By concentrating resources on growing taller, apical
dominance is an evolutionary adaptation that increases the
plant’s exposure to light
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Important where vegetation is dense.
Removing the terminal buds usually stimulates growth of the
axillary buds.

Branching is important for increasing exposure the environment
Modified Roots, Stems, and Leaves
 Modified roots:
 Some plants have unusually large taproots that store food in
carbohydrates such as startch:
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Carrots, turnips, sugar beets, and sweet potatoes
Sugar Beet
Modified Roots, Stems, and Leaves
 Modified Stems:
 Stolon
“runner”; has a horizontal stem that grows along the ground
surface
 Plantlets form at nodes along their length, enabling a plant to grow
asexually
 Example: strawberry
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Rhizomes
Look like large, brownish, rootlike structures
 Horizontal stems that grown just below or along the soil surface
 Store food, and having buds, can also spread and form new plants
 Potato plant has enlarged structures specialized for storage called
tubers (the potatoes we eat)

Modified Roots, Stems, and Leaves
 Modified stems (continued)
 Bulbs

Modified stems that are also used for underground food storage
(onions)
Modified Roots, Stems, and Leaves
 Modified Leaves:
 Tendrils
Tips coil around a stem, help plants climb
 Examples: grapevines, peas

Tendril- Pea Plant
Plant Tissues in Stems and Leaves
 Each plant organ- root, stem, or leaf- is made up of
three tissue systems:
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Dermal
Vascular
Ground tissues
Plant Tissues in Stems and Leaves
 Dermal Tissue
 Forms an outer protective covering.
 Acts as first line of defense against physical damage and
infectious organisms.
 Consists of a single layer of tightly packed cells called the
epidermis:
Epidermis of leaves and most stems is covered with a waxy layer
called cuticle, which helps prevent water loss.
 Typical dicot leaf also has pores on its epidermis called stomata
 which allow CO2 exchange between the surrounding air and the
photosynthetic cells inside the leaf.
 Surrounded by guard cells:

• Regulate the size of the stoma
Plant Tissues in Stems and Leaves
Plant Leaf
Plant Tissues in Stems and Leaves
 Vascular Tissue:
 Made up of:
Xylem
 type of vascular tissue that is made up of cells that transport
water and dissolved ions from the roots to the leaves
 Phloem
 type of vascular tissue that is made up of cells that transport
sugars from leaves or storage tissues to other parts of the plant

Plant Tissues in Stems and Leaves
 Vascular Tissue (continued):
 In the stem..

Vascular tissue forms vascular bundles
 Dicots arranged in a circle
Plant Tissues in Stems and Leaves
 Vascular Tissue (continued):
 In the leaf…
Vascular tissue form network of veins
 In the veins, the xylem and phloem are continuous with the
vascular bundles of the stem.
 Allows them to be in close contact with photosynthetic tissues,
ensuring water and mineral nutrients from the soil are supplied,
and that sugars made in the leaves are transported throughout
the plant

Plant Tissues in Stems and Leaves
 Ground Tissue (continued):
 Accounts for the bulk of a young plant, by filling in spaces
between the epidermis and vascular tissue.
 Functions include photosynthesis, storage, and support.
 Ground tissue inside vascular tissue is called pith
 Ground tissue external to vascular tissue is called cortex
Dicot Stem
Plant Tissue in Stems and Leaves
 Ground Tissue (continued):
 Ground tissue of dicot stems…
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
consists of both a cortex region and pith region
Ground tissue of the leaf…

Is called Mesophyll :
 Sandwiched between the upper and lower epidermis
 Consists mainly of photosynthesis cells
 Loosely arranged to provide air spaces which CO2 and O2 can
circulate
 Main location of photosynthesis
Plant Growth
 Growth in plants is made possible by tissues called
meristems.
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A meristem consists of cells that divide frequently,
generating additional cells.
Some products of this division remain in the meristem and
produce still more cells, while others differentiate and are
incorporated into tissues and organs of the growing plant.
Plant Growth
 Apical Meristems
 Meristems at the tips of roots and in the buds of shoots
 Cell division in the apical meristems produces the new cells
that enable a plant cell to grow in length primary growth
 Enables roots to push through the soil and allows shoots to
increase exposure to light and CO2.
 Growth occurs behind the root tip in three zones of primary
growth:
Zone of cell division, zone of elongation, and zone of maturation
 Zone of maturation brings about the three tissue systems
(dermal, ground, and vascular)

Plant Growth
Primary Growth of a Root
Plant Growth
 Lateral meristems
 Associated with the increase in thickness of stems and roots
secondary growth
 Caused by the activity of two cylinders of dividing cells that
extend along the length of roots and stems:
Vascular cambium
 Secondary growth adds layers of vascular tissue on both sides of
the vascular cambium wood
 Cork cambium
 Outer cambium that forms the secondary growth of the
epidermis cork

Control of Plant Growth
 Auxin is a term used for any chemical substance
that promotes seedling elongation.
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Apical meristem at the tip of a shoot is a major site of auxin
synthesis.
As auxin moves downward, it stimulates growth of the stem by
making cells elongate.
Concentration of auxin determines its effect
Too low to stimulate shoot cells will cause root cells to elongate
 High conc. stimulates shoots cell and inhibits root cell elongation.
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Stimulates stem elongation and root growth, differentiation,
and branching.
Control of Plant Growth
 Auxins also play a part in phototropism, an occurrence
that involves plants bending or moving away from light.
 The shoot tip is responsible for directional movement by
the plant in response to sunlight, as this is the area where
auxins can be found.
 Sunlight eradicates auxin, meaning that the part of the
shoot tip of the plant which is receiving direct sunlight will
have the least amount of auxin.
 The extra auxin present on the shaded side promotes more
cell division and elongation, causing the plant to bend
towards the sunlight after this lop-sided growth.
Control of Plant Growth
Cells on the
darker side are
larger and have
elongated faster;
causes the shoot
to bend towards
the light.
Effect of Auxin on
Phototropism
If a plant receives
sunlight uniformly
from all sides or
is kept in the
dark, the cells all
elongate at a
similar rate.
Transport in Plants
 Several factors necessary for plant growth:
 CO2 from airabsorbed by leaves
 O2 from air or soilabsorbed by leaves or roots
 H2O from soil absorbed by the roots
 Minerals from the soil absorbed by the roots
 Sugars are made in the leaves from the absorbed molecules and
ions and used to build the plant’s body and provide energy
Solute Uptake From The Roots
 Mineral ions from the soil can get into the root of
plants by three different ways:

1. Diffusion

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2. Fungal hyphae
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If the concentration of certain ions is lower inside of the root hair
cells, they can simply diffuse into the root hair cells from the soil
Some plants live in a symbiotic relationship with fungi and use
fungal hyphae to increase the surface of the root even more. The
combination of plant root and fungal fibers are called
mycorrhiza. The fungus benefit from a constant supply of sugar
while the plant benefit from the increased surface area that the
fungal hyphae provide, they also excrete growth factors and
antibiotics
3. Mass flow of water into the root can also carry ions passively
in dissolved form
Solute Uptake From the Roots
 Roots hairs are extensions of epidermal cells that
cover the root and form a huge surface area
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Allows the plant to absorb the water and minerals it needs for
growth
Watery solution has to be transported from the soil to
epidermal cells to cortex of the root to the xylem (waterconducting vascular tissue)
Plasma membrane of the xylem cells are selectively permeable,
which helps regulate the mineral composition of a plant’s
vascular system.
Solute Uptake From the Roots
 Two possible routes to the xylem:
 Intracellular route
 Extracellular route
Solute Uptake From the Roots
 Intracellular route:
 A.k.a. Symplatic route
 Cells within roots are connected via plasmodesmata (channels
through the walls of adjacent cells) which allows for a
continuum of living cytoplasm among the root cells
 Once inside epidermal cells, solution can move inward from cell
to cell without crossing membranes
Solute Uptake From the Roots
 Extracellular route:
 Solution moves inward within the hydrophillic walls and
extracellular spaces of the root cells but does not enter the
cytoplasm of the epidermis or cortex cells.
 Solution passes through no plasma membranes, and there is
no selection of solutes until they reach the endodermis.
 Endodermis has a waxy barrier called the Casparian strip
which stops water and solutes from entering the xylem.
 Water and ions are forced to cross a plasma membrane into an
endodermal cells, then are discharged into the xylem.
Solute Uptake in the Roots
 In a real plant…
 Water and solutes rarely follow just the two kinds of routes
 May take a combination of these routes, and may pass through
numerous plasma membranes and cell walls en route to the
xylem.
 All water and solutes must cross a plasma membrane at some
point.
Transpiration
 Why transpiration?
 As a plant grows upward toward sunlight, it needs to get water
and minerals from the soil.
 Must be able to transport resources from the roots to thrive.
Transpiration
 Xylem tissue is made of two types of conducting
cells: tracheids and vessel elements.
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When mature, but types of cells are dead, consisting only of
cell walls, and both are in the form of very thin tubes that are
arranged end to end.
Because the cells have openings in their ends, a solution of
water and inorganic nutrients, called xylem sap, can flow
through these tubes.
Xylem sap flows all the way up from the plant’s roots through
the shoot system to the tips of the leaves.
Transpiration
Transpiration
 Forces that push xylem sap against gravity are:
 Root pressure
Root cells actively pump inorganic ions into the xylem, and the
root’s endodermis holds the ions there
 As ions accumulate in the xylem, water tends to enter by osmosis,
pushing xylem sap upward ahead of it.
 Can push sap up a few meters
 For the most part, however, xylem sap is not pushed from below by
root pressure by pulled upward by the leaves.


Transpiration
The pulling force caused by the loss of water from the leaves and
other aerial parts of a plant.
 Water molecules leave the plant through the stoma of the leaf by
diffusion.
 When the stoma is open, water concentration is higher in the
plant cells than in the surrounding atmosphere.

Transpiration
 Properties of water stimulate transpiration:
 Cohesion
Sticking together of molecules of the same kind.
 Because water is polar, they are attracted to each other by
hydrogen bonds
 Water molecules form continuous strings in xylem tubes,
extending all the way from the leaves down to the roots.

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Adhesion
Sticking together of molecules of a different kind.
 Water molecules tend to adhere via hydrogen bonds to
hydrophillic cellulose molecules in the walls of xylem cells.

Transpiration
 Transpiration-Cohesion-Tension Mechanism:
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Before a water molecule can leave the leaf, it must break off from the end of
the string
It is pulled off a steep diffusion gradient between the moist interior of the leaf
and the drier surrounding air.
Cohesion resists the pulling force of the diffusion gradient, but it is not strong
enough to overcome it.
The molecule breaks off, and the opposing forces of cohesion and
transpiration put tension on the remainder of the string of water molecules.
As long as transpiration continues, the string is kept tense and is pulled
upward as one molecule exits the leaf and the one right behind it is tugged up
into its place.
Adhesion pulls the remaining water molecules upwards from the root against
the downward pull of gravity.
Process does not require energy from the plant, they are all extended by
physical properties of water and the surrounding molecules.
Transpiration
 Summary of Transpiration-Cohesion-Tension
Mechanism:

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Transpiration exerts a pull that is relayed downward along a
string of water molecules held together by cohesion and helped
upward by adhesion.
Transpiration is an efficient means of moving large volumes of
water upward from roots to shoots.
http://www.phschool.com/science/biology_place/labbench/la
b9/transpull.html
Guard Cells
 PROBLEM! Photosynthesis requires large leaf
surfaces
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Results in constant transpiration and water loss
If soil dries out, plants wilt and eventually die
 SOLUTION! Leaf stomata can open and close via the
control of guard cells.

Guard cells control the opening of a stoma by changing shape,
widening or narrowing the gap between the two cells.
Abscisic Acid
 Absicisic acid causes the closing of stomata.
 ABA crucial for plants to withstand drought.
 When the plant starts to wilt, ABA accumulates in the leaves
and causes stomata to close.
Abiotic Factors that Affect Transpiration
 Light intensity
 Temperature
 Wind
 Humidity
 **We will perform a lab to see how these factors
affect Transpiration! 
Adaptations of Xerophytes
 Plants adapted to dry conditions are called xerophytes.
 Adaptations include:
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thick, waxy cuticles  reduces water loss through the cuticle
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Reduced number of stomata  reduces the number of pores through
which water loss can occur
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Prickly pear , ivy, sea holly
Prickly pear, Nerium
Stomata sunken in pits, grooves, or depressions, leaf surfaces
covered with fine hairs, massing of leaves into a rosette ground level
 moist air is trapped close to the area of water loss, reducing the
diffusion gradient and therefore the rate of water loss

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Sunken stomata: Pinus sp, Hakea sp.
Hairy leaves: lamb’s ear
Leaf rosettes: dandelion, daisy
Adaptations of Xerophytes
Hakea
Lamb’s Ear
Prickly Pear
Daisy
Adaptations of Xerophytes

Adaptations include:
 Stomata closed during the light, open at night CAM metabolism: CO2 is fixed during
the night, water loss in the day is minimized.
 CAM plants, American aloe, pineapple, Kalanchoe, Yucca
 When the weather is hot and dry, keeps its stomata closed most of the time, conserving
water. At the same time, it continues making sugars by photosynthesis .Have an enzyme
(very high affinity of CO2) that fixes carbon into a four-carbon compound, which acts as
a shuttle to transfer CO2 around the leaf.
 a C4 plant; corn and sugar cane
 Leaves reduced to scales, stem photosynthetic, leaves curled, rolled, or folded when
flaccid reduction in surface area from which transpiration can occur
 Leaf scales: broom; Rolled Leaf: marram grass
 Fleshy or succulent stems; Fleshy or succulent leaves when readily available, water is
stored in the tissues for times of low availability
 Fleshy stems: candle plant; Freshly leaves: Bryophyllum
 Deep root system below the water tableRoots tap into the lower water table
 Acacias, oleander
 Shallow root system absorbing surface moisture roots absorb overnight condensation
 Most cacti
Adaptations of Xerophytes
Broom
Bryophyllum
Phloem transport
 Main function of phloem is to transport the products
of photosynthesis.
 Phloem contains cells called sieve-tube members
arranged end to end as tubes, with sieve-tube plates
in between
 Phloem sap (sugary solution may contain
inorganic ions, amino acids, hormones, and mostly
sucrose) moves freely from one cell to the next
 Example: Maple syrup!
Phloem Transport
 Phloem sap moves through the plant in various
directions, in contrast to the xylem sap which only
flows upward from the roots.
 Sieve tubes always carry sugars from a sugar
source (organ that is the net producer of sugar,
mostly the mature leaf) to a sugar sink (an organ
that is net consumer or storer of sugar; examples
include growing roots, buds, stems and fruits, also
tubers or bulbs )
Phloem Transport
Phloem Transport
 Each food-conducting tube in phloem tissue has a
source end and sink end, but these may change with
the season or the developmental stage of the plant
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A storage organ, such as a tuber or bulb, may be a source or
sink depending on the season.
When stockpiling carbohydrates in the summer, it is a sugar
sink. After breaking dormancy in spring, it is a source as its
starch is broken down to sugar, which is carried to the growing
tips of the plant.
Phloem Transport
 The phloem sap moves by pressure flow
mechanism:
1.At the sugar source, sugar is loaded into a phloem tube by active
transport. Sugar loading at the source end raises the solute
concentration inside the phloem tube.
 2.The high solute concentration draws water into the tube by
osmosis. The inward flow of water raises the water pressure at the
source end of the tube.
 3.At the sugar sink (a beet root, for example), both sugar and
water leave the phloem tube. The exit of sugar lowers the sugar
concentration in the sink end; the exit of water lowers the
hydrostatic pressure in the tube.
 4.The lower water pressure and sugar concentration causes the
phloem sap to move toward the sugar sink.

Phloem Transport
 The pressure flow mechanism explains why phloem
sap always flows from a sugar source to sugar sink,
regardless of their locations in the plant.
Sexual Life Cycle of a Flowering Plant
 From an evolutionary viewpoint, all the structures
and functions of a plant can be interpreted as
mechanisms contributing to reproduction.

An oak tree is merely an acorn’s way of making more acorns.
Reproductive Structures of Angiosperms
 Flowers are the reproductive shoots of angiosperms.
 Four types of modified leaves called floral organs:
 Sepals: enclose and protect the flower bud, usually green and
more leaf-like than the other floral organs.
 Petals: often colorful and advertise the flower to pollinators
 Stamens: reproductive organ containing sperm
 Carpels: reproductive organ containing egg
Reproductive Structures of Angiosperms
 Stamen
 Consists of a stalk (filament) tipped by an anther
 Anther
 Contain sacs in which meiosis occurs and in which pollen is
produced. Pollen grains house the cells that develop into
sperm.
 Carpel
 Has a long slender neck (style) with a sticky stigma at its tip
 Stigma
 Landing platform for pollen
Reproductive Structures of Angiosperms
 Ovary
 Base of the carpel, contain ovules.
 Develops into a fruit, which protects the seed and aids in
dispersing.
 Ovules
 Found within ovary, contain a developing egg and supporting
cells.
 Where fertilization occurs, which develops into a seed
containing the embryo.
 Seed germinates (begins to grow) in a suitable habitat; the
embryo develops into a seedling; and the seedling grows into a
mature plant.
Reproductive Structures of Angiosperms
Pollination
 Pollination
 the first step to fertilization
 transfer of pollen from anther to stigma
 utilize wind, insects, birds, or other animals
Fertilization
 After pollination, double fertilization occurs:
 1. the pollen grain germinates on the stigma
 2. Pollen tube forms and grows downward into ovary.
 3. Pollen tube then reaches ovule and discharges two sperm
near the embryo sac.
 4. One sperm fertilizes the egg, forming the zygote.
 5. Another contributes its haploid nucleus to the large diploid
central cell of the embryo sac, forming a triploid (3n) nucleus.
This gives rise to a food-storing tissue called endopserm.
Fertilization
Seed Dispersal
 After fertilization, the ovule, containing the triploid central cell and the
zygote, begins developing into a seed.
 The results of embryonic development in the ovule is a mature seed.
 Flowering plants have evolved many ways to ensure
their seeds are dispersed.


In some cases the seed itself is the agent of dispersal, but often
it is the fruit.
Chief agents of seed dispersal are wind, water, and animals.
Wind: such seeds have wing-like or feathery structures that catch
the air currents and carry the seeds long distances.
 Water: many seeds, including those that lack special buoyancy
mechanisms
 Animals: seeds have hooks or barbs that catch animal hair, sticky
secretions that adhere to skin or hair, or fleshy fruits that are eaten
leaving the seed deposited in feces

Seed Dispersal
Wind
Wind
Water
Animals
 After fertilization, ovule with triploid central cell and zygote,
develops into a seed.

Endosperm is used to nourish embryo
 Results of embryo development is a mature seed
 A seed is an entire reproductive unit, housing the
embryonic plant in a state of dormancy.
Dicot Seed Structure
 External Structures:
 Testa, or tough, protective seed coat that encases the embryo
and its food supply
 Internal Structures:
 Embryo is an elongated structure with two fleshy cotelydons.
 Embryonic root develops just below the point at which the
cotyledons are attached to the rest of the embryo.
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Embryonic shoot, develops just above the point of attachment,
with embryonic leaves
Micropyle: A minute opening in the ovule of a seed plant through
which the pollen tube usually enters.
Seed contains no endosperm because it’s been absorbed by cotyledons during
seed formation.
Nutrients pass from cotyledons to embryo during germination
Dicot Seed Structure
Seed Germination
 The germination of a seed is commonly used to
symbolize the beginning of life, but as you now
know, the seed already contains a miniature plant,
complete with embryonic root and shoot.

…think about that the next time you make popcorn!
 Thus, at germination, the plant does not begin life
but rather resumes the growth and development that
was temporarily suspended during seed dormancy.
Seed Germination
 Begins when…
 1.seed takes up water
 2.hydrated seed expands, rupturing its coat.
 3.inflow of water triggers metabolic changes in embryo that
make it start growing again
 4.A hormone called gibberllin forms in the embryo’s cotelydon.
This stimulates the production of amylase, which catalyses the
breakdown of starch to maltose. This subsequently diffuses to
the embryo for energy release and growth.
Seed Germination
Seed Germination
 Conditions needed seed germination:
 oxygen, moisture, temperature, and sometimes
Oxygen, which increases respiration, and correct temperature
settings are needed to activate growth enzymes.
 Providing the right amount of moisture to the growing medium is
important in that if there is too much water, and oxygen is limited,
it will prevent the normal germination process
 Although most seeds do not need light to germinate, a few do such
as lettuce and begonias.

Photoperiodism in Plants
 Photoperiodism is the response of a plant to the
relative lengths of daylight and darkness.
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
Flowering is a photoperiodic activity; individuals of a single
species will all flower at around the same time, even if there
germination and maturation dates vary.
Flowering is actually controlled by night length, not day
length.
Photoperiodism in Plants
 The exact onset of flowering varies depending on
whether the plant is a:

Short-day (long-night) plants
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

Generally flower during the late summer, fall, or winter when light
periods shorten.
Examples: chrysanthemums and poinsettias
Long-day (short-night) plants


Usually flower in late spring or early summer when light periods
lengthen.
Examples: spinach, lettuce, iris, and many cereal grains
Photoperiodism in Plants
 Photoperiodic activites are controlled through the
action of a pigment called phytochrome:


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Acts as a signal for some biological clocks in plants and is also
involved in other light initiated responses, such as
germination, shoot growth, and chlorophyll synthesis.
Is a light-absorbing blue-green pigment (absorbs red light)
universal in vascular plants.
Red light (660nm), a component of white daylight, is the most
effective wavelength for interrupting night length and affecting
flowering in short-day and long-day plants.
The effect of a flash of red light that interrupts a period of
darkness can be reversed by a subsequent flash of far-red light
(730nm)
Photoperiodism in Plants

Phytochrome has two forms: Pfr and inactive Pr
Pr absorbs red light, Pfr absorbs far red light
 When Pr absorbs red light, it is quickly converted to Pfr, and when
Pfr absorbs far-red light, it is quickly converted to Pr
 Pfr predominates in daylight, but reverts back to Pr form in the
dark.
 The plant measures daylight (or rather night length) by the
amount of phytochrome in each form.
 Mainly in the Pr form during the night. The sudden conversion
of Pr to Pfr, due to the absorption of sunlight, signals dawn.

Photoperiodism in Plants
Photoperiodism in Plants
 Pfr is a promoter of flowering in long-day plants and
an inhibitor of flowering in short-day plants