soils and mineral nutrition -...
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Essential Elements
• Original research don by Julius von Sachs 1860 using hydroponics
– Using various solutions found ones that supported plant life –Sachs found several elements that were needed in relatively large amounts and called these major or macro essentialelements – those found to be needed in very small amounts were called minor, micro essential, or trace elements
– Criteria for Essentiality
• The element must be necessary for complete, normal plant development through the life cycle
• The element itself must be necessary, and no substitute can be effective
• The element must be acting within the plant not outside it
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Mineral Deficiency Diseases
• Causes of Deficiency Diseases
– Desert soils
• Osmotic drought• Some plants adapted to absorb both water and solutes – eventually
secretes minerals out via glands on leaves• Other plants precipitate excess salts as crystals
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Mineral Deficiency Diseases
– Serpentine Soils
• Those deficient in calcium
• If a plant is sensitive the these low amount it usually becomes diseased or is out competed by other plants
– Non-native Plants, Crops or Ornamentals
• Selected for high yields
• Require nitrogen and other mineral enrichment (fertilization)
• Harvesting crops – What effect does this have on soil nutrient content?
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Mineral Deficiency Diseases
• Symptoms of Deficiency Diseases –
– Irrespective of the element most plants demonstrate chlorosis (yellowing of the leaves, leaves become brittle and dry)
– Nitrogen and or phosphorous deficiency causes leaves to become brown or purplish with the accumulation of xanthocyanins
– Some leaves demonstrate necrosis, dead patches of cells
• Potassium deficiency leads to dead leaf tips and margins
• Manganese deficiency leaf tissue between veins dies
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Mineral Deficiency Diseases
• Mobile and Immobile Elements – problem seen in parts of plants of different ages – for some once deposited in plant tissue cannot move to other parts of the plant
– Immobile Elements – boron, calcium, iron
• Once picked up by plant are incorporated into plant tissues
• Plants parts being produced at the time utilize these elements
• Eventually elements are depleted from the soil
• Once elements are removed from the soil older parts of the planthave none to use
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Mineral Deficiency Diseases
– Mobile Elements – chlorine, magnesium, nitrogen, phosphorous, potassium, and sulfur
• Once depleted from soil older leaves give up their nutrients to younger leaves
• What is the adaptive significance of this?
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Soils and Mineral Availability
• Two important aspects of rock
– Cyrstalline in nature - if essential elements are part of the crystal structure then they are not available to the plant
– Water can be trapped in the crystal and made unavailable
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Soils and Mineral Availability
• Weathering of rock produces soil
– Physical Weathering
• Breakdown of rock via physical action
– Wind– Water movement– Temperature changes
• Produces soil particles of different sizes
– Coarse sand – 2 - .2 mm– Fine sand - .2 - .02 mm– Silt - .02 - .002 mm– Clay - <.002 mm, form micelles
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Soils and Mineral Availability• As seen earlier texture effects water and mineral holding capacity
– Sands very well aerated
– Water passes through them too easily giving it a poor field capacity
– Chemical Weathering
• Typically the result of acids produced via decay
– Organic acids
– Carbon dioxide
– Where warmth, moisture and large amounts of decaying matter weathering takes place very rapidly producing – releases bound nutrients
• Not only decreases particle size but also changes soil chemistry
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Soils and Mineral Availability» Positive ions are released leaving negatively charged soil particles behind
» In sands the surface area to volume ratio is small but in smaller particles this has a greater effect – positively charged ions can bind to the particles helping the soil retain these nutrients rather than having them be leached away
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Soils and Mineral Availability
• Cation Exchange
– Cations are bound to soil particles and must the removed for use
– Roots and root hairs via respiration produce carbon dioxide –Results in the production of what?
– Acids from other sources also aid cation exchange
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Soils and Mineral Availability
• Soil Acidity – important in
– Cation exchange
– Retention of cations in the soil
– Solubility of certain elements
• Acidic soils – following minerals are soluble, can reach toxic levels
– Aluminum– Manganese
• Alkaline soils – certain minerals become insoluble and unavailable
– Iron– Zinc
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Soils and Mineral Availability
– On average best pH between 6.5 – 7.0
– Certain plants have adapted to particular pH’s
• Endodermis and Selective Absorption of Substances
– Once again, substances can enter the cells of roots by passing through the cell membranes of the cells (selectivity of membraneplays a roll) and via the apoplast (no control)
– What is the control of substances entering the vascular tissues of roots for transport?
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Soils and Mineral Availability• The presence of the Casparian strip of the endodermis prevenst
free passage from the apoplast – substances must pass through the cell membranes that are highly selective
• Mycorrhizae and the Absorption of Phosphorous
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Nitrogen Metabolism
• Nitrogen for the most part exists as a gas, N2, and is fairly non-reactive – for organisms to use it it must be converted to an active form – three methods
– Nitrogen Fixation – conversion of nitrogen to nitrate, nitrite or ammonium
• Man can produce nitrate or ammonium from nitrogen industrially –expensive, energy intensive
• Natural methods
– Lightning
• Bacterial Fixation
– Enzyme nitrogenase converts N2 into NH3 (ammonia) once in the cell gains a proton and becomes ammonium NH4
+ - enzyme is very sensitive to, and reacts with oxygen making it ineffective
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Nitrogen Metabolism– Nitrogen Fixing microorganisms
» Free living in soil, contribute nitrogen to soil upon their death - Nostoc(cyanobacterium) other bacteria, Azobacter, Clostridium, Kelbsiell
» Symbiotic bacteria – best known Rhizobium – as mentioned earlier assists in pioneering plants
– Nitrogen Reduction – typically nitrogen is available to roots in the form of nitrate (the result of microorganisms oxidizing ammoniumfor energy) – as a result plants must reduce nitrate back to ammonium for their use – to do this they must
• Donate 8 electrons for each nitrogen atom – this requires a large amount of energy – two steps
– Step 1 -
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Nitrogen Metabolism– Step 2 – nitrite is reduced to ammonium
» Not well understood» Requires NADPH and NADH – can’t be used to make ATP
– Nitrogen Assimilation – using of ammonium to make organic molecules
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Storage of Minerals in Plants
• For the most part in plants minerals are stored in solution within the large central vacuole of plant cells
• Seeds differ in that they must be light weight, while still possessing the nutrients necessary to produce an entirely new plant
– Stores amino acids by crystallizing them into a structure calledthe protein body
• Protein body may also contains crystals of a substance called phytin, which stores cations such as Mg2+, Ca 2+, Zn2+ and K+, which it donates upon germination