Enzyme activator in plant metabolism
The availability of zinc in the soil is strongly influenced by the pH value. The content of exchangeable zinc decreases with increasing pH and is already very low at pH 6. Zinc activates or is a component of various enzymes and therefore influences a wide range of metabolic processes in the plant. As an essential component of RNA polymerase, for example, it influences RNA and protein formation. In the event of a zinc deficiency, the entire plant will lag behind in growth and is referred to as "dwarfism".
Zinc is more plant-available at low pH values
The zinc content of unpolluted soils ranges between 10 – 80 mg/kg and the Zn content of sandy soils is generally lower than that of loamy soils. Freely available zinc in the soil solution binds mainly to the organic matter in the soil. Furthermore, it can be found adsorbed onto iron, manganese, and aluminium oxides or strongly bound to the lattice of clay minerals and silicates. Additional immobilization of zinc occurs when the sulfate and phosphate content in the soil solution are excessive.
The availability of zinc is strongly affected by the pH and the total Zinc content of the soil. The proportion of exchangeable zinc decreases with increasing pH and is already strongy reduced at pH 6. With increasing pH, the affinity of zinc to manganese oxide and iron oxide increases strongly.
Under anaerobic conditions, the hardly soluble zinc sulfide can be precipitated. This withdraws zinc from plant nutrition. Zinc displacements with the leachate are only significant on acidic soils.
Zinc is important for regulation of plant growth
Zinc is taken up by plants from the soil solution either as the Zn2+ ion (at low pH) or as the zinc hydroxide ion (at higher pH values). Plants grown in acid conditions of less than pH 6 are rarely short of zinc since the availability under such conditions increases strongly. Zinc activates or is a component of several enzymes and therefore affects many metabolic processes in the plant.
Functions of zinc in the plant:
- As an essential component of RNA polymerase which catalyzes RNA synthesis, which in turn affects production of proteins.
- As a component of enzymes, zinc catalyzes the synthesis of fructose-6-phosphate, an important metabolite in glycolysis and therefore photosynthesis.
- Is essential for the stability of ribosomes.
- Affects the indole-3-acidic acid content which is important for regulation of plant growth.
Zinc surplus in the plant
An excess of zinc can be toxic in plants although the tolerance levels are usually high. Some plants are able to store surplus zinc in their vacuoles.
Consequences of zinc excess:
- Inhibition of root development
- Chlorosis seen on the younger leaves
- Induced iron deficiency
Zinc deficiency results in stunted growth and white leave tips
- The leaves are small and their tips are often white. The whole plant is retarded in growth (dwarfism).
- In fruit, rosette leafiness occurs as a result of compressed internodes. Branch growth is stunted and young shoots die. The leaves fall off early.
- In grapevines, there is increased development of stingy shoots and grapes remain small.
- Chlorotic spots with dead zones appear on older and middle leaves.