Light The Provider of Life

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Light The Provider of Life

• Through a series of nuclear reactions occurring
  within the sun, mass is converted into energy
• Em c2
• Agriculture is a system of exploiting solar
  energy through photosynthesis
• Yield is dependent upon the size and efficiency
  of the photosynthetic system
• Pigment excitation is dependent upon the
  interaction between a photon and a pigment, a
  measure of light used in photosynthesis is often
  based on photon flux density

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Light Quantification

• Electromagnetic Theory Light travels through
  space as a wave, and the number of waves passing
  a given point in a certain time interval is a
  frequency
• Quantum Theory Light travels in a stream of
  particles called photons. The energy present in
  one photon is called a quantum
• Photon Flux Density Number of photons striking a
  given surface area per unit of time

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Solar Radiation

• Maximum solar radiation intensity occurs at 500
  nm
• Plants appear to have adapted to utilize solar
  radiation between 400 and 700 nm
• Solar radiation level decreases as it passes
  through the atmosphere due to absorption and
  scattering
• In addition, other factors that influence solar
  radiation include
• Angle of suns rays on that spot
• Daylength
• The amount of atmosphere the radiation passed
  through as a function of the angle of the suns
  rays
• The number of particles in the atmosphere (e.g.,
  dust, condensed water particles such as fog or
  clouds)
• Other minor factors such as fluctuations in solar
  output, distance from the earth to the sun, etc.

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• Of the solar radiation absorbed during the
  daytime by a crop surface
• 75 - 85 Used to evaporate water
• 5 - 10 Sensible heat storage in the soil
• 5 - 10 Sensible heat storage in the atmosphere
  by convection processes
• 1 - 5 Goes into photosynthesis
• Wavelengths between 400 and 700 nm are most
  efficiently used in photosynthesis (Pn)
• Photosynthetically Active Radiation
• (PAR µE/m2/s)
• Photosynthetic Photon Fluence Rate
• (PPF µmol/m2/s)

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Absorption and Fate of Light Energy

• Pigment Biological compound that absorbs light
• Absorption Extremely fast process (10-15 s)
• Energy of the absorbed photon is transformed to
  an electron in the molecule
• Energy of the electron is elevated from the
  ground (nonexcited) state to an elevated level
  known as the excited or singlet state
• A photon can only be absorbed if its energy
  content matched the energy required to raise the
  electron to a higher, allowable energy state
• An excited molecule has a very short lifespan
  (10-9 s) and must rid itself of any excess energy
  and return to ground state

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• Dissipation of excess energy
• 1. Thermal deactivation (loses excitation energy
  as heat)
• 2. Fluorescence (emission of a photon of light by
  the excited molecule)
• 3. Inductive resonance or radiation less transfer
  (very efficient but require pigment molecules to
  be in close proximity)
• 4. Triplet energy state (more stable energy state
  and is sufficiently long to allow photochemical
  reactions to occur)

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• Absorption and Action Spectrum
• Function of wavelength
• Action spectrum specific for an individual
  molecule

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Selected Photoreceptors

• Chlorophyll
• Porphoryn head
• Made of 4 nitrogen containing pyrrole rings
• Long hydrocarbon tail (phytol tail)
• Chlorophyll molecule is completed with the
  addition of the magnesium ion chelated to the 4
  nitrogen atoms
• Four known types of chlorophyll (a,b,c,d)
• Absorption spectra is very specific

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Carotenoids

• Lipid soluble
• Includes carotenes and xanthophylls
• Carotenes Predominately orange/red-orange
• B carotene major carotenoid in algae and higher
  plants
• May protect chlorophyll by absorbing excess blue
  light and combining with oxygen to form
  xanthophylls

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Flavonoids

• Include the pink, purple, scarlet, and blue
  anthocyanins
• Water soluble pigments and are found
  predominately in vacuolar sap
• May protect the underlying tissue from UV-B
  radiation
• Attractants to insects (UV and visible spectrum)?

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Phycobilins

• Serve as accessory light-harvesting pigments
  and/or critical regulatory system in green plants
• Three phycobilins are involved in Pn
  (phycoerythrin, phycocyanin, and
  allophycocyanin), and the 4rth (phytochromobolin)
  is an important photoreceptor
• Differ from chlorophyll in that the tetrapyrole
  group is covalently linked to a protein structure

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Phycobilins (cont.)

• Phytochromobolin exist in 2 forms that are photo
  reversible
• (i) P660 (Pr) absorbs maximally at 660 nm
• (ii) Absorption at 660 nm converts it to a second
  far-red pigment (P735) or (Pfr)
• Absorption at 735nm converts the protein back to
  P660
• Pr is believed to be an active form of the
  pigment responsible for initiating a wide range
  of photomorphogenic responses

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Photosynthetic Apparatus

• Light Reaction
• Chloroplast
• Chloroplast lamellae (membranes)
• Packed full of photosynthetic pigments
• Stroma lamellae (double lamella)
• Grana lamellae (stacked lamella)
• Stroma
• A less dense fluid filled area where the
  reduction of CO2 occurs
• The transformation of light energy to chemical
  energy (photophosphorylation) occurs in the
  lamellae

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• Light reaction (cont.)
• Consists of the oxidation of water and production
  of chemical potential or reduced
• Nicotinamide adenine dinucleotide phosphate
  (NADPH) and the phosphorylation of adenosine
  diphosphate (ADP) to adenosine triphosphate (ATP)
• NADPH is one of the most powerful reductants
  (acceptors of electrons and suppliers of hydrogen
  ions) known in biological systems
• ATP is synonymous with available energy in the
  biological system (when a phosphate group is
  released from ATP, energy is also released)
• The released phosphate, attaching to some
  molecule (phosphorylation) by an energy input,
  raises the energy of the molecule and allowing it
  to undergo even more biochemical reactions
• Both NADPH and ATP are needed to convert CO2 to
  organic molecules

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Electron Transport System

• Fairly well understood
• 2 reaction centres exist where energy from
  absorbed photons are used to drive the system
• After a pigment has absorbed a photon of light,
  the energy lifts an electron from a ground to an
  excited energy state
• While in the excited state the pigment molecule
  can donate and accept electrons from other
  molecules
• Photosystem II catalyses the removal of electrons
  from water molecules, and these electrons are
  accepted by a substance labelled Q.

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Electron Transport System (cont.)

• Photosystem I uses more energy absorbed from
  photons to catalyse the removal of electrons from
  Q
• This sets up the energy required for
  photophosphorylation (ATP formation) and the
  reduction of NADP to NADPH
• Remember light reactions transform light energy
  to the short-term chemical energy of ATP and
  NADPH
• ATP and NADPH are used to reduce CO2 to stable
  organic forms from which dry weight results
• Electron Transport and Weed Control
• Triazine herbicides and derivatives of urea
  (diuron) act by interfering with the electron
  transport chain (bind the Q site of PSII)
• Viologen dyes diquat and paraquat Act by
  intercepting electrons on reducing side of PSI

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Carbon Dioxide Fixation

• Agriculture is based on yield or weight of crop
  products
• Dry matter production is dependent upon the
  balance of CO2 (Pn) uptake and CO2 evolution
• During growth, respiration accounts for 25 to 30
  of total photosynthesis
• When respirationgtPn ?

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Carbon Dioxide Fixation (cont.)

• The pathway by which all eukaryotic organisms
  incorporate CO2 into carbohydrate is known as
  carbon fixation or the photosynthetic carbon
  reduction (PCR) cycle
• Pathway was determined through the use of
  radiolabelled CO2 (14C)
• CO2 fixation is catalysed by the enzyme ribulose
  bis-phosphate (RuBP) carboxylase oxygenase
  (RuBISCO)
• RuBISCO is the most abundant protein in the world
  accounting for 50 of protein in leaves

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Carbon Dioxide Fixation (cont.)

• 5 carbon sugar ribulose 1,5 bisphosphate (RuBP)
  is the acceptor molecule for which a 3 carbon
  molecule is made
• ATP produced in photophosphorylation is used to
  convert ribulose-5-phosphate to RuBP which is
  extremely unstable and is quickly hydrolysed into
  2 molecules of 3-PGA
• After CO2 fixation, ATP along with the reduced
  nucleotides from the light process, change the
• 3-phosphoglyceric acid (3-PGA) to
• 3-phosphoglyceraldehyde (3-PGald).

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Carbon Dioxide Fixation C4 Plants

• In 1966, Hatch and Slack presented evidence that
  another pathway for CO2 fixation exists.
• 1st product from the mesophyll is a 4C compound
• Pathway encorporates CO2 using phosphoenolpyruvate
  (PEP) carboxylase enzyme
• ATP produced in photophosphorylation is used to
  convert pyruvate to PEP

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Carbon Dioxide Fixation C4 Plants (cont.)

• The PEP (3C) is carboxylated to three four-carbon
  acids (oxaloacetate, malate, and aspartate)
• These acids are converted to the vascular sheath
  cells where they are converted to pyruvate
• In the change to pyruvate, a carbon is released
  that is converted, either by addition to RuBP or
  by addition to a two-carbon molecule to 3-PGA by
  RUBP carboxylase
• After 3-PGA is produced, the Calvin cycle is
  operative

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Comparing C3 and C4 Species

• Anatomical differences
• L C4 species have chloroplasts in the vascular
  sheath cells, C3 species do not
• L Chloroplasts in mesophyll of C3 and C4 are
  structurally similar but no starch is produced
  in C4 plants (just 4C compounds)
• L Chloroplasts in vascular sheath cells of C4
  species are larger and have less developed grana
  than in mesophyll cell chloroplasts (since Calvin
  cycle is operative, they store starch)
• Remember the morphological differences?
• Previously covered material

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Crassulation Acid Metabolism Plants

• Occurs predominately in succulent plants
• Adapted to arid conditions where low
  transpiration is a survival necessity
• Stomata are closed during the day and open at
  night to absorb CO2.
• Domestic examples of CAM plants
• - Pineapple
• - Agave (sisal, henequen) and prickly pear
• Fix CO2 into 4-C acids with PEP carboxylase only
  at night when stomata are open and energy
  required comes from glycolysis

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Factors Essential for Photosynthesis

• Light
• As light levels increase Pn gradually increases
  until CO2 uptake equals CO2 evolution
• Therefore, CER (carbon exchange rate) 0
• Most C3 species including grapes reach light
  saturation before full sunlight

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• Carbon Dioxide Concentration
• Dry air contains 78 N2, 21 O2, 0.93 argon,
  0.034 CO2 (340 ppm), and traces of other gases
• 85 to 95 of plants dry weight is dependent upon
  CO2 uptake in photosynthesis
• Greenhouse effect?
• CO2 absorbs infrared bands of light, earth should
  retain more heat
• Influence on plant physiology? Pn?

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• Temperature
• Light reaction (photophosphorylation) is
  independent upon temperature in the temperature
  range in which plants grow.
• CO2 fixation is enzymatically controlled and
  dependent upon temperature
• Increases with temperature until enzyme
  denaturation occurs

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• Water
• See the water relations material for the course
• Substrate for Pn
• Transpiration accounts for 99 of all water used
  by plants
• 1 is used to hydrate the plant, maintain turgor,
  and make growth possible
• Leaf Age and Mineral Status
• Major rate influencing the senescence rate is
  mineral nutrient status of the leaf
• Lower rates have been associated with
  deficiencies of N, Mg. Fe, P, and Ca

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Photosynthate Utilization by the Plant

• After Pn, formation of a hexose sugar
• Many further changes occur
• Interconvert from glucose to fructose
• Combine to form sucrose for translocation
• Polymerize in chloroplast
• Transformed to compounds such as cellulose