Zinc deficiency in soils, crops and humans

W. AhmAd1, m.J. WAtts2, m. ImtIAz3, I. Ahmed4, m.h. zIA2 5*

1 Faculty of Agriculture, Food and Natural Resources, the University of sydney, NsW, 2006, Australia2 Inorganic Geochemistry Laboratories, British Geological survey, Keyworth, Nottingham, NG12 5GG, United Kingdom3 soil science division, Nuclear Institute for Agriculture, tandojam, sindh, Pakistan4 National Institute for Genomics & Advanced Biotechnology, National Agricultural Research Centre, Park Road, Islamabad-45500, Pakistan5 Research & development section, Fauji Fertilizer Company Limited, 93-harley street, Rawalpindi Cantt., Pakistan

Keywords: bioavailability, crop yield response, human Zn malnutrition, soil application, zinc fertilizer

Introduction. – zinc (zn) is a micronutrient essential for nor-mal healthy growth and reproduction of plants, animals and humans. In plants, zn plays a key role as a structural constituent or regulatory co-factor of a wide range of different enzymes (Barak and helmke, 1993). these enzymes are important in biochemical pathways concerned with carbohydrate metabolism, photosynthesis and in the conversion of sugars to starch. In addition, zn is also vital for protein metabolism, auxin metabolism, pollen formation, maintenance of the integrity of biological membranes and those related to the resistance to infection by certain pathogens (Rashid, 1996; Alloway, 2008). In soils, the total concentration of zn was reported to be in the range of 10-300 mg zn kg-1 (Kiekens, 1995). however, total zn content is not a reliable index to reflect the capacity of soil to supply zn for plant uptake. A very small proportion of the total zn content of a soil (< 1 mg zn kg-1) is present in the soil solution (Kabata-Pendias and Pendias, 1992) that governs zn supply to crop plants. the critical soil zn concentration range for most crops has been reported between 0.5-2.0 mg zn kg-1 for dtPA and 0.5-3.0 mg zn kg-1 for mehlich-1 (sims and Johnson, 1991).

zinc exists in five forms for plant uptake: a) as a free and complexed ion in soil solution; b) as a non-specifically adsorbed cation; c) as an ion occluded mainly in soil carbonates and Al oxide; d) in biological resi-dues and living organisms; e) as lattice structures of primary and second-ary minerals (Iyenger et al., 1981; Neilsen et al., 1986). zinc is taken

Agrochimica, Vol. LVI - N. ?? ?? 2012

* Corresponding author: munirzia@gmail.com

Received December 25, 2011 – Received in revised form April 24, 2012 – Accepted May 20, 2012

W. AhmAd et AL.2

up as zn2+ or as zn(Oh)2 at high ph. In case of low concentrations of zn in the soil solution, uptake is mainly through direct root contact and is metabolically controlled. the phytosiderophores are released from the roots as a result of zn deficiency which can chelate zn for onward absorption by crop plants. zinc deficiency has been identified through-out the world. zinc deficiency in crops resulting in heavy yield losses was reported in Afghanistan (Alloway, 2008), Australia (takkar and Walker, 1993), Bangladesh (Alloway, 2008), Canada (ILzRO, 1975), China (Xie et  al., 1998), Great Britain (Alloway, 2008), turkey (Yakan et al., 2000), India, (singh, 2001), UsA (sillanpaa, 1982; Leon et  al., 1985), Iraq (sillanpaa, 1990), Pakistan (hamid and Ahmad, 2001; sillanpaa, 1990) and syria (sillanpaa, 1982; CImmYt, 2011).

zinc deficiency is common throughout the arid and semi-arid regions of the world (takkar and Walker, 1993) due to low zn solubility and high zn fixation under such conditions (donner et al., 2010). A signifi-cant amount of zn is present in the soil matrix, but only a small fraction of that is available to plants (Rahmatullah et al., 1988). several soil factors and conditions may render soils deficient in total and available zn. Weathered parent material, nature of clay minerals, alkaline ph, sandy texture, high salt concentrations, calcareousness, waterlogging or flooding, organic matter content, high magnesium and/or bicarbon-ate concentrations (also in irrigation water), more nutrient uptake than application, intensive cultivation and the use of high analysis fertilizers (i.e., poor in micronutrients) are considered to be the major factors asso-ciated with the occurrence of zn deficiency (Alloway, 2008; 2009). Variations in soil ph, lime content, organic matter, clay type and the amount of applied phosphorus fertilizer can significantly affect the zn bioavailability (Adiloglu and Adiloglu, 2006). Apart from other fac-tors, zn deficiency is one of the factors responsible for low yield. the introduction of high yielding crop varieties in the past and their imbal-anced fertilization also contributed towards zn deficiency in many parts of the world. the problem of zn deficiency, especially in the developing world, has been furtherly aggravated due to a lack of information on zn sensitivity and by growing cultivars, which are highly susceptible to zn deficiency. In a recent study, Broadley et al. (2010) observed a wide variation in shoot zinc concentrations within Brassica oleracea L. genotypes. however, local environment factors had a profound effect on shoot zinc, and its (shoot zinc) heritability was relatively low compared

A GLOBAL ReVIeW ON zINC deFICIeNCY 3

to a previous study on shoot Ca and mg contents by the same authors (Broadley et  al., 2008). Remarkable insights into the evolutionary potential of plants to respond to elevated soil zn have recently been made through detailed anatomical, physiological, chemical, genetic and molecular characterizations of the brassicaceous zn hyperaccumulators Thlaspi caerulescens and Arabidopsis halleri (Broadley et al., 2007)

diagnosis and correction of zinc deficiency. – a) Zinc defi-ciency and crop response. – Compared with other micronutrients, defi-ciencies of zn are more widespread. several soil types having ph above 6.0 and low in organic matter are prone to deficiency of this micronutri-ent (mortvedt and Gilkes, 1993). About 70% of agricultural soils of Pakistan have been reported to be deficient in zn (hamid and Ahmad, 2001). In China, the order of importance for potential deficiencies was reported to be: zn 51% > mo 47% > B 35% > mn 21% > Cu 7% > Fe 5% (zou et al., 2008). In India, analysis of 14,863 soil samples from all over the country showed that 49% of soils were potentially deficient in zn, 33% in B, 12% in Fe, 11% in mo, 5% in mn and 3% in Cu (singh, 2008). Alloway (2008; 2009) reported widespread deficiencies of zn throughout India, Pakistan (50-70% of soils), Bangladesh (~2 m ha of paddy soils), China, Japan, Philippines (> 8 m ha) and the United states. In the United states, zn deficiency for one or more crops was reported for 30 states (Berger, 1962). According to an estimate, 50% of soils in turkey (eyupoglu et al., 1994), 30-70% of soils in India (takkar, 1991) and about 70% cultivated soils in Pakistan were zn deficient (Rashid et al., 1988). Also these soils are alkaline, calcareous in nature. the combination of high ph, CaCO3 together with low organic matter was discussed as major factors lowering zn availability to plant roots (Cakmak et al., 1996). the crops grown on such soils produce a lower yield due to the lower availability of zn.

stagnant yields of major crops have been ascribed to imbalanced use of fertilizers and micronutrient deficiencies, particularly for zn and B (Kausar et  al., 2001). A reduction in yield of approximately 28% for maize was recorded when zn was omitted from the fertilizer treat-ments applied to zn deficient soils (Qayyum et al., 1988; Rashid and Qayyum, 1991). during field trials, sillanpaa (1982; 1990) found that all 9 sites in different regions of Pakistan were deficient in zn. highly calcareous soils of Pakistan with an average ph of 8.2 actually give rise to zn deficiency in cereals, fibres and newly planted fruit trees. Rashid

W. AhmAd et AL.4

(2006) estimated a substantial potential net economic benefit from the use of zn fertilizers in zn-deficient crops.

b) Sources, rates, methods and timing of Zn application. – there are five major sources of zn, namely: a)  zinc sulphate monohydrate (znsO4.h2O), which mostly contains approximately 35% total zn, of which 98% is water-soluble; b) Ammonical zinc sulphate znsO4-Nh3 complex (zn 10-15%); c) zinc oxide (znO), a common inorganic salt of zn derived from many industrial processes. znO normally contains 70 to 80% zn but is less soluble than znsO4; d) zinc oxysulfate (znOx), formed by adding h2sO4 to zn feedstocks. these feedstocks are com-monly znO industrial by-products. the water solubility of these fertil-izer materials is variable and is related to the amount of h2sO4 added during the manufacturing process. mostly it contains 18-50% zn; and e) zinc edtA (znedtA). A chelate is a liquid zn fertilizer with 9% total zn that is 100% water soluble. Raven and Loeppert (1997) sum-marised zn concentrations from different sources of fertilizers available worldwide (tab.  1). Among these, znsO4 is commonly used in rice, wheat, barley, potato, citrus, deciduous fruits, and many other crops. Generally, a zn source must be 40-50% water-soluble to be an efficient zn fertilizer (slaton et al., 2005a; 2005b). soil-applied zn has a rela-tively long residual effect (Rashid and Ryan, 2004). soil application of zn is occasionally less effective for fruit crops, because the roots of some fruit crops occupy deep soil layers and zn does not easily transfer

table 1. – Zinc contents in some commonly used fertilizer.

Fertilizer typte Zn Contents (mg kg-1)

Ammonium sulfate 6.40 ± 0.56monoammonium phosphate 10.3 ± 2.6diammonium phosphate 386 ± 17triple superphosphate 61.3 ± 4.2Potassium chloride 4.59 ± 0.58Potassium-magnesium sulfate 8.75 ± 0.79North Carolina rock phosphate 382 ± 16tilemsi rock phosphate 78.8 ± 6.0dolomite 8.01 ± 0.93Austinite 563 ± 26milorganite 450 ± 21

modified from Raven and Loeppert (1997).

A GLOBAL ReVIeW ON zINC deFICIeNCY 5

downwards between soil layers. thus the deficiency of zn can be better controlled by making up deficiency doses through soil and followed by maintenance doses through foliar feeding (zia et al., 2006).

Positive responses to zn addition on numerous crops have been documented in many countries. zinc concentrations in roots, leaves, and stems can be increased through the application of zn-fertilisers. Root zn concentrations of up to 500 to 5000 mg kg-1 dry matter, and leaf zn concentrations of up to 100 to 700 mg kg-1 dry matter, can be achieved without loss of yield when zn-fertilisers are applied to the soil (White and Broadley, 2011). Broadcast applications of zn sources should be incorporated into the soil because zn movement in soil is restricted. many researchers have investigated crop response to different sources of zinc to determine an economical source of zn fertilizer. schulte and Walsh (1982) showed that znsO4, znO and zn frits were equally effective for a wide range of crops with bank and broadcast applications. however, approximately twice as much zn was required for broadcast compared to banded applications. mengel (1980) concluded that zn oxide must be finely ground to be effective for correcting zn deficien-cies. Brown (1973) reported that the chelate, znedtA, was 2.0 to 2.56 times more effective per unit of zn than znsO4 fertilizer for edible beans and sweet corn. Relatively small amounts of zn that are normally required to provide a significant improvement in zn status of an annual crop, namely 1–2 kg zn ha-1, are in broad agreement with such recovery rates. On average, recommended zn application rates range from 1 to 11 kg ha-1 (snyder and slaton, 2002). Pakistan Agricultural Research Council recommends a dose of 2.5 kg ha-1 elemental zn for major crops like cotton, rice and wheat (NFdC 1998). For many crops, soil applica-tion of 5 to 11 kg zn ha-1 is recommended for 3 to 4 crop seasons to sus-tain crop production. Based on an average of several studies worldwide, the residual effect of zn fertilizer application persists for three years (zia et al., 2006).

Compared with the broadcast technique (10 kg zn ha-1), seed enrich-ment by applying 20 kg zn ha-1 to the nursery bed gave a greater yield increase in the rice crop grown in a variety of soils differing in zn sta-tus (Rashid et al., 1999). Growers can confidently apply zn fertilizer solutions or granules to the soil surface without incorporation before emergence, with recommended rates (11 kg zn ha-1) of granular zn pre-ferred for alkaline, zn-deficient soils (slaton et al., 2005a). ekiz et al. (1998) found no significant difference in crop yield when zn doses were

W. AhmAd et AL.6

applied in the range of 7-21 kg ha-1, indicating that 7 kg zn ha-1 would be sufficient to overcome zn deficiency.

Foliar application of zn in fruit trees has several advantages over soil application that include high effectiveness and a rapid plant response. however, foliar absorbed zn is not easily translocated in plants, which necessitates repeated spray applications. the addition of urea to zn sprays may improve zn absorption. some fungicides like dithane m-45 and zineb also contain zn, and therefore, recommended foliar doses should be adjusted accordingly. due to the restricted mobility of zn in plant tissues and keeping in view their physiological character, authors of this paper are of the view that as the orchard crops try to accumu-late maximum amounts of essential nutrients before flower formation. therefore, micronutrient foliar sprays should be made preferably after fruit harvest and before flower formation in addition to recommended deficiency doses already applied through soil. zinc foliar sprays applied before anthesis may be most beneficial in terms of yield in citrus and other fruit plants like grapes (Neilsen and Neilsen, 1994).

the amelioration of zn deficiency using chelated zn in a wide range of crops is not common due to reservations regarding its efficacy and the relative economic cost compared to other approaches. Chelated zn sourc-es, which are more expensive, are usually applied at 1-2 kg ha-1, so that lit-tle residual zn is available for the next crop. In chelated forms, metal ions are less likely to react and are more likely to be available for plant uptake. In the fertiliser business markets, especially in developing countries, some products are organically complexed micronutrient sources, but sold and labelled as organic chelates. such products are formed by reacting metallic salts with various organic industrial by-products (e.g. sugar cane molasses, by-products of wood pulp industry). the term “complexes” is used because the structure of these by-products is not well defined and there is no evidence that the resulting product has true chelate properties. Producers of organic sources generally claim a 10:1 advantage of organic sources versus inorganic sources of micronutrients. however, most of the research work reported that for znedtA, a true organic chelate, there is approximately a 3:1 to 5:1 advantage. the efficacy of chelated micronu-trients was only well proven for iron (tandon, 1995). Foliar application of the micronutrients (including zn) in the form of sulphates at higher application rates might be more effective than the chelates due to a lower cost (modaihsh, 1997). this conclusion was based on trials using wheat crops grown over calcareous soil where, opposite to chelated forms, appli-

A GLOBAL ReVIeW ON zINC deFICIeNCY 7

cation of the micronutrients in sulphate form generally resulted in higher concentrations of these elements in grain.

therefore, to apply the correct dose of zn fertilizer and to distribute it uniformly, extreme care is required. In comparison, banded soil appli-cations or foliar sprays, require higher rates than for broadcast applica-tions. since zn is immobile in plants, zn deficiency can occur in crops growing in soils with marginal zn levels during peak growing periods (vegetative, flowering and seed development stages), and so a steady supply throughout the growing season is essential. during restricted root activity, foliar fertilization is an effective way to supply zn to plants (mortvedt, 1994).

Yield response of selected crops to zn fertilization. – a) Rice. – Rice (Oryza sativa L.) is the staple food of more than one-half of the world’s population. On a global basis, rice provides 21 and 15% per capita of dietary energy and protein, respectively (Van Breemen and Quijano, 1980). Based upon responses to the zn appli-cation, widespread deficiencies throughout India, Pakistan (50 to 70% of soils) Bangladesh (~2 m ha of paddy soils), China, taiwan and UsA have been extensively reported (Alloway, 2008; 2009). severe zn deficiency causes loss of grain yield, and rice grains with low zn content contribute to human nutritional zn deficiencies (Johnson-Beebout, 2009). many alkaline silt loam soils (ph  >  7.0) from Arkansas, California, Louisiana, mississippi, missouri, and texas states have shown a significant positive response to zn application. In the United states, zn is often applied to these (and occasionally to clayey) soils at 1 to 11 kg zn ha-1, depending on the zn source and time/method of application (snyder and slaton, 2002).

zinc fertilization significantly affected grain yield (an increase by 12 to 180%) at all the tested sites within Us as compared with the unfertil-ized. Whereas, zn application time, averaged across zn sources, sig-nificantly affected rice grain yield at only one of the tested sites, which had severe zn deficiency (slaton et al., 2005a; 2005b). the authors noted that zn applied either at the pre-plant incorporation stage or at the delayed pre-emergence stage produced similar yields that were greater than zn applied at post-emergence (before flooding at the four-leaf) stage. zinc solutions sprayed at 1.1 to 2.2 kg zn ha-1 generally produced yields that were comparable with yields from granular fertilizers applied at 11.2 kg zn ha-1.

W. AhmAd et AL.8

Prado et al. (2008) studied the effect of seed application of differ-ent zn sources (znsO4 and znO) and concentrations (0, 1.0, 2.0, 4.0 and 8.0 g kg-1 of seed) on the initial nutrition and growth of the highland rice cultivar. the results showed that znsO4 yielded greater production of total dry matter in relation to znO. the application of 3.92 g zn kg-1 of seed, using the znsO4 source provided the greatest increment in dry matter (48% higher) than a control treatment.

Van Asten et al. (2004) reported that application of zn fertilizer (10 kg zn ha-1) on 29 farmed fields of Burkina Faso eradicated low pro-ductive spots in rice, (attributed to zn deficiency) and increased yields from 3·4 to 6·0 t ha-1. In India, shivay et al. (2008a) conducted experi-ments on the relative yield and zn uptake by rice from znsO4 and znO coatings onto urea fertilizer. sufficient supply of zn under submerged conditions in rice increased the weight of filled grains, the ratio of grain to straw and reduced the percentage of empty grains.

Field trials conducted at varied soils conditions exhibited the preva-lence of zn deficiencies in paddy fields (NFdC, 1998). however, the application of zn on such soils improved both the yield and quality of rice grains. For example, rice responded to zn application in 80% of the field experiments carried out in six districts of Punjab (PARC, 1986). data pertaining to extensive field trials (201 experiments on different representative soils) revealed an average increase in paddy yield (zn application at 2.5-10 kg ha-1) from 12% (coarse grained rice) to 10% (fine grained rice) over the zero-zn (control). the additional benefit of zn fer-tilizer use (value cost ratio – VCR) has been reported to be 3.5:1 in coarse rice varieties and 6:1 in case of fine rice varieties (NFdC, 1998). Relative susceptibility of local rice varieties to zn deficiency was also studied and confirmed in Pakistan, with the aim of incorporating the informa-tion of germplasm in future varietal development programmes (Kausar et  al., 1976). In thailand, heavy applications of phosphate fertilizer (200 mg P kg-1 soil) combined with 10 mg zn kg-1 soil (in the form of znsO4 or zn edtA) increased grain and straw yield on sandy loam soils and also reduced the percentage of unfilled grains (Osotsapar, 1999). Nammuang and Ingkapradit (1986) explored two different types of zn fertilizer (znsO4 and zn edtA) and found that they had different effects on P uptake in rice. zinc sulfate reduced phosphorus uptake and the P content of the rice shoot, while zn-edtA increased uptake of P.

In rice, zn uptake efficiency correlates with exudation rates of low-molecular weight organic anions and a substantial proportion of the

A GLOBAL ReVIeW ON zINC deFICIeNCY 9

phenotypic variation in zn uptake efficiency is under genetic control (hoffland et al., 2006). Rice genotypes differ in response to zn appli-cation on deficient soils throughout the world. A long-term sustainable solution to zn deficiency is the development of rice genotypes with superior zn efficiency. In India, sudhalakshmi (2007) worked on the screening of 56 rice genotypes at different zn levels znsO4. the author observed that zn inefficient genotypes had the largest decrease in shoot/root ratio, in stark contrast to those zn efficient genotypes which main-tained almost the same ratio.

b) Wheat. – Wheat (Triticum aestivum L.) is amongst the major cereal crops grown in almost every part of the world. Around 10% of the world’s wheat is produced in regions with a mediterranean-type climate, with predominantly calcareous soils (Lantican et al., 2002). Wheat grown on calcareous soils is also highly prone to zn deficiency (Alloway, 2008; 2009). Wheat as a species is shown to have a relative-ly low susceptibility, but some wheat varieties are more susceptible than others. In general, durum wheat (Triticum durum desf.) is less tolerant to zn deficiency than bread wheat (Triticum aestivum L.) (Cakmak, 2008). A mean yield increase of 43% was obtained in bread wheat when zn was applied at 10 kg zn ha-1 (Cakmak, 2008). A large number of studies have confirmed genotypic differences among wheat cultivars for their response to zn application (dong et  al., 1995; Rengel and Romheld, 2000; Imtiaz et al., 2006) that can be attributed to the differ-ent uptake capacity of zn and adaptability of wheat cultivars to zn defi-cient soils. zn-efficiency factors exist in wheat and zn-efficient wheat is necessary for full exploitation of stored water at depth in nutritionally depleted subsoil. these efficiency factors could give a breakthrough in yield potential of semiarid, high soil ph and dryland salinity prone areas (Graham et al., 1992).

Application of zn in the form of zn-enriched urea considerably increased productivity, zn uptake and efficiency of an aromatic rice-wheat cropping system in a multi-year experiment on a sandy clay-loam soil (shivay et al., 2008b). According to Ozkutlu et al. (2006), fertilization with zn-humate eliminated zn-deficiency symptoms and enhanced dry matter production by 120% in wheat as compared with control (zero-zn). In this study, zn-humate and znsO4 were similarly effective in increasing dry matter production in wheat. the authors con-cluded that under zn-deficient soil conditions, plant growth and yield could be maximized by the combined positive effects of zn and humic

W. AhmAd et AL.10

acids. Wheat and other crops response to zn application in India is sum-marised in table 2.

A large number of field experiments conducted at research stations and on farmers’ fields have verified the yield increases in response to zn application in other parts of the world. Increase in wheat grain yield with zn application generally ranged from 10 to 15% over control (zero-zn). Average yield increment with zn was reported to be 403 kg wheat grains per hectare or 13% over unfertilized (control). the economic response of zn application has been reported both in irrigated and rain-fed wheat. A summary of 19 field trials revealed that the economics of wheat response to zn fertilisation (VCR) varied from 3:1 to 11:7 with average VCR of 7:1 (NFdC, 1998). Rafique et al. (2006) have shown that the soil-applied zn increased wheat grain yield up to 12% over con-trol (zero-zn) in alkaline zn-deficient “typic haplustalfs”. the VCR for this study was reported to be 4:1. the authors further concluded that zn content in mature grain is a good indicator of soil zn availability status, and plant tissue critical zn concentration ranges appeared to be 16-20 mg zn kg-1 in young whole shoots, 12-16 mg zn kg-1 in flag leaves, and 20-24 mg zn kg-1 in mature grains. A summary of response of wheat cultivars to the application of zn in the form of varied concen-tration in wheat leaves compared with one another and control (zero-zn) is shown in table 3.

Yilmaz et al. (1997) monitored the effect of zn application meth-odology on grain yield and concentrations of zn in three wheat cultivars on severely zn-deficient calcareous soils in turkey. Application of zn significantly increased grain yield in all cultivars. the authors concluded that when high grain yield and high zn concentration in grains are desired, soil+leaf application of zn is the most effective method of zn application. Chaudhry and Loneragan (1970) studied the effects of

table 2. – Crop response to Zn application-India.

Crop Fieldexperiments

% trials in response to grain yield (kg ha-1) range of

Average response (kg ha-1)

< 200 200-500 > 500

Wheat 2453 39 37 24 380Rice 2289 23 43 34 760maize 285 47 21 32 670

source: Phillips (2004).

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zn fertilizers on dry matter yields and on zn absorption and distribution in wheat grown in acid loamy sand. zinc deficiency depressed vegeta-tive growth and delayed maturity; however, at maturity the deficiency enhanced straw yields and depressed grain yields. the authors suggested that nitrogen fertilizer could induce copper and zn deficiencies in cereal crops growing on soils with marginal Cu or zn supplies. zinc fertilizers would aggravate the effect of N on Cu deficiency, and Cu fertilizers would aggravate the effect of N on zn deficiency.

Brennan (1996) studied availability of past and current applica-tions of zn fertilizer using single super phosphate for the grain produc-tion of wheat in Australia. the results showed that where zn fertilizer had been applied previously, applications of high levels of N fertilizer to cereal crops did not require further applications of zn if super phosphate (400-600 mg zn kg-1) was used instead of di-ammonium phosphate fer-tilizer. this was because of trace amounts of zn in rock phosphate used to make super phosphate. In another study, Brennan (2001) found that whilst monitoring the residual value of zn fertilizer relative to fresh zinc oxide applied, the level of zn contamination in the single super phosphate application, provided adequate zn to wheat for up to 13 years after application; thus depicting good residual value. the results were similar to those of takkar and Walker (1993) and Brennan (1996). singh and Abrol (1985) in a field study found that the application of zn at 2.25 kg ha-1 to both rice and wheat crops or an annual application of 4.5 kg zn ha-1 only to rice provided the optimum yields for the rice-wheat rotation. Additional higher zn applications (up to 27 kg zn ha-1)

table 3. – Response of different wheat cultivars (Triticum aestivum L.) to Zn application.

Sr. no. Cultivar name Control + Zn All cultivars showed a posi-tive response to zn application as depicted from the increase in zn contents of the leaves

1 Chakwal-86 8.5 23.92 Inqalab 13.9 27.13 Pasban 7.0 20.14 Rawal-87 10.5 20.35 Pothwar 13.2 22.56 Rohtas 12.0 25.57 Bakhtawar 14.0 22.38 Khunduru 12.6 23.59 Pak-81 10.0 19.710 Pirsabak 12.6 21.1

Adapted from Imtiaz et al. (2006).

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increased the availability of zn in the soil and its content in the plants, but did not increase crops yields. Bansal et al. (1990) reported critical deficiency levels of zn for wheat as 0.60 mg kg-1 in the soil and 19 µg g-1 dry matter in 45-day-old plants in field experiments at 26 sites. Further field trials at nine locations with varying levels of zn deficiency showed successive increases in the grain yield of wheat with increases in zn application, thus emphasising the need for zn fertilisation when wheat is grown on zn-deficient soils.

Radford et al. (1995) also emphasised that better crop management including zn fertilisation can significantly enhance wheat yields. Foliar application of the zn in znsO4 form generally resulted in higher concen-trations of these elements in wheat grain than when the chelated forms were applied (modaihsh, 1997). torun et al. (2001) grew 25 wheat cultivars on zn-deficient and B-toxic soil and reported that zn fertilisa-tion significantly increased grain yield of all cultivars for two years. On an average, increases in grain yield were 37% in the first and 40% in the second year. despite large genotypic variation in zn efficiency, shoot zn concentrations under zn-deficient conditions did not differ among zn-efficient and zn-inefficient cultivars. ekiz et  al. (1998) studied effects of zn fertilization (0, 7, 14, 21 kg zn ha-1 as znsO4.7h2O) on grain yield and concentration and content of zn in two bread wheat and two durum wheat in zn-deficient calcareous soils. the results demon-strated existence of a large genotypic variation in zn efficiency among and within wheat genotypes. Increasing doses of zn application resulted in significant increases in the concentration and content of zn in shoot and grain. the results also suggested that plants become more sensitive to zn deficiency under rain-fed rather than irrigated conditions.

c) Maize/Corn. – maize (Zea mays L.) belongs to the gramineae family. It ranks second (after wheat) in world cereal production and was reported to be highly susceptible to zn deficiency. zinc deficiency in sweet corn exhibits a white coloration in the top leaf also known as “white bud”. the plants are stunted and internodes are short. For maize, zn deficiency has been reported to occur mainly in irrigated, alkaline, calcareous, and in sandy soils of low organic matter content. the United states and China contribute a major share towards world maize pro-duction at 43 and 10%, respectively. the prevalence of zn deficiency in maize fields was reported for Canada, United states (30 states), Australia, France, India and the northern part of China (ILzRO, 1975; Xie et al., 1998; Alloway, 2008).

A GLOBAL ReVIeW ON zINC deFICIeNCY 13

For maize, a number of cases of increased yield and positive response to zn application was reported throughout the world with no exception to the United states and Pakistan. A seven year field experi-ment was conducted near Ithaca, New York to determine the corn yield response to zn fertiliser (typic Fragiochrepts) that was moderately low in organic matter, moderately high in P and K, whilst very low in zn. Corn responded to zn fertiliser in 6 out of the 7 yrs of the study where severe zn deficiency symptoms were observed before the start of the experiment (Carsky and Reid, 1990). zn fertilisation increased maize yields in the range of 14 to 45% compared to a control on representa-tive soils (NFdC, 1998). the economics of zn use was very good as the VCR ranged from 5:1 to 22:1 with an average of 9:1. the aver-age increase in yield was calculated to be 18% and the benefits were observed on a wide range of soils. zn application rates and methods affect yield worldwide (Prasad and sinha, 1981; Kanwal et al., 2009; López-Valdivia et al., 2002). singh (2003) explained the role of dif-ferent zn fertilisers, rates and methods of application in diversified yield records in corn (tab. 4). the authors concluded that znsO4, applied as a basal dose, was a better source of zn, since it enhanced corn yield sig-nificantly for both soil series compared to znO and teprosyn fertilizers used as an alternate source of zinc.

d) Cotton. – Cotton (Gossypium hirsutum L.) is an important fibre crop grown throughout the world. several factors contribute to a low crop yield, including zn as micronutrient. Alloway (2008) regarded this crop as zn sensitive, and it is therefore highly responsive to added zn. It suffers from zn deficiency particularly on alkaline calcareous soils (Rashid et  al. 1997). zinc application on such soils yielded positive results, with success stories of yield increases contributing to sustainable

table 4. – Effect of Zn-sources and mode of application on grain yield of corn.

Source fertilizer ModeZn rate (kg ha-1)

Grain yield (t ha-1)

Haplustert Calciorthent

teprosyn (300  g zn+ 200 g P2O5/L)

seed coating 0.06 5.04 3.40

znO (80% zn) seed coating 0.13 5.19 3.09znsO4.7h2O (21% zn) Basal (soil) 5.00 5.74 3.69

source: singh (2003).

W. AhmAd et AL.14

agriculture. zinc deficiencies have also been reported extensively in a number of Us states including California, Louisiana, Arizona and texas as well as New south Wales in Australia, India, Pakistan, China, egypt, turkey and a number of other countries (Alloway, 2008).

In India on several zn deficient soils, application of different sources of zn fertilisers znsO4, znO, farm manure) enhanced cotton yields in the range of 7.2 to 47.6% over the control treatment (Brar et al., 2008). In an egyptian study on cotton (Gossypium barbadense cv. Giza 80), the application of zn produced a significant influence on the seed index, seed cotton yield and lint cotton yield (elwan et al., 2002). shukla and Raj (1987) reported significant differences between cotton geno-types for zn uptake and its utilisation at different growth stages. Irshad et al. (2004) also documented significant differences in growth response of cotton cultivars to added zn fertilizer. Li et al. (1991) evaluated the cotton (Gossypium hirsutum L.) response to zn fertilisation in terms of plant growth and development and yield components in both field trials and pot experiments. Application of znsO4 promoted nutrient (N, P, and K) uptake, increased dry matter production and improved cotton quality. An Australian study found that a continuous supply of zn throughout the season was necessary on a “cracky clayey, high ph soil” (Constable et  al., 1988). this study confirmed that the redistribution of zn from leaves was insufficient to sustain boll requirements on alkaline soils. zn fertilisation (znsO4) on a wide range of soils has been demonstrated to be beneficial for cotton in connection with both yield increases and quality improvement. In the Punjab, an increase in seed cotton yield with zn fertilization ranged from 6 to 13% over the control (NFdC, 1998). In ten field experiments conducted during 1995-96, the increase was 5 to 17% (mean 9%), whereas for seven follow-on experiments (1996-97) the increase was observed to be in a range of 3 to 10% (mean 6%). In contrast, field trials in Faisalabad, exhibited a 50% increase in seed cot-ton yield with 5 kg zn ha-1 (NFdC, 1998). On calcareous, alluvial soils of this country, the use of zn fertilisers is highly profitable with a benefit cost ratio of 15:1 for soil application and 30:1 for foliar spray (Rashid and Akhtar, 2006).

e) Soybean. – soybean (Glycine max L.) belongs to the family Leguminosae. It is among the most important oilseed crops containing 18 to 22% oil, 85% of which as unsaturated fatty acids, and is widely used for biodiesel production. the United states, China, India and Indonesia are the leading soybean growing countries. the occurrence

A GLOBAL ReVIeW ON zINC deFICIeNCY 15

of zn deficiency in this crop, based on responses at multi-location field experiments, has been reported for many countries like India, Pakistan, Australia, Canada, China, thailand and United states. After maize, soybean is seriously affected by zn deficiency in the United states. the main symptoms of zn deficiency in soybean are characterised by stunted plants, interveinal chlorosis, development of necrosis, scarce flowering, malformed pods and delayed maturation (sinclair, 1993). Generally, zn deficiency is a common problem for this crop, especially when grown on alkaline calcareous and sandy soils of low organic matter con-tents. soybean is known to respond positively to zn application on such deficient sites of the world (singhal and Rattan, 1999; moraghan and helms, 2005; Ozkutlu et al., 2006).

Very few field responses of soybean crops to zn fertiliser have been reported, mainly because of genotypic differences. For example, Rose et al. (1981), moraghan (1984) and Bank (1982) provided clear evi-dence that the response of soybean to zn fertiliser is strongly affected by cultivar. seed zn data indicated that zn deficiency had possibly affected seed yields of some soybean genotypes at the lower soil-zn site (moraghan and helms, 2005). In addition, zn humate eliminated zn-deficiency symptoms and enhanced dry matter production by 50% in soybean. zinc-humate and znsO4 were similarly effective in increasing soybean dry matter (Ozkutlu et al., 2006). In another experiment in India using “typic Ustifluvent”, zn sources (znsO4, zn-edtA, znO and znCO3) were used at 0 to 10 mg zn kg-1 soil. With increasing zn application rates, there was an increase in seed yield, zn contents in seed and shoot tissues and in the amount of available zn in soil after harvest (singhal and Rattan, 1999). substantial yield increase (11%) has been observed over control treatment for soybean at a site having zn status of 0.6 mg zn kg-1 (dtPA extractable) in response to zn fertilisa-tion (NFdC, 1998).

f) Potato. – Potato (Solanum tuberosum L.) belongs to solanaceae family and is relatively sensitive to zn deficiency. evidence for the occurrence of zn deficiency in potatoes is based on responses at farmers fields in Pakistan (zanoni and delobel, 1993), hungary (szakal and szalka, 2008), Bangladesh (Alloway, 2008), Norway (Aasen, 1987), in addition, to India, China, Australia, Canada and the United states (Alloway, 2008; 2009). zn deficiency in potatoes gives rise to stunted plant growth in young fern leaves, with early symptoms similar to leaf role. deficient plants are more rigid having shorter upper internodes

W. AhmAd et AL.16

(Alloway, 2008). most of the potato fields surveyed for fertility status have been reported to be deficient in zn. therefore, appreciable crop yield increases (22%) under local conditions were observed in a number of trials at the farmers’ fields (NFdC, 1998).

Imbalanced use of fertilisers is one of the main reasons for the low yield of potato, whilst the balanced use of micro and macronutrients, including zn, could considerably increase potato yields. In Bangladesh, yield increases (5-15%) in farmers fields that were previously zn defi-cient were obtained for potato and other field crops (wheat, maize, sugarcane, pulses, mustard and vegetables) with the application of zn fertilisers. the authors were of the opinion that the zn deficiency prob-lem was likely to increase in the future because of greater crop intensi-fication and transformation of more dry land to wet land by irrigation in floodplain agriculture. In Norway, on a zn deficient heavily limed sandy loam soil, foliar application of 1% znsO4 solution increased potato yield by 200%. the best results of zn application were recorded after 4 to 5 weeks from emergence (Aasen, 1987). In another study, the applica-tion of zn was found to improve yield and other chemical parameters in potatoes on an alluvial danube zn deficient soil in hungary (szakal and szalka, 2008). the results of an experiment from Iran revealed that zn application in the form of znsO4 was most effective in improving potato yield (3.5 t ha-1) over control plots (zero-zn). It was further noted that the quality of potato fertilised with znsO4 applied at 30  kg  ha-1 was improved, compared to other sources of zn (znO) (Farajnia and Ardalan, 1998).

g) Citrus spp. – Citrus production is one of the world’s largest agri-cultural industries. Citrus plants are sown in more than 125 countries in the belt within 35° latitude north and south of equator (duncan and Cohn, 1990). traditionally farmers apply fertilisers according to the requirement of intercropping rather than according to the timing of nutri-ent requirements by the citrus tree. Very few farmers apply fertilisers directly to citrus trees by spreading uniformly under the tree canopy, but not on the drip line. It is well known that zn is required by citrus more than other micronutrients (Yaseen and Ahmad, 2010). zinc deficiency is the most prevalent nutritional disorder in citrus orchards worldwide (srivastava and shyam, 2009). deficiency was reported in citrus belts of Western and south Australia, California, Florida, hawaii, Louisiana states of the United states, Brazil, Iran, India, Nepal and Pakistan (Alloway, 2008; Andersen, 2007; holloway et  al. 2008). Citrus

A GLOBAL ReVIeW ON zINC deFICIeNCY 17

trees often show interveinal chlorosis in response to zn deficiency (Chang, 2000).

the management strategy of zn deficiency is still governed by the efficacy of two conventional methods of zn supply to plants via soil or foliar fertilisation. In India, multi-year (2004-2007) experiments were conducted on “haplustert soil” to investigate the responses of citrus to zn application. the results revealed that both the methods of zn application influenced yield and other major fruit quality parameters like juice, acidity, and tss (srivastava and shyam, 2009). In Iran, Kangarshahi et al. (2007) studied the effect of rates and methods of zn application on yield and fruit quality of satsuma mandarin which responded well to zn as foliar application (4  kg  znsO4 per 1000  L water). Both fruit yield and leaf zn concentrations were significantly increased in addition to increased average fruit weight (23% more over zero-zn). swietik (1995) conducted a study on sour orange (Citrus aurantium L.) seedlings to accentuate the effect of zn application on the plants sensitivity to high B concentration in the root environment. the author concluded that the observed B toxicity symptoms in zn-deficient citrus could be mitigated with zn applications which are of potential practical importance as B toxicity and zn deficiency are simultaneously encountered in some soils of semiarid zones. In addition to other fac-tors, micronutrient deficiency (including zn and B) was also considered among constraints that were hampering citrus yield in the citrus belt of Pakistan (Johnson, 2006). Khattak (1995) reported a 26% increase in fruit yield of citrus at mardan and 16% at Peshawar with a foliar spray of 0.1% zn. soil applied zn may or may not prove to be effective in fruit trees, particularly in the short term. therefore, its foliar applica-tion is recommended for quick results. however, trees with citrus blight also show similar leaf symptoms to those caused by zn deficiency. therefore, citrus orchards obviously need careful investigation before treatment owing to the similarity in symptoms with mn and Fe defi-ciency and ‘citrus blight’ (Alloway, 2008).

h) Groundnut/Peanut. – Groundnut (Arachis hypogaea L.) belongs to the Leguminosae family. About 90% of the world’s total groundnut production is from Asian and African countries. the leading groundnut growing countries include India, China and the United states. zinc deficiency in groundnuts is often associated with high soil CaCO3, high soil ph, and high soil indigenous P levels (Alloway, 2008). the main symptoms of zn deficiency are restricted development of new leaves

W. AhmAd et AL.18

and decreased internode length. Accumulation of reddish pigment in stems, petioles and leaf veins has been reported in zn deficient plants. zinc deficiency causes reduced pegging without appearance of distinct leaf symptoms (ILzRO, 1975). Rashid et al. (1997), based on multi-location trials, have reported that 60% of the groundnut grown fields are deficient in zn. Positive responses of groundnut to zn fertilisation have also been recorded in India, with pod yield increases (shankar et al., 2004; Polara et  al., 2009-10a; 2009-10b). Nayak et  al. (2009) con-ducted field trials in two different farmers’ fields under two agroclimatic zones of Orissa (India) with different treatments of zn. they showed that application of zn to soil and seed dressing on entisols and Alfisols increased the pod yield, shelling outturn and oil content compared to the control. Previously, sahu et  al. (1998) reported that soil applica-tion of zn and other micronutrients registered the highest net return (Us$ 222 ha-1) and increased the benefit-cost ratio. the authors recom-mended that NPK fertilizer along with soil application of zn and other micronutrients were required to increase the productivity of groundnut. In egypt foliar spraying with zn had a significant effect on groundnut growth, yield and its components as well as seed quality (Gobarah et al., 2006). moreover, extensive research is needed to cope with zn deficiency in groundnuts and the viable solution to this deficiency seems to lie in the screening of the available germplasm.

i) Alfalfa. – Alfalfa (Medicago sativa L.; also called Lucerne) is one of the most important forage crops worldwide. It is well adapted to a wide range of agro-climatic conditions and soils. despite of medium level sen-sitivity to zn (Rashid and Fox, 1992), zn deficiency is a factor limiting sustainable production of alfalfa. zinc deficiencies first appear as bronze-coloured specks around the margins of the upper leaves. As the deficiency progresses, the bronze spots become white and the leaves die. Under severe deficiency conditions, the bronze spots appear across the whole surface of the leaflets (Alloway, 2008). Although not common, zn defi-ciency in alfalfa was found to be responsible for reduced seed production. On average, 433 g zn ha-1 is removed from soil through a single harvest of this crop (tandon, 1995). zinc was reported to influence herbage yield, nodulation, disease severity and leaf drop of alfalfa cultivars (Grewal, 2001). In Australia, deficiencies of zn were found in alfalfa in New south Wales and Victoria (Alloway, 2008) thereby affecting yield components. In India, zn nutrition for alfalfa has shown increased forage and seed yield (hazra and tripathi, 1998; Patel and Patel, 2003). similarly, in a

A GLOBAL ReVIeW ON zINC deFICIeNCY 19

Chinese study, zn fertiliser application increased yield and crude protein content of this crop (Liu and zhang, 2005). Adequate zn supply sig-nificantly enhanced the leaf area, leaf to stem ratio, biomass production of shoots, and roots, succulence of plants and zn concentration in leaves of alfalfa (Grewal and Williams, 2000). In a recent study, Ceylan et al. (2009) reported that different zn doses had significant effects on herbage, hay, dry matter, crude protein and zn contents of alfalfa. the highest yields were obtained with 80 kg ha-1 zn. through this application, increase in yields were determined as 30.9% in herbage, 34.7% in hay and 32.1% in dry matter yield compared to controls (zero-zn).

zinc deficiency and crop diseases. – zinc sufficiency was report-ed to increase insect pest and disease resistance in crop plants. however, limited published research is available on this subject. In some cases, applied zn aided plant resistance to virus-induced zn deficiency (Graham, 1983). earlier, zn deficiency had also been reported to be involved in the induction of speckle-bottom symptoms in potato (Cipar et  al., 1974). sparrow and Graham (1988), while growing wheat plants at three levels of zn, monitored the extent of colonisation above the point of infection. the disease incident decreased significantly with an increasing level of zn supply. however, colonisation of the seminal or secondary roots was not affected by zn supply, nor was the incidence of infected plants.

Application of zn fertiliser to wheat reduced the severity of take-all (a disease caused by fungus) and improved the wheat grain yield (Brennan, 1992). however, where zn is adequately supplied for plant growth and yield, further additions of zn fertilisers may not affect the severity of take-all. Limited research work indicated a positive effect of zn sprays on fruit calcium content that resulted in less bitter pit (e.g., in apple). however, the results of other investigations do not confirm these findings (Yogaratnam and Johnson, 1982). Grewal et  al. (1996) showed that zn-efficient wheat genotypes were less susceptible to crown rot disease in wheat caused by Fusarium graminearum in soils with low native zn concentrations. they suggested that grow-ing zn-efficient genotypes and the judicious use of zn fertiliser on zn-deficient areas where crown rot is a problem may yield sustain-able wheat production by reducing the severity of the disease. the plant vigour was also promoted. A similar relation of zn application in curing the problem of rust has also been reported (NIAB, 2007).

W. AhmAd et AL.20

In summary, in most cases zn application reduced disease severity in different crop species throughout the world which might be due to toxic effects of zn on the pathogens directly and not through the plants metab-olism (Graham and Webb, 1996). Furthermore, during zn deficiency in plants, reducing sugars in plant tissues are accumulated (Romheld and marschner, 1990), oxidative damage to the membrane increases due to the presence of superoxide radicals (Cakmak, 2000), thus pathogenecity is favoured (marschner, 1995; mengel and Kirkby, 2001).

Role of zinc in human health. – zinc is an essential trace ele-ment also for human health. humans require at least 25 elements for their well-being (stein, 2010). the dietary sources of most of these elements are crop plants. Unfortunately, mineral malnutrition is prevalent in both developed and developing countries and it is estimated that up to two-thirds of the world’s population might be at risk of deficiency in one or more essential mineral element (White and Broadley, 2009). this is consid-ered to be one of the most serious challenges to mankind. In the past, the focus of agronomists and policy makers was on crop yields. Consequently, crop nutritional quality was neglected and this phenomenon is presumably responsible for mineral malnutrition in humans (Welch and Graham, 2000). the most common deficient elements in human diets are Fe, zn, I, se, Ca, mg and Cu (stein, 2010). the elements like zn, Fe and Cu are as crucial for human health as organic compounds such as carbohydrates, fats, protein and vitamins. zinc deficiency was identified as the most prevalent of all of these elements and has been identified with increasing concern.

there are more than 300 enzymes involved in key metabolic proc-esses in humans which contain zn. therefore, an adequate zn intake is essential for normal healthy growth and reproduction (FAO/WhO/IAeA, 1996). the detoxification of highly aggressive free radicals and structural and functional integrity of biological membranes also critical-ly require zn (Cakmak, 2000). hence, any alteration in zn homeostasis or any decrease in zn status in the human body may pose a number of cellular disturbances and impairments such as: i) retardation of mental development; ii) immune dysfunctions and high susceptibility to infec-tious diseases, and iii) stunted growth of children (Black, 2003). the most vulnerable to zn malnutrition are young children and women of child bearing age (dickinson et al., 2009).

the recommended daily dietary intake of zn for young adults is reported to be 15 mg per day (Kiekens 1995). Intake less than this rec-

A GLOBAL ReVIeW ON zINC deFICIeNCY 21

ommended amount can cause slow physiological processes and results in many diseases. hence, the zn deficiency has received global attention (IzINCG, 2004) particularly due to deficiency in young children below 5 years of age. diseases in infants and children, caused by zn deficiency, include diarrhoea, pneumonia, stunted linear growth, weak immune sys-tem and retarded mental growth (Gibson et al., 2008). zinc deficiency in pregnant women can lead to these problems and even mortality in infants.

Interest in zn nutrition of humans followed the discovery (Pories and strain, 1966) of the role of zn in wound healing. Prasad et al. (1961) were the first to establish the relationship between dwarfism and hypogonadism due to zn malnutrition in boys in the middle east. zn deficiency in grown-up children and adolescent males causes retarded growth and dwarfism, retarded sexual development, impaired sense of taste and poor appetite and mental lethargy (hambridge, 2000). It is estimated that 2 billion people in Asia and 400 million people in sub-saharan Africa could be at the risk of low zn intake (IRRI, 2007). Approximately 40 to 50% of the low income population of Pakistan, especially women and children are suffering from zn deficiency (Rashid, 2005). through the application of disability adjusted life years (dALYs), it was estimated that zn deficiency in India is a highly rel-evant health problem and responsible for a loss of 2.8 million dALYs (stein et al., 2007). metal homeostasis research in plants can lead to nutrient-rich food and higher yielding crops.

Approaches to overcome zinc deficiency in humans. – there are two popular approaches to overcome zn deficiency in humans: a)  biofortification, which can increase the zn content of food grains, which could be achieved by breeding crop cultivars with higher zn concentration in grains or by fertilising crops with zn and b) nutraceu-tical, through dietary supplements or by diversification of the diet by including more fruit, vegetables, and animal products, such as meat, fish, eggs. Use of various common agricultural measures such as soil or foliar-applied fertilisers, biofertilisers, soil amendments, crop and cul-tivar selection and rotations, may increase micronutrient concentration in grain, depending on soil type, crop and other factors (Rengel et al., 1999). subsequently, this necessitates the design of specific agricultural measures for a particular crop-soil-zn combination that will be effective in increasing zn concentration in grain. these increases would have to be achieved without a concomitant loss of yield, if farmers in devel-

W. AhmAd et AL.22

oping countries are to adopt the recommended agricultural measures (Alloway, 2009). soil application of zn as znsO4 has been reported to be an economical and effective method not only for increasing yields, but also grain zn density as well (hemantaranjan and Garg, 1988). there are, however, two concerns: 1)  increased levels of zn fertilisa-tion beyond a certain critical point caused vegetative (Nambiar and motiramani, 1983) and grain yield decline (Rengel and Graham, 1995), because of zn toxicity; and 2) applying heavy metals to agricul-tural environments in increasing amounts should be viewed with some apprehension. Although the judicious use of organic and mineral fertilis-ers may increase micronutrient density in grain destined for human con-sumption, it should be kept in mind that an increase may not be uniform in various parts of the grain. For example, T. aestivum pericarp accu-mulates proportionally more zn than other parts of the grain (Pearson et al., 1996). the processing of grains may need to be adjusted to allow maximum benefits from field fortification of grains to improve entry into the food chain. here is a brief overview of above given approaches:

A) Biofortification approach. – zinc deficiency to some extent can be cured by zn supplementation and improvement in dietary composi-tion. It is better to increase the zn content in cereals, the staple food as a matter of fact in the entire south and south east Asia. this can be achieved by biofortification (enrichment of grains with micronutrients [zn]) of food grains either by developing crop cultivars with high con-centration of zn in grains or by adequate zn fertilisation of crops grown on zn-deficient soils. A concentrated effort for testing of various agri-cultural practices in their capacity to increase micronutrient density in staple foods grown in the field is required. Advantages accrued through increasing micronutrient density in grains via agricultural means may need to be accompanied by changes in the milling practices to achieve full benefits of field fortification of staple crops. Animals can also suffer from zn malnutrition (Prasad, 2010).

Biofortification/breeding strategies and zn (plus other micronutri-ents) enriched fertiliser application are amongst the strategies to cope with zn deficiency. Crop breeding strategy is, however, a long term process requiring different types of efforts and significant resources. A successful breeding programme for bio-fortified cereals with micronu-trients is much dependent on the amount of plant available micronutri-ents pools in the soil. developing crop cultivars with increased grain zn density is a new approach and has received considerable attention

A GLOBAL ReVIeW ON zINC deFICIeNCY 23

in recent years (Pfeiffer and mcClafferty, 2007). Rice is one of the priority crops for enhancement of the nutritional factors such as vitamin A, zn and Fe through international schemes such as harvest Plus and humanitarian Board (Gregorio et  al., 2000). Approximately 1500 lines were screened for zn tolerance at the International Rice Research Institute in Philippines (Guerta-Quijano, 2002). Recently, bioavail-ability of Fe in rice was increased by inserting a gene for heat resistant phytase from fungal sources that degrades phytate in plants (Bhat and Vasanthi, 2005) which may also increase the zn bioavailability in rice. Considerable progress has been made by the plant physiologists in identifying mechanisms for efficient zn genotypes (singh et al., 2005; Broadly et  al., 2007). zn concentrations in fruits, seeds, and tubers are severely limited by low zn mobility in the phloem and zn concen-trations higher than 30 to 100 mg kg-1 dry matter are rarely observed. however, genetically modified plants with improved abilities to trans-locate zn in the phloem might be used to biofortify these phloem-fed tissues (White and Broadley, 2011). In a four year study, White et al. (2012) evaluated the biofortification of potatoes genotypes, whilst using foliar zinc application techniques, they observed only a doubling in tuber zinc concentration, despite achieving a 40-fold increase in shoot zinc concentration (compared to un-fertilised controls) suggesting lim-ited mobility of zn in the phloem.

Addition of zn with regular fertilisers is beneficial for improving plant yield and health and also grain nutrient density. soil-applied zn could be used to increase the concentration of zn in the edible portions of crops, such as grains. In many cases, it can also result in increased yields rendering excess levels of zn to accumulate in soils. this field fortification of crops with zn would need to be accompanied by changes in milling practices to maximise its benefits in terms of dietary availabil-ity (Rengel et al., 1999). Good responses to zn fertilisation have been reported for a number of crops including rice and wheat (Katyal and Rattan, 2003; Prasad, 2006). zn fertilisation is now recommended for most of the regions of the world. shivay et  al. (2008a; 2008b) reported that zn application to soil as znsO4 or zn-enriched/coated urea increased both yield and zn concentration in rice and wheat grain. thus adequate fertilisation of food crops can partly improve zn intake by humans. however, this might not be true for soils highly deficient in zn as under such conditions additional zn supply improves straw and grain yield with an increase in straw zn concentration, but grain zn concentra-

W. AhmAd et AL.24

tion remains unchanged (Wissuwa et al., 2008). Biofortification of rice and wheat grains with zn was reported to save 0.6 to 1.4 million dALYs each year in India. the long-term implications of zn malnutrition for an individual/population, and success of biofortification demands joint efforts at the interface between agricultural and nutritional scientists.

the plant breeding may provide a way of getting high yielding cere-al crops with adequate levels of bioavailable zn in their grain (Cakmak et al., 2010). Indians suffer from zn malnutrition due to the majority of them being vegetarian (Prasad, 2010). south Asian countries and China are better placed in this regard, due to a wider range of foods including several marine products. Animal products such as meat, fish and poultry contain more zn than cereals and do not have phytates. the best way to provide enough zn is to increase the intake of proteinaceous foods such as meat, fish, poultry, cheese, and milk. Increasing the proportion of fruit and vegetables in diets can contribute towards meeting zn requirements (Paul et al., 2006).

B) Nutraceutical approach. – this approach is also commonly referred as dietary zn supplementation. A daily dose of 14 to 40  mg per day was found to considerably reduce the incidence of diarrhoea in infants in India (sachdev, 1988; dutta, 2000), Bhutan (Bhatnagar, 2004), Indonesia (hidayat et al., 1998), Bangladesh (Roy, 1997) and Nepal (Chandyo, 2005). An Indonesian study even suggested a reduc-tion in infant mortality due to zn supplementation (Prasad, 2010). Oral doses of zn can accelerate the healing of chronic skin diseases (hussain, 1969). similarly, zn supplements improved the growth and height of children from low income families in Guatemala, Bangladesh, Iran, turkey, ecuador and Chile (Rivera et  al., 1998; Gibson et  al., 2000). A nutraceutical approach requires programmes for raising public awareness and sustained funding from concerned quarters, which is not easy. Also, such programmes are most likely to help the easily accessi-ble urban population. strict medical supervision needs to be undertaken during fortification of human diets with zn, since excessive consump-tion may lead to toxic effects (Prasad, 2010).

In summary, to tackle zn malnutrition, zn deficiency should be addressed simultaneously in soils, crops and humans. therefore, by increasing zn concentration in plants, both by increasing zn contents in soil or by tailoring the plants appropriate in zn deficient soils would increase zn concentrations in animal products through to human con-sumption. the challenge remains for plant scientists to develop cultivars

A GLOBAL ReVIeW ON zINC deFICIeNCY 25

that respond to zn fertiliser with higher grain yield and higher zn con-centrations when grown in soils with low native zn status.

Conclusions and future research needs. – the use of increased amounts of nitrogenous and phosphatic fertilisers with high yielding hybrid varieties of rice, wheat, maize and other crops, often causes or exacerbates zn deficiency where the plant available zn levels in soils are marginal. thus the prevention and/or correction of zn deficiency in crops have a considerable effect on yield and quality of production. Variable increases in yields (120% in wheat, 48% in rice, 50% in cot-ton, 18% in maize, 22% in potato, 30% in alfalfa, 50% in soybean and 26% in citrus) were observed with zn fertilisation. however, these yield responses can suffer from a number of edaphic and climatic factors. moreover, source, rate, formulation, time and method of zn application and proper balancing of zn with other nutrients in soil are all impor-tant to enhance crop productivity, quality of grains and zn uptake on zn-deficient soils. Although zn deficiency causes characteristic visible symptoms in plants, this often only occurs in cases of relatively severe deficiency. Where the deficiency is more marginal, yields and crop qual-ity may be reduced, or impaired, due to hidden zn deficiency without the development of obvious symptoms in the crop. the hidden deficiencies are always to increase the cost of production and are of greater eco-nomic importance than severe deficiencies showing obvious symptoms. Whenever there are clear zn deficiency symptoms in crops, farmers would be able to take preventive measures and carry out a corrective treatment. however, hidden deficiencies cannot be recognised for sev-eral growing seasons and the farmers will not realise that reduction in yields is due to zn deficiency. On a global scale, reduced crop produc-tion due to zn deficiency will increase imports of grain in countries unable to correct soil-zn deficiencies, which will be an extra burden on the exchequer of many countries, particularly in the developing world.

this review reports that both soil and foliar application methods of zn are effective in ameliorating zn deficiency disorders. higher the VCR value was also accounted for by many researchers where zn deficiency is one of the constraints for low yield. Based on intensive local research experience it is now proven that soil applied zn leaves a residual effect on succeeding crops grown in the same field. soil application of zn at relatively high rates produced residual benefits for 4 to 5 years. the increase in concentration and uptake of zn in horticultural and agronomic

W. AhmAd et AL.26

crops due to zn fertilization is usually parallel to yield increase. zinc deficiencies can be overcome by diagnosing it at a very early stage and possible diagnosis models for zn deficiency symptoms may be evaluated to develop specific heuristics for the domain of application.

Research on the recycling of crops such as rotation systems rather than a mono-cropping culture can generate useful information for zn management. screening of cultivars of major crop species for zn effi-cient strains is likely to be used in plant breeding. this would enable crop genotypes to be matched to soils and reduce the requirement for zn fertilisers. management decisions for the use of zn fertilisers should consider both immediate and long-term effects of zn fertiliser on crop yield, quality of products and economics.

the data generated on the comparative susceptibility behaviour of both agronomic and horticultural crops would be useful for farmers and extension workers for their proactive measures to ameliorate zn defi-ciency. Furthermore, under the changed scenario of climate, research should be conducted on wider scale investigating the shift of low land rice to normal cultivation trend so that negative impacts of zn defi-ciency on this crop due to submerged conditions could be minimised. Collaborative research involving agricultural scientists from various disciplines can provide a sustainable tool for tailoring the crop species to the inherent zn supplying capacity of soils. there is a need to explore the response of zn on different soils; feasibility studies of standardisa-tion of soil test procedures and protocols for major soil types and the related critical values for all types of crops under different growing conditions. this would be a value added research paradigm for the next century to play its role in mitigating food insecurity through sustainable agriculture.

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summary – zinc deficiency is most prevalent in alkaline/calcareous, inherently low organic carbon, heavily limed, light textured, salt prone, and waterlogged soils. In this review the diagnosis and correction of zn deficiency in several important crops in a wide range of soils has been discussed. Crop yield increases up to 120% in wheat, 48% in rice, 50% in cotton, 18% in maize, 22% in potato, 30% in alfalfa, 50% in soybean and 26% in citrus orchards have been reported with application of zn by using appropriate rates, methods (soil or foliar) and sources (such as znsO4). Critical soil zn concentration range for most crops has been reported between 0.5-2.0 mg kg-1 for dtPA and 0.5-3.0 mg kg-1 for mehlich-1. In general, soil application of 5-11 kg zn ha-1 is recommended for 3-4 crop seasons to sustain crop production. In the past, the focus of agronomists and policy makers has been on crop production, rather than crop nutritional quality. this phenom-enon contributed to zn malnutrition in humans. the two most common approaches to overcome zn deficiency in humans are nutraceutical, and bio-fortification, which have been addressed in detail.