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Physiological role of zinc and its deficiency consequences in plants

Zinc is an essential micro‐nutrients required in optimum quantity to harvest the potential of the plants and to get the maximum yield. Its deficiency is a major yield reducing factor especially in the several Asian countries. Thirty percent soils and more than two billion people in the world are deficient in Zinc. The optimum status of zinc in soils is 1.17 ppm and about 70 per cent of cultivated area of Pakistan is considered as zinc deficient, and its deficiency is third most serious crop nutritional problem after N and P deficiency. Soils of Pakistan are generally alkaline in reaction, calcareous in nature which further reduces the availability of zinc which led to zinc deficiency in cereals especially in the rice tract. This zinc deficiency affects the crop yield and quality which ultimately affects the human health. Almost every third child and 40 per cent of mothers in Pakistan are suffering from Zn deficiency. Required Zn intake depends on gender and growth stage. Generally, it is 10 mg Zn d−1 for adult women and 12 mg Zn d−1 for men. However, women during pregnancy and lactation require up to 14 mg Zn d−1.
Zinc plays an important role in different metabolic processes in plant and its deficiency in crop plants results in significant decreases in both productivity and nutritional quality of the produce. Zinc is part of more than 300 enzymes. Zinc deficiency limits photosynthetic CO2 fixation due to the susceptibility of some photosynthetic enzymes to Zn deficiency and impairments in the utilization of photo assimilates. Zinc also improves human immune function, cognitive development and resistance to some infectious diseases, including diarrhea.
Plant nutrients can be made available by distinctive nutrient application techniques; these consist of soil application, seed treatments and foliar spray. All of these application processes have some advantages and limitations. Foliar application technique increased the concentration of micronutrient such as iron. The application of Zn fertilizer is a common agricultural practice in lowland rice production to correct Zn deficiency and increase grain yield. Fertilizer placement is an important approach. However, it involves high application cost and risk of nutrient loss by fixation or leaching. Nutrients required in small quantities by the plant may accumulate in soil to toxic level and nutrients availability is influenced by soil chemical and physical properties. However, seed priming is an awesome scientific approach to improve the crop performance even in the Zn deficient soils. Seed priming refers to the pre-sowing technique of moderately hydration of seeds to a point at which germination-related metabolic processes begin but radical emergence does not occur. In cereals especially rice, seed treatment with Zn substantially improved growth and grain yield, which was cost effective and economically more viable than soil application. Similarly, seed priming with CaCl2 helps in improving emergence time, final emergence percentage, seedling fresh weight and seedling dry weight, number of tillers, number of panicle, 1000-kernal weight, kernel yield, minimum sterile spikelet production and maximum normal kernel in rice.
However, bio-fortification is a new promising technique to combat the nutrient deficiency, is the enrichment of the grain with zinc, can be achieved through conventional breeding, genetic modifications or by agronomic practices, and is considered a promising and cost-effective approach to combat malnutrition. To sum up it, it is concluded that bio-fortification is a new effective approach to increase the concentration of nutrient not only to feed the world but also to combat the diseases associated with its deficiency across the globe.  
The author is M.Sc. (Hons.) Agronomy, University of Agriculture, Faisalabad, Pakistan.


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