Zinc

Zinc in the soil

The zinc content of unpolluted soils ranges between 10 – 80 mg kg^-1 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. In addition, it can be found adsorbed onto iron, manganese and aluminium oxides or strongly bound to the lattice of clay minerals and silicates. Additional immobilisation of zinc occurs when the sulphate 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 greatly reduced at pH 6. With increasing pH, the affinity of zinc to manganese oxide and iron oxide increases strongly. Under anaerobic conditions, zinc can be precipitated into the barely soluble sulphide form which is largely unavailable to plants Zinc can be leached from the soil but this process generally only occurs in acidic soils.

Zinc in the plant

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 greatly. 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 deficiency symptoms

• Leaves are small and their tips are often white. The entire plant is often stunted (dwarfism).
• In fruit crops, ‘rosette’ or ‘little leaf’ development occurs because of jammed internodes. The growth of sprigs is inhibited and young shoots die. Premature leaf senescence can also occur.
• Grape vines develop an increased number of Geiztrieben and the grapes stay small.
• Older and middle leaves display chlorotic spots with necrotic areas.

Zinc toxicity

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. Zinc toxicity results in:

• Inhibition of root development
• Chlorosis seen on the younger leaves.
• Induced iron deficiency.