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Watercolor Pansy

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This can be separated into two steps movement through the soil to the root, ... amount of total plant C at any point in time, but they turn over very rapidly. ... – PowerPoint PPT presentation

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Title: Watercolor Pansy


1
Nutrient Acquisition and Assimilation
2
  • 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.)

3
(primarily)
(primarily)
From Taiz and Zeiger, Plant Physiology
4
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

5
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
6
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.
7
In natural systems, most nutrients in plants are
available through ecosystem recycling this is
especially true for N
From Lambers, Chapin, Pons p. 241
8
  • 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.

9
  • 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

10
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
11
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
12
Movement of nutrients in soil
13
nutrients get to roots by three mechanisms
  • Diffusion
  • Mass Flow
  • Root (or mycorrhizal) growth to intercept
    nutrients

14
  • 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

15
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
16
The diffusivity of nutrients in the soil is also
described as soil mobility
17
From Lambers, Chapin and Pons p. 243
18
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
19
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
20
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.
21
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).
22
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)
23
Nutrient uptake by roots is influenced by
channels, carrier proteins, and pumps in cell
membranes.
24
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.
25
Nutrient Assimilation
26
  • 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

27
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)
28
Plants take up N either as ammonium (NH4) or
nitrate (NO3-)
29
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-

30
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.
31
  • 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

32
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.
33
  • 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

34
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.
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