The Function of Nutrients in Plants

Marschner provides an excellent account of the function of all the nutrients (Marschner Chapters 8 ,9 and 10). 
This section of the course aims to provide a structure for a comparative approach to thinking about the importace of the different nutrients. 

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 

Nitrogen 

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 pK_a values >9. In a heterocyclic ring the pK_a 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. 
 

Sulphur 

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. 
 
 
 

Phosphorus 

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. 
 

Potassium 

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. 
 

Calcium 

A COMPARISON OF MANGANESE AND MAGNESIUM 

Manganese 

The most important Mn soil fractions are Mn²⁺ and the Mn oxides. 

These can be presented in the form of a cycle: 
 

Magnesium  

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. 

Nutrient availability 

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. 
 
 

Magnesium availability. 

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. 

Plant Uptake 

Manganese 

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. 

Magnesium 

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. 

Growth 

Manganese 

Shoot growth decreases with Mn concentration. But a clear indication of a single toxic concentration is absent. 
 

 

Magnesium 

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: 
 

1. Manganese toxicity is dependant on magnesium content and not on the absolute concentration of manganese.
2. The presence of magnesium increases the tolerance of tissues to manganese. 
3. There is some control on manganese uptake or translocation exerted by 

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. 

Biochemical Functions 

Manganese 

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: - 
 

1. Electron transport in photosystem II 
2. Sequential reduction of nitrate 
3. Co-factor in the biosynthesis of secondary metabolites including lignin and IAA. IAA oxidase increases under Mn deficiency. 

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. 

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 O_A and O_B (quenchers present in lipoproteins). 

Magnesium 

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. 
 

1. In photosynthesis it is a key component of chlorophyll (15-20% of Mg content of the shoot). 
2. Co-factor in many anabolic and catabolic processes. 
3. Stabilizes the structure of ribosomes. 

Enzymic reactions that require magnesium or are promoted by it, fall into three groups: 

1. Transfer of phosphate or nucleotides: eg. phosphates, kinases, ATPases, synthesases, nucleotide transferases The magnesium forms a bridge between the pyrophosphate structure of ATP or ADP and the enzyme molecule. The active form of the co-factor is MgATP^2- 
2. Transfer of the carboxyl group: e.g. carboxylases, decarboxylases.  Light triggers the transport of Mg from the thylakoid compartment into the stroma due to hydrogen ion pumping in the opposite direction. Mg²⁺ increases the enzyme affinity for CO₂ and the V_max for the process increases. NB: Ca²⁺ in the chloroplast is tightly bound and so doesn't move. This is an example of the importance of the mobility of magnesium.
3. Other reactions: e.g. dehydrogenases, mutases, lyases.