Plant Structure and Function II - Ecol 182

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Plant Structure and Function II - Ecol 182
4-19-2005
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This angiosperm adapted to steam-side conditions
cannot conduct water in the presence of any
drought
This gymnosperm can continue to conduct water
under extreme drought
This angiosperm is slightly more drought tolerant
than the streamside tree
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Mechanisms of cavitation

• Desiccation-induced vulnerability to cavitation
  is a function of air entry from pit membrane
• size and number of pits becomes the important
  traits
• Freeze-thaw induced vulnerability occurs due to
  insoluble gases in sap that form bubbles under
  repeated low temperature conditions
• Differences in xylem diameter is important

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Ring Porous Trees Vessels confined to spring
wood embolized vessels cannot be re-filled
water transport is dependent upon new spring wood
construction Diffuse Porous Trees Vessels occur
uniformly throughout the annular ring re-filled
can occur over the winter
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So..water transport

• Vessel diameter and pit membrane density
• (why do North American desert species tend to
  have both reduced vessel diameter AND pit
  membrane density?)
• 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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• Importance of water potential components
• Positive pressure in a cell (turgor) allows for
  expansion

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How tall can trees become? the importance of
components of leaf water potential Coastal
Redwoods
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How tall can trees become? the importance of
components of leaf water potential Coastal
Redwoods
Variation in leaf form has been hypothesized to
be a function of light environment
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How tall can trees become? the importance of
components of leaf water potential. Minimum
Turgor required for growth is 2 MPa (which is
compensated for at about 125 m)
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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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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

• 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

• Pressure flow model of transport
• Source sieve tube cells have a greater sucrose
  concentration that surrounding cells
• water enters by osmosis.
• causes greater pressure potential at the source,
  so that the sap moves by bulk flow towards the
  sink.
• Sucrose is unloaded actively at the sink,
  maintaining the solute and water potential
  gradients.

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Table 36.1 Mechanisms of Sap Flow in Plant
Vascular Tissues
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The Acquisition of Nutrients

• All living things need raw materials from the
  environment.
• Carbon derives from CO2 in the air
  (photosynthesis).
• Hydrogen comes from water.
• Carbon, oxygen, and hydrogen are fairly
  plentiful.
• Nitrogen is in relatively short supply for
  plants.
• Nitrogen enters living forms first in bacteria,
  which can convert N2 in air to forms that are
  useful to plants.
• Other mineral nutrients essential for life
  include sulfur, phosphorus, potassium, magnesium,
  and iron.
• Plants take up most nutrients as dissolved
  solutes in the water of the soil, the soil
  solution.

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Nutrient classification Table 37.1

• 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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Soils and Plants

• The structure of many soils changes with depth,
  revealing a soil profile.
• Most soils have two or more horizontal layers,
  called horizons.
• Minerals tend to leach, or be carried away by
  water from the upper horizons, and sink into
  deeper horizons.
• Soil scientists recognize three major horizons
• A, the topsoil
• B, the subsoil
• C, the parent rock

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Figure 37.3 A Soil Profile
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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)
• 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 vs. diffusion nutrient delivery

• Nutrient uptake is a function of BOTH plants
  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

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Figure 37.4 Ion Exchange
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Nitrogen acquisition

• Nitrogen is the nutrient that plants require in
  the greatest quantity
• N 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 (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

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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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Figure 37.9 Carnivorous Plants
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• Variation in Acquisition a cost / benefit
  function of availability
• Assimilation costs
• 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
• Advantages of each strategy shift with the
  availability of the different nitrogen forms
• Advantages shift with the varying limitations by
  water, carbon and nitrogen

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Integrating nitrogen acquisition into a
whole-plant function perspective

• 75 of leaf N is located within chloroplasts
  (most in PSN function)
• 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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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