Zinc is one of the required micronutrients, that is necessary for enzymes that playing a role in lipids metabolism. co-factor for more than 300 enzymes involved in the plant metabolisms (McCall, et al., 2000; Hafeez et al., 2013; Sadeghzadeh, 2013). More than 30 percent of soils have Zn deficiency for plant’s growth and development (Hacisalihoglu ; Kochian, 2003; Rehman et al., 2012; Hafeez, et al., 2013), the hazard of Zn deficiency involved approximately 60–70 percent of people in Africa and Asia (Gibson, 2006; Farooq et al., 2018). However, the frequency of Zn deficiency in rice is more than other crops, more than 50 percentage of the crop globally faced to this disturbance. Thus, Zn deficiency is found to be one of the main nutritional deficiency that limiting production of rice (Dobermann ; Fairhurst, 2000; Rehman et al., 2012; Ghoneim, 2016; Farooq et al., 2018).
In general, Zn deficiency is envisaged in sandy soils, calcareous soils, peat soils, and soils which is contain high silicon and phosphorus (Alloway, 2008; Hafeez et al., 2013). Zinc deficiency bring out several symptoms that become visible usually 2 to 3 weeks after transplanting, with expanding brown spots and veins in leaves that can wholly coat older leaves, smaller and chlorosis leaves, spikelet sterility, plants remain stunted, whereas in severe cases, the plants may die, while recover plants after 4–6 weeks will show considerable lag in maturity and decline in yield (Dobermann ; Fairhurst, 2000; Mustafa et al., 2011; Hafeez et al., 2013; Prasad, et al., 2016). Zinc seems to affect the capacity for absorption of water and transport in plants and also decrease the contrary impact of short periods of salinity and heat stress (Hafeez et al., 2013). Chemical properties of soil such as pH, organic matter, redox potential, pedogenic oxide and soil sulfur contents play a serious role in regulating Zn solubility in soils (Alloway, 2009).
Nowadays, water deficiency caused turn towards water saving techniques in agriculture, from flooded to aerobic rice systems (Farooq, et al., 2009; Farooq, et al., 2011). Notably, authorization of these water-saving approaches may decrease Zn availability (Gao, et al., 2006). Zinc deficiency takes place in both conventional flooded (Dobermann ; Fairhurst, 2000; Quijano-Guerta et al., 2002; Jan, et al., 2015; Farooq et al., 2018) and direct seeded aerobic rice production systems (Gao et al., 2006). Therefore, it is deduced that rice is mainly cultivating under submerged conditions that are the main cause of Zn deficiency because in this kind of conditions potential of redox will reduce and the formation of irresoluble Zn compounds enhanced like Zn(OH)2 formed owing to enhance in pH, ZnCO3 formed because of the partial pressure of CO2 and ZnS formed because of intense decrease conditions (Jan et al., 2015). Zinc is uptake in plant in the Zn2+ ion form in early stages of growth that is extremely phytotoxic. It was showed that Zn2+ has an important role in the photosynthetic system. particularly, it participates in the reducing the activity of photosynthesis through the disintegration of chlorophylls in lichens (Rout ; Das, 2003; Meng et al., 2017). The principals of resistance mechanisms of Zn deficiency in plants are still weakly comprehended. Multiple potential mechanisms have been recommended to increase tolerance of Zn deficiency: (1) increased availability of Zn in the soil for root uptake, (2) increased absorption of Zn by roots and further translocation and re-translocation from old tissues, (3) cellular homeostasis to hold a higher Zn concentration in the cytoplasm, and (4) efficient use of Zn in active tissues and cells (Hacisalihoglu ; Kochian, 2003).
Application and interactions of Zinc with other nutrients
There are various methods of Zn fertilizer in rice consisting of soil application, foliar spray or seed treatments (Fageria et al., 2002; Johnson, et al., 2005; Ghoneim, 2016; Farooq et al., 2018). Under conventional flooded production systems, soil application is the main method for Zn supply (Dobermann ; Fairhurst, 2000; Rehman et al., 2012; Farooq et al., 2018). The appropriate Zn sources to soil application seem to be an alternative tactic to develop availability of Zn. In general, ZnSO4 is the most broadly applied Zn source for its low cost and high solubility (Ghoneim, 2016), and with compare to other sources (ZnO and Zn-EDTA) the application of ZnSO4 is the best (Cakmak, 2008). Beside, ZnSO4 is raised as best for the seed preparing to construct the grain Zn focus in poor Zn soils as opposed to soil requisition and foliar (Yilmaz et al., 1997; Rengel, et al., 1999; Cakmak, 2008; Jan et al., 2015).
Different approaches of Zn application may have various results in various rice production systems. For instance, soil application enhanced rice yields more than using a foliar spray of Zn under conventional flooded systems (Ghoneim, 2016). Whereas, in dry seeded aerobic rice contrary of this issue was illustrated (Ram, et al., 2015; Ghoneim, 2016). Khan et al. (2003) on an alkaline calcareous soil demonstrated with each of the application approaches paddy yield was increased, but a higher enhance was with Zn soil application compared to the foliar application or root dipping. Also, soil application of Zn (10 kg/ha) improved grain Zn concentration and grain yield in comparison with foliar application (Rana ; Kashif, 2014). Several studies have done to compare soil, foliage and seed treatments of Zn application in rice (Phattarakul et al., 2012; Imran, et al., 2015; Farooq et al., 2018).
The application of Zn by foliar application, soil application, seed priming or seed coating examined by Farooq et al. (2018), they found enhanced grain Zn concentration and grain yield of both puddled transplanted flooded rice and dry seeded rice grown. There were slight differences in the yield under different application methods, nevertheless, Zn concentration of grain was least with seed coating and always highest or equal highest with soil application (Farooq et al., 2018). On the other hand, some researchers believed that foliar application of Zn is more efficient in comparison with soil application owing to the fact that there are more chances for the losses in soil application owing to Zn adsorption and precipitation. Further, applied of Zn at the primary stages and panicle initiation is more important in rice. also there are some possible problems in this approach like rainfall wash off solution, quick drying of spray solution, low penetration rate in thick leaf and incomplete translocation in the leaf of plant (Jan et al., 2015). Arif, et al. (2006) reported that of micronutrients application as foliar enhanced spikelets per spike and 1000-kernels weight. These results are in line with finding of Ghani, et al. (1990) and Naik ; Das, (2007); who showed that soil application of zinc enhanced 1000-kernel weight of rice. Zinc application to nursery had no meaningful effect on grain yield. Zinc solution sprayed on rice seedlings three weeks after transplanting was the most impressive post transplanting treatment to regain its deficiency. Foliar spray can be used effectively to cope with the micronutrients deficiency problem in sub-soil (Mustafa et al., 2011).
Application of nitrogen fertilizer on rice grain Zn-concentration has also demonstrated contradictory results, generally, enhancing nitrogen application adversely influences grain Zn (Kutman, et al., 2010; Shi et al., 2010). Synergistic impacts of zing and nitrogen interaction have been shown in rice (Lakshmanan, et al., 2005). Kutman, et al. (2011) reported that N increased Zn absorption via roots and its translocation to the shoot as well. Nonetheless, high levels of nitrogen leading to extreme vegetative growth rate, which can persuade Zn deficiency in plants under Zn deficient soils (Ozanne, 1955; Prasad et al., 2016).
Phosphor application not only reduced exchangeable Zn and water soluble, but simultaneously enhanced bound forms of soil Zn (Mandal ; Mandal, 1990). Also, phosphor application reduced the root and shoot Zn concentration. Other studies displayed that P application effecting Zn uptake of rice and translocation to shoots (Haldar ; Mandal, 1981; Chatterjee, et al., 1982; Lal, et al., 2000; Rehman et al., 2012; Prasad et al., 2016). Several macronutrient elements, consisting magnesium, calcium, sodium and potassium are known to prevent the uptake of zinc via plant roots in solution culture experiments, but in soils experiment their major effect seems to be through their effect on soil pH (Alloway, 2008). Haldar ; Mandal (1981) showed that Zn application decreased concentration of Fe and Cu, but enhanced Mn in rice roots and shoots. A depletion in Zn concentration owing to iron fertilization was shown in rice (Verma ; Tripathi, 1983; Prasad et al., 2016).
The effort to cope with Zn deficiency
Deficiency of Zinc could be reformed via applying Zn compounds to the plant or soil, but the high cost correlated with adding Zn fertilizers in adequate quantities to cope with Zn deficiency, especially in the places where the poor farmers are planting, thus, adding the fertilizer is not affordable for poor farmers. It has therefore been recommended that breeding efforts should strengthen by researchers to develop the Zn deficiency tolerance rice cultivars (Singh, et al., 2005; Wissuwa et al., 2006). Existence of great genotypic diversity in terms of Zn grain concentration (Gregorio, 2002; Shi, et al., 2009;), and various genotypic behavior to Zn deficiency also shows the possibility of breeding using conventional methods to develop of high-yield rice varieties with suitable grain Zn concentration (Ismail et al., 2007; Wissuwa, et al., 2008). It seems that high grain Zn trait is strongly connected with aroma, although (Gregorio, 2002; Welch ; Graham, 2004). Several reports indicated a significant adverse correlation among yield and grain Zn concentration in rice (Jiang et al., 2008; Wissuwa et al., 2008), but a positive association among grain Zn concentration and grain yield was considered on Zn deficient soil (Gregorio, 2002) and also in various panel of landraces and aromatic rice in Zn adequate conditions non-significant relation were observed among grain Zn and yield (Swamy et al., 2016).
Zinc-regulated transporter, iron-regulated transporter like proteins (ZIPs) generally contribute to metal-ion homeostasis by moving cations into the cytoplasm (Colangelo and Guerinot, 2006).
Rice wild relatives are the great sources of high grain Zn. Wild species of rice including O. nivara, O. latifolia, O. rufipogon, O. granulate, and O. officinalis, also possess high amounts of Zn rather than cultivated rice (Banerjee, et al., 2010; Anuradha et al., 2012). Gregorio, (2002) also found aromatic rice had high Zn in comparison with non-aromatic rice. It has been reported that the amounts of Zn in three IR64 mutant genotypes including; M-IR-180, M-IR-175, and M-IR-49 in polished rice was more than compared to IR64. These mutants could be exploited in breeding programs for Zn deficiency and understanding of Zn demonstration mechanisms (Swamy et al., 2016).
Genetic studies of rice genome revealed that rice has nine heavy metal ATPases (HMA) genes. Three of these genes including OsHMA1-3 have important role in transporting of Zn (Miyadate et al., 2011).
Mapping quantitative trait loci (QTLs) for Zn deficiency tolerance is one method to cope with these constraints. By detecting QTLs related to symptoms of Zn deficiency, it will eventually be possible to analyze the entirely Zn deficiency response into different genetic factors, that have associated with tolerance mechanism (Wissuwa et al., 2006). A genome wide association mapping detected meaningful SNPs on chromosomes 3 and 9, correlated via grain Zn (Norton et al., 2014). Rice varieties like IR64, NSICRc222, BR29, Swarna, BR11, PSBRc82, Ciherang, BR28, and Swarna Sub1 that has been improved at IRRI, have high Zn material background (Swamy et al., 2016). Likewise, considering analysis of 21 metal genes in 12 rice genotypes, 39 SSR markers and 179 novel SNPs were detected for grain Zn (Banerjee et al., 2010). Furthermore, various grain Zn trait associations and SSR markers have also been demonstrated in various populations and germplasm of rice (Brar, et al., 2015; Swamy et al., 2016). Several mapping populations have been utilized in mapping studies for grain Zn (Norton et al., 2010; Zhang et al., 2011; Anuradha et al., 2012).
The detected grain Zn QTLs on chromosomes 7, 11, 12 are suit targets for marker assisted selection program. Three studies have found 53 QTL for Zn tolerance by RIL populations (Zhang, et al., 2013; Liu et al., 2016), the robust QTL qZNT-1 on chromosome 1 at interval markers XNpb93-C3029C justified 21.9 percentage of phenotypic variance (Dong et al., 2006), It is intelligibly obvious that high grain Zn QTLs are spread over the genome and observed to co-locate with other mineral elements QTLs for the grain. The region on chromosome 5 (QSdw5) at interval 17.3–19.5Mb (Zhang, et al., 2013) and qFRSDW11 on chromosome 11 between C11S49-C11S60 (Liu et al., 2016) were identified for Zn and Fe stresses. It seems that there is a genomic overlap in tolerance to Zn deficiency and Fe toxicity in rice.