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Fluid Journal : Fall 2011
4 The Fluid Journal Fall 2011 Summary: It is estimated that zinc (Zn) deficiency affects, on average, one-third of the world’s population and is known to be the major reason for high incidence of micronutrient malnutrition in human populations. Soil Zn deficiency is also an important constraint to crop production and nutritional quality of grains. Based on a range of studies, the average concentration of Zn in whole grain wheat in various countries ranges between 20 to 35 mg Kg-1 , which is inadequate for human nutrition. Evidence is available showing that seedlings derived from seeds with low concentrations of Zn are highly susceptible to biotic and abiotic stress conditions during seed germination and early growth stages. These results indicate that improving Zn concentration of seeds/grains is also important for better agronomic performance of seedlings. Additionally, biofortification of food crops with Zn is considered a useful intervention against problems in human nutrition by using agricultural tools such as breeding and fertilization. Biofortification strategy is also proving highly promising as a cost effective and long-term solution to Zn deficiency in human populations. Developing new Zn-dense genotypes by using plant breeding, however, takes a long time. The success of a breeding program depends on a sufficient amount of readily available pools of Zn in soil solution. Zinc deficiency represents a common micronutrient deficiency problem in human populations. The Fluid Journal • Official Journal of the Fluid Fertilizer Foundation • Fall 2011 • Vol. 19, No. 4, Issue # 74 Dr. Ismail Cakmak Zinc: A Key Nutrient in Crop Production Zinc has particular physiological functions in all living systems, such as: • Maintenance of structural and functional integrity of biological membranes • Detoxification to highly toxic oxygen- free radicals • Contribution to protein biosynthesis and gene expression. Among all metals, Zn is needed by the largest number of proteins. Zinc-binding proteins make up nearly 10 percent of the proteomes in eukaryotic cells, indicating that at least 2,800 proteins are Zn dependent. About 36 percent of the eukaryotic Zn proteins are involved in gene expression. Its deficiency, therefore, results in diverse impairments in biological systems. Zinc deficiency represents a common micronutrient deficiency problem in human populations, resulting in severe impairments in human health. Major health complications caused by Zn deficiency include: • Impairments in brain function • Weakness in immune system to deadly infections • Alterations in physical development. Zinc deficiency is known to be responsible annually for deaths of nearly 450,000 children under age five. Analyses under the 2008 Copenhagen Consensus identified Zn deficiency, together with vitamin A deficiency, as the top priority global issue. It is estimated that Zn deficiency affects, on the average, one-third of the world’s population, ranging from 4 to 73 percent in different countries. Low dietary intake is known to be the major reason for the high incidence of Zn deficiency in human populations, particularly in those regions where soils are low in available Zn and cereal grains having low Zn concentration are the major source of calorie intake. Soil Zn Deficiency Soil Zn deficiency represents an important constraint to crop production and nutritional quality of grains. Nearly half of the cultivated soils are affected by low levels of plant available Zn, especially calcareous soils of arid and semi-arid regions. Major soil factors resulting in adverse impacts on solubility of Zn in soils include high pH, low organic matter, low soil moisture, and high metal oxides with large fixing capacity for Zn (Figure 1). Concentration. Since food crops, particularly cereal crops, are inherently low in grain Zn concentration, growing them on potentially Zn-deficient soils further reduces Zn concentration of food crops and thus dietary intake of Zn by human populations. Based on a range of reports and survey studies, the average concentration of Zn in whole grain wheat in various countries ranges between 20 to 35 mg kg-1 , which is not adequate for human nutrition with Zn. Same situation applies to rice and maize, which contain even less Zn that wheat. In the case of Zn deficient soils, the reported Zn concentrations for wheat are much lower and range between 5 to 15 mg kg-1 . These values indicate high urgency of biofortification of food crops with Zn. Yields. Soil Zn deficiency has severe impacts on crop yields. In certain regions with very low plant-available Zn in soils (DTPA-Zn around 0.1 mg kg-1), cereal production is not economical, thus Zn fertilization is necessary to obtain a proper yield. As shown in Central Anatolia, application of Zn fertilizers in such soils enhances grain yield by a factor of 6 to 8 to around 2,000 kg ha-1 . In general, soils containing less than 0.5 mg DTPA-extractable Zn are considered Part 1