Plant Structure and Function II and a half Ecol 182 4212005

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Plant Structure and Function II (and a half) -
Ecol 182 4-21-2005
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How tall can trees grow? the importance of
components of leaf water potential Coastal
Redwoods Reduction in leaf size occurs due to
the inability of cell osmotic potential to
overcome the effects of gravity and maintain
turgor pressure
How do plant connect many modular parts?
Transport in phloem and xylem active versus
passive processes. Driving force for flow is
derived from either the development of tension
(xylem) or positive pressure (phloem).
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Nutrient Dynamics (outline)

• Nutrient availability
• Sources of nutrients
• Direct and indirect controls over sources
• Nutrient Uptake
• Plant and environmental interactions
• Nutrient Return from the plant to the soil
  (cycling)
• Ecological and environmental processes

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Nutrient availability

• Where do most nutrients come from in terrestrial
  ecosystems?
• What is the big deal about the observation that
  there is high nutrient availability in mineral
  and organic forms compared to the rather low
  nutrient concentration of the soil solution?
• What environments would have low nutrient
  availability (and why)?

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Nutrient uptake root absorption of ions

• Nutrient delivery from the soil depends on
• Diffusible nutrient concentration ion-specific
  rates of transport
• Nitrate is fast and phosphate and potassium are
  slower (why?)
• Ions of nutrient are primarily taken up by a
  purely passive process
• Following the concentration charge gradients
• Nutrient uptake is a function of BOTH plants
  soils and includes two processes (1) mass flow
  and (2) diffusion
• Mass flow is rapid, whereas diffusion is measured
  in mm per day
• When mass flow rates are low, nutrient
  concentration at the root surface decrease to
  below that of the surrounding soil
• These depletion zones create concentration
  gradients that drives diffusion of nutrients in
  the soil

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Nutrient uptake maintenance of important
gradients
Most nutrients are taken up in ionic form, that
require compensatory balances in the cell (1)
role of the primary vacuole and (2)
co-transporters
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Figure 37.4 Ion Exchange
Metabolic products can be used by roots to
scavenge for nutrients in the soil CO2 effects
and pH and direct organic acid dumping
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Nutrient uptake - nitrogen acquisition

• N can limit growth in natural systems (why is N
  important?)
• The carbon expended acquiring N can make up a
  significant fraction of the total energy a plant
  consumes
• Growth and maintenance of absorbing organs
  (usually roots)
• Transport of minerals against a concentration
  gradient
• Assimilation of N in leaves
• Plants have developed several approaches to
  nitrogen acquisition, including (in order of
  costs)
• Root absorption of inorganic ions ammonium and
  nitrate
• Fixation of atmospheric nitrogen
• Mycorrhizal associations
• Carnivory

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• NITROGEN FIXERS
• Associations with bacterial symbionts (Rhizobium)
  that allow for the use of atmospheric nitrogen
• These plants incur the expense of (1)
  constructing root nodules (locations of
  symbiosis) and (2) providing bacterial symbionts
  with carbon compounds

2 Ammonia (NH3) molecules
Addition of 6 protons by nitrogenase enzyme
(reduction)
N2 (gas)
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• MYCORRHIZAL ASSOCIATIONS allow greater soil
  exploration, effectively increases absorbing
  surface area of the plant
• Endomycorrhizae fungus penetrates root tissue
• Ectomycorrhizae fungus forms a sheath over root
• Costs (C compounds) can be extensive
• 15 of total net primary production

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Figure 37.9 Carnivorous Plants
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Recall Nutrient availability in most
ecosystems is a function of recycling of
organic matter
Big Point Tight coupling of nutrient cycling
in an ecosystem and the functional diversity of
dominant plant species
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Responses to Environmental Challenges - Ecol 182
4-21-2005
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Photosynthetic Pathway Variation

• Focus until now has been on C3 photosynthesis
• C3 reflects the first intermediate of the Calvin
  cycle 3-phosphoglycerate
• CAM and C4 photosynthesis represent alternative
  physiological syndromes.
• Evolution of variation in biochemistry of
  photosynthesis has important ecological
  implications
• Resulting from the inefficiencies of RUBISCO

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RUBISCO and Photorespiration

• Rubisco is both a carboxylase and an oxygenase,
  adding either CO2 or O2 to RuBP.
• These two reactions compete with each other.
• O2 competitively inhibits CO2 fixation.
• RuBP O2 phosphoglycolate 3PG.
• Glycolate diffuses into peroxisomes, where it is
  converted to glycerate and CO2
• Photorespiration uses the ATP and NADPH produced
  in light reactions, but releases CO2.
• Photorespiration rate is a function of CO2
  O2.
• O2 becomes high when stomata close.

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Photosynthetic Pathway Variation

• C3 CO2 fixation by Rubisco PGA
  (phosphoglycerate a 3 carbon sugar) is the
  product of initial carboxylation
• C4 CO2 fixation by PEP carboxylase to produce
  OAA (oxaloacetate a 4 carbon acid) transport
  within the leaf, decarboxylation and re-fixation
  by Rubisco.
• CAM (crassulacean acid metabolism) CO2 fixation
  by PEP carboxylase to OAA, but results in the
  production of malic acid in the vacuole during
  the day and de-carboxylation and fixation at
  night

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C3, C4 and CAM

• C4 photosynthesis results in CO2 concentrating at
  the site of carboxylation, by a spatial
  specialization of biochemistry Kranz Anatomy
  (Bundle Sheath Design)
• CAM photosynthesis results in CO2 concentration
  at the site of carboxylation by a temporal
  specialization of biochemistry

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Spatial separation of activity C4 Photosynthesis
Mesophyll CO2 concentration 100 mmol mol-1
Outside leaf CO2 Conc. 370 mmol mol-1
Bundle Sheath CO2 concentration 2000 mmol mol-1
C3
C3
PCR
CO2
PEP
CO2
CO2
C4
CHO
C4 (mal)
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Temporal separation of activity CAM
photosynthesis
Dark CO2 concentration 150 mmol mol-1
Light CO2 concentration 4000 mmol mol-1
CHO
CHO
PEP
PCR
CO2
CO2
C3
C4 (mal)
CO2
C4
Acidification of Vacuole
Mal
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Figure 40.14 Stomatal Cycles
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Functional consequences of C4 and CAM

• Light-use efficiency
• Water-use efficiency
• Nitrogen-use efficiency

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Light-use efficiency
At temperature optima
C4
C3 in the absence of photorespiration
Leaf Photosynthetic rate
C3
Photosynthetic Photon Flux Density (PPFD) (light
intensity)
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Light-use efficiency
C3 where photorespiration is negligible
C4
QUANTUM YIELD OF PHOTOSYNTHESIS
C3
TEMPERATURE (10 TO 40oC)
Quantum yield of photosynthesis is initial slope
of the light response curve (previous slide)
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Water-use efficiency

• Water-use efficiency Photosynthesis /
  Transpiration
• C4 and CAM photosynthesis change the denominator
  (photosynthetic activity - increase CO2 flux)
• Secondarily, reductions in transpiration occur
  from
• Reductions in stomatal conductance
• Temporal dynamics of stomatal behavior
• Consequences
• C3 plants 1 gram biomass per 650-800 grams
  water transpired
• C4 plants 1 gram biomass per 250-350 grams
  water transpired
• CAM plants 1 gram biomass per 100-200 grams
  water transpired

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Nitrogen-use efficiency

• Rubisco in C3 plants represents up to 30 of
  total leaf N (and sometimes up to 50)
• C4 plants have 3 to 6 times less Rubisco than C3
  plants resulting in a smaller N requirement in
  leaves
• (120-180 mmol N m-2 and 200-260 mmol N m-2
  respectively)
• CAM is more variable in N requirements and
  investmentsbut succulence is such a different
  morphological characteristic

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Ecological consequences - separation of seasonal
activity of C3 and C4 species

• Tall-grass prairie (mixed C3 C4)
• Early versus late season separation, substantial
  competition

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Ecological consequences - separation of seasonal
activity of C3 and C4 species

• Mojave desert (dominated C3)
• Infrequent summer water availability (El Niño
  years)

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Ecological consequences - separation of seasonal
activity of C3 and C4 species

• Deserts with bimodal rainfall (e.g., Sonoran)
• Two distinct floras (mixed C3 C4)

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Ecological consequences - separation of seasonal
activity of C3 and C4 species

• Deserts with summer only rainfall (e.g.,
  Chihuahuan)
• C4 flora only (or nearly)

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Evolutionary pressures leading to C4 CAM

• Seems logical that selection for increased
  water-use efficiency or nitrogen-use efficiency
  would be the main selective pressures

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Factors favoring C4 evolution

• Low CO2 concentrations (low CO2/O2)
• Starting around 40 million years ago
• Increased photorespiration in C3 plants
• Secondarily
• Higher day temperatures
• Limited water

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Evolution of CAM

• Selection favoring maximizing carbon assimilation
• Extending the period of assimilation when
  availability of daytime CO2 is low cycling and
  epiphytes
• Predictability of water availability?
• Succulence as an afterthought? Ultra-structural
  constraint?
• Increasing stress tolerance and recovery?

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Combinations of photosynthetic strategies

• Mesembryanthemum crystallinum
• (ice plant a halophyte)
• Induction of CAM during drying
• Maintenance of photosynthetic rate

• Quillwort Isoetes howelii
• Vernal pool plant
• CAM, but switches to C3 when pool dries out

• Aquatic grass Orcuttia californica uses C4,
  with submerged and aerial leaves

• Aquatic plant Eleocharis acicularis uses C4
  when submerged, shifts to C3 when exposed to the
  atmosphere

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General framework for thinking about how
photosynthesis and stress interact in different
environments
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• Circles evergreen leaves
• Squares deciduous
• Triangles succulent
• Primary life history / physiological strategies
  for desiccation
• Tolerance - evergreen
• Avoidance - deciduous
• Escape annuals / succulent

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Figure 40.7 Desert Annuals Evade Drought
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Figure 40.9 Opportune Leaf Production
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General framework for thinking about how
photosynthesis and stress interact in different
environments
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Seasonality of Water-use niche partitioning,
facilitating coexistence
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Organ pre-formation and plant size which
growth form above benefits from increases in size
(with respect to reproductive output)?