The impact of foliar applied zinc fertilizer on zinc and phytate accumulation in dorsal and ventral grain sections of four thai rice varieties with different grain zinc

Highlights

• Major route of Zn transport into the endosperm is the vascular bundles in dorsal side.

• Foliar Zn affected on grain Zn differently among grain section and varieties.

• The depression of phytate concentration in rice by foliar Zn application.

Abstract

This study investigated the effect of foliar applied zinc (Zn) on the distribution of Zn and phytate in rice grain between four Thai rice varieties that differ in grain Zn. Foliar Zn application at 0.5% ZnSO₄ was applied at flowering and the early milky stage compared with non-foliar applied Zn. Among the high-yielding, low grain Zn varieties (CNT1 and RD21), foliar applied Zn increased Zn concentration in both dorsal and ventral sections of unpolished rice by up to 17.7 and 14.3%. In the low-yielding, high grain Zn varieties (KPK and NR), Zn concentration increased by 11% in the dorsal section of NR, but no effect was found in both sections of KPK. In polished rice, the Zn concentration increased by 20% in both sections but it was increased only in the ventral section of KPK and CNT1 by 21.0% and 25.0% respectively, while there was an increase of 12.5% in the dorsal section of RD21. The phytate in the seed fractions was measured as an indication for Zn bioavailability within humans. A lower phytate concentration was observed after foliar Zn application in both unpolished and polished rice, indicating the potential for a higher bioavailability of Zn in the rice grain.

Introduction

Around 2 billion people are considered to be zinc (Zn) deficient and efforts are underway to biofortify staple food crop with Zn, to try and reduce the prevalence of this deficiency. Within rice consumption regions, the number and severity of Zn deficient people is reported to be between 34 and 73% of the total population and this is attributed to the low concentration of Zn in rice grain (White and Broadley, 2005). Zn deficiency leads to increased morbidity and exasperates ailments such as diarrhea, pneumonia and malaria and results in the increased burden of disease (Hotz and Brown, 2004).

Genotypic variation for Zn in unpolished rice has been found among different rice germplasm, with reports of 14–58 mg kg⁻¹ in rice germplasm at IRRI (Welch and Graham, 2004) and 17–50 mg kg⁻¹ from local upland rice germplasm from Thailand (Jaksomsak et al., 2015). Even though, some genotypes have been considered as high in grain Zn concentration, they often still do not meet the target of at least 28 mg kg⁻¹ in polished rice (Bouis et al., 2011) and this is because there is a loss of Zn during the polishing process.

Agronomic biofortification is an approach through the application of Zn fertilizer and this strategy is widely accepted because it is a short-term, practical approach to lifting grain Zn concentration in the cereals (Cakmak, 2008). The role of applied Zn fertilizer on the accumulation of Zn in cereal grain has been investigated by soil and foliar Zn application at different growth stages and soil Zn fertilizer application was less effective in enhancing grain Zn in rice when compared to wheat (Wissuwa et al., 2008, Cakmak et al., 2010a). By contrast, foliar Zn application at the reproductive stage, efficiently improved grain Zn in the whole grain and penetrated to increase Zn in the endosperm and embryo (Cakmak et al., 2010a). In rice, Phattarakul et al. (2012) and Boonchuay et al. (2013) reported that 2 applications of foliar Zn spray at flowering and early milky stages increased the Zn concentration from 66 to 264% in unhusked grain and from 25 to 55% in polished rice, respectively, even though grain yield was not affected by foliar application of Zn.

Studies have shown that Zn accumulates in the bran layer, pericarp and aleurone and embryo of rice grain and this Zn is removed during the polishing process (Saenchai et al., 2012, Lu et al., 2013). Ideally, there should be a greater deposition of Zn deeper into the endosperm. Pathways of Zn transport into rice grain reflect the delivery and distribution of Zn. The sole vascular bundle locating on the dorsal ridge of the seed pericarp layer is the major path for entry of Zn into the grain but this is isolated from the endosperm and embryo (Oparka and Gates, 1981, Zhang et al., 2007). The dorsal location of the vascular bundle is why Zn was more abundant in the aleurone layer in the dorsal and lateral side than on the ventral side during seed development (Iwai et al., 2012). The uneven distribution of Zn between dorsal and ventral sections was also observed between rice varieties (Jaksomsak et al., 2014). Even though, foliar Zn application improved both unhusked and unpolished rice Zn concentrations, the effect on the distribution of Zn into the dorsal and ventral sections has not yet been reported in different rice varieties.

This study aimed to investigate the distribution of Zn between the dorsal and ventral grain sections in both unpolished and polished rice after two foliar Zn application, at flowering and the early milk stage between different Thai rice varieties that normally have a range of grain Zn concentration. This will be useful for the biofortification strategy to improve Zn in polished rice which is the major form of rice eaten by rice consumers.