Application of zinc improves the productivity and biofortification of fine grain aromatic rice grown in dry seeded and puddled transplanted production systems
Introduction
Rice (Oryza sativa L.) fulfils about 21% of the global energy and protein requirements of the human population and feeds more than half of the world population (McLean et al., 2002). About 60–70% of people in Asia and sub-Saharan Africa are at risk of Zn deficiency (Gibson, 2006), which is equivalent to 2 billion people in Asia and 400 million in sub-Saharan Africa (IRRI, 2006). More than 4% of the worldwide mortality and morbidity in children under five years and 16 million of the global disability-adjusted life years are caused by Zn deficiency (Black et al., 2008, Walker et al., 2009).
Zinc has multiple roles in the core biochemical processes in plants including enzyme activation, protein synthesis, starch, auxin and nucleic acid metabolism, and pollen development (Marschner, 1995, Cakmak, 2000, Chang et al., 2005). More than 50% of field crops are sensitive to Zn deficiency, but Zn deficiency in rice is more common than that observed in other field crops (Dobermann and Fairhurst, 2000, Fageria et al., 2002, Quijano-Guerta et al., 2002).
Zinc deficiency occurs in both conventional flooded (Dobermann and Fairhust, 2000) and direct-seeded aerobic (Fageria, 2000, Gao et al., 2006) rice production systems. Various soil factors such as pH, redox potential and the concentrations of Zn, P, Fe and Mn in the soil solution affect plant availability of Zn in paddy fields (Mandal et al., 2000, Alloway, 2009, Fageria et al., 2011). For instance, Zn precipitates as ZnS at low redox potential, as Zn(OH)2 with increases in soil pH, and as ZnCO3 in calcareous soils (Bostick et al., 2001), which decreases plant Zn availability (Quijano-Guerta et al., 2002, Johnson-Beebout et al., 2009).
Physical or economic water scarcity, increasing labour costs, and labour shortages are driving change from conventional puddling, transplanting and flooding (TR) of rice to labour-saving and irrigation water-saving systems such as dry seeding with alternate wetting and drying water management (Farooq et al., 2011). This shift alters soil water content and redox potential, and thus other soil properties including Mg:Ca ratio, bicarbonate concentration and soil organic matter status (Rehman et al., 2012), which influence plant Zn availability (Gao et al., 2006, Gao et al., 2012, Rehman et al., 2012, Guo et al., 2016). Plant uptake of Zn is mainly controlled by diffusion (Marschner, 1995). Thus, the lower soil water content in dry-seeded aerobic rice (DSR) compared with flooded rice may reduce the transport of Zn to roots (Yoshida, 1981). Likewise, the increased soil redox potential (Gao et al., 2002) under aerobic conditions may cause iron (Fe) and manganese (Mn) oxides to form, onto which Zn might be adsorbed (Guo et al., 2016).
Grain biofortification is an economical and viable option for improving Zn levels in grain and reducing the wide-scale nutritional disorders induced by Zn deficiency in rice, the staple food crop in much of Asia. The two main approaches for grain biofortification are breeding (Cakmak, 2008, Phattarakul et al., 2012, Johnson-Beebout et al., 2016) and micronutrient fertilisation. Micronutrient fertilisation is a cost-effective approach for increasing Zn concentration in grains (Cakmak, 2008, Phattarakul et al., 2012).
In rice, Zn fertiliser may be delivered through soil application, as a foliar spray or as seed treatments (Johnson et al., 2005). Soil application is the principal method for Zn supply in conventional flooded production systems (Dobermann and Fairhurst, 2000, Khan et al., 2003, Naik and Das, 2007). However, the high cost of chelated-Zn fertilisers, high application rates required, and Zn binding in the soil make soil application uneconomical in some situations (Jiang et al., 2008, Stomph et al., 2011). Foliar sprays may be a cost-effective alternative (Stomph et al., 2011). Different methods of Zn application may have different outcomes in different rice production systems (Rehman et al., 2012). For example, in conventional flooded systems, rice yields increased more with soil application of Zn than with a foliar spray (Ram et al., 2015, Ghoneim, 2016). In contrast, in dry-seeded aerobic rice, foliar spray improved zinc levels more than soil application, improved grain yield (Abilay and De Datta, 1978) and increased kernel Zn concentrations (Ram et al., 2015, Ghoneim, 2016). In another study on rice, soil application of Zn (10 kg ha−1) improved grain yield and grain Zn concentration compared to foliar application (Rana and Kashif, 2014).
Micronutrient delivery of Zn as a seed treatment is another viable option (Farooq et al., 2012). For seed priming, seeds are soaked in aerated micronutrient solution followed by redrying to the original seed weight (Farooq et al., 2012, Rehman et al., 2016). For seed coating, the target material adheres to the seed surface as an outer covering (Farooq et al., 2012, Rehman et al., 2016).
Several published studies have compared soil, foliage and seed treatments of Zn application in rice (e.g., Slaton et al., 2001, Khan et al., 2003, Phattarakul et al., 2012, Imran et al., 2015). However, no study has been carried out to compare the influence of various Zn application methods on the productivity, profitability and grain Zn concentrations of rice grown in DSR and TR production systems. For this study, we hypothesised that Zn application would improve paddy yield and grain Zn concentration in rice grown in different production systems and that DSR would be more responsive to Zn than TR because of the drier soil conditions. The specific objective of this study was to determine the most effective and economical way of applying Zn to improve paddy yield and grain Zn concentration of fine grain rice grown in DSR and TR production systems.
Section snippets
Site, soil and climatic conditions
This study was conducted at two locations—the Agronomic Research Farm, University of Agriculture Faisalabad (31°N, 73°E, 184.4 m asl), Pakistan, and in a farmer’s field in the district of Sialkot (32.51°N, 74.53°E, 256 m asl)—in 2013 and 2014. At both sites, there were two production systems: puddled transplanted flooded rice (TR) and dry-seeded rice with alternate wetting and drying water management (DSR).
The soil at Faisalabad was a sandy loam from the Lyallpur series with pH 7.6, 0.38% soil
Yield and yield components
The interaction between rice production systems and Zn application methods (PS × T) was significant for grains per panicle, productive tillers, biological yield, grain yield and harvest index at Sialkot but only for productive tillers at Faisalabad (Table 3, Table 4). At both sites, the number of grains per panicle, grain yield and harvest index were significantly higher in TR than DSR, and at Faisalabad 1000 grain weight was also significantly higher in TR than DSR.
At Faisalabad with DSR, soil
Discussion
The results supported the hypothesis that Zn application would improve paddy yield and grain Zn concentration in rice grown in different production systems, but the results did not support the hypothesis that DSR would be more responsive to Zn application than TR. All Zn treatments increased biological and paddy yields of both DSR and TR at both sites in consecutive years. The increase in yield was associated with increased panicle production, the number of grains per panicle and grain weight (
Conclusions
The application of Zn using any of the methods tested (soil application, foliar application, seed priming or seed coating) increased grain yield, grain Zn concentration and profitability in of both puddled transplanted flooded rice (TR) and dry seeded rice grown with alternate wetting and drying water management (DSR). There were only small differences in the yield response to method of Zn application, however, grain Zn concentration was always highest or equal highest with soil application,
Acknowledgments
We are thankful to the Higher Education Commission of Pakistan (Grant No. 1871) for support to conduct this study.
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