Zinc deficiency is a global nutritional problem in agricultural soils (Figure 1*) and human populations (Figure 2*) in large number of countries that calls for closer cooperation between agronomists, plant breeders, human nutritionists, the fertilizer industry and farmers. This was a key conclusion of Zinc Crops 2007, an international conference that took place in Istanbul, Turkey on May 24-26.
Zinc deficiency affects, on average, one-third of the world's population, ranging from 4 to 73 % in different countries (Hotz and Brown, 2004, Food Nutr Bull 25: 94-204). Children are particularly sensitive Zn deficiency. It has been estimated that Zn deficiency is responsible for deaths of nearly 450,000 children under age 5 annually (Black et al., 2008, Lancet 371: 243-260). Major health problems associated with Zn deficiency include i) impairments in brain function and mental development, ii) disturbances in immune system and susceptibility to deadly infectious diseases and iii) delays in physical development (e.g., stunting).
The recent analyses made under the Copenhagen Consensus in 2008 identified Zn deficiency, together with vitamin A deficiency as the top priority global issue, and concluded that elimination of the Zn deficiency problem will result in immediate high impacts and high returns for humanity in the developing world. (www.copenhagenconsensus.com).
Nearly 50 % of the cereal-cultivated soils have Zn deficiency problem, causing decreased crop yields and grain nutritional quality. Cereal grains are a major calorie source in the developing world where cereal-based foods provide nearly 50 % of the daily calorie intake on average, likely increasing to more than 70 % in the rural regions. Since cereal grains have inherently low in Zn concentrations, growing them on micronutrient deficient soils further decreases grain Zn and thus the daily intake of Zn through diets.
Based on a range of reports and survey studies, the average concentration of Zn in whole grain of wheat in various countries range commonly between 20 to 35 mg kg-1 (Rengel et al., 1999, Field Crops Res. 60: 27-40; Cakmak et al., 2004, Sci. Plant Nutr. 50:1047-1054) which are not adequate for better human nutrition with Zn. 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 (Erdal et al., 2002, J. Plant Nutr. 25:113-127; Cakmak et al., 2010, Cereal Chem.87:10-20). Consequently, reliance on a high proportion of cereal-based diets may induce Zn deficiency-related health problems. Increasing concentrations of Zn in cereal grains is, therefore, an important global humanitarian challenge.
Agricultural Solutions to Zinc Deficiency Problem
Plant breeding for increased Zn-density in cereal crops is a realistic and a cost-effective approach, being used as an intervention to reduce Zn deficiency-related health problems in human-beings (visit: www.harvestplus.org). The plant breeding strategy, however, is a long-term process, and might be affected by adverse soil chemical factors (e.g., extreme pH values, low organic matter and low soil moisture) which limit plant availability of Zn in soils. Maintenance of sufficient amount of plant-available Zn in soils may, therefore, be required, and this can be effectively realized by applying Zn fertilizers to soil and/or foliar (Cakmak, 2008, Plant and Soil, 302:1-17).
Application of Zn-containing fertilizers (agronomic biofortification) offers a rapid solution to the problem, and represents an important complementary approach to on-going breeding programs for developing new genotypes with high Zn density in grain. Maintenance of adequate amount of readily available Zn in soils ensures healthy root growth, and thus, better acquisition of Zn from soil. Zinc has relatively high phloem mobility, and foliarly applied Zn is, therefore, easily translocated into grain. Increasing number of evidence is available showing that application of Zn to foliage represents a rapid approach to enrichment (biofortification) of cereal grains with Zn, especially in the case of wheat. Use of Zn-containing fertilizers can also contribute to crop production.
* Fig. 1: Global distribution of Zn deficiency-affected areas (Alloway, 2008)
** Fig. 2: Prevalence of countries which are under risk of zinc deficiency (http://www.izincg.org/)