Photobiology

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Photobiology

• 3rd Year Student of biophysics

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Prepared ByProf. Dr. Mohammed Naguib Abd
El-Ghany Hasaneen

• Professor Of Plant Metabolism And Biotechnology
• Academic Year
• 2005 - 2006

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Contents

• Introduction
• Radiation
• Visible light
• Ultraviolet light
• Ultraviolet light damage
• Phytochrome concept
• Distribution and translocation of phytochrome
• Physiological effects of phytochrome

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Introduction
Light in Plants
We see visible light (350-700 nm) Plants sense
Ultra violet (280) to Infrared (800) Examples
Seed germination - inhibited by light Stem
elongation- inhibited by light Shade avoidance-
mediated by far-red light There are probably 4
photoreceptors in plants We will deal with the
best understood PHYTOCHROMES
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A Primer on Radiation
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• Some important plant responses to radiation
• (light is only one form of radiation)
• Photosynthesis
• Photomorphogenesis
• Photropism Photoperiodism
• Energy balance/temperature
• respiration
• enzyme activity
• transpiration
• UV-responses
• mutagenesis

(note that there is a much more detailed table
and discussion of responses of plants to light in
chapter 1 of Hart Light and Plant Growth)
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In what form does energy from the sun travel to
Earth?

• Energy travels to Earth in the form of
  electromagnetic waves
• Electromagnetic waves are classified according to
  wave length
• Radiation is the direct transfer of energy by
  electromagnetic waves

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Most of the energy from the sun reaches Earth in
the form of

• Visible light
• Infrared radiation
• A small amount of ultraviolet radiation

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• The different colors of light make up the visible
  spectrum.
• Red has the longest wave length
• Violet has the shortest wave length

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Infrared radiation has the following properties

• Wavelengths longer than red light
• It is not visible
• It can be felt as heat
• Used to warm food or baby chicks in an incubator

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Ultraviolet light has the following properties

• Wave lengths shorter than violet light
• Can cause skin damage
• Can cause eye problems

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Radiation and radiation laws
The way we describe and quantify radiation, and
the units used, vary depending on the kind of
process were interested in
Properties of radiation that are important to
plants include Quality, Quantity, Direction
(including diffuse vs. direct) and Periodicity.
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Radiation quality (or color, for visible light)
is a function of its wavelength (or frequency)
distribution
Note these two charts are arrayed in opposite
directions one by increasing wavelength/decreasi
ng energy and the other by increasing
frequency/increasing energy
The symbol l is often used for wavelength
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Radiation measurements

• Radiation quantity is measured in one of three
  ways, depending on the application
• Quantum measurements (numbers of photons)
• Radiometric measurements (amount of energy)
• Photometric measurements (light intensity, based
  on human perception)

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The amount of radiation is expressed as fluence
(also known as density quantity per area), rate
(also known as flux quantity per time) or
fluence rate (also known as flux density amount
per area per time)
Parameter Term Energy units Quantum units
Quantity per area fluence J m-2 mmol m-2
Quantity per time rate (or flux) J s-1 (watt) mmol s-1
Quantity per area per time fluence rate (or flux density) W m-2 mmol m-2 s-1
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For studies of photosynthesis and
photomorphogenesis, the quantity of radiation is
usually measured in quantum units (quantum flux
density quantum fluence rate) mmol m-2
s-1 usually, only the visible, or
photosynthetically active part of the spectrum is
measured, or in the case of photomorphogenesis,
only specific wavelengths
Note that mol refers to a mole of photons, and
that 1 mol photons1 Einstein. A quantum is one
indivisible package of radiation, or one photon.
PPFD photosynthetically active photon flux
density PAR photosynthetically active
radiation (400-700 nm)
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For energy balance studies, radiation is
measured in radiometric units, for
example Watts m-2
(note 1 Watt 1 Joule s-1)
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Radiometric and quantum units are interconverted
based on the amount of energy in photons.
See link from website to working with light or
p. 28 of the handout by Hart or any good
reference on radiation for more information on
this conversion)
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Because most light sources contain a wide range
of wavelengths, it is difficult to convert
precisely between quantum and radiometric units.
Usually an approximation is used that assumes a
typical distribution of wavelengths for a
particular light source
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All objects emit radiation (i.e., they radiate)
as a function of their temperature (in addition
to the emissivity of the material). Temperature
affects both the amount and the quality
(wavelength) of radiation emitted.
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The bulk of solar radiation is shortwave
(visible plus near infrared)
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Spectral Quality

• visible 400-700 nm, about 45 of incident
  insolation
• solar IR 700-5000 nm, about 46 of incident
• UV 190-400 nm, about 9 of incident

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• Restating this as a rough rule of thumb
• When the sky is clear, the photosynthetically
  active part of the solar spectrum accounts for
  about HALF of the total solar energy, IR accounts
  for the other half

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Radiance vs. Irradiance Radiance is the
radiation that is emitted from an
object Irradiance is the radiation that
impinges upon an object
In this case, radiation is commonly described as
a flux (rate), or amount per unit time. This
could be either a radiant flux or a quantum flux
In this case, radiation is commonly described as
a flux density, or amount per unit time per unit
area. Again, the flux could be quantified either
with either radiometric or photometric units.
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Irradiance usually has both direct and diffuse
components
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The amount of energy in direct-beam irradiance is
strongly affected by the angle between the
surface and the beam
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Solar angle and leaf angle can have a very big
influence on irradiation, dramatically affecting
photosynthesis, transpiration and leaf temperature
definitions Heliotropic sun
tracking Paraheliotropic leaf stays parallel to
direct beam of sun Diaheliotropic leaf stays
perpendicular to direct beam
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Connections between matter and energy
A short, painless review of simple organic
chemistry to develop the connection between
cycles of organic biomass and cycles of energy
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(Inorganic not a hydrocarbon. This is a highly
oxidized form of carbon)
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general deterioration of 4 green
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shade from trees and tower
general deterioration of 4 green
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shade from trees and tower
poor air circulation from trees and shrubs
general deterioration of 4 green
concentrated traffic between trap and green
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shade from trees and tower
poor air circulation from trees and shrubs
general deterioration of 4 green
poor internal and surface drainage
concentrated traffic between trap and green
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shade from trees and tower
poor air circulation from trees and shrubs
general deterioration of 4 green
poor internal and surface drainage
concentrated traffic between trap and green
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shade from trees and tower
poor air circulation from trees and shrubs
delicate turfgrass
hot, humid microenvironment
general deterioration of 4 green
O2-deficient rootzone
poor internal and surface drainage
concentrated traffic between trap and green
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Wavelength - ENERGY

• Photons in short wavelengths pack a lot of energy
• Visible light (400-750nm)
• 1 mole of photons 250kJ energy
• Ultraviolet light (lt 400 nm)
• 1 mole of photons 500 kJ energy
• Photons in longer wavelengths do not
• Infrared radiation (gt750 nm)
• 1 mole of photons 85 kJ energy

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• What happens when sunlight hits the wall of a
  building?
• Some reflected back to space (no effect) (this
  depends upon the COLOR of the wall!)
• Most is absorbed. Then what?
• Absorption of radiation makes the temperature of
  the object rise
• How hot?
• The hotter ? the more radiation emitted (as
  infrared)
• Heats until energy in energy out
• Or energy absorbed energy re-radiated

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The Thermal Environment

• Energy is gained and lost through various
  pathways
• radiation - all objects emit electromagnetic
  radiation and receive this from sunlight and from
  other objects in the environment
• conduction - direct transfer of kinetic energy of
  heat to/from objects in direct contact with one
  another
• convection - direct transfer of kinetic energy of
  heat to/from moving air and water
• evaporation - heat loss as water is evaporated
  from organisms surface (2.43 kJ/g at 30oC)
• change in heat content metabolism - evaporation
  radiation
• conduction convection

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Organisms must cope with temperature extremes.

• Unlike birds and mammals, most organisms do not
  regulate their body temperatures.
• All organisms, regardless of ability to
  thermoregulate, are subject to thermal
  constraints
• most life processes occur within the temperature
  range of liquid water, 0o-100oC
• few living things survive temperatures in excess
  of 45oC
• freezing is generally harmful to cells and tissues

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So how do organisms regulate temperature?

• Manipulating the energy balance equation!
• Net radiation
• Color, Orientation to sun, Minimizing/maximize IR
  losses (insulation)
• Conduction
• Use warm or cool surfaces
• Convection
• Minimize or maximize exposure to wind or water
  (boundary layers, exposure, immersion)
• Evaporation
• Minimize or maximize evaporation to control heat
  loss
• Metabolism Generate or limit generation of heat!
• These can be morphological, physiological, or
  behavioral adaptations

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Conserving Water in Hot Environments

• Animals of deserts may experience environmental
  temperatures in excess of body temperature
• evaporative cooling is an option, but water is
  scarce
• animals may also avoid high temperatures by
• reducing activity
• seeking cool microclimates
• migrating seasonally to cooler climates

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Conserving Water in Hot Environments

• Desert plants reduce heat loading in several ways
  already discussed. Plants may, in addition
• orient leaves to minimize solar gain
• shed leaves and become inactive during stressful
  periods

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The Kangaroo Rat - a Desert Specialist

• These small desert rodents perform well in a
  nearly waterless and extremely hot setting.
• kangaroo rats conserve water by
• producing concentrated urine
• producing nearly dry feces
• minimizing evaporative losses from lungs
• kangaroo rats avoid desert heat by
• venturing above ground only at night
• remaining in cool, humid burrow by day

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Tolerance of Freezing

• Freezing disrupts life processes and ice crystals
  can damage delicate cell structures.
• Adaptations among organisms vary
• maintain internal temperature well above freezing
• activate mechanisms that resist freezing
• glycerol or glycoproteins lower freezing point
  effectively (the antifreeze solution)
• glycoproteins can also impede the development of
  ice crystals, permitting supercooling
• activate mechanisms that tolerate freezing

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Organisms maintain a constant internal
environment.

• An organisms ability to maintain constant
  internal conditions in the face of a varying
  environment is called homeostasis
• homeostatic systems consist of sensors,
  effectors, and a condition maintained constant
• all homeostatic systems employ negative feedback
  -- when the system deviates from set point,
  various responses are activated to return system
  to set point

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Temperature Regulation an Example of Homeostasis

• Principal classes of regulation
• homeotherms (warm-blooded animals) - maintain
  relatively constant internal temperatures
• poikilotherms (cold-blooded animals) - tend to
  conform to external temperatures
• some poikilotherms can regulate internal
  temperatures behaviorally, and are thus
  considered ectotherms, while homeotherms are
  endotherms

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Homeostasis is costly.

• As the difference between internal and external
  conditions increases, the cost of maintaining
  constant internal conditions increases
  dramatically
• in homeotherms, the metabolic rate required to
  maintain temperature is directly proportional to
  the difference between ambient and internal
  temperatures

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Limits to Homeothermy

• Homeotherms are limited in the extent to which
  they can maintain conditions different from those
  in their surroundings
• beyond some level of difference between ambient
  and internal, organisms capacity to return
  internal conditions to norm is exceeded
• available energy may also be limiting, because
  regulation requires substantial energy output

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Partial Homeostasis

• Some animals (and plants!) may only be
  homeothermic at certain times or in certain
  tissues
• pythons maintain high temperatures when
  incubating eggs
• large fish may warm muscles or brain
• hummingbirds may reduce body temperature at night
  (torpor)

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What are energy units?

• 1. umoles of photons per meter squared per
  second
• umol m-2 s-1
• Watts per meter squared W m-2
• Sunny day in Colorado solar input
• 2200 umol m-2 s-1
• 1100 W m-2
• Why no time unit for W? (W 1 J s-1)
• Can you convert between the two units?
• Not quite since the conversion depends on
  wavelength

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Infrared Light and the Greenhouse Effect 1

• All objects, including the earths surface, emit
  longwave (infrared) radiation (IR).
• Atmosphere is transparent to visible light, which
  warms the earths surface.

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Infrared Light and the Greenhouse Effect 2

• Infrared light (IR) emitted by earth is absorbed
  in part by atmosphere, which is only partially
  transparent to IR.
• Substances like carbon dioxide and methane
  increase the absorptive capacity of the
  atmosphere to IR, resulting in atmospheric
  warming.

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Greenhouse Effect - Summary

• Greenhouse effect is essential to life on earth
  (we would freeze without it), but enhanced
  greenhouse effect (caused in part by forest
  clearing and burning fossil fuels) may lead to
  unwanted warming and serious consequences!

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Ozone and Ultraviolet Radiation

• UV light has a high energy level and can damage
  exposed cells and tissues.
• Ozone in upper atmosphere absorbs strongly in
  ultraviolet portion of electromagnetic spectrum.
• Chlorofluorocarbons (formerly used as propellants
  and refrigerants) react with and chemically
  destroy ozone
• ozone holes appeared in the atmosphere
• concern over this phenomenon led to strict
  controls on CFCs and other substances depleting
  ozone

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Clouds

• What happens on a cloudy day?
• Less radiation comes in
• What happens on a cloudy night?
• Less radiation goes out

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The Absorption Spectra of Plants
Plants Respond to Light

• Various substances (pigments) in plants have
  different absorption spectra
• chlorophyll in plants absorbs red and violet
  light, reflects green and blue
• water absorbs strongly in red and IR, scatters
  violet and blue, leaving green at depth

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Plants Respond to Light
Photomorphogenesis, Phototropism, Photoperiodism
Phytochrome responses (red/far red) flowering
and dormancy branch patterns root growth
Blue light responses stomatal opening
phototropism chloroplast orientation
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• Photomorphogenesis.
• nondirectional, light-triggered development
• red light changes the shape of phytochrome and
  can trigger photomorphogenesis

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Phototropisms

• Phototropic responses involve bending of growing
  stems toward light sources.
• Individual leaves may also display phototrophic
  responses.
• auxin most likely involved

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Carbon vs. Energy

• Plants convert LIGHT energy into CHEMICAL energy
• They use the chemical energy to take CO2 from the
  atmosphere, and turn it into glucose, and other
  C-structures.

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Seed location?
Red light from sun penetrates to seed.
No light from sun to this deep seed.
Seed germinates.
No germination.
Red light to seed near surface
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Sun Exposure and UV damage

• Sunshine, essential for life, strikes the earth
  in rays of varying wavelengths. Long rays
  (infrared) are unseen but felt as heat.
  Intermediate length rays are visible as light.
  Shorter rays (ultraviolet) are also invisible and
  are further divided into the following groups
• Ultraviolet (UVA) rays are beneficial in low
  doses, but may increase the chance of cancer in
  high doses. UVBs are primarily responsible for
  sunburn and cancerUVCs are the shortest and most
  dangerous UV rays contain enough energy to
  damage DNA in living skin and eye cells. DNA
  controls the ability of cells to heal and
  reproduce. The ozone layer allows life to
  flourish by passing the longer, beneficial
  wavelengths and effectively blocking almost all
  UVC, some UVB and a little UVA.

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Phytochrome
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The Pigment That Controls Growth and Flowering In
Many Plants

• phytochrome

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What Is Phytochrome ?

• Phytochrome is a pigment found in some plant
  cells that has been proven to control plant
  development.
• This pigment has two forms or phases in can
  exist in. P-red light sensitive (Pr) and P far
  red light sensitive (Pfr) forms.
• The actual plant response is very specific to
  each specie, and some plants do not respond at
  all.

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The structure of Phytochrome
A dimer of a 1200 amino acid protein with several
domains and 2 molecules of a chromophore.
Chromophore
660 nm 730 nm
Pr Pfr
Binds to membrane
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Signal Transduction of Phytochrome
Membrane
Pfr
Ga
G protein a subunit
Pr
Cyclic guanidine monophosphate
Guanylate cyclase
cGMP
Ca2/CaM
Calmodulin
CAB, PS II ATPase Rubisco
FNR PS I Cyt b/f
CHS
Anthocyanin synthesis
Chloroplast biogenesis
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How Phytochrome Works
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Light-Regulated Elements (LREs) e.g. the
promotor of chalcone synthase-first enzyme in
anthocyanin synthesis
Promoter has 4 sequence motifs which participate
in light regulation. If unit 1 is placed upstream
of any transgene, it becomes light regulated.
Unit 1
Transcription Factors
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Light-Regulated Elements (LREs)

• There are at least 100 light responsive genes
  (e.g. photosynthesis)
• There are many cis-acting, light responsive
  regulatory elements
• 7 or 8 types have been identified of which the
  two for CHS are examples
• No light regulated gene has just 1.
• Different elements in different combinations and
  contexts control the level of transcription
• Trans-acting elements and post-transcriptional
  modifications are also involved.

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Which Wavelengths Are Photoperiodic?

• The length of the night period plays a major role
  in determining which wavelength will be
  effective, as the phytochrome pigment tends to
  revert to Pr during long periods of darkness.
• Thus the length of exposure to light in a
  building, or if outdoors, the seasonal light
  changes, affect how long the plants perceives
  each form of phytochrome.

R FR
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Photoperiodic Response Its all about
Preferences! Long Day Plants flower when there
is adequate PR Short Day Plants flower when
there is adequate Pfr
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phytochrome
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phytochrome
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phytochrome
Long-Day Plants Need Low Pr!
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phytochrome
Long-Day Plants Need Low Pr!
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phytochrome
Reproductive (Flowering)
Short-Day Plant Need Low Pfr!
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phytochrome
Short-Day Plant Need Low Pfr!
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phytochrome
Short-Day Plants Need Low Pfr!
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phytochrome
Black Cloth
Short-Day Plants Need Low Pfr!
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phytochrome
Night lighting disrupts reversion to Pr and
maintains vegetative status!
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Light Interruption of Darkness Affects
Short- and Long-Day Plants Differently
24-hour day cycle
Critical day length
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The Phytochrome System Works Within The Apical
Meristem
Photoperiodicresponses are triggered in the
meristem (both apical and axillary), long
before the new branches develop. We can
control development !
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To lengthen the night, plants are covered with a
blackout shade cloth. Applied in late afternoon
and removed in the morning (5 pm to 8 am)
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Photoperiodic shade cloth
Light penetration through the shade cloth should
not be more than 2 fc in order to prevent delay
in flowering and/or disfigured flowers.
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SUPPLEMENTAL LIGHTING

• Light sources.

• incandescent lamps emit large amounts of red
  light and are good for lighting mums (standard
  mum lighting)
• mums flower when the day length decreases to
  13.5 hrs or less
• whenever the day length is longer than 14.5 hrs
  plants remain vegetative
• split each long night in two short nights with
  supplemental light to prevent flowering

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DAILY DURATION OF LIGHT

• The length of day has an effect on two plant
  processes
• time of flowering
• plant maturity
• This light-induced response is called
  photoperiodism, and plants that flower under only
  certain day-length conditions are called
  photoperiodic.

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Plants Respond to Gravity

• Gravitropism is the response of a plant to the
  earths gravitational field.
• present at germination
• auxins play primary role
• Four steps
• gravity perceived by cell
• signal formed that perceives gravity
• signal transduced intra- and intercellularly
• differential cell elongation

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The pigment phytochrome

• Detects R and FR light
• Provides information about environment
• Answers 3 questions for plant
• Am I in the light?
• Do I have plants as neighbors or above me?
• Is it time to flower?

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Why bother?

• Seeds store materials to start growth
• Must reach light before running out of stored
  materials
• Small seeds
• Need to be very near surface
• Often need light for germination
• Germinating plants straighten open leaves at
  surface, too

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Plant neighbors?
Far red reflected from other plants.
Red absorbed by other plants.
Far red enriched neighbors
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Why does this matter?

• Neighboring plants are threats
• Might grow taller, shade you
• Solution
• Grow at least as tall as neighbors
• Need to know that you have neighbors
• Isolated plants typically shorter than crowded
  plants
• Other reasons, too

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Under other plants?
Far red reflected from other plants or
transmitted.
Red absorbed by other plants.
Far red enriched understory
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Why important?

• Best growth strategy for understory plants is
  different than for plants in open
• Need to know whether
• Shaded by other plants
• OR
• Just cloudy
• OR
• Late in day (low light)

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Right time to flower?

• Unreliable indicators of time of year
• Temperature
• Moisture
• Light levels
• Reliable length of day/night
• Varies with season
• Varies with latitude
• Detected by phytochrome

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Phytochrome has 2 forms

• Red-absorbing phytochrome
• Far red absorbing phytochrome
• Interconverted
• Two forms of the same compound
• Total amount same

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In red light
Pfr
Pr
Pfr
Pr absorbs red light, changes to Pfr form.
Pfr doesnt absorb red light, stays the same.
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In far red light
Pr
Pfr
Pr
Pfr absorbs far red light, changes to Pr form.
Pr doesnt absorb far red light, stays the same.
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In pure light
Pfr
Pr
In pure red light, all the phytochrome
ends up in the Pfr form.
In pure far red light, all the phytochrome ends
up in the Pr form.
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Sunlight
Mostly red A little far red
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In sunlight
In sunlight most P gets converted to Pfr form.
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Start of night
Most P in Pfr form.
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In the dark
Pfr form changes gradually to Pr form.
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After a short night
Pfr still left.
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LDP SNP

• Needs short night
• Needs Pfr still present at end of night
• Pfr promotes flowering for LDPs

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Later in the night
More Pfr changes to Pr.
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After a long night
All the Pfr is gone.
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Day dawns
Most P gets converted to Pfr form again.
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SDP LNP

• Needs long night
• Needs Pfr gone at end of night
• Pfr inhibits flowering for SDPs

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LDP SDP
Long day Pfr left at end of short night. Pfr
promotes flowering for LDPs. Pfr inhibits
flowering for SDPs.
Short day Pfr gone at end of long night. No Pfr
to promote flowering for LDPs. No Pfr to inhibit
flowering for SDPs.
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Waiting for the right time

• Plants grow leaves until it is time to flower
• LDPs wait until the day is long enough
• Really night short enough
• Some time before June 21
• SPDs wait until the day is short enough
• Really night long enough
• Some time after June 21
• Flower opening happens later

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Day neutral plants

• Flower when mature enough
• Maybe other environmental signals (temp?)
• Day length (dark length) doesnt matter

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Through the year
Specific flowers at specific times.
May
September
July
August
October
June
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Phytochrome tells plants

• If they are near the surface
• About their plant neighbors
• Whether it is time to flower
• And lots more

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References

• http//www.abdn.ac.uk/sms/ugradteaching/GN3502/GN3
  502_0732005_1.ppt
• http//www.warnercnr.colostate.edu/class_info/by22
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  ,20part202202004.ppt
• http//www.coe.unt.edu/ubms/documents/classnotes/S
  pring2006/256,1,Sensory Systems in Plants
• http//128.192.110.246/pthomas/Hort3140.web/Phytoc
  hrome20lecture.ppt
• http//fp.uni.edu/berg/pp/downloads/PhytochromeAct
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• http//www.fsl.orst.edu/bond/fs561/lectures/radia
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• http//www.coe.unt.edu/ubms/documents/classnotes/S
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