Plant Physiology

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Plant Physiology

• Mineral Nutrition

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Mineral Nutrition in plants

• Plants are
• Capable of making all necessary organic compounds
  from inorganic compounds and elements in the
  environment (autotrophic)
• Supplied with all the carbon, hydrogen, and
  oxygen they could ever need (CO2, H2O)
• Required to obtain all other elements from the
  soil so in a sense plants act as soil miners.

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Mineral Nutrition in plants

• The study of how plants obtain, distribute,
  metabolize, and utilize mineral nutrients.
• Mineral An inorganic element
• Acquired mostly in the form of inorganic ions
  from the soil
• Nutrient A substance needed to survive or
  necessary for the synthesis of organic compounds

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Classifying mineral nutrients

• Amount required or present in plant tissue
• Metabolic need for the mineral nutrient
• Biochemical function(s) for the mineral nutrient
• Mobility within the plant

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Mineral macronutrients
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Mineral micronutrients
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Essentiality of mineral nutrients

• Essential Universal for all plants
• Absence prevents completion of life cycle
• Absence leads to deficiency
• Required for some aspect of mineral nutrition
• Beneficial Often limited to a few species
• Stimulates growth and development
• May be required in some species
• Examples Na, Si, Se

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Essentiality of mineral nutrients

• There are four basic groups
• Group one
• Forms the organic components of plants
• Plants assimilate these nutrients via biochemical
  reactions involving oxidation and reduction
• Group two
• Energy storage reactions or maintaining
  structural integrity
• Present in plant tissue as phosphate, borate or
  silicate esters
• The elemental is bound to OH group of an organic
  molecule

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Biochemical functions of mineral nutrients
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Essentiality of mineral nutrients

• Group three
• Present in plant tissue as either free ions or
  ions bound to substrates such as the pectin
  component of the plant cell wall
• Of particular importance are their roles as
• Enzyme cofactors
• In the regulation of osmotic potentials

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Biochemical functions of mineral nutrients
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Essentiality of mineral nutrients

• Group four
• This last group has important roles in reactions
  involving electron transfer.
• Some also involved in the formation/regulation of
  plant growth hormones Zinc
• The light reaction of photosynthesis - Copper

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Biochemical functions of mineral nutrients
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Techniques used to study plant nutrition
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Nutrient deficiencies

• Mineral nutrient deficiencies occur when the
  concentration of a nutrient decreases below this
  typical range
• Deficiencies of specific nutrients lead to
  specific visual, often characteristic, symptoms
  reflective of the role of that nutrient in plant
  metabolism

Chlorosis
Necrosis
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Nutrient deficiency v. sufficiency
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Patterns of deficiency

• The location where a deficiency reflects the
  mobility of a nutrient
• Nutrients are redistributed in the phloem
• Old leaves mobile
• Young immobile

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Patterns of deficiency
Older leaves on celery turning yellow while the
growing points in the center remain green.
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How are mineral nutrients acquiredby plants?

• Uptake through the leaves
• Artificial called foliar application. Used to
  apply iron, copper and manganese.
• Associations with mycorrhizal fungi
• Fungi help with root absorption
• Uptake by the roots

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The soil affects nutrient absorption

• pH affects the growth of plant roots and soil
  microbes
• Root growth favors a pH of 5.5 to 6.5
• Acidic conditions weathers rock and releases
  potassium, magnesium, calcium, and manganese.
• The decomposition of organic material lowers soil
  pH.
• Rainfall leaches ions through soil to form
  alkaline conditions

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The soil affects nutrient absorption

• Negatively charged soil particles affect the
  absorption of mineral nutrients
• Cation exchange occurs on the surface of the soil
  particle
• Cations (ve charged ions) bind to soil as it is
  ve charded
• If potassium binds to the soil it can displace
  calcium from the soil particle and make it
  available for uptake by the root

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Plant roots the primary route for mineral
nutrient acquisition

• Meristematic zone
• Cells divide both in direction of root base to
  form cells that will become the functional root
  and in the direction of the root apex to form the
  root cap
• Elongation zone
• Cells elongate rapidly, undergo final round of
  divisions to form the endodermis. Some cells
  thicken to form casparian strip
• Maturation zone
• Fully formed root with xylem and phloem root
  hairs first appear here

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Root absorbs different mineral ions in different
areas

• Calcium
• Apical region
• Iron
• Apical region (barley)
• Or entire root (corn)
• Potassium, nitrate, ammonium, and phosphate
• All locations of root surface
• In corn, elongation zone has max K accumulation
  and nitrate absorption
• In corn and rice, root apex absorbs ammonium
  faster than the elongation zone does
• In several species, root hairs are the most
  active phosphate absorbers

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Why should root tips be the primary site of
nutrient uptake?

• Tissues with greatest need for nutrients
• Cell elongation requires Potassium, nitrate, and
  chlorine to increase osmotic pressure within the
  wall
• Ammonium is a good nitrogen source for cell
  division in meristem
• Apex grows into fresh soil and finds fresh
  supplies of nutrients
• Nutrients are carried via bulk flow with water,
  and water enters near tips
• Maintain concentration gradients for mineral
  nutrient transport and uptake

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Root uptake soon depletes nutrients near the roots

• Formation of a nutrient depletion zone in the
  region of the soil near the plant root
• Forms when rate of nutrient uptake exceeds rate
  of replacement in soil by diffusion in the water
  column
• Root associations with Mycorrhizal fungi help the
  plant overcome this problem

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Mycorrhizal associations

• Not unusual
• 83 of dicots, 79 of monocots and all
  gymnosperms
• Ectotrophic Mycorrhizal fungi
• Form a thick sheath around root. Some mycelium
  penetrates the cortex cells of the root
• Root cortex cells are not penetrated, surrounded
  by a zone of hyphae called Hartig net
• The capacity of the root system to absorb
  nutrients improved by this association the
  fungal hyphae are finer than root hairs and can
  reach beyond nutrient-depleted zones in the soil
  near the root

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Mycorrhizal associations

• Vesicular arbuscular mycorrhizal fungi
• Hyphae grow in dense arrangement , both within
  the root itself and extending out from the root
  into the soil
• After entering root, either by root hair or
  through epidermis hyphae move through regions
  between cells and penetrate individual cortex
  cells.
• Within cells form oval structures vesicles
  and branched structures arbuscules (site of
  nutrient transfer)
• P, Cu, Zn absorption improved by hyphae
  reaching beyond the nutrient-depleted zones in
  the soil near the root

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Nutrients move from fungi to root cells

• Ectotrophic Mycorrhizal
• Occurs by simple diffusion from the hyphae in the
  hartig net to the root cells
• Vesicular arbuscular mycorrhizal fungi
• Occurs by simple diffusion from the arbuscules to
  the root cells
• Also, as arbuscules are degenerating as new ones
  are forming, the nutrients may be released
  directly into the host cell

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Manipulating mineral transport in plants

• Increase plant growth and yield
• Increase plant nutritional quality and density
• Increase removal of soil contaminants (as in
  phytoremediation)

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Periodic table of plant mineral nutrition
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Vesicular-arbuscular mycorrhizaHighly colonized
root of maize dyed with trypan blue. Mycorrhizal
formations are clearly visible 1) vesicles 2)
arbuscules
Ectomycorrhiza root tip of Pinus nigracolonised
by ectomycorrhizal fungus