Watercolor Pansy

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Nutrient Acquisition and Assimilation
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• Nutrient acquisition (or uptake) is the net
  movement of nutrients from the soil into the
  plant. This can be separated into two steps
  movement through the soil to the root, and
  transport (active or passive) into plant cells.
• Nutrient assimilation is the incorporation of
  mineral nutrients into organic compounds
  (including pigments, enzyme cofactors, lipids,
  nucleic acids, amino acids, etc.)

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(primarily)
(primarily)
From Taiz and Zeiger, Plant Physiology
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Ion concentrations in solution vs. root sap of
beans and maize grown in aquaculture illustrate
three important principles of mineral nutrition
1) Selectivity of ion uptake, 2) Accumulation of
ions, 3) genetic differences
From H. Marschner, 1986, Mineral Nutrition of
Higher Plants, p. 8

• How does this table illustrate selectivity? How
  does it show and selectivity?
• Which nutrients show the greatest accumulation
• Where are there genetic differences

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Nitrogen, Potassium and Phosphorus (N, P and K)
are usually the most important nutrients listed
in fertilizers. Why not Ca and Mg, given that
they are a much bigger component of plant biomass
than P?
(primarily)
From Taiz and Zeiger , Plant Physiology
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Important principle The rate of nutrient
acquisition and assimilation must increase in
proportion to the rate of growth in order to
maintain the concentration of nutrients in plant
biomass.
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In natural systems, most nutrients in plants are
available through ecosystem recycling this is
especially true for N
From Lambers, Chapin, Pons p. 241
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• in woody perennials, plant mobile nutrients
  (N, K, Mg) are retranslocated from leaves into
  storage before leaf senescence. The storage
  can be in other foliage (in evergreen plants), in
  bark or wood (especially in ray parenchyma
  cells), and in roots. The need for these
  nutrients for new growth can be largely met
  through internal recycling, especially in
  nutrient poor soils.

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• Some Implications
• 1. Removing all or most of the live biomass from
  an ecosystem can severely deplete the supply of
  plant-mobile nutrients this is especially true
  for nutrients that have low natural supply
  rates (like nitrogen, in most ecosystems)
• 2. Disturbance without removal of live biomass
  can lead to a flush of plant-mobile nutrients

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Nutrient acquisition is determined by- the
rate of mineralization in the soil- Nutrient
movement to the roots (soil mobility)-
Absorptive surface area (roots and mycorrhizae).
Well discuss this more in the 2nd half of
todays lecture.- Root membrane transport
processes
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Only a small fraction of the nutrient supply in
soil is dissolved in soil water (less than 0.2).
Most of the rest is bound in organic detritus,
humus, and relatively insoluble inorganic
compounds or is adsorbed onto soil colloids.
Implication the mineralization rate is strongly
coupled to biochemical properties and biological
activity in the soil. (But plants can have quite
a bit of influence over this).
See Larcher, 1995, The utilization of mineral
elements , in Physiological Plant Ecology
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Movement of nutrients in soil
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nutrients get to roots by three mechanisms

• Diffusion
• Mass Flow
• Root (or mycorrhizal) growth to intercept
  nutrients

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• The importance of diffusion vs. mass flow of
  nutrients in soils depends on the plants needs
  for the nutrient, the transpiration rate, and the
  diffusion coefficient for the nutrient

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diffusion coefficients in the soil are
similar to the concept of conductivity we
discussed earlier they relate the diffusion
velocity to the concentration gradient
From Lambers, Chapin and Pons p. 243
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The diffusivity of nutrients in the soil is also
described as soil mobility
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From Lambers, Chapin and Pons p. 243
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Nutrient availability in soil is strongly
dependent on pH (note that phosphorus has a
particularly narrow mobility range)
From Lambers, Chapin and Pons p. 240
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Nutrient depletion zones develop around roots
depending on the relative rates of nutrient
supply vs. plant uptake
Relative concentration of nutrient in soil
Increasing distance from root
Root surface
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Absorptive surfaces
Plants can enhance nutrient acquisition of
nutrients by increasing fine root proliferation,
root hairs, or mycorrhizae. These mechanisms are
especially important for nutrients with low
diffusion coefficients (e.g. phosphorus). Many
gymnosperms have no root hairs, but they all
regularly form mycorrhizal associations.
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Root hairs, and even fine roots, account for a
very small amount of total plant C at any point
in time, but they turn over very rapidly. So the
standing biomass of roots is a poor indicator
of the plants allocation below ground
Most of the fine roots of most forest trees are
concentrated in the upper 15 cm of soil (thats
where the nutrients are), although deep roots of
some species may penetrate many meters deep.
(a 16-year-old apple tree Pallardy, Physiology
of Woody Plants, 2008, p. 28).
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Membrane transport
Up to this point weve talked about how nutrients
are made available at the surface of roots. But
how do nutrients enter roots against a
concentration gradient?
(you know it cant be by diffusion for most
nutrients, because the concentration in the roots
is higher than the concentration in the soil.
And the movement is against an electrochemical
potential for most anions. Nutrient uptake must
involve energy input if the movement is against a
concentration gradient)
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Nutrient uptake by roots is influenced by
channels, carrier proteins, and pumps in cell
membranes.
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Most carrier proteins are inducible protein
synthesis increases when supply of the nutrient
in the plant or in the soil is low BUT for
plants adapted to low nutrient soils, uptake rate
of immobile nutrients (PO4-2, K) is low. These
plants are inherently slow growers.
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Nutrient Assimilation
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• N and S assimilation -- nitrate and sulfate are
  first reduced, then incorporated into amino acids
  (very energy demanding!)
• Or N is symbiotically fixed (key enzyme is
  nitrogenase).
• Phosphorus is readily incorporated into organic
  material directly as phosphate
• Cations (Ca, K, Mg etc.) easily form complexes
  with organic molecules

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Common inorganic forms of nitrogen N2 (N
N) di-nitrogen (gas) NH3 ammonia NH4 am
monium NO2- nitrite NO3- nitrate
Nitrogen fixation
nitrification
N-mineralization
Organic compounds
N-assimilation (in soil microbes this is called
immobilization)
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Plants take up N either as ammonium (NH4) or
nitrate (NO3-)
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Uptake of Ammonium vs. Nitrate

• Most crop plants and deciduous trees use nitrate
• Many coniferous trees (e.g., white spruce,
  lodgepole pine, western hemlock) and ericaceous
  shrubs use ammonium especially those that tend
  to grow in acid soils. Other conifers can use
  either ammonium or nitrate, but appear to grow
  better on nitrate (e.g., Douglas-fir, western
  redcedar)
• undisturbed coniferous forest ecosystems are
  generally high in NH4 and low in NO3-

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Ammonia (and the ammonium ion) is toxic to living
cells it destroys membrane potentials
To avoid the toxicity problem, if plants take up
ammonium, they quickly assimilate it into organic
material or convert it to NO3- for transport.
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• Nitrate is reduced to Nitrite (NO2-) by the
  enzyme nitrate reductase, and then nitrite is
  reduced to ammonium by the enzyme nitrite
  reductase (requires equivalent of 12 ATPs per N
  atom).
• Nitrate reductase activity is highly regulated
  stimulated by the presence of nitrate and (in
  leaves) by light and carbohydrates

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Nitrate can be assimilated into organic material
either in roots or leaves Some sources say that
N-reductase activity of woody plants is primarily
in roots, but actually many temperate species
have more N-reductase activity in leaves than in
roots.
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• Advantages to reduction of N in leaves vs roots
  
• in abundant light, reduction in leaves is more
  efficient (and, in fact, could utilize excess
  light energy! This is a form of photochemical
  quenching!)
• assimilation by roots prevents excessive
  build-up of NO3- in leaves

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Finally Ammonium is incorporated into amino
acids through the work of two important
enzymes, GS glutamate synthase. Produces the
di-amino acid glutamine from ammonium and
glutamate (glutamate is a simple amino
acid) GOGAT Glutamate synthase. Takes the
amino group back off of glutamine and adds it on
to a organic acid, oxoglutarate, to produce two
molecules of glutamate.