Biofortification of Cereal Grains with Zinc by Applying Zinc Fertilizers

Publiceret Januar 2009

Zinc is an essential micronutrient for biological systems. One of the critical physiological roles of Zn in biological systems is its role in protein synthesis and metabolism. In biological systems Zn is required by largest number of proteins.  It has been estimated that nearly 2800 human proteins are capable of binding Zn which corresponds to 10 % of human proteome (Andreini et al., 2006).  Almost 40% of the Zn-binding proteins are transcription factors needed for gene regulation and the 60% enzymes and proteins involved in ion transport (Andreini et al. 2006). Zinc is also a critical micronutrient required for structural and functional integrity of biological membranes and for detoxification of highly aggressive free radicals (Cakmak, 2000). Any alteration in Zn homeostasis or any decrease in Zn  concentration of human body  will, therefore, result in number of cellular disturbances and impairments such as i) immune dysfunctions and high susceptibility to infectious diseases, ii) retardation of mental development and iii) stunted growth of children (Black et al., 2008).

In recent years Zn deficiency receives an increasing attention not only by nutritionists and medical scientists, but also by economists and social scientists.    Zinc deficiency problem has been recognized as a very serious threat to human health, especially to child health. Together with vitamin A deficiency, Zn deficiency has been considered as the top priority problem facing the world currently by eight worldwide distinguished economists (including five Nobel Laureates) at the Copenhagen Consensus Conference (www.copenhagenconsensus.org). The economists believe that elimination of Zn deficiency problem  will result in very high and quick impacts for humanity and global stability. Children are particularly sensitive to Zn deficiency. Zinc deficiency has been shown to be a major cause of child death in the world, and responsible for nearly 450,000 deaths in children under 5 years of age, that corresponds to 4.4 % of the deaths of children less than 5 years of age globally (Black et al., 2008). 

Zinc Deficiency: A Global Nutritional Problem

It is estimated that nearly half of the world population is affected from Zn deficiency as a consequence of low dietary intake of Zn. High consumption of cereal-based foods with low amount and bioavailability of Zn seems to be major reason for high prevalence of Zn deficiency in human populations, especially in developing world.  In most of the developing countries cereal grains, especially wheat and rice, contributes to about 70 % of the daily calorie intake. Today, billions of poor people rely on cereal-based foods as their predominant calorie and protein source.  Cereals are, however, inherently very low in concentrations of Zn to meet daily requirement of humans (Cakmak, 2008). The Zn deficiency problem in cereal grains is aggravated by growing cereal crops on potentially Zn deficient soils.  Soil Zn deficiency is also a well-documented problem that reduces crop production. Under Zn-deficient soil conditions, plants show a high susceptibility to environmental stress factors such as drought stress and pathogenic infections, and develop severe symptoms such leaf necrosis and stunting growth (Figure 1).  Consequently, Zn deficiency results in significant decreases in plant performance to grow and yield better, as shown in various countries such as in India, Pakistan, Australia and China (Alloway, 2007).

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Figure 1: Effect of zinc fertilizer application on wheat growth on a zinc-deficient soil in Central Anatolia (see for further detail: Cakmak et al., 1999)

It is estimated that about 50 % of the cereal-cultivated soils globally contain low amounts of plant available Zn, leading to further decreases in grain Zn concentration. Figure 2 shows the regions/countries where Zn deficiency has been reported as an important micronutrient deficiency in soils and an important constraint to crop production.  It is, therefore, not surprising why a widespread Zn deficiency in human beings generally occurs in the regions where soils have Zn deficiency problem and cereals are major source of daily calorie intake (Cakmak. 2008).

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Figure 2: Global distribution of Zn-deficiency affected regions (Alloway, 2007)

Increasing Grain Zinc: A Global Challenge

Currently, increasing Zn concentration of cereal grains is a big global challenge. Among the interventions to improve Zn nutrition of human beings, plant breeding strategy is becoming a widely accepted, cost-effective and easily affordable solution in the target populations. Currently, numbers of breeding programs are on-going to develop new cereal genotypes with high genetical ability to absorb Zn (and also other micronutrients such as Fe) from soil and accumulate in grain at desired levels for human nutrition. HarvestPlus Biofortification challenge program  (www.harvetsplus.org) is the leading global program aiming at improving stable food crops with Zn, Fe and vitamin A by using plant breeding strategy (Pfeiffer and McClafferty, 2007). This program is, currently, progressing well with very promising results.

Breeding program is, however, a long-term process requiring number of crossing and back-crossing activities over many years, and its success depends on the stability of the targeted micronutrient trait under various environmental conditions. In addition, the cultivated soils have number of chemical and physical problems which reduce chemical solubility and availability of Zn in soils and thus restrict ability of roots to absorb adequate amount of Zn from soils. Among the chemical factors, high soil pH, low soil moisture and  low organic matter are the most critical factors reducing solubility and root absorption of Zn. For example, an increase in soil pH from 6 to 7 is responsible for about 30-fold decrease in solubility of Zn in soil which in turn results in significant decreases in plant concentrations of Zn. Similar impairments in root absorption of Zn also take place in soils with limited moisture level and low amounts of organic matter. In Turkey, Zn deficiency is a well-documented problem in Central Anatolia (Cakmak et al., 1999) where annual rainfall is too low (around 300 mm) and wheat production is dominating. In this region, based on the collected 78 soil samples, the mean value of soil pH is extremely high (7.9) ranging between 7.5 to 8.1 and the content of soil organic matter is very low (averaged 1.5 %). Very similar soil conditions have been also reported for majority of cereal-cultivated soils in many countries, such as in China, India, Iran, Pakistan and Australia.

As expected, under soil conditions having such adverse chemical problems, the genetic capacity of the newly developed (biofortified) plant genotypes to absorb adequate amount of Zn from soil and accumulate in grain may not be expressed at desired level. It is, therefore, important to maintain adequate amount of available Zn in soil, and this can be achieved by applying Zn-containing fertilizers.

Agronomic Biofortification

Application of Zn fertilizers to soil and/or foliar seems to be a practical approach to improving grain Zn concentration (e.g., agronomic biofortification). Very recently, a global zinc fertilizer project has been initiated, so called HarvestZinc project (www.harvestzinc.org) under HarvestPlus program.  This project aims at evaluating the potential of Zn-containing fertilizers for increasing Zn concentration of cereal grains (e.g., wheat, rice and wheat) and improving crop production in different target countries (e.g., India, China, Pakistan, Thailand, Turkey, Mozambique, Zimbabwe and Brazil). The zinc fertilizer strategy represents an important complementary approach to on-going breeding programs for developing new genotypes with high Zn density in grain. As described in HarvestZinc project (www.harvestzinc.org), biofortification of cereal grains through use of Zn fertilizers (e.g., agronomic biofortification) is required for i) keeping sufficient amount of available Zn in soil solution, ii)  maintaining adequate Zn transport to the seeds during reproductive growth stage and iii) optimizing the success of biofortification of staple food crops with Zn through use of breeding tools.

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Figure 3: Efect of various Zn application methods on grain zinc concentration of wheat grown in Central Anatolia (see Yilmaz et al., 1997 for further details)

Increasing evidence is available indicating that soil and/or foliar applications of Zn fertilizers greatly contribute to grain Zn concentrations (Cakmak, 2008). In the past, numerous studies have been published on the role of soil- and foliar-applied Zn fertilizers in order to correct Zn deficiency and increase yield. However, there are only few studies that investigated the effects of Zn fertilizers on grain-Zn concentrations (or in other edible parts). Zinc sulfate (ZnSO4) is the widely applied source of Zn because of its high solubility and low cost. In Central Anatolia, application of ZnSO4 fertilizers was very effective in increasing grain Zn concentration of wheat. Applying Zn fertilizer into soil doubled grain Zn concentrations (Figure 3). As presented in Figure 3, foliarly-applied Zn resulted in much greater increases   in grain Zn concentration than the soil application of Zn. It seems that combined application of soil and foliar Zn fertilizers is the most effective way to maximize grain Zn accumulation. Besides improving grain Zn concentrations, these soil or foliar Zn applications resulted also in significant increases in plant growth (Figure 4) and grain yield in various locations in Central Anatolia (Cakmak et al., 1996).

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Figure 4: Effect of foliar applied Zn on growth of barley plants in Central Anatolia (see Cakmak et al., 1996 for further information)

Due to significant effects of Zn fertilizers on grain yield production in Central Anatolia, farmers showed a growing interest in Zn-containing fertilizers in Turkey since the mid of 1990s. In the past 10-15 years increasing amount of Zn-supplemented fertilizers has been produced and applied in Turkey, especially in Central Anatolia.  The total amount of Zn-containing compound fertilizers applied in Turkey increased from zero in 1994 to a record level of 400 000 tonnes per annum (Figure 5). Use of such high amounts of Zn-containing fertilizers increases in grain Zn concentration, and obviously contributes to human health and nutrition in Turkey, especially in rural areas where wheat provides more than 50 % of the daily calorie intake (Cakmak, 2008).  Little information is, however, available about the effectiveness of Zn-containing compound fertilizers in improving grain Zn concentrations in other countries. In India, Zn-enriched urea fertilizers are becoming an important source for Zn application to wheat and rice. Applying Zn-coated urea fertilizers (up to 3 % Zn) increased both grain yield and  grain Zn concentration in rice (Shivay et al., 2008; Table 1). 

Treatments

Zn Added

Grain Yield

Zn Concentration

 

(kg ha-1)

(ton ha-1)

(mg kg-1 DW)

Prilled Urea

-

3,87

27

0.5% ZEU

1,3

4,23

29

1.0%

2,6

4,39

33

2.0%

5,2

4,60

39

3.0%

7,8

4,76

42

Table 1: Effect of Zn-enriched urea (ZEU)  (up to 3 % Zn in urea) on grain yield and grain Zn concentrations of aromatic rice grown in India. Data show average values of 2-year field trials. For more details see Shivay et al. (2008).

Recent studies also indicate that intercropping systems contribute to grain Zn and Fe concentrations. Various field tests in China with peanut/maize and chickpea/wheat intercropping systems showed that gramineaceous species are highly beneficial in biofortfying dicots with micronutrients. In the case of chickpea/wheat intercropping, Zn concentration of the wheat grains was 2.8-fold higher than those of wheat under monocropping (Zuo and Zhang, 2009). In many wheat-cultivated countries, continuous wheat cropping is a widely used cropping system. Inclusion of legumes in the crop rotation system may contribute to grain concentrations of wheat plants.  Elevated soil organic matter content of soils up to a certain level improves solubility and root uptake of Zn, especially in alkaline soils. There are several reports on combined applications of Zn fertilizers together with organic materials (like farmyard manure and green manures) being particularly effective in facilitating Zn uptake by roots and correcting Zn deficiency (Cakmak, 2008). 

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Figure 5: Use of Zn-containing NP and NPK fertilizers in Turkey (source: Turkish Ministry of Agriculture and TOROS Fertilizer and Chemical Industry, 2007)

Conclusion

Agronomic approaches such as application of Zn-containing fertilizers appear to be a rapid and simple solution to the Zn deficiency problem.  Combination of breeding and fertilizer strategies is an excellent complementary approach to alleviate zinc-deficiency related problems in human nutrition. New research programs are needed to develop or improve Zn application methods in terms of form, dose, and application time of Zn fertilizers. It is important to highlight that use of agronomic biofortification approach to improve grain Zn concentrations might be limited in various developing countries/regions, because resource-poor farmers (e.g., in Africa) cannot afford application of mineral fertilizers, especially micronutrient fertilizers. Under such situations plant breeding becomes a high priority approach to the problem

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