Chlorosis - yellowing of leaves; often interveinal; in some species, young leaves are the most affected, but in others both old and new leaves are chlorotic;
Necrotic spots - death of leaf tissue on areas of chlorosis;
Bronzing of leaves - chlorotic areas may turn bronze coloured;
Rosetting of leaves - zinc-deficient dicotyledons often have shortened internodes, so leaves are clustered on the stem;
Stunting of plants - small plants may occur as a result of reduced growth or because of reduced internode elongation;
Dwarf leaves - small leaves that often show chlorosis, necrotic spots or bronzing;
Malformed leaves - leaves are often narrower or have wavy margins.
Soil conditions
Zinc deficiency is common in many different types of soil; some soils have low total zinc concentrations, and others have low plant-available zinc due to strong zinc sorption. Soils low in organic matter, and compacted soils that restrict root proliferation also have a high risk of zinc deficiency. Application of phosphorus fertilizers has frequently been associated with zinc deficiency; this may be due to enhanced sorption by clay minerals, suppression of vesicular arbuscular mycorrhizae and/or immobilization of zinc in plant tissues. Liming of soils also frequently induces zinc deficiency by increasing zinc sorption.
Zinc requirements
Zinc is an essential micronutrient which means it is essential for plant growth and development, but is required in very small quantities. Although zinc requirements vary among crops, zinc leaf concentrations in the range 20 to 100 mg/kg are adequate for most crops.
Treatment
Zinc sulphate or zinc oxide can be applied to soils to correct zinc deficiency. Recommended applications of actual zinc range from 5 to 100 kg/hectare but optimum levels of zinc vary with plant type and the severity of the deficiency. Application of zinc may not correct zinc deficiency in alkaline soils because even with the addition of zinc, it may remain unavailable for plant absorption. Foliar applications of zinc as zinc sulphate or as zinc chelate are also widely used, especially with fruit trees and grape vines. Zinc can also be supplied as a seed treatment, or by root-dipping of transplant seedlings.
Functions
Zinc occurs in plants as a free ion, as a complex with a variety of low molecular weight compounds, or as a component of proteins and other macromolecules. In many enzymes, zinc acts as a functional, structural, or regulatory cofactor; a large number of zinc-deficiency disorders are associated with the disruption of normal enzyme activity. Zinc deficiency also increases membrane leakiness as zinc-containing enzymes are involved in the detoxification of membrane-damaging oxygen radicals. Zinc may be involved in the control of gene expression; it appears important in stabilizing RNA and DNA structure, in maintaining the activity of DNA-synthesizing enzymes and in controlling the activity of RNA-degrading enzymes.
