The Function of Nutrients in Plants
Most nutrients become constituents of organic components of the plant: proteins, enzymes, etc, where they are directly or indirectly involved in metabolic function. Potassium and chloride are the only nutrients that are not found as constituents of organic molecules (Marschner p 229).
Marschner provides an excellent account of the function of all the nutrients
(Marschner Chapters 8 ,9 and 10).
A classification of plant nutrients according to their biochemical
significance (after Mengel & Kirkby)
Nutrients can also be differentiated according to the form in which they participate in reactions.
The form of nutrients taking part in key processes within plants
Cl is important in the process of oxygen release in photosystem II.
The activation of important organic ligands tends to be associated with particular metal ions, although the degree of specificity is variable.
Organic ligands and the ions most commonly associated with them
Comments on specific nutrients
NitrogenThe nitrogen content of plants is normally between 20 and 50 g kg-1, and it is taken up as NO3- or NH4+. Alexander et al.(J. Pl. Nutr., 14:31-44, 1991) showed that there was improved shoot dry weight and grain yield when corn plants received 31% of their total N supply as NH4+ rather than 96% as NO3- and 4% as NH4+.
Effect of N source on dry matter production and grain yield (after
Alexander et al, 1991).
Organic nitrogen occurs almost exclusively in the reduced state. NO3- ions are reduced in the plant by the enzymes nitrate reductase and nitrite reductase. The reduction tends to take place in the root when the external supply is limited. However, when the supply is abundant, the capacity for nitrate reduction in the roots becomes a limiting factor and an increasing proportion of the total nitrogen is translocated to the shoots in the form of nitrate. Reduction and assimilation of nitrate requires a lot of energy (23% of energy released in root respiration) compared with assimilation of NH4+ ions (14% of energy released in root respiration). The overall equation is:
Nitrate reductase is found in the cytoplasm, but nitrite reductase is localized in plastids (chloroplasts in leaves and pro-plastids in roots).
In C4 plants the reduction of nitrate occurs in the mesophyll cells not in the cells of bundle sheaths.
At neutral pH in the aqueous phase, primary aliphatic amines tend to be protonated at pKa values >9. In a heterocyclic ring the pKa is less so the N can serve as a ligand for complexing metals.
N also readily participates in hydrogen bonding with other nucleophiles eg. in the DNA helix and in proteins. Hence it contributes to the secondary and tertiary structure of macromolecules.
N can also induce structure because of the peptide bond which has a
more limited rotation than either the ether or ester linkages.
SulphurOrganic sulphur is also commonly present in the reduced state as sulphydry1 (SH group) or disulphide (S-S bond)
Oxidised sulphur is found in the phospholipid 'sulphoquinovosyldiglyceride' of chloroplast membranes.
Organic sulphates may serve to enhance the water solubility of organic
compounds which may be important in the enhancement of cellular osmotica
under saline stress.
PhosphorusPhosphorus concentrations in plants normally is between 2 and 5 g kg-1, and uptake is as H2PO4- or HPO42-.
At neutral pH, phosphate exists in almost equal parts of the mono- and di-valent anions, so contributing to the buffering capacity of the cell.
Orthophosphate can be condensed to give polyphosphates linked through
oxygen eg. ADP and ATP. Their stability appears to reside in the strength
of the phosphoryl bond which delocalizes the electrons.
PotassiumPotassium concentrations in plants normally is between 20 and 50 g kg-1.
The role of K+ has much in common with Mn and Mg, being concerned
with enzyme activation, membrane transport processes, anion neutralization,
osmotic potential adjustments, transport of NO3-N in the xylem.
CalciumCompared with magnesium the ionic activity of Ca in the cytoplasm is low. It is only very slowly mobile (immobile) in the phloem and in the symplast. There is a critical requirement for Ca in the cell wall and on the exterior surface of the plasmalemma.
A COMPARISON OF MANGANESE AND MAGNESIUMSource of the ions
The most important Mn soil fractions are Mn2+ and the Mn oxides.
These can be presented in the form of a cycle:
Mg is present in easily weathered ferromagnesian minerals eg. biotite, serpentine, hornblende and olivine.
Also its found in secondary minerals such as chlorite, illite and montmorillonite.
It can be present as MgCO3 (dolomite is CaC02 .MgCo3).
In some arid or semi-arid soils MgSO4 is also present.
Manganese availability is altered by:
soil pH, organic matter content, microbial activity, soil water content
If the pH rises, Mn complexes with organic matter. Exudation of organic anions and H+ increases Mn solubility (see diagram above), as can ammonium sulphate for the same reasons.
In highly anaerobic conditions, eg paddy soils, reducing conditions prevail so increasing Mn availability.
Oxidation of Mn by microbes reduces availability so soil sterilization can increase availability.
It is not generally possible to identify levels of Mn in the soil that
indicate deficiency or toxicity. Generally it is a matter of treating the
crop once the symptoms of deficiency appear.
pH has little effect on magnesium availability.
Leaching is the most important factor reducing the availability of magnesium.
Rates of leaching of plant nutrients from soils of different texture
Rates of leaching of plant nutrients from grazed grassland
Soil tests are available which indicate ranges over which crop growth may be restricted by the soil available content of nutrients. The availability of Mn is determined following extraction with 0.1M phosphoric acid, and Mg following extraction with neutral ammonium acetate.
Significant tissue test values for magnesium and manganese
A minimum soil test of 20 for Mg is considered essential in Ontario for corn. The critical soil test for manganese is 16 for small grains and soybeans, but no critical value has been identified for corn.
Mn enters the symplast along electrochemical potential gradients. Both Ca and Mg reduce manganese uptake into the cytoplasm. Transfer from cytoplasm into the vacuole requires energy.
Mg also enters root cells along electrochemical gradients, and uptake can be competitively inhibited by Ca2+, K+ and NH4+. In contrast NO3- stimulates uptake.
Although Mg can inhibit Mn uptake, the Mn uptake characteristics show a dependence on Mg for translocation to the shoot. K+ can have a similar effect on the translocation of Mg.
Shoot growth decreases with Mn concentration. But a clear indication
of a single toxic concentration is absent.
Growth generally increases with Mg.
The variability in the impact of the two ions Mn and Mg on growth can
be greatly diminished and a strong empirical relationship with growth can
be obtained when the ratio of the two ions is used as the independent variable.
There are several possible consequences of such a relationship:
In addition the ratio of the two ions might provide a means of testing for magnesium and manganese fertilizer additions, especially if there is any relationship between the concentration ratio in the soil and in the plant. In fact there is a clear relationship between the concentration ratio in the solution (soil or culture solution) and the ratio in the plant.
The lightest of the transition metals Mn Fe Cu and Zn required as micronutrients. It forms strong complexes with ligands to form metallo-proteins.
Mn has a profound influence on 3 metabolic function: -
Mn is also important for the structural integrity of chloroplast lamellae where it is bound to a protein.
Mn forms weak bonds so it can substitute for Mg in reactions with -O-
ligands in phosphates.
The water splitting enzyme.The evolution of oxygen from water involves the removal of four electrons.
2H2O O2 +4H+ +4e-
The electrons are then passed via a donor molecule 2 (now identified as a plastoquinone cation radical) to P680, (photo-reactive chlorophyll a) and thence to phaeophytin and OA and OB (quenchers present in lipoproteins).
Much of the Mg is diffusible although increasing amounts are bound in cell walls and on the outer layers of the plasmalemma as the Mg content in the soil increases. Some is associated with non-diffusible anions.
The main functions are, superficially, somewhat similar to manganese.
Enzymic reactions that require magnesium or are promoted by it, fall into three groups:
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