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Fluid Journal : Fall 2011
5 The Fluid Journal Fall 2011 potentially Zn deficient that may respond well to Zn fertilizers. Low concentration of Zn in seeds also has negative impacts on growth of plants in Zn deficient soils. 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. Seeds with high nutrient density, especially with micronutrients, contribute greatly to better agronomic performance of seedlings, besides its positive impacts on human nutrition. In the future, particular attention should be paid, therefore, to routine seed analyses for composition of mineral nutrients. Harvesting seeds with high nutrient density represents an important challenge for both better human nutrition and better seedling vigor. Problem solving Supplements. Currently, various strategies are being discussed to alleviate Zn deficiency as it relates to problems in human nutrition. Giving Zn supplements to the target populations or fortification (artificial enrichment) of foods with Zn is considered one useful intervention against the problems. Although these approaches are effective in reducing the extent of the problem, these are not always affordable long-term nor easily accessible to target populations living in rural areas of developing countries. Plant breeding. Alternatively, agriculture offers simple and cost-effective solutions to the problem. Plant breeding and agronomy represent cost effective strategies to alleviate micronutrient malnutrition problems by increasing grain concentrations of micronutrients and their daily intake through diets. It is well-documented that plant genotypes are highly different in use of poorly soluble sources of micronutrients in soils and translocation of micronutrients into grain. For example, in case of Zn, genotypes of a given crop species show impressive genetic variation for Zn accumulation in grain, especially wild and primitive forms of food crops. Such large natural variations in seed concentrations of Zn can be exploited under breeding programs to improve modern cultivars with high concentrations of Zn (e.g., genetic biofortification). The genetic biofortification strategy is a highly Dr. Cakmak is a Professor at Sabanci University, Istanbul, Turkey. promising, cost-effective and long-term solution to Zn deficiency problems in human populations. Currently, impressive progress is being made under different breeding programs in improving stable food crops with high concentrations of micronutrients, especially under the HarvestPlus program (www.harvestplus. org), which is established under the Consultative Group on International Agricultural Research. The Harvest Plus program uses plant breeding tools to improve stable food crops with Zn, Fe, and vitamin A, and to contribute to human health globally. The main sponsor of this global program is the Bill and Melinda Gates Foundation. Cultivars. Developing new genotypes by using the plant breeding approach takes a long time however, plus the success of a breeding program depends on a sufficient amount of readily available pools of Zn in soil solution. High Zn deficiency incidence in human populations is observed mainly in the regions where soils are very low in plant- available (chemically soluble) Zn. The majority of cereal-cultivated soils globally has a number of adverse soil chemical factors (i.e., high pH values, low soil moisture, and low organic matter) that can potentially diminish the expression of high grain Zn trait and limit the capacity of newly developed (biofortified) cultivars to absorb adequate amounts of Zn from soils and accumulate in grain. For example, among the soil chemical factors, soil pH plays a decisive role in chemical solubility and root uptake of Zn. In a pH range between 5.5 and 7.0, Zn concentration in soil solution is decreased up to 45-fold for each unit increase in soil pH. This increases risk for inducing Zn deficiency problems in plants and leading to low yield and simultaneously low Zn concentrations in grain. Increasing cultivation of high-yielding cultivars may further contribute to the extent of Zn deficiency in soils by progressively depleting available soil Zn pools. This depletion of available Zn pools by large off-take in agricultural produce may occur to a greater extent in soils with low Zn solubility. Intensification of farming by introducing high-yielding cultivars contributes not only to Zn depletion in the soil but also to dilution of Zn in the harvested parts of plants such as seeds/ grains. Increasing evidence is available showing that selection of modern cultivars with high yield capacity over more than 100 years caused a clear decline in grain concentrations of minerals, especially micronutrients. Figure 1. Main soil factors affecting solubility and root uptake of Zn in soils (Cakmak, 2008). This is Part 1 of a two-part series. Part 2 will appear in the Winter 2012 issue and cover Zn fertilizer strategies for improving yield and grain Zn concentrations. “Improving zinc concentration improves seedling performance.”