Plant Structure and Function I - Ecol 182

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Plant Structure and Function I - Ecol 182
4-14-2005
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The Angiosperms Flowering Plants

• A number of synapomorphies, or shared derived
  traits, characterize the angiosperms
• They have double fertilization (upcoming figure).
• They produce triploid endosperm.
• Their ovules and seeds are enclosed in a carpel
  (modified leaf).
• They have flowers (modified leaves).
• They produce fruit (at minimum mature ovary and
  seed).
• Their xylem contains vessel elements (specialized
  H2O transport) and fibers (structural integrity).
• Their phloem contains companion cells (assists
  with metabolic issues associated with transport).

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Angiosperm vascular systems

• Xylem in angiosperms consists of vessel elements
  in addition to tracheids
• Vessel elements also conduct water and are formed
  from dead cells.
• Vessel elements are generally larger in diameter
  than tracheids and are laid down end-to-end to
  form hollow tubes.
• Sieve tube elements (Phloem) in Angiosperms are
  stacked, similar to xylem
• Have adjacent companion cells that retain all
  organelles
• Companion cells may regulate the performance of
  the sieve tube members through their effects on
  active transport of solutes

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Figure 35.10 Evolution of the Conducting Cells
of Vascular Systems
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Figure 35.11 Sieve Tubes
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Angiosperms Flowering Plants

• Monocots - a single embryonic cotyledon (grasses,
  cattails, lilies, orchids, and palms)
• Eudicots - two cotyledons, and include the
  majority of familiar seed plants
• Additional clades - water lilies, star anise, and
  the magnoliid complex
• Big question in plant evolution what is the
  basal angiosperm?

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Plant Structure and Function

• Uptake and Movement of Water and Solutes
• Transport of Water ( Minerals) in the Xylem
• Transpiration and stomatal regulation of
  water-loss (use)
• Translocation of Substances in the Phloem

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General problem of water in plant function

• Need for H2O for
• photosynthesis,
• Solute transport,
• temperature control,
• internal pressure for growth
• Plants obtain water and minerals from the soil
  via the roots
• in turn roots extract carbohydrates other
  important materials from the leaves.
• Water enters the plant through osmosis
• but the uptake of minerals requires transport
  proteins.

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Plant function in the context of the
soil-plant-atmosphere continuum
Plants bridge the steep potential energy gradient
between the soil and the air use it as a
mechanism for water and solute transport
But.
- The soil is not an endless supply of water!

• Compromises between biomechanics, size and
  growth rate
• set the stage for catastrophic loss of water
  transport

• Decreases in leaf water content result in stress
  that does
• not allow for growth and may result in
  mortality

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Uptake Movement of Water Solutes in Plants

• Osmosis is the diffusion of water through a
  membrane primary means of water transport in
  plants
• Water movement across a membrane is a function of
  osmotic potential, or solute potential.
• Potential refers to the potential energy
  contained in the system measured
• Dissolved solutes effect the concentration of
  water (changing the potential energy).
• Greater solute concentration results in a more
  negative solute potential and a greater the
  tendency of water to diffuse to the solution.

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Figure 5.8 Osmosis Modifies the Shapes of Cells
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Uptake Movement of Water Solutes in Plants

• Water potential is the tendency of a solution to
  take up water from pure water (Y).
• Water potential of a system is the sum of the
  negative solute potential (ys) and the (usually
  positive) pressure potential (yp) along with
  other potentials.
• y ys yp
• Each component is measured in megapascals (Mpa).

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Figure 36.2 Water Potential, Solute Potential,
and Pressure Potential
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Figure 36.4 Apoplast and Symplast routes of
water movement from the soil into the plant
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Figure 36.5 Casparian Strips
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Transport of Water and Minerals in the Xylem

• The adhesion-cohesiontension theory of water
  movement
• Water vapor concentration is greater inside the
  leaf than outside, so water diffuses out through
  stomata
• (this is transpiration).
• Tension develops in the mesophyll drawing water
  from the xylem of the nearest vein into the
  apoplast surrounding the mesophyll cells
• Removal of water from the veins establishes
  tension on the entire volume of water in the
  xylem, so the column is drawn up from the roots.

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Figure 36.8 The TranspirationCohesionTension
Mechanism

• Hydrogen bonding results in cohesion (sticking
  of molecules to one another).
• The narrower the tube, the greater the tension
  the water column can stand.
• Maintenance of the water column also occurs
  through adhesion of water molecules to the walls
  of the tube.

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Transport of Water and Minerals in the Xylem

• The key elements in water transport in xylem
• Transpiration
• Tension
• Adhesion / Cohesion
• The adhesioncohesiontension mechanism does not
  require energy.
• At each step, water moves passively toward a
  region with a more negative water potential.
• Mineral ions in the xylem sap rise passively with
  the solution.
• Transpiration also contributes to the plants
  temperature regulation, cooling plants in hot
  environments.

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Figure 36.9 A Pressure Bomb
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Why is there a disconnect (temporally) between
leaf, root and soil?
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Short and long-term responses to water limitation
When water is withheld the pressure potential
of the cells declines (hours to days) and rates
of cell expansion are reduced (long-term).
-Rates of photosynthesis declines (stomata close-
short). -New leaves are smaller, with smaller
cells (long). -Profound change in patterns of
allocation (long).
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Regulation of Transpiration by Stomata

• Leaf and stem epidermis has a waxy cuticle that
  is impermeable to water, but also to CO2.
• Stomata, or pores, in the epidermis allow CO2 to
  enter by diffusion.
• Guard cells control the opening and closing of
  the stomata.
• Most plants open their stomata only when the
  light is intense enough to maintain
  photosynthesis.
• Stomata also close if too much water is being
  lost.

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Figure 36.11 Stomata (Part 2)

• Stomatal aperture is regulated by controlling K
  concentrations in the guard cells.
• Blue light activates a proton pump to actively
  pump protons out of the guard cells. The proton
  gradient drives accumulation of K inside the
  cells.
• Increasing K concentration makes the water
  potential of guard cells more negative, and water
  enters by osmosis.
• The guard cells respond by changing their shape
  and allowing a gap to form between them.
• Abscisic acid (a stress hormone) can invoke
  this stomatal closure in addition to blue light
• Changes in guard-cell photosynthesis can also
  invoke this stomatal response

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Leaf temperature
VPD
CO2 demand
PPFD
Transpiration
Stomatal Conductance
Hydraulic resistance
Soil Y
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Types of stomatal responses

• Isohydric species control gas exchange such
  that daytime leaf water status is unaffected by
  soil water deficits. (Primarily responds to ABA)
• Anisohydric species exhibit decreases in leaf
  water potential proportional to changes in soil
  water potential. (responds to both ABA and Yleaf)

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Conceptual understanding of stomatal function

• Optimization theory (Cowan 1977) stomata work
  to optimize or maximize water exchanges for
  carbon dioxide
• Long-distance transport hypothesis Tyree and
  Sperry stomata regulate water loss to maintain
  long-distance water and nutrient transport
• Operate to avoid of catastrophic xylem
  dysfunction (cavitation), that occurs through the
  development of excessive tension.

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Cavitation or Embolism

• Breakage of the xylem water column
• Entry of air into the conduit
• Primarily through the pit membrane
• Large tensions in the xylem stream
• Species and individuals differ in their
  vulnerability to cavitation trade-offs produced
  relative to water flow rates

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Mechanisms of cavitation

• Desiccation-induced vulnerability to cavitation
  through air entry from pit membrane
• size and number of pits becomes the important
  traits
• EVEN THE WIDEST VESSELS IN RING-POROUS TREES ARE
  SUFFICIENTLY NARROW TO PREVENT BREAKING OF A
  WATER COLUMN.(Sperry 1995)
• Other mechanisms besides vessel diameter alone
  are important in determining drought stress
  tolerance

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Mechanisms of cavitation

• Freeze-thaw events dissolved gases in sap are
  insoluble in ice and form bubbles under repeated
  low temperature conditions
• DIFFERENCES IN CONDUIT DIAMETER CAN AFFECT THE
  POTENTIAL FOR GAS EMBOLISMS FORMING FROM GAS
  MOVING OUT OF SOLUTION

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Constraints on water transport when embolism
occurs differences in phenology and distribution
Ring Porous
Vessels confined to spring wood (uniform
distribution) Oaks (Quercus)
Emboli zed vessels cannot be re-filled water
transport is dependent upon new spring wood
construction (which tent to have large vessels)
Diffuse porous
Vessels occur uniformly throughout the annular
ring re-filling can occur over the winter.
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So..water transport

• Vessel diameter and pit membrane density
• (why do desert species tend to have both reduced
  vessel diameter AND pit membrane density?)
• Constraints from the interaction of water stress
  and temperature stress affect vulnerability to
  cavitation
• Implications for plant functional strategies and
  controls over the distribution of plants

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Think about this figure as a general example of
how soils and plants interact in all different
ecosystems
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Safety Margins
Mesic species with the ability to recover each
night operate close to the xylem tensions that
cause 100 cavitation Xeric species that do not
have that opportunity to recover operate with a
much larger safety margin
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Mesic habitats
Variation within and between species associated
with variation in PSN capacity, leaf N content,
leaf morphology/ontogeny
Variation between species associated with
adaptation to aridity
Range of leaf conductance
Arid habitats
Photosynthetic capacity
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Translocation of Substances in the Phloem

• Sugars, amino acids, some minerals, and other
  solutes are transported in phloem and move from
  sources to sinks.
• A source is an organ such as a mature leaf or a
  starch-storing root that produces more sugars
  than it requires.
• A sink is an organ that consumes sugars, such as
  a root, flower, or developing fruit.
• These solutes are transported in phloem, not
  xylem, as shown by Malpighi by girdling a tree.

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Figure 36.12 Girdling Blocks Translocation in
the Phloem
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Translocation of Substances in the Phloem

• Translocation (movement of organic solutes) stops
  if the phloem is killed.
• Translocation often proceeds in both directions
  both up and down the stem simultaneously.
• Translocation is inhibited by compounds that
  inhibit respiration and the production of ATP.

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Translocation of Substances in the Phloem

• Plant physiologists have used aphids to collect
  sieve tube sap from individual sieve tube
  elements.
• An aphids inserts a specialized feeding tube, or
  stylet, into the stem until it reaches a sieve
  tube.
• Sieve tube sap flows into the aphid. The aphid
  is then frozen and cut away from its stylet,
  which remains in the sieve tube.
• Sap continues to flow out the sieve tube and can
  be collected and analyzed by the physiologist.

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Figure 36.13 Aphids Collect Sieve Tube Sap
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Translocation of Substances in the Phloem

• There are two steps in translocation that require
  energy
• Loading is the active transport of sucrose and
  other solutes into the sieve tubes at a source.
• Unloading is the active transport of solutes out
  of the sieve tubes at a sink.

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Translocation of Substances in the Phloem

• Sieve tube cells at the source have a greater
  sucrose concentration that surrounding cells, so
  water enters by osmosis. This causes greater
  pressure potential at the source, so that the sap
  moves by bulk flow towards the sink.
• At the sink, sucrose is unloaded by active
  transport, maintaining the solute and water
  potential gradients.
• This is called the pressure flow model.

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Figure 36.14 The Pressure Flow Model
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Table 36.1 Mechanisms of Sap Flow in Plant
Vascular Tissues
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Translocation of Substances in the Phloem

• If the pressure flow model is valid, two
  requirements must be met
• The sieve plates must be unobstructed.
• There must be effective methods for loading and
  unloading the solute molecules.
• The first condition has been shown by microscopic
  study of phloem tissue.
• Mechanisms for loading and unloading the solutes
  exist in all plants.

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Translocation of Substances in the Phloem

• Sugars and other solutes produced in the
  mesophyll cells leave the cells and enter the
  apoplast.
• The solutes are then actively transported to
  companion cells and phloem tubes, thus reentering
  the symplast.
• The passage of solutes to the apoplast and back
  to the symplast allows for selectivity of solutes
  to be transported.

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Translocation of Substances in the Phloem

• Secondary active transport loads the sucrose into
  companion cells and sieve tubes.
• Sucrose is carried across the membrane by
  sucroseproton symport. For this symport to work,
  the apoplast must have a high concentration of
  protons.
• These protons are supplied by a primary active
  transport the proton pump.

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Translocation of Substances in the Phloem

• Many substances move from cell to cell within the
  symplast through plasmodesmata.
• The plasmodesmata participate in the loading and
  unloading of sieve tubes.
• Solutes enter companion cells by active transport
  and move into the sieve tubes through
  plasmodesmata.
• At sinks, plasmodesmata connect sieve tubes,
  companion cells, and the cells that will receive
  the solutes. Plasmodesmata in sink tissues are
  abundant and allow large molecules to pass.

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Nutrient classification

• Amount
• Macronutrients (H,C,O,N,K,Ca,Mg,P,S)
• Micronutrients (Cl,B,Fe,Mn,Zn,Cu,Mo)
• Function
• Constituents of organic material (C,H,O,N,S)
• Osmotic potential or contribute to enzyme
  structure/function (K,Na,Mg,Ca,Mn,Cl)
• Structural factors in methalloproteins
  (Fe,Cu,Mo,Zn)

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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 sources for plants

• Mineral nutrients in the soil
• 98 bound in organic matter (detritus), humus,
  and insoluble inorganic compounds or incorporated
  in minerals
• NOT DIRECTLY AVAILABLE TO PLANTS
• 2 is absorbed on soil colloids
• These are positively charged ions
• 0.2 is dissolved in the soil water
• Usually negatively charged, nitrates and
  phosphates

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Soil Colloids

• Ion exchangers
• Exchange capacity depends upon surface area
• Clay (montmorillonite) 600 800 m2 g-1
• Many humic substances 700 m2 g-1
• Retain charged substances (mainly cations, but to
  a lesser extent, anions)

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Soil Colloids

• Adsorptive binding of nutrient ions result in
• Nutrients freed by weathering and decomposition
  are collected and protected from leaching
• Concentration in soil solution remains low and
  constant
• Removes a potential osmotic effect
• Adsorbed nutrient ions are readily available to
  plants

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Nutrient uptake

• Conditions that affect nutrient content in the
  soil
• Soil texture (clay content)
• Soil organic matter content
• Soil water content (precipitation)
• Soil temperature

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Environments that tend to result in low nutrient
contents

• Sandy soils low clay content and thus
  inadequate exchange capacity
• High rainfall excessive leaching of nutrients
• Low rainfall inadequate soil moisture for
  organic matter decomposition
• Cold soils low decomposition low root
  respiration and thus low nutrient uptake
• Waterlogged soils inadequate oxygen for root
  respiration and decomposition

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Ion uptake by roots

• The rate at which nutrients are supplied to a
  plant depends on
• The concentration of diffusible minerals in the
  rooted soil strata
• Ion-specific rates of diffusion and mass
  transport
• Nitrate is fast and phosphate and potassium are
  slower (diffusivities)
• Ions of nutrient salts are taken up by a purely
  passive process
• Following the concentration and charge gradients
  between the soil solution and the interior of the
  root

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Mass flow versus diffusion nutrient delivery

• Nutrient uptake is a function of BOTH plants and
  soils and includes two processes (1) Mass Flow
  and (2) Diffusion
• Mass flow in soils is a rapid process, whereas
  diffusion is only measured in mm per day in soils
• Where mass flow is insufficient to satisfy plant
  demand, ion concentrations at the root surface
  are reduced below that of the surrounding soil
  volume
• Zones of depletion create concentration gradients
  that drives diffusional processes in the soil (as
  a function of soil water content)

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Nutrient Uptake

• Absorption of nutrient ions from soil solution
• NO3-, SO42-, Ca2, Mg2 (lt1000 mg l-1)
• K (lt100 mg l-1)
• PO42- (lt1 mg l-1)
• Exchange absorption of adsorbed nutrient ions
• Release of H and HCO3- as dissociation products
  of the CO2 resulting from respiration
• Mobilization of chemically bound nutrients
• H ions and organic acids, nutrients fixed in
  chemical compounds are liberated and form
  chelated complexes

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Nitrogen acquisition

• Nitrogen is the mineral nutrient that plants
  require in the greatest quantity and that most
  frequently limits growth in both agricultural and
  natural systems
• The carbon expended in acquiring nitrogen can
  make up a significant fraction of the total
  energy a plant consumes
• Plants have developed several approaches to
  nitrogen acquisition, including
• Root absorption of inorganic ions ammonium and
  nitrate
• Fixation of atmospheric nitrogen
• Mycorrhizal associations
• carnivory

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Nitrogen acquisition consists of

• Absorption bringing N from the environment into
  the plant
• Translocation moving inorganic N within the
  plant
• Assimilation converting N from inorganic to
  organic forms

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• Carbon costs for N absorption include
• Growth and maintenance of absorbing organs
  (usually roots)
• Transport of minerals against a concentration
  gradient
• Assimilation of N in leaves

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• Variation in Acquisition a cost / benefit
  function of availability
• Variation in N acquisition additional carbon
  costs for other absorbing organs
• NITROGEN FIXERS
• Some plants have developed associations with
  bacterial symbionts that allow for the use of
  atmspheric nitrogen
• These plants incur the expense of (1)
  constructing root nodules (locations of
  symbiosis) and (2) providing bacterial symbionts
  with carbon compounds

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• Variation in Acquisition a cost / benefit
  function of availability
• Variation in N acquisition additional carbon
  costs for other absorbing organs
• MYCORRHIZAL ASSOCIATIONS
• Associations with fungi that allow greater soil
  exploration
• Endomycorrhizae fungus penetrates root tissue
• Ectomycorrhizae fungus forms a sheath over root
• Effectively increases absorbing surface area
• Costs (carbon compounds) can be extensive 15
  of total net primary production in a Fir species

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• Variation in Acquisition a cost / benefit
  function of availability
• Assimilation costs
• N-FIXATION, MYCORRHIZAL ASSOCIATIONS and
  CARNIVORY
• Transfer N to host as amino acids
• Conversions
• Ammonium to amino acid 2 electrons and 1 ATP
• Nitrate assimilation 10 electrons and 1 ATP
• N-fixation 4-5 electrons and 7-10 ATP per
  nitrogen atom
• Costs increase from MYCORRHIZAE to CARNIVORY to
  AMMONIUM to NITRATE to NITROGEN FIXATION
• Fraction of carbon budget spent on nitrogen
  acquisition (absorption, translocation, and
  assimilation)
• 25-45 for ammonium
• 20-50 for nitrate
• 40-55 for N fixation
• 25-50 for mycorrhizae

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• Variation in Acquisition a cost / benefit
  function of availability
• Advantages of each strategy shift with the
  availability of the different nitrogen forms
• Advantages shift with the varying limitations by
  water, carbon and nitrogen
• Gaseous N is always abundant but has a high
  carbon cost
• In environments where N limits growth more than C
  or H2O, N fixation becomes advantageous (early
  successional sites)

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• Variation in Acquisition a cost / benefit
  function of availability
• Preference for ammonium versus nitrate
• Substantial species-specific variation
• Mixtures of ammonium and nitrate are requirements
  for many species flexible N acquisition plans?

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Nitrogen allocation

• 75 of leaf N is located within chloroplasts
  (most in PSN function)
• Recall, leaf age / leaf thickness /
  photosynthetic capacity / leaf nitrogen
  relationships

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Emergent patterns at ecosystem scale
Evergreen Forests
Productivity (g m-2 y-1)
Deciduous forests
N uptake (g m-2 y-1)
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Integrating nitrogen acquisition into a
whole-plant function perspective

• Processes / factors to consider
• Water-use
• Photosynthetic gas exchange
• Root shoot allocation
• Reproduction
• Stress tolerance
• Competition

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Whole plant integration function in the context
of life-history
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Nutrient Dynamics (outline and big deals)

• 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
• Complexity of cycling

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Big Point Tight coupling of nutrient cycling
in an ecosystem and the functional diversity of
dominant plant species
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Whole plant integration function in the context
of life-history
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Resource flow and growth rate

• Inputs of resources govern growth potential (not
  necessarily growth rate)
• Plants adjust allocation schedules to match
  resource supply rates (or loss rates) (e.g.,
  adjustment of sources and sinks)

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Theory of allocation

• Major assumption
• Finite supply of resources
• Distributed among
• Growth
• Maintenance / defense
• Reproduction
• Key to linking life history theory and physiology
  (the basis for ecophysiology)

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Relative conducting abilities of aboveground and
belowground structures surface area
characteristics Biomass is often used as a proxy
of this allocation of energy to function (both
surface exchange capacity and rates of
exchange) Compensatory changes in each exchange
surface result in different patterns of growth
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Trade-offs

• Resources are allocated among COMPETING functions
• Generates trade-offs
• Not always true
• Photosynthetic fruit
• Stems as biomechanical support
• Vegetative reproduction

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How do plants know that a neighbor is near?

• Reduction in resource availability?
• Reduction in PAR
• Reduction in nutrients, etc.
• Cryptochrome and phytochrome pigments
• perceive red / far-red ratios of radiation

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Phytochromes

• Plants growing in closed-spaced rows or high
  densities receive lower red / far-red ratios than
  sparse populations
• Red light is absorbed to a greater extent by
  plant tissue than far-red light
• Reductions in R / fR promote stem growth (height)
• Species specific

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How do plants sense their neighbors?

• Chemical signals
• Jasmonate herbivory induced and may influence
  neighbors (illicit a defensive response)
• Ethylene often a senescence inducing hormone

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How do plants sense their neighbors?

• Microclimate manipulation
• Differential heat exchange
• Eucalyptus seedlings surrounded by grass see a
  lower minimum air temperature inducing stress
• Belowground interactions (black box!)

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Interactions among species

• From a physiological viewpoint .to understand
  the mechanistic basis for patterns
• Competition
• Occurs between individuals using a common
  resource pool
• similar to our understanding of allocation
  dynamics within a plant

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Theories of competitive mechanisms

• Phillip Grime (1977) relative growth rate
  defines competitive potential
• High RGR facilitates rapid growth and allows a
  species to dominate space and acquire resources

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Theories of competitive mechanisms

• David Tilman (1988) ability to tolerate the
  drawdown of resources to some critical level
• Species that can reduce a critical resource to a
  level not tolerated by neighbors is competitively
  superior
• Described by R
• R is the level at which growth matches loss for
  any given speciesit also varies by species

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Theories of competitive mechanisms

• Tilman and Grime do not present competing
  hypotheses, but each has slightly different
  implications
• Dependence on stable-state dynamics
• Resource levels
• Species composition

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Resource competition

• Depletion of a shared limiting resource occurs
  by
• A species effectively removing the resource from
  the environment
• A species tolerating relatively low resource
  environments
• The physiological underpinnings of these two
  strategies are quite different but as a result of
  physiological trade-offs, these strategies may be
  highly correlated

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Trade-offs

• Two major physiological trade-offs
• Between rapid growth to maximize resource
  acquisition versus resource conservation through
  reductions in tissue turnover (recall Chapin N
  figure)
• Between allocation to roots to acquire water and
  nutrients versus allocation to shoots to capture
  light
• Because of these trade-offs, there are not
  competitively superior species for all
  environments

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What keeps a species from dominating an
environment

• Some environments are dominated by a single
  species
• Some environments have significant environmental
  heterogeneity that influence the costs / benefits
  of these trade-offs
• This influences competitive ability