Role of Zinc in Plant Nutrition- A Review

collaboration between all and the with pertaining to the first of the manuscript. YMK the literature and improved its English. All the final manuscript. ABSTRACT Zinc is plant micronutrient which is involved in many physiological functions its inadequate supply will reduce crop yields. Zinc deficiency is the most wide spread micronutrient deficiency problem, almost all crops and calcareous, sandy soils, peat soils, and soils with high phosphorus and silicon are expected to be deficient. Zinc deficiencies can affect plant by stunting its growth, decreasing number of tillers, chlorosis and smaller leaves, increasing crop maturity period, spikelet sterility and inferior quality of harvested products. Beside its role in crop production Zn plays a part in the basic roles of cellular functions in all living organisms and is involved in improving the human immune system, due to its insufficient intake, human body will suffer from hair and memory loss, skin problems and weakness in body muscles.


INTRODUCTION
Zinc is essential for the growth in animals, human beings, and plants it is vital to the crop nutrition as required in various enzymatic reactions, metabolic processes, and oxidationreduction reactions. In addition, Zn is also essential for many enzymes which are needed for nitrogen metabolism, energy transfer and protein synthesis. Zinc deficiency not only retards Review Article growth and yield of plants, but it also has effects on human beings. More than 3 billion people worldwide are suffering from Fe and Zn deficiencies, and this condition is particularly widespread in areas where population is heavily dependent on an unvaried diet of cerealbased foods, in which Fe and Zn are stored almost exclusively in the husk, and are therefore lost during milling and polishing [1,2].

Importance of Zn in Humans
Zinc deficiency is common in humans, animals and plants. More than 30% world's population suffers from Zn deficiency [3]. Zinc plays a part in the basic roles of cellular functions in all living organisms and is also involved in improving the human immune system. The optimum dietary intake for human adults is 15 mg Zn per day. Zinc acts as a catalytic or structural component in various body enzymes.
Unsatisfactory intake and improper absorption of Zn in the body may cause deficiency of Zn. Due to Zn deficiency; the human body will suffer from hair and memory loss, skin problems and weakness in body muscles. Further insufficient intake of Zn during pregnancy also causes stunted brain development of the fetus. Infertility has also been observed in Zn deficient men. Zinc deficiency may cause congenital diseases like Acrodermatitis enteropathica [4,5,6].
As per recommendations an average male need 11 mg of Zn daily while an average female needs 9 mg of Zn. During pregnancy and lactation, the female needs 13 mg to 14 mg of Zn daily. Infants from 7 months to 3 years need 3 mg, 4 to 8 years need 5 mg and children from 9 to 13 years need 8 mg of Zn daily [7,8] . Zinc is stored in the rice husks and grains and with the consumption of this cereal human zinc deficiency can be decreased. The foods rich in Zn are beef, pork, chicken, and breakfast cereals, nuts like roasted peanuts, almonds, walnuts, oats and dairy products like yogurt, cheese and milk [9].

Role of Zinc in Plants
The Zn plays very important role in plant metabolism by influencing the activities of hydrogenase and carbonic anhydrase, stabilization of ribosomal fractions and synthesis of cytochrome [10]. Plant enzymes activated by Zn are involved in carbohydrate metabolism, maintenance of the integrity of cellular membranes, protein synthesis, regulation of auxin synthesis and pollen formation [11]. The regulation and maintenance of the gene expression required for the tolerance of environmental stresses in plants are Zn dependent [12]. Its deficiency results in the development of abnormalities in plants which become visible as deficiency symptoms such as stunted growth, chlorosis and smaller leaves, spikelet sterility. Micronutrient Zn deficiency can also adversely affect the quality of harvested products; plants susceptibility to injury by high light or temperature intensity and to infection by fungal diseases can also increase [11,12]. Zinc seems to affect the capacity for water uptake and transport in plants and also reduce the adverse effects of short periods of heat and salt stress [13,14,15,16]. As Zn is required for the synthesis of tryptophan which is a precursor of IAA, it also has an active role in the production of an essential growth hormone auxin [17,18]. The Zn is required for integrity of cellular membranes to preserve the structural orientation of macromolecules and ion transport systems. Its interaction with phospholipids and sulphydryl groups of membrane proteins contributes for the maintenance of membranes [12,19,17,20,14].
The Zn-finger transcription factors are involved in the development and function of floral tissues such as anthers, tapetum, pollen and pistil secretory tissues in many plant species, it is likely that VvZIP3 may play a key role in both flower and normal fruit development [21,22]. The alteration of VvZIP3 expression during flowering and fertilization can modify the distribution and availability of Zn, hence affects normal reproductive development [23].

Zinc Deficiency in Soil
Zinc deficiency can be found in every part of the world and almost all crops respond positively to application of Zn [3]. Normal soils inherit their trace elements which include Zn primarily from the rocks through geochemical and pedochemical weathering processes. Besides mineralogical composition of the parent material, the total amount of Zn present in the soil is also dependent on the type, intensity of weathering, climate and numerous other predominating factors during the process of soil formation [24]. Meanwhile, high pH and high contents of CaCO 3 , organic matter, clay and phosphate can fix Zn in the soil and give rise to the reduction of available Zn [25]. Soils derived from granite and gneiss can be low in total Zn [26]. Similarly, total Zn is low in highly leached, acid, sandy soils such as the ones found in many coastal areas. Quartz in the soil dilutes Zn from it because the reported concentrations of Zn in quartz are very low, which ranged from 1.0 µg g -1 to < 5 to 8 µg g -1 [27,28]. According to the Food and Agriculture Organization (FAO), about 30% of the cultivable soils of the world contain low levels of plant available Zn [29]. The lowest Zn concentrations were always found in Spodosols (28 µg g -1 ) and luvisols (35 µg g -1 ), while higher levels were found in fluvisols (60 µg g -1 ) and Histosols (58µg g -1 ) [30].
Generally, Zn deficiency is expected in calcareous soils, sandy soils, peat soils, and soils with high phosphorus and silicon [31,17]. The submerged soils are well recognized for the lack of Zn availability to the plants; particularly due to the reaction of Zn with free sulphide [32]. Flooding and submergence bring about a decline in available Zn because of the changes in pH value and the formation of insoluble Zn compounds. Meanwhile, the insoluble Zn compounds formed are likely to be with Mn and Fe hydroxides from the breakdown of oxides and adsorption on carbonates, specifically magnesium carbonate. Under the submerged conditions for rice cultivation, Zn is transformed into amorphous sesquioxide precipitates or franklinite; ZnFe 2 O 4 [33]. Zinc deficiency causes multiple symptoms which usually appear 2 to 3 weeks after transplanting of rice seedlings, with leaves developing brown blotches and streaks that may fuse to entirely cover older leaves, and plants remain stunted, whereas in severe cases, the plants may die, while those which recover will show substantial delay in maturity and reduction in yield [34,35,36]

Zinc under Submerged Soil Conditions
Zn deficiency is very common under flooded soil conditions. In acidic soils, Zn is precipitated as Zn (OH) 2 and as ZnS in sulfur-rich and alkaline soils. The availability and solubility of Zn decreases while pH increases. The oxides of Mn and Zn along with CaCO 3 or MgCO 3 are strongly absorbed by Zn under submerged condition. Whereas in calcareous soils, HCO 3 is the predominant anion, which mainly reduces Zn transport from root to shoot, but not so much the Zn uptake by roots. Under anaerobic condition Zn forms an insoluble Znphosphate. Under this condition plant roots will not take up the soluble Zn from the Zn solution as required by the plant. Under submerged conditions, when organic acid concentration increased, the Zn uptake is reduced and this effects the plant growth. The Zn uptake is also reduced under acidic rhizosphere condition due to the release of H + from the roots and the surplus intake of cat ions over anions. Under acidic rhizosphere conditions, Zn is released from acid-soluble fractions (e.g., absorbed Zn, organic matter or Fe (OH) 3 ) and is available for plant uptake. Generally, rice plants absorb most Zn from solubilization in the rhizosphere because the available Zn in soil is very low in flooding condition [37].

Soil Factors Associated with Zn Deficiency
All types of soil may be affected by Zn including: loams, sands, clays ( with all classification), loess, alluvium, and soils formed from basalt, sandstone, granite, volcanic ash and many other rocks. In general, soils of arid and semi arid regions and the slightly acidic, leached soils of warm and tropical climates are most inclined to Zn deficiency, however, crops are not equally susceptible to Zn deficiency and at the same soil some crops may suffer from Zn deficiency while others are not affected. Major Zn deficiency causes include: (i) Soils of low Zn content (Parent material), (ii) soils with Restricted Zones, (iii) pH, (iv) soils low in organic matter, (v) Microbially inactivated Zn, (vi) Cool soil temperature, (vii) Plant species and genotypes (viii) High level of available phosphorus and (ix) Effects of nitrogen [38,39,40,31]. Zn deficiency problems may occurs in soils with the subsequent characters; (a) strongly alkaline in reaction (b) high phosphorus status by application of phosphatic fertilizers may reduce use of zinc (c) leached sandy soils (d) acid soils of low total Zn status developed on highly weathered parent material (e) calcareous soil (f) peat and muck soils (g) permanently wet (water logged) and (h) high bicarbonate and magnesium in soils or irrigated water [17,41].

Parent material of soils and Zn content
The amounts of Zn in unpolluted soils typically are lower than 125 ppm [42,43]. The major factors affecting the concentration of Zn in soils is the concentration of Zn in soil parent material. The soils derived from gneisses and granites can be low in total Zn and also those originating from sandstone and limestone had lower Zn contents [44,40]. Quartz (sand) in the soils also dilutes soil Zn as concentrations of Zn in quartz are very low which range between 1 -8 µg g -1 [28]. Also total Zn is low (< 30 µg g -1 ) in highly leached acid sands. Zinc deficiency may occur in such soils which are inherently low in Zn. The total Zn concentrations in soils vary between 10 to 300 µg g -1 with an average of 50 µg g -1 [39]. But the average available Zn varied from 1 to 3 µg g -1 (extracted by dithizone). The problem is that only a small amount of soil Zn is available to the crop because of one or more adverse factors. The remainder of the total Zn is fixed in the soil in an insoluble or unexchangeable form and difficult to make available to crop [45].

Soil pH
Zinc availability is highly dependent on pH. When the pH is above 6, the availability of Zn is usually very low. The availability of Zn in alkaline soils is reduced due to lower solubility of the soil Zn. The concentration of Zn in the soil solution decreases from 10 -4 (6.5 µg g -1 ) to 10 -10 M (0.007 µg L -1 ) with an increase from pH 5 to pH 8 [30]. Thus it is more probable that Zn deficiency will occur in alkaline rather than acidic soils. The solubility constant values of ZnCO 3 and hydroxides suggest that a soil having high pH would usually contain a small amount of available Zn. In the case of soils characterized by high contents of hydroxyl (OH -) ions, it is difficult to get a crop response even to applied Zn. The lower availability of Zn under alkaline conditions is attributed to the precipitation of Zn as Zn (OH) 2 or ZnCO 3 [46,24]. The higher carbonate contents in alkaline soils also absorb Zn and hold it in an unexchangeable form [47]. All these factors contribute to the low availability of Zn at higher pH values. Liming of acidic soils increases pH and also the Zn fixing capacity, particularly in soils with high P levels [17]. The movement of Zn in limed soils is considerably lower than in acidic soils so that absorption of Zn by the crop may be low. Liming can thus reduce the Zn uptake [48] and induce Zn deficiency [49].

Soil organic matter
Low organic matter contents in soils give rise to Zn deficiency as it is observed that available Zn increases with increase in organic matter in soil. Soil organic matter is an important soil constituent which originates from decomposition of animal and plant products. The most stable organic compounds in soil are humic substances such as humic and fulvic acids. Both of these substances contain a relatively large number of functional groups (OH, COOH, SH) which have a great affinity for metal ions such as Zn 2+ . Fulvic acids mainly form chelates with Zn over a wide pH range and increases the solubility and mobility of Zn [30]. Simple organic compounds such as amino acids, hydroxy acids and also phosphoric acids are effective in complexing Zn, thus increasing its mobility and solubility in soils [40]. An increase in the organic matter contents of a soil will increase its Zn availability; however, if the organic matter content in soil is too high, like in peat and muck soils, this can also contribute to Zn deficiency due to the binding of Zn on solid state humic substances [50].

Soil texture
Lighter textured soils (sands) contain low levels of Zn. Finer texture soils like clay have higher CEC values and therefore have highly reactive sites and can retain more Zn than lighter textured soils [46]. Therefore heavier textured soils with larger CEC have higher capacities for Zn adsorption than light textured soils [45]. Consequently, Zn deficiency is more likely to occur in sandy than clayey soils. Clay soils adsorb Zn and this adsorption is controlled by CEC and pH [51]. Reddy et al. [52] showed that a certain portion of the Zn adsorbed on the clay was not exchangeable but was acid soluble. This portion of Zn was not available to the plants. [53] found that kaolonite fixes less Zn than bentonite or illite. Thus clays such as bentonite and illite with higher CECs contribute to the fixing of Zn more strongly, thus making it unavailable to plants.

Phosphate fertilizers
Soils with higher phosphate levels, either from native P or due to application of phosphate fertilizers, can cause Zn deficiency stress in crops [31]. Heavy application or prolonged use of phosphatic fertilizers reduces Zn uptake by plants. This effect may be due to the physiological imbalances within the plant [54]. Zinc deficiency due to phosphorus application is termed "P-induced Zn deficiency" [55].

Soil flooding
Zinc deficiency is more often associated with flooded soil than dry soils. For example, rice plants under submerged conditions suffer from Zn deficiency in calcareous soils. But wheat grown in the same soil following rice grows normally [56]. Zinc deficiency due to flooding was a result of Zn reaction with free sulphide [32]. Under the submerged conditions of rice cultivation, Zn is changed into amorphous sesquioxide precipitates or franklinite; ZnFe 2 O 4 [33,57]. Thus a delay in Zn application until after flooding for rice minimizes Zn fixation by sesquioxides [58].

Soil temperature
In warm and moist soils, Zn uptake was higher in rice than in maize (Zea mays L.) [59]. Temperatures below 16ºC during growth caused decreased Zn uptake in maize tops [60]. It appears that Zn deficiency was associated with cool and wet seasons. Soil temperature effects appear to be due to the rate of Zn mineralization [61]. Other factors which can cause Zn deficiency in plants are high light intensity and long day-lengths [62]. Besides the natural soil and environmental factors, soil management practices carried out by man often causes Zn deficiency beside this plants can also suffer from Zn deficiency under adverse climatic conditions such as drought or compaction [31].

Zinc Interaction with Other Nutrients
Interactions occur between the micronutrients and some macronutrients. 'Interaction' may be defined as "an influence, a mutual or reciprocal action of one element upon another in relation to plant growth" [54]. Another factor is the differential response of plants to one element in combination with varying levels of a second element applied simultaneously i.e. the two elements combine to produce an added effect not due to each of them acting alone [54]. Such interactions may take place in the soil and within the plant. These interactions should be taken into account when providing adequate micronutrient supply to plants. Other nutrients may interact with Zn by affecting its availability from soils and its status in the plant throughout the growth process, especially Zn absorption, distribution or utilization. These interactions may enhance or reduce plant growth as a response to Zn. Where an interaction does occur, it is necessary for the diagnosis and treatment of Zn deficiency to identify the factors and its sites and modes of action [63]. Some important interactions of Zn with other nutrients will be discussed below.

Phosphorus-Zn interactions
The study of the interaction between P and Zn started in1936 [64] and till now, this important plant growth disorder is still under investigation. The interaction is usually termed 'P-induced-Zn deficiency'. This disorder in plant growth is associated with high levels of available P or with application of P to soil. The Zn deficiency symptoms can be prevented by the application of Zn fertilizers. The actual causal relationship and mechanisms are still not fully understood. In general, four possible causes have been considered responsible for Pinduced-Zn deficiency. These include (i) a P-Zn interaction in soil; (ii) a slower rate of translocation of Zn from the roots to shoot; (iii) a simple dilution effect on Zn concentration in plant tops due to growth responses to P; (iv) a metabolic disorder within plant cells related to an imbalance between P and Zn [54].
It was suspected that formation of an insoluble Zn 3 (PO 4 ) 2 in the soil reduced the Zn concentration in soil to deficient levels. But these suspicions were disproved and further observed that Zn 3 (PO 4 ) 2 was a good source of fertilizer for sorghum [65]. The investigation of this precipitation as a mechanism that causes Zn deficiency continued till 1970. [66] reported that maximal or near maximal yields were found with legumes at 0.05 µM Zn in flowing culture and with cereals at 0.01 µM. This evidence indicates that precipitation of Zn 3 (PO 4 ) 2 is not involved in P-induced -Zn deficiency. Many researchers have reported that applied P accentuated Zn deficiency symptoms in plants [67,68]. The higher P levels in soil reduced the Zn concentrations in the plant tops and also reduced total Zn contents [55,69]. These scientists suggested that P-Zn antagonism existed in the roots of the plants. Other studies suggested that although P decreased the Zn concentrations in the tops, the total Zn contents either increased or remained the same [70,71]. The cause of this P-induced-Zn deficiency has been suggested to be due to interference by P with the uptake, translocation, or utilization of Zn [72].

Nitrogen-Zn interactions
Zinc deficiency can be increased or ameliorated in plants with the application of nitrogen fertilizers. The interactions resulting from the effects of N application helps to promote plant growth and, to a lesser extent, in changing the pH of the root environment since application of N promotes the growth of plants, it is possible to find positive interactions between increasing levels of Zn and N fertilizers [17]. It was reported that wheat grown on N deficient soil with adequate levels of all nutrients except N and Zn, did not respond to Zn application in the absence of NH 4 NO 3 fertilizer, however, a strong response to Zn application was observed in the presence of N fertilizer [73].
On the other hand, in soils low in Zn and high in fertility, N fertilizers have ameliorated (or intensified) Zn deficiency by affecting Zn absorption through changing pH [74]. As ammonium ions have an acidifying effect, ZnSO 4 application with concurrent dressings of N were very effective in controlling Zn deficiency where ZnSO 4 alone had no effect [74]. It was also observed that NH 4 + salts inhibited Zn absorption from low Zn 2+ concentration, in a short term study with wheat. Ammonium ions inhibited Zn 2+ absorption more strongly than alkali and alkaline earth anions, but were competitive with alkali and alkaline earth cat ions. So NH 4 + effect would be diminished by relatively high concentrations of competing ions in soil. Thus any direct effect of NH 4 + on Zn absorption would disappear [75].

Macronutrient cations-Zn interaction
Macronutrient cations such as Ca, Mg and K inhibit the absorption of Zn by plants from solution. They need to be considered when interpreting the results of solution culture experiments involving Zn nutrition, however, in soil they seem to be less effective in the inhibition of Zn absorption compared to the effects of their salts on soil pH. Zinc concentrations were highest in legumes grown in solution culture at constant pH with the lowest Ca level at which the plants were not Ca deficient. Zinc concentrations progressively decreased with increasing Ca concentrations in solutions [76]. This finding that Ca inhibits Zn absorption was in accord with a short term study conducted by [75]. They found that increasing concentrations of Ca (NO 3 ) 2 from 0 mM to 40 mM inhibited the rate of Zn absorption by wheat seedlings in a non-competitive manner, however, higher Ca concentrations (100mM) had no additional effect on Zn absorption. This inhibition was attributed to Ca as varying the anions and had little effect on Zn absorption, whereas substituting other cat ions for Ca had similarly negative effect.
In soils, the effects of Ca compounds on Zn nutrition are variable, due to the effects of its salts on soil pH. Zinc concentrations in plants growing in soil treated with CaSO 4 (which decreased the soil pH from 5.6 to 4.8) increased slightly but decreased strongly when an equivalent amount of CaCO 3 was applied (which increases the soil pH from 5.7 to 6.6) [77].The macronutrient cat ions K, NH 4 and Mg all inhibited the rate of Zn absorption strongly from solutions of low Ca concentrations; with increasing Ca concentrations, the inhibitory effects weakened and in the case of two ions (K, Mg) tested at sufficiently high Ca concentration (2.5-10 mM) eventually disappeared, suggesting that they operate through the same mechanism as Ca [75].

Copper-Zn interactions
It was reported that Cu and Zn may interact in several ways: Zn strongly depresses Cu absorption, Cu competitively inhibits Zn absorption and Cu nutrition affects the redistribution of Zn within plants [63]. A very strong Cu-Zn antagonism has been observed in wheat growing on soils deficient in Cu and Zn [56]. In N-Cu-Zn experiment, [73] found that N fertilizer increased grain yield in the absence of Zn and diluted Cu concentrations to deficiency levels in plant. Addition to that, Zn along with N fertilizer intensified the Cu deficiency so severely that grain yield was lower than in the control plants (without NH 4 NO 3 ). In this case, Zn intensified Cu deficiency in plants by depressing Cu uptake. This may be a result from competitive inhibition of Zn on Cu absorption [78]. The competitive inhibition of Cu 2+ ion on Zn 2+ absorption has been established in short term studies [79]. While Zn severely depressed Cu uptake by wheat, Cu did not depress Zn absorption in the same experiment. The reason for the difference in soil and solution culture results may be the form of these ions present in the soil and solution. In solution studies, the Cu and Zn were present as divalent ions whereas in most of the soils they are predominantly present as complex forms and a much higher proportion of Cu is complexed compared to Zn [80]. So Zn 2+ activity would be much higher than Cu 2+ activity at the absorbing sites making it an effective competitor in Cu absorption and making its absorption less sensitive to competition from Cu [63].

Iron-Zn interaction
The interaction between Zn and Fe is also complex like P-Zn interaction. The increased application of Zn had little effect or decreased Fe concentrations in the shoot [81,82]. In the same way, higher levels of Fe generally have only a depressive effect on Zn concentration in plant tissues [83], although it has been shown to increase have no effect on or to decrease the rate of Zn absorption by plant roots. These conflicting reports are probably due to differences in experimental details, especially in plant species and the concentration, ionic state and complexation of Fe [79].
Iron (Fe 2+ ) at low concentrations (10 µM) had no effect on the rate of Zn absorption by wheat seedlings from solutions containing 1 or 10 µM Zn and 50 mM Ca(NO 3 ) 2 [72]. But at higher concentrations (100 µM Fe 2+ ), and at concentrations likely to occur in flooded rice soils, Fe completely suppressed the Zn absorption by rice seedlings from a solution of 0.05 µM ZnCl 2 with no Ca [79]. Iron deficiency increased Zn concentrations in shoots of plants [84] and also the rate of Zn absorption in both dicotyledonous plants [85] and grasses [83]. In dicotyledonous plants, the mechanism for increasing Zn absorption is probably the acidification of the rhizosphere resulting from Fe deficiency [62]. For grasses, the release of phytosiderophores under Zn deficiency is responsible for the higher Zn absorption rate as phytosiderophores have enhanced the mobilization of Zn from calcareous soils [83,86]. In a similar way, under Zn deficient conditions, Fe accumulated in the shoots of Zn deficient navy beans and corn plants are possibly due to the acidification of the rhizosphere and the release of reductants and phytosiderophores [87,88].

Dilution Effect
When the rate of plant growth is faster than the rate of uptake of a particular nutrient, the concentration of the nutrient is "diluted" in the plant [55,54]. These researchers also showed that in wheat and bean the yield and total Zn content increased with P application while the Zn concentrations in plants decreased. A response in yield was found for applied P, so a dilution effect on Zn is largely accountable for this effect. In general, this interaction occurs when the soil is deficient in P and/or slightly deficient in available Zn. The growth rate increases due to applied P but the rate of Zn uptake does not increase fast enough to maintain Zn concentration in plants. In some cases of P-induced-Zn deficiency the dilution effect only partially explains the data. The applied P reduces the Zn concentration in the tops of the plant while the yield response to P is minimal [55].

Distribution of Zn in Plant Roots and Tops
Zn is an essential micronutrient involved in a wide variety of physiological processes [12,89,90,91,92,93]. Zn uptake varies among plant species and is determined by the composition and concentration of the growth media. Zn uptake occur as divalent cation or as complexes with organic ligands and display a linear pattern with its concentration in the nutrient solution of soils [19], roots load it via xylem to the shoot tissues [93]. The Zn translocation to roots xylem occurs via symplast and apoplast but its high levels have also been detected in the phloem, denoting that this metal is translocated through both xylem and phloem tissues [18,93,94].

ZINC EFFICIENCY
Zn efficiency can be defined as "the ability of plants to maintain high yields in soils with low Zn availability". Many mechanisms are perhaps involved in Zn efficiency [95]. Depending on the nature of experiments and plant species, the most significant mechanisms may be Zn utilization in tissues and Zn uptake [96,97]. Under Zn deficiency, Zn-efficient genotypes have a high activity of Cu/Zn anhydrase [98,99,100] and carbonic anhydrase [101,102]. Zn efficiency and Zn uptake are very susceptible for plant growth and its total content in soil is influenced by several soil properties like pH, CaCO 3 , organic matter content, crop, as well as cultivars and nutrient interactions in soil environment. There is no precise mechanism used in determining Zn efficiency is available so far; however, several crops have been evaluated for their Zn efficiency like: beans [87,103,96], wheat [104,100] and rice [105,106,107,108,109].

Effect of Zn Deficiency on Plants and Its Correction
Zinc is an essential micronutrient for plant growth and plays an important role in the catalytic part of several enzymes [110] its deficiency will result in stunted growth. Many researchers observed that Zn is closely related to the nitrogen metabolism pathway of plants, thus causing a reduction in protein synthesis for Zn deficient plants. Zinc deficiency significantly affects the root system including root development [111]. Zinc deficiency affects the absorption of water and nutrients from soil and thus resulting in growth and yield reduction in the plant. [112] indicated that the flowering and fruiting process were greatly reduced under severe Zn deficiency.
Zinc deficiency is one of the major constraints in world food production. Identification of Zndeficient areas, and causes would help in planning the appropriate strategies to correct these Zn deficiencies. Although Zn is widely used as a fertilizer, but efficient and economical methods to correct its deficiency on a long term basis and in a specific cropping system is desirable. Zinc deficiency can be corrected through the application of Zn fertilizers, recycling crop residues, natural organic manures and cultivation of Zn efficient genotypes [113]. Zinc fertilizers are broadcast and sprayed onto topsoil, banded in the seedbed, applied as foliar sprays, used as seed treatment and in the case of transplanted rice seedlings, roots of these seedlings are dipped into Zn before transplanting. Zinc sulphate is the commonly used fertilizer compound (ZnSO 4 _7H 2 O containing 26% Zn, or ZnSO 4 -H 2 O containing 37% Zn). Other Zn compounds are Zn chloride (ZnCl 2 ), Zn nitrate (Zn (NO3)2), Zn oxide (ZnO), Zn oxy-sulphate and Zn-coated urea [114].

Zinc Deficiency Symptoms in Plants
Visual symptoms of Zn deficiency in plants are fairly characteristic and are relatively easy to identify. These distinctive symptoms are useful for recognizing acute Zn deficiency and for indicating Zn responsive soils, but not the hidden or marginal deficiencies. The most common symptoms of Zn deficiency include: stunted growth, shortened internodes and petioles, and small malformed leaves (little leaf) which results in the "rosette" symptom in the early growth stages of dicotyledons and "fan shaped" stems in monocotyledons [115]. The deficiency symptoms first appear on young leaves as Zn is immobile under conditions of deficiency. These leaves remain small, cup upward and develop interveinal chlorosis and necrotic spots on the upper leaf surfaces which later join to each other to form brown necrotic and brittle patches. The necrosis is often more noticeable on middle aged leaves which eventually wilt, bend and collapse [116]. Zinc deficiency is typically patchy, even within a single field and symptoms develop rapidly but depend greatly on the degree of stress [117]. Enzyme activity, like ribonuclease activity or carbonic anhydrse activity can be used as an index for precise information. This is particularly important in the initial stages of growth when micronutrient requirements of plants are very low and the total contents of Zn fail to provide precise information about the hidden deficiency. Zinc deficient plants are unthrifty, lack vigor; give patchy appearance with short and thin stems. In young plants interveinal areas are with dark brown necrotic lesions [41].
The visible symptoms in rice are: wilting due to loss of turgidity in the leaves, basal chlorosis of the leaves, delayed development of the plants, "bronzing" of the leaves and in some cases death of the rice seedling. The common symptoms of Zn deficiency in rice are: chlorosis in the mid rib at the base of the youngest leaf within 2-4 weeks after sowing or transplanting and the appearance of brown spots on the older leaves. The spots enlarge, coalesce and give the leaves a brown color. Zinc deficient plants show stunted growth and reduced tillering. If the deficiency is not too severe the plant may recover after 4-6 weeks but maturity is delayed and yields of susceptible cultivars are reduced [118]. The most noticeable symptom is the plant's loss of turgidity, where plants fall over and float on the surface of the water. The basal leaves become pale green and after 3-7 days the leaves become chlorotic. It is important to note that visual symptoms of Zn deficiency in rice vary, to a certain extent, with soil type, cultivar and growth stages. Symptoms can be mistaken for those of N, Mg, Mn or Fe deficiencies which are often combined with Zn deficiency, making it difficult to distinguish between the symptoms of the two. Therefore, plant analysis is required for confirmation [37,17,31].

ZINC CRITICAL LEVELS IN PLANTS
Zinc deficiency is one of the major constraints in world food production. It is therefore essential to identify the Zn-deficient areas, and the different causes of deficiency. It would help in planning the appropriate strategies to correct these Zn deficiencies. Although Zn is being used as a fertilizer, an understanding of efficient and economical methods to correct Zn deficiency on a long term basis and in a specific cropping system is desirable. It was reported by [50] that there is a strong relationship between Zn concentration in tissues with the growth and yield of crops. The critical limits of Zn in plants indicates deficiency as suggested by [37] are :< 10 mg kg -1 definite Zn deficiency, 10-15 mg kg -1 very likely,15-20 mg kg -1 likely and >20 mg kg -1 unlikely (sufficient). In most crop species leaf sufficiency range for Zn 15 to 50 ppm in the dry matter of mature plants and in most cases 15 ppm Zn is considered as critical value [41].

ZINC TOXICITY
The threshold of Zn toxicity varies among plant species, time of exposure to Zn stress and composition of the nutrient growth medium. Plant growth inhibition extends in E. maculata and E. urophylla by five weeks after addition of 400-1600 mM ZnSO4, whereas Pisum sativum became inhibited after 1000 μM Zn application [119,90]. Photosynthesis is strongly affected in plants exposed to heavy metals excess. High Zn concentrations in plants can cause phytotoxicity. The yield may be reduced when plant leaf Zn concentrations reaches about 300 -1000 µg Zn g -1 .A typical phytotoxicity critical concentration is about 500 µg Zn g -1 [120]. The best way to identify Zn deficiency in crops is the determination of Zn concentrations in tissues, however, the results should be interpreted in full recognition of the interaction of Zn with other nutrients because the deficiency of one nutrient may result in excess accumulation of other nutrients by a plant [50].

EFFECT OF ZN ON MICROBIAL ACTIVITY
Microorganism requires various nutrients for their growth and metabolism. Among the nutrients, Zn is an element present in the enzyme system as co-factor and mental activator of many enzymes [121]. Zinc might limit the growth of bacteria at higher levels (>13.60 mg kg -1 ). Furthermore, cell growth as well as microbial populations and their activity in soil were badly affected under high levels of Zn [122,123].

CONCLUSION
Our extensive review of literature has shown that Zn is very essential plant nutrient for all types of crops. It is deficient in all parts of the globe with different types of soils. Under these conditions application of Zn fertilizer is necessary for healthy crop growth and higher yields. Soil and foliar applications of Zn fertilizer are recommended for correcting deficiencies. Soil dressings of Zn chelates, sulfates and oxides should be broadcast and mixed in the soil. Soil applied Zn had residual effects for subsequent crops but foliar sprays have no residual effect and fresh applications must be made to each crop.