Plant Ecology - Chapter 2

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Plant Ecology - Chapter 2

• Photosynthesis Light

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Photosynthesis Light

• Functional ecology - how the biochemistry and
  physiology of individual plants determine their
  responses to their environment, within the
  structural context of their anatomy and morphology

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Photosynthesis Light

• Functional ecology - closely related to
  physiological ecology, which focuses on
  physiological mechanisms underlying whole-plant
  responses to their environment

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Photosynthesis Light

• Photosynthesis is a package deal
• How much light
• Competitors
• Limitations (pollution, pathogens)
• Herbivores
• Plants must cope with multiple items at same time

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Process of Photosynthesis

• Biochemical process to acquire energy from sun,
  carbon from atmosphere
• 2 parts
• Capture of energy (light reactions)
• Storage of energy into formed organic molecules
  (carbon fixation)

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Process of Photosynthesis

• Reactions take place in chloroplasts
• Light reactions on thylakoid membranes
• Carbon fixation (Calvin cycle) within the stroma

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Process of Photosynthesis

• Light reactions involve pigment molecules
• Many forms of chlorophyll
• Accessory pigments (carotenoids and xanthophylls
  in terrestrial plants)

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Process of Photosynthesis

• Pigment molecules arranged into two molecular
  complexes
• Photosystems I and II
• Capture energy (form ATP, NADPH) plus generate
  oxygen

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Process of Photosynthesis

• Energy captured from light reactions powers the
  Calvin cycle
• Captured energy ultimately stored in chemical
  bonds of carbohydrates, other organic molecules

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Rates of Photosynthesis

• Gross photosynthesis - total amount of carbon
  captured
• Cellular respiration - organic compounds broken
  down to release energy
• Net photosynthesis - gross photosynthesis minus
  respiration

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Rates of Photosynthesis

• Basic limiting factor - amount of light energy
  reaching thylakoid membranes
• Darkness - loss of energy due to respiration -
  giving off CO2
• Low light - respiration plus some photosynthesis
  - giving off and taking up CO2
• Compensation point

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Rates of Photosynthesis

• Strong light - respiration plus photosynthesis -
  giving off and taking up CO2, up to a point
• Maximum rate of photosynthesis, despite further
  increase in light energy

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Rates of Photosynthesis

• Different plants have different photosynthetic
  responses to same light intensity
• Some do better under low light, others strong
  light
• Habitat - shade vs. sun
• Some can shift light compensation point to deal
  with changes in light availability (lots in
  spring, less in summer in shade)

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Quality of Light

• Light quality (availability of different
  wavelengths) can limit rate of photosynthesis
• Blue and red wavelengths are captured
  preferentially
• Green wavelengths are discarded (green plants)

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Global Light Availability

• Tropical latitudes - day and night equal
• Polar latitudes - continuously light at
  midsummer, continuously dark at midwinter
• Maximum sunlight energy greater in tropics than
  polar regions

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Global Light Availability

• Maximum sunlight energy greater at high altitudes
  than at sea level
• Damaging UV-B radiation greater in tropics than
  polar regions, high elevations vs. low elevations
• Biochemical protection flavonoids to absorb,
  antioxidant and DNA repair enzymes

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CO2 Uptake Limitations

• CO2 diffusion from surrounding air into leaf and
  into chloroplast
• Leaf conductance - rate at which CO2 flows into
  the leaf
• Mostly under control of stomata

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CO2 Uptake Limitations

• Stomata open, close to maintain water balance
  (seconds, minutes)
• Stomata change as leaf morphology, chemistry
  change (days, months)
• Natural selection modifies (100s, 1000s of years)

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CO2 Uptake Limitations

• Controlling water loss is main reason why plants
  restrict their CO2 uptake
• Huge amount of air required for photosynthesis -
  2500 L air for each gram of glucose produced

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CO2 Uptake Limitations

• Stomata can be very dynamic, opening and closing
  constantly to regulate CO2 and water loss
• Much variation even within same leaf
• Patchy closure also common in stressed plants

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Variation in Photosynthetic Rates Habitats

• Photosynthetic rates vary within and among
  habitats
• Correlated with species composition, habitat
  preferences, growth rates

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Variation in Photosynthetic Rates Habitats

• Photosynthetic rates may be unrelated to species
  distributions, populations processes
• Other important components of photosynthesis
  total leaf area, length of time leaves active,
  maintained

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Photosynthetic Pathways

• Carbon fixation done using 3 different pathways
• C3
• C4
• CAM (crassulacean acid metabolism)

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Photosynthetic Pathways

• C3 and C4 named for 3-carbon and 4-carbon stable
  molecules first formed in these pathways
• CAM named after plant family Crassulaceae where
  it was first discovered

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Photosynthetic Pathways

• Most plants use C3 photosynthesis, and plants
  that use it are found everywhere
• C4 and CAM are modifications of C3, and evolved
  from it

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Photosynthetic Pathways

• C3 CO2 joined to 5-carbon molecule with assist
  from the enzyme RuBP carboxylase/oxygenase -
  rubisco
• Rubisco probably most abundant protein on earth,
  but does its job very poorly

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Photosynthetic Pathways

• Rubisco inefficient at capturing CO2
• Also takes up O2 during photorespiration
• O2 uptake favored over CO2 uptake as temperatures
  increase
• Limits photosynthesis
• Plants must have HUGE amounts of rubisco,
  especially those in warm, bright habitats, to
  compensate for poor performance

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Photosynthetic Pathways

• Increases in atmospheric CO2 concentrations
  should allow C3 plants to increase rates of
  photosynthesis

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Photosynthetic Pathways

• C4 photosynthesis contains additional step used
  for initial CO2 capture
• 3-carbon PEP (phosphoenol-pyruvate) CO2
  4-carbon OAA (oxaloacetate)
• Catalyzed by PEP carboxylate

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Photosynthetic Pathways

• PEP carboxylate only captures CO2
• Higher affinity for CO2 than rubisco
• Not affected by warmer temperatures
• Decarboxylation (CO2 removal) process allows
  standard Calvin cycle (including rubisco)

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Photosynthetic Pathways

• C4 requires special leaf anatomy
• Spatial separation of C4 and C3 reactions
• Rubisco exposed only to CO2, not O2 in atmosphere
  like in C3 plant

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Photosynthetic Pathways

• C4 Mesophyll cells for carbon fixation, bundle
  sheath cells for Calvin cycle - keeps O2 away
  from Calvin cycle
• C3 Mesophyll cells for carbon fixation and
  Calvin cycle - allows O2 access to Calvin cycle

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Photosynthetic Pathways

• C4 plants generally have higher maximum rates of
  photosynthesis, and have higher temperature
  optima

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Photosynthetic Pathways

• C4 plants generally do not become
  light-saturated, even in full sunlight
• Also have better nitrogen use and water use
  efficiencies because of reduced needs for rubisco
  (1/3 to 1/6)

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Photosynthetic Pathways

• Requires additional energy to run C4 pathway, but
  easily compensated for by photosynthetic gains at
  high light levels
• Very successful in warm, full-light habitats,
  e.g., deserts

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Photosynthetic Pathways

• CAM photosynthesis - Crassulacean acid metabolism
• Uses basically same biochemistry as C4, but in
  very different way
• Rubisco found in all photosynthetic cells, not
  just bundle sheath cells

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Photosynthetic Pathways

• CAM uses temporal separation of light capture,
  carbon fixation rather than spatial separation as
  in C4
• CO2 captured at night, converted into organic
  acids

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Photosynthetic Pathways

• During daylight, organic acids broken down to
  release carbon, used normally in Calvin cycle
• Stomata remain closed during day

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Photosynthetic Pathways

• CAM plants have thick, succulent tissues to allow
  for organic acid storage overnight
• Tremendous water use efficiency (stomata closed
  during heat of day)

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Photosynthetic Pathways

• Some CAM plants not obligated to just CAM
• Can use C3 photosynthesis during day if
  conditions are right, to achieve higher rates of
  photosynthesis
• CAM cant accumulate carbon as fast as C3 or C4
  plants, lowering rate of photosynthesis

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C3, C4, and CAM

• C3 plants most abundant ( of species, total
  biomass)
• More CAM species than C4 species
• CAM plants less abundant than C4 in biomass,
  worldwide distribution

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C3, C4, and CAM

• Half of grass species are C4
• Dominate warm grassland ecosystems
• Warm, bright conditions where C4 is favored

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C3, C4, and CAM

• CAM plants typically are succulents in desert
  habitats, or

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C3, C4, and CAM

• Epiphytes growing on trees in tropics or
  subtropics
• Both types experience severe water shortages

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C3, C4, and CAM

• Phenology - seasonal timing of seasonal events
• C3 plants typically more springtime, vs. C4
  plants being mostly summer

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C3, C4, and CAM

• C4 grasses are most common where summer
  temperatures are warm in N. America

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C3, C4, and CAM

• C3 grasses - cool, winter-moist
• C4 grasses - warm, summer-moist

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C3, C4, and CAM
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Sun Shade Leaves
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Sun Shade Leaves
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Sun Shade Leaves
Higher light saturation levels Greater maximum
photosynthetic rates
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Species Adaptations-Sun
Solar tracking increases light availability
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Species Adaptations-Shade
Velvety, satiny leaf surfaces, blue iridescence
on leaf enhance available light
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Species Adaptations-Shade
Shade species use brief sunflecks with high
efficiency stomata open slow loss of
photosynthetic induction
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Species Adaptations-Ecotypes?
Genetically distinct populations of same
species adapted to low- and high-light
conditions? Phenotypic plasticity
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Daylength

• Flowering, seed dormancy, seed germination, other
  physiological responses of plants controlled by
  daylength (actually nightlength)
• More reliable predictor of seasonal change than
  temperature
• Ratio of two forms of phytochrome A controlled by
  length of dark period