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Plant Nutrition

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I.  Plants require at least seventeen essential nutrients

A.       The chemical composition of plants

1.       Aristotle thought that the soil provided the substance for plant growth and the leaves were used to protect the fruit.

2.       Jean-Baptiste van Helmont postulated that plants grew from the water in the soil.

3.       Stephen Hales said that plants were nourished by air.

4.       Plants do extract minerals from the soil.

5.       About 80% of herbaceous plants are water.

6.       Water can truly be considered a nutrient because it provides the plant with the hydrogen necessary for photosynthesis.

7.       More than 90% of the water absorbed is lost through transpiration.

8.       More than 50 elements have been identified in plants.

B.       Essential nutrients

1.       An essential nutrient is a particular chemical element that is required for the plant to grow from a seed and complete the life cycle by producing more seeds.

2.       Hydroponics have been used to identify many of these essential nutrients.

3.       Macronutrients are elements required by plants in relatively large amounts.

i.         carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, calcium, potassium, magnesium (See p. 713 for uses)

4.       Micronutrients are elements needed in small amounts.

i.         iron, chlorine, copper, manganese, zinc, molybdenum, boron, and nickel.

ii.        Function as cofactors of enzymatic reactions.

iii.      Because of enzymatic activity, they are found in small amounts but can weaken or kill a plant if not present.

II.  The symptoms of mineral deficiency depend on the function and mobility of the element

A.       Symptoms depend on function.

B.       Lack of magnesium causes leaves to yellow because it is found in chlorophyll.

C.      Iron causes the same problem but is a cofactor to the production of chlorophyll.

D.      Symptoms also depend of mobility.

1.       If a mineral moves freely about the plant, the symptoms will be found in the older organs first because the newer parts have “drawing power”.

2.       If a mineral does not move freely, the symptoms will be found in the younger part of the plant because the older tissues may have adequate amounts stored in the plant.

E.       Deficiencies of nitrogen, potassium, and phosphorus are the most common.

F.       Shortages in micronutrients are less common and tend to be geographically localized.

III.  Soil characteristics are key environmental factors in terrestrial ecosystems.

G.      The texture and chemical composition of soils

1.       Soil is weathers of solid rock.

2.       Water that seeps into crevices and freezes in winter fractures the rock and acids dissolved in the water also help to break it down.

3.       Organisms accelerate the decomposition.

4.       Topsoil is a mixture of decomposed rock of varying texture, living organisms, and humus (made of decayed organic materials).

5.       The topsoil and other distinct soil layers or horizons are often visible in vertical profile.

6.       The texture of topsoil depends on the size of its particles which are classified from coarse sand to microscopic clay particles.

7.       The most fertile soils are usually loams made up of a mixture of sand, silt, and clay.

i.         Have enough fine particles to provide a large surface area for retaining minerals and water

ii.        Also provides air spaces continuing oxygen that can be used for cellular respiration.

8.       If soil does not drain, the roots suffocate because the air is replace by water and mold growth.

9.       Soil also has an astonishing number of organisms and these affect the physical and chemical properties of the soil.

i.         Earthworms aerate the soil by burrowing and adding mucus that holds fine soil particles together.

ii.        Bacteria change the mineral composition of the soil by fixation.

10.    Humus is the decomposing organic material formed by the action of bacteria and fungi on dead organisms, feces, fallen leaves, and other organic refuse

i.         Prevents clay from packing

ii.        Allows soil to stay crumbly and allows for air pockets and water retention.

H.      The availability of soil water and minerals.

1.       After a heavy rainfall, water drains away from the larger spaces of the soil but smaller spaces retain water because of its attraction for the soil particles which have electrically charges surfaces.

i.         Some adheres to hydrophilic soil particles that cannot be extracted by plants.

ii.        In the tiniest spaces of the soil a film of water can be found.  It is not tightly bound and is not pure water but called soil solution.

iii.      This soil solution contains many dissolved minerals.

2.       Many minerals in soil are positively charged like potassium, calcium, and magnesium and adhere to the negatively charged soil particles.

i.         This prevents leaching during heavy rain.

ii.        Positively charged minerals are made available to the plant when hydrogen ions in the soil displace the mineral ions from the clay particles.

iii.      This is called cation exchange and is stimulated by the roots themselves.

IV.  Soil conservation is one step toward sustainable agriculture

A.       It may take centuries for a soil to become fertile through decomposition but human mismanagement can destroy that fertility within a few years.

B.       Agriculture is unnatural.

C.      Fertilizers

1.       Romans used manure to fertilize their crops.

2.       Native Americans buried fish along with seeds when they planted corn.

3.       Most farmers now days use fertilizers that have been mined or processed.

4.       Fertilizers are usually enriched in nitrogen, phosphorus, and potassium.

5.       Manure, fish meal, and compost are organic fertilizers but must be processed before their inorganic molecules can be used.

6.       To conserve these fertilizers, the pH of the soil must be paid attention to.

i.         Acidity not only affects cation exchange but also influences the chemical form of all minerals.

D.      Irrigation

1.       The availability of water most often limits the growth of plants.

2.       Irrigation can help but it also puts a huge drain on water resources.

i.         It can also make the soil very salty and then infertile.

ii.        Slat makes the soil solution hypertonic to root cells so the lose water instead of gaining it.

E.       Erosion

1.       Topsoil is lost to wind and water erosion.

2.       Certain precaution can help reduce this by planting trees as a windbreak or alfalfa or wheat provides good ground cover for the soil.

3.       Sustainable agriculture is a commitment that are conservation-minded, environmentally safe, and profitable.

V.  The metabolism of soil bacteria makes nitrogen available to plants.

A.       Nitrogen is the one that most limits the growth of plants and the yields of crops.

1.       Important ingredient of proteins, nucleic acid, and other important organic molecules.

2.       Plants cannot use nitrogen in its gaseous form and it must be converted to ammonium or nitrate.

3.       Nitrogen-fixing bacteria restock nitrogenous minerals in the soil by converting N2 to NH3 by nitrogen fixation.

B.       Nitrogen fixation

1.       Conducted by nitrogen fixing prokaryotes.

2.       Complicated processes

3.       Uses nitrogenase that catalyzes the entire reaction sequence that reduces N2 to NH3 by adding electrons and hydrogen ions.

4.       ATP expensive

C.      Symbiotic Nitrogen fixation

1.       Plants of the legume family have built in sources of nitrogen fixation.

2.       Their roots have swellings called nodules composed of plant cells that contain the bacteria Rhizobium.

3.       Inside the nodule the bacteria forms bacterioids which are contained within the vesicles formed by the root cell.

4.       This is a mutualistic relationship.

5.       Bacteria supply the legume with nitrogen and the plant provides the bacteria with carbohydrates and other organic compounds.

6.       Leghemoglobin is an iron-containing protein that binds reversibly to oxygen for the intense respiration that must take place for nitrogen fixation.

7.       It also keeps the concentration of free oxygen low because nitrogenase is inhibited by oxygen.

8.       Most of the ammonium is used to make amino acids that are transported to the shoot and leaves via the xylem.

9.       The bacteria can over produce ammonium that is released into the soil to help other nonlegume plants.

10.    Other plants may use other nitrogen fixing organisms.

VI.  Improving protein yield of crops is a major goal of agricultural research

A.       Plant breeding to improve the protein content of plants.

B.       Improving productivity of symbiotic nitrogen fixation by using enzymes other than nitrogenase.

C.      Improving the ability to induce root nodule formation in nonlegumes.

D.      Genetic engineering

VII.  Predation and symbiosis are evolutionary adaptations that enhance plant nutrition

A.       Parasitic plants

1.       May be photosynthetic but use haustoria to tap into the xylem of the plant they are growing on.

2.       Epiphytes are sometimes mistaken for parasites.

3.       Epiphytes nourish itself but grows on the surface of another plant.

4.       It is anchored to the plant but gets its water and minerals from rain that falls on the leaves.

B.       Carnivorous Plants

1.       Plants that occasionally feed on animals.

2.       Make their own carbohydrates from photosynthesis but obtain some of their nitrogen and minerals by killing and digesting insects.

3.       Use traps that contain glands that secrete digestive juices.

C.      Mycorrhizae

1.       Mutualistic associations between roots and fungi

2.       Fungi secrete growth factors that stimulate the roots to grow and branch.

3.       As the root develops into the mycorrhizae, the fungus either sheathes the root and extends hyphae among the cortex cells or invades the root cells themselves.

4.       The fungi are more efficient in absorbing these minerals and secrete acid that increases the solubility of some minerals.

5.       The fungi provides the minerals and the plant provides the carbohydrates.