Ch' 2 Photosynthesis Ps and Light

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Ch. 2 Photosynthesis (Ps) and Light
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CAM photosynthesis

• Also minimizes FFF (fatal flaws of fotosynthesis)
• CAM Crassulacean Acid Metabolism
• Crassulaceae family of succulent plants first
  known for this pathway
• Acid Early scientists noted the leaves of these
  plants become acidic at night

Burros tail (Sedum morganianum)
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Leaf anatomy

• Succulent (fleshy) leaves (or photosynthetic
  stems)
• Mesophyll not in layers
• Cells large, with large central vacuoles
• Veins with small vascular bundle sheath cells

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CAM pathway

• Uses same enzymes and similar chemical reactions
  to C4 pathway
• Separates C fixation and photosynthesis in time
  rather than space

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CAM pathway

• Night Stomata open, CO2 taken up, fixed using
  PEP carboxylase to make 4 C acid (often malic
  acid).

Malic acid
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CAM pathway

• Night Stomata open, CO2 taken up, fixed using
  PEP carboxylase to make 4 C acid (often malic
  acid).
• Acid stored in large central vacuole of leaf cell
• Leaf pH goes down during night (acid drop)

Acid drop in skateboarding
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CAM pathway

• Day Stomata close. 4 C acid removed from vacuole
  to chloroplast, reaction reversed to release CO2
• Photosynthesis by Calvin Cycle during day

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CAM advantages

• 1) Uses PEP carboxylase to fix C no
  photorespiration, higher affinity for CO2

photorespiration
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CAM advantages

• 1) Uses PEP carboxylase to fix C no
  photorespiration, higher affinity for CO2
• 2) Stomata open at night temperature lower,
  humidity higher. Less water lost

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CAM advantages

• 1) Uses PEP carboxylase to fix C no
  photorespiration, higher affinity for CO2
• 2) Stomata open at night temperature lower,
  humidity higher. Less water lost
• 3) Do Calvin cycle during day with stomata
  closed. Seals CO2 in leaf, gives low O2CO2
  ratio, minimizes photorespiration

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CAM pathway
Day
Night
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CAM variation

• Dependence of CAM plants on CAM varies
• Some only do CAM when under stress (do C3 other
  times)

Aeonium arboreum
Fig. 2.14 More succulence, More CAM
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CAM variation

• Some plants do carbon cycling capture CO2 from
  respiration during night and use it during day
  for Calvin cycle
• Ex, Welwitschia mirabilis (Namib desert of S.
  Africa)
• Produces only 2 leaves!

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CAM variation

• Ex, Welwitschia mirabilis (Namib desert of S.
  Africa)
• Gymnosperm does carbon cycling

Some plants may be 1,000 yr old
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Comparing C3, C4, CAM plants

• Taxa most plants (99) do C3
• C4 plants 0.4 of flowering plants
• Examples
• Some grasses Sugar cane (Saccharum spp.)

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Amaizing Facts

• Corn or maize (Zea mays)

another corny Boyd joke.....
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Comparing C3, C4, CAM plants

• Crabgrass (Digitaria)

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Comparing C3, C4, CAM plants

• Some Andropogon species
• Ex, Andropogon gerardii (big bluestem)
• Dominant grass in tallgrass prairie of US
• Also A. virginicus here in SE

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Comparing C3, C4, CAM plants

• Dicots
• Kudzu (Pueraria montana)

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Comparing C3, C4, CAM plants

• Dicots
• Some saltbush species (e.g., Atriplex canescens)
• Found in salty places in hot US deserts

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Comparing C3, C4, CAM plants

• Typical C4 plant habitat
• Low elevation and low latitude areas
• Warm temperatures and high light levels
• In temperate areas
• winter/spring is C3 season
• summer is C4 season

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Comparing C3, C4, CAM plants

• Ex, of grass flora that is C4 in N. America

Fig. 2.18
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Comparing C3, C4, CAM plants

• CAM plants included in 20 families of flowering
  plants
• Examples many cacti (saguaro, cholla)

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Comparing C3, C4, CAM plants

• CAM plants included in 20 families of flowering
  plants
• Examples Agave species

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CAM and Epiphytes

• Epiphytes plants that grow on other plants
• most abundant in tropical rain forests

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CAM and Epiphytes

• Present in other areas as well (ex, Alabama)
• Spanish moss (Tillandsia usneoides)
• CAM flowering plant in pineapple family
  (Bromeliaceae)

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Comparing C3, C4, CAM plants

• Examples some Crassulaceae (Diamorpha smallii)

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Comparing C3, C4, CAM plants

• Examples many Orchidaceae (esp. epiphytic ones)
• Ex, Phalaenopsis (moth orchids)

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Comparing C3, C4, CAM plants

• CAM plants
• Succulent terrestrial plants in deserts
• Epiphytes in trees in tropical/subtropical areas

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Comparing C3, C4, CAM plants

• Trait C3 C4 CAM
• Maximum Ps rate 10X 20X 1X

Fig. 2.12
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Comparing C3, C4, CAM plants

• Trait C3 C4 CAM
• Maximum growth rate 50X 200X 1X
• (optimal conditions)

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Comparing C3, C4, CAM plants

• Trait C3 C4 CAM
• Optimum Ps temperature Lo Hi Intermed.

Fig. 2.11
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Comparing C3, C4, CAM plants

• Trait C3 C4 CAM
• Water use efficiency 1X 2X 20X
• (g CO2 gained per
• Kg water lost)

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Applied Aspects

• What crops are best adapted to hot, dry areas?
  C4!
• Corn/maize
• Sorghum
• Millet

millet
sorghum
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Applied Aspects

• Other crops?
• Grain amaranth (C4 dicot) (Amaranthus cruentus)

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Applied Aspect

• Genetic engineering to give C3 plants C4
  properties
• Adapt them to warm high-light climates
• Example rice (C3) grass
• People from some Asian countries get 70 of
  calories from rice
• 70 of worlds 1.3 billion poor live in Asia

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Applied Aspect

• Researchers have put maize Ps genes into rice
• Ex, PEP carboxylase
• Have shown greater yields (10-30)

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C3 variation

• Note that C3 plants can vary in Ps abilities

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C3 variation

• Note that C3 plants can vary in Ps abilities
• Between species range from heliophytes to
  sciophytes
• Heliophytes adapted for high light intensity
• Sciophytes adapted for low light intensity
  (shady) habitats
• Ex, Oxalis oregona from redwood forest floor

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C3 variation

• Within species sun vs shade leaves
• Typically, sun leaves
• are thicker
• are smaller
• are more deeply dissected/lobed
• have greater stomatal density

European beech
Sun (left) and shade (rt) leaves of Quercus
pagoda (cherrybark oak)
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C3 variation

• Light curves show ability of sun/shade leaves to
  use different light levels
• Works similarly for heliophytes/sciophytes
• Note sun leaves can use more light than shade
  leaves
• Have higher light saturation points (light level
  where net Ps is maximum)

Fig. 2.22
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C3 variation

• Note shade leaf has lower light compensation
  point
• Light compensation point light level at which
  net Ps 0 (break even point)
• Why? Note Rs rate is lower for shade leaves.
  Takes less light for them to reach compensation
  point

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CO2 compensation points

• CO2 level at which net Ps is 0 (break even
  point for CO2 concentration in leaf)
• Note C3 plant value around 100 ppm, C4 is zero!
• Why? Recall PEP carboxylase with greater affinity
  for CO2 than Rubisco

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CO2 compensation points

• Corn (C4) vs bean plant (C3) in sealed jar (no
  CO2 added to support new growth)
• Who will live longer??

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CO2 compensation points

• Corn (C4)!

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Carbon isotope ratios

• Recall isotope different versions of an element
• C has 6 protons, and often has 6 neutrons
• Thats carbon-12 (98.89 of C)
• But can have 6 protons and 7 neutrons (carbon-13)
  (1.11 C)
• Or 6 protons and 8 neutrons (carbon-14)

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Carbon isotope ratios

• Enzyme affinity for substrate effects
  selectivity for isotopes
• If affinity low, lighter CO2 bound more (more
  discrimination)
• If affinity high, less discrimination (ratio in
  fixed carbon more closely mirrors atmosphere
  ratio)
• How document ratio? delta C-13 value
• Expressed in parts per thousand (ppt)
• The more negative the delta C-13 value, the more
  discrimination between isotopes

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Carbon isotope ratios

• Recall carbon fixing enzymes have different
  affinities for CO2
• Rubisco (fixes C in C3 plants) affinity lesser
• PEP carboxylase (fixes C in C4 plants) affinity
  greater
• C3 plant tissue
• delta C-13 values -24 to -34 ppt
• C4 plant tissue
• delta C-13 values -10 to -20 ppt