The nutrient ions present in the seed embryo plus those released in digestion of food reserves enable the embryonic axis to start growing. With the extension of the seedling roots, the internal supplies are supplemented by absorption of nutrient ions from the soil. The adequacy of the reserves varies widely. With some micronutrients (eg., Cu, Mo) the seed supply might be sufficient for the entire growing season, while Ca commonly becomes limiting after 2-3 days unless supplied from the soil. Cell composition includes a basic level of nutrient elements, and an increment of growth requires an increment of nutrient ions to supply these. The agricultural fertilizer industry is built on the premise that an increment of growth should never fail for want of nutrient ions.
Essential elements are defined as those which are necessary for a plant to complete its life cycle (normal growth and reproduction), and for which no other element can substitute. An essential element is either a constituent of an essential metabolite or is needed for an enzymatic function. Either criterion is sufficient to demonstrate essentiality.
With these elements and sunlight (and CO2 and H2O) most plants can synthesize all the compounds they require. However, plants always contain other elements, such as Na and Si, that have structural and physiological roles in certain species, but can be "replaced" by other elements (eg., in some plants Rb or Na can replace K when K is limiting).
More realistically, we are concerned with mineral nutrition rather than essentiality: The mineral nutrition of a plant is a function of all the elements it contains which participate in metabolism.
Classification of minerals:
For completeness C, H and O are included here since they make up the bulk of a plants mass (average protein composition is 51% C, 25% O, 16% N, 7% H, 0.4% S, and 0.4% P).
Carbon (C):Basic structural element of life. No substitute.
Although not very plentiful in the earth's crust (<0.1%), carbon is one of the most abundant elements in living things. It occurs in plants combined with hydrogen and oxygen, and in their geological derivatives, petroleum and coal, where it is combined mostly with hydrogen in the form of hydrocarbons. Carbon also occurs in the atmosphere as CO2, and in rocks as carbonate minerals such as limestone.
Carbon is the lightest element with 4 valence electrons, the maximum number for atoms bonding with themselves. Carbon has both acidic and basic properties (receives and donates electrons in bond formation). Valence electrons are directed to the points of a tetrahedron, providing for 3 dimensional C-C bonding and hence complex molecular structure. Double bonds force planar structures and rings can be formed. Thus, strong stable C-C bonding is capable of producing large molecules of varied shape and complexity.
C-C and C-H bounds are high in energy which can be made available by oxidation in e- transfers (respiration). Stable covalent bonding to O, N, S and PO4 makes complex polymer formation possible, and gives rise to hydrophilic structures. Without these, C compounds are hydrophobic, making membranes possible.
CO2 is a gas, easily available for photosynthesis (SiO2, a rock, is the closest counterpart)
Oxygen is the most abundant element in the earths crust on the basis of both mass and number of atoms (49.5% of the mass of the earths crust is oxygen atoms). In the free state oxygen occurs in the atmosphere as O2 molecules (21% of air by mass). In the combined state, oxygen occurs in many minerals, living things and water.
Highly electronegative. O2 (O=O) is a gas: a mobile, penetrating oxidizing agent.
When covalently bonded it tends to draw electrons to itself, permitting polarization of molecules, H-bonding, and formation of acidic groups (eg., COOH --- COO- + H+). Important as a divalent "bridge" in polymer formation: polysaccharides, lipids, and nucleic acids.
Important as a ligand in metal binding (shares e- in metal chelates)
Almost all of the oxygen found in plants is derived from CO2 via photosynthesis. A little enters from H2O during oxidative metabolism. A few reactions incorporate O2.
Most abundant element in the universe. In the earth's crust hydrogen is third in abundance on an atom basis. On a mass basis, it is ninth in order of abundance and contributes only 0.88% of the mass of the crust. Free, uncombined hydrogen is very rare. However, combined hydrogen is quite common (eg., water, and organic compounds).
Supplied in the mobile oxidized form of H2O, and made available as a reducing element by photosynthesis. Forms covalent bonds with the electronegative elements C, N, O and H.
When combined with strongly electron negative elements such as oxygen, the proton (H+) can be dissociated from the electron (e-), and transported across membranes (eg., creating a protonmotive force). As the simplest and lightest cation, H+ has a unique role in energy transduction.
Pretty important for hydrogen bonding!
Lightest element with 5 e- in outer orbital shell, 3 valence electrons and one unshared pair. Makes + charged groups possible.
In resonating rings, acts as the focal point for redox reactions (NAD+). Amine N is important in complexing metals (eg., binding Fe in cytochromes, or binding Mg in chlorophyll). Acts as a donor atom in many enzymatically catalyzed reactions.
In peptide bond formation the -C--N- is constrained in its rotation and permits a planar structure important to helix formation and H-bonding.
N can be viewed as a C substitute which introduces an essential distortion into the symmetry of C, providing compounds with additional properties of coordination, basicity, charge, chemical reactivity, oxidation-reduction properties, and structure. Without these distortions, there could be no life as we know it.
In living things, N is found almost exclusively in the fully reduced state. Most of the N absorbed from the soil by higher plants is in the fully oxidized form of NO3, and must be reduced for assimilation (we will discuss the reduction reactions in more detail in association with photosynthesis). Two enzyme complexes are involved, one in the cytoplasm, the other in plastids. If available, plants will absorb and assimilate ammonium (NH4+).
Participates in metabolism by forming water-stable phosphate esters and anhydrides. In these forms P has several fundamental roles: linkage (as in nucleic acids), substrate mobilization (particularly of non-polar compounds, energy conservation (phosphorolysis instead of hydrolysis conserves bond energy), source of free energy in bond formation, H+ pumping, etc.
Reactivity during enzymatic catalysis is provided by binding to a divalent cation (primarily Mg++), bringing the ==O groups into a plane and introducing enough polarization for nucleophilic attack (electron donation). Mg++ (or Mn++) is a required cofactor in reactions involving phosphate transfer. Mg++ also commonly neutralizes polyphosphate compounds.
Reduced S can be viewed as a less electronegative O substitute (-SH vs. -OH) forming more stable complexes with certain metals (Cu and Fe containing metalloproteins important for electron transfer reactions). Also, disulfides are more stable than dioxides (or peroxides), permitting -SH participation in redox reactions (-SH + HS- ---- -S-S-). SH groups are also unlikely to form hydrogen bonds. Sulfhydryl groups (SH) can be the reactive sites of enzymes or coenzymes (Coenzyme A). Sulfhydryl groups are important for protein conformation.
Sulfate (SO4=) from the soil is the primary source of S, although some SO2 is absorbed from the atmosphere (too much SO2 can be quite toxic to plants. Sulfate reduction is very energy intensive and occurs mainly in chloroplasts (we will see this later along with photosynthesis).
K+ is an activator of many enzymes that are essential for photosynthesis and respiration, and it also activates enzymes needed to form starch and proteins.
K+ is quite mobile in the plant, presumably because there are many membrane carrier systems adapted to K+. It is so abundant that it is a major contributor to the osmotic potential of cells and therefore to their turgor pressure. K+ regulation of osmotic potentials forms the basis for turgor movements in plants (eg., stomate opening, leaf movements). K+ serves as a counter ion during movement of other ions: it moves with anions and as a counter-current to ion fluxes like H+.
Important component of cell walls. It stabilizes the polysaccharides by forming intermolecular complexes with -COO- groups of pectins. Calcium is also important for maintaining the integrity of membranes, especially the plasma membrane.
Free calcium concentration in the cytosol is normally very low, about 10-7 M. Some hormonal or environmental signals raise the free Ca++ concentration to 10-6 to 10-5 M, which activates certain enzymes. (The increase is brought about by increased influx or release from vacuoles). Because changes in calcium are associated with hormonal and environmental signals it is often referred to as a secondary messenger.
Involved in most reactions involving ADP and ATP. Activates enzymes for DNA and RNA synthesis. Constituent of chlorophyll. Activates key enzymes involved in CO2 fixation. Magnesium also has structural roles in membranes, especially in organelles.
Iron forms a locus for electron transfer in many enzymes (eg., cytochromes, peroxidases, catalyses). It is also required for chlorophyll synthesis. Iron is a difficult cation for plants to handle since it readily precipitates. Internally it is thought to be transported in the form of chelates with organic acids such as citrate.
Copper is an important component of several critical enzymes (eg., plastocyanin for photosynthesis and cytochrome oxidase for respiration).
It is a key component of nitrate reductase where it functions as an e- carrier for nitrate reduction. It is also important in organisms that can carry out nitrogen fixation (from N2).
A major role for manganese is in the removal of electrons from water during photosynthesis (water oxidation). Manganese also is essential in respiration and nitrogen metabolism. It can function effectively in some metal catalyzed enzymatic reactions which require magnesium.
However, boron is found in cell walls complexed with raffinose-containing polymers. It is also found in phloem complexed with sorbitol. In the absence of boron, meristems stop growing. In addition, pollen tubes can't elongate without boron. Some research suggests a role for boron during synthesis of nucleic acids.
Plants frequently contain a good deal of chloride but very little is required as a nutrient. It has important functions in photosynthesis. It may play a general role in maintaining electrical equilibrium.
Sodium (Na+): Essential for some halophytes.
Sodium can replace potassium where it is deficient. Exact functions unknown. May be important for maintaining electrical equilibrium.
Silicon (Si4+): Abundant in soils.
Absorbed from soils as silicic acid (H4SiO4). Is used by some plants to strengthen cell walls (eg., rice, oats, equisetum).
Cobalt: Not required by plants, but required by the bacteroids of root nodules which fix N2, and thus indirectly in nitrogen nutrition.
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