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Chapter 12 & 13Transport, Soil and
Mineral Nutrition
Topics
Methods of transport Xylem transport Phloem transport Soils properties and nutrient absorption Macro and micro essential nutrient elements Too much or too little nutrients Mobile or immobile nutrients within the plant Deficiency symptoms Special adaptations in N-poor soils Plant mineral storage
Short distance - Diffusion, Osmosis, and Active Transport
Diffusion = random movement of particles from areas of high concentration to low concentration
Diffusion of water through a selectively permeable membrane = osmosis selectively permeable membranes allow only certain
substances to pass through water molecules pass through all membranes, but pass
more rapidly if the membrane has protein channels called aquaporins
To move molecules against their gradient, energy (via ATP) is necessary - this is active transport
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Water Potential - Water has free energy, capacity to do work,
chemical potential Chemical potential of water = water potential
(symbolized as ) When water adheres to a substance, the water
molecules form hydrogen bonds with the material and are not as free to diffuse as are other water molecules
So, waters capacity to work has decreased when in solutions
Water moves from higher to lower
Guard Cells For guard cells to open, K+ are actively transported
from surrounding cells into them Guard cell becomes more negative and the adjacent cells
become less negative; results in a net movement of water into the guard cell
Guard cells become turgid and swollen, bending and opening the pore due to uneven thickening of guard cell wall
Once open, pumping stops and water movement brings guard cells and adjacent cells into water potential equilibrium, and net water movement stops
Guard Cells The process is reversed for the
stomatal pore to close
Guard cells of fully opened and fully closed stomata are both in equilibrium with surrounding cells, even though they all have different internal conditions
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Control of Water Transport - Guard Cells
Numerous mechanisms have evolved that control stomatal opening and closing
If the leaf has an adequate moisture content, then light and carbon dioxide are the normal controlling factors Blue light triggers stomatal opening Decrease in internal carbon dioxide concentration may
lead to stomatal opening Decreased air humidity - high wind - may close stomata
partially High T leads to stomatal closure e.g. CAM plants
Control of Water Transport - Guard Cells
These mechanisms in healthy plants are completely overridden by a much more powerful mechanism triggered by water stress
Roots under water stress synthesize hormone, abscisic acid (ABA) transported to leaves, which immediately causes guard cells to close the stomatal pore (ABA is synthesized by apical buds and senescing tissues too)
In water stress - pores are closed even under blue light and low concentrations of CO2
Long-Distance Transport: Xylem
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Transpiration loss of water vapor mainly thru stomata for nutrient uptake and cooling
Casparian strip forcesselective absorption of solutes (keep unwanted solutes out) and help hold water in xylem
Transpiration generates tension on soil-plant-atmosphere water path water flows along water potential gradient
XylemTransport Tensioncohesionadhesionmodel
Leaf= 1.5 MPa
Atmosphere= 80 MPa
Stem= 0.7 MPa
Rootxylem Root= 0.4 MPa
Soil water= 0.1 MPa
Coleus Plant
Active loading by STM/CC/P complex, and polymer trapping in STM at source/leaf greater sugar conc. In STM waterabsorption from xylem increased turgor pressure mass flow toward sink -active and passive unloading in sink along pressure gradient pressure flow hypothesis by Ernst Mnch for phloem transport
Sieve tuberunning throughlength of plant
XYLEM PHLOEM
Companion cellSieve tube element
Direction ofwater movementDirection ofsucrose movement
Phloem transport - Mnch Pressure Flow hypothesis
Soil has both abiotic (chemical + physical) and biotic properties - minerals, water, air, T, flora and faunaRight soil is crucial for plants
Supplies minerals Holds water Supplies air, T to roots Acts as a matrix that
stabilizes plants Harbors nitrogen-fixing
bacteria, mycorrhiza, other microbes
Animals for plants
Soils and Plants
SoilParticles
Sizerange(mm)
Sand,Coarse
2.0 0.2
Sand,Fine 0.2 0.02
Silt 0.02 0.002
Clay(micelles)
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Soils and Mineral Availability: CE CO2 from root respiration reacts with soil water to produce
carbonic acid H+ from carbonic acid disrupt cations from soil micelle
(negatively charged mineral/clay matrix or organic matter) Roots cannot absorb cations directly from micelle cation
exchange is crucial
Essential Elements Research in mineral
nutrition involves growing the plant in hydroponic solution in which the chemical composition is carefully controlled e.g. except one element see picture
Elements that are necessary for plant growth = essential elements/nutrients
Essential Elements Macro - needed in large
amounts Micro - needed in smaller
amounts
Criteria for essentiality Must be needed for normal
plant development through a full life cycle
No substitute can be effective Must be acting within the plant,
not outside it
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Too Much
Salty regions - some excrete salt from salt glands on leaves
Desert soils sometimes too much minerals too alkaline too negative water potential
Toxicity caused by elevated levels of single minerals: Aluminum toxicity in acid soils High levels of heavy metals on
mine tailings, polluted soils
Too LittleSome soils - low concentrations of certain essential elements - plants are unable to thrive on them
Deficiency diseases are most commonly encountered in
crop plants or ornamentals
Harvesting crops leads to soil depletion
Fruits, seeds, tubers, and storage roots often have the
greatest concentration of minerals in a plant
Symptoms of DeficiencyOne symptom common in many elements = chlorosis
Leaves lack chlorophyll, tend to be yellowish, and are often brittle and papery
Deficiencies of either nitrogen or phosphorus cause accumulation of anthocyanin - coloration
Leaves become dark green or purple
Lack of potassium or manganese causes necrosis Patches of tissue die
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Mobile and Immobile Elements Chlorine, magnesium, nitrogen, phosphorus (picture
below), potassium, and sulfur - mobile elements After been incorporated into a tissue, they can still be
translocated to younger tissue If soil is exhausted - salvaged and moved to growing regions
Mobile and Immobile Elements Boron, calcium, and iron (picture below) are
immobile elements They remain in place after being incorporated into
plant tissue. In deficient soils - newer tissues show symptoms
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Nitrogen from Animals Soils in bogs and swamps have very little nitrogen
available because of nitrifying and denitrifying bacteria
Many bog-adapted, carnivorous plants get reduced nitrogen by catching animals
HydnophtumA mutualistic ant plant
Storage of Minerals within PlantsAll plant parts (except seeds) store minerals in soluble form in central vacuoles
Nitrogen is converted to compounds with multiple amino groups
Phosphates, sulfates, and other mineral nutrients - simply sequestered in the same forms in which they are used
Seeds store minerals as polymerized forms, usually in protein bodies