Zinc Localization and Speciation in Rice Grain Under Variable Nutrient Limitation Conditions

Abstract Background and Aims Severely low soil nutrient status and malnutrition or "hidden hunger" are two serious global problems. The consumption of rice constitutes approximately 20% of human caloric intake. Trace elements like zinc (Zn) is essential nutrient for rice growth, and to the populations depend on rice staples. This research examines the speciation of Zn in rice, how that speciation is impacted by soil fertility and its potential effect on malnutrition. Methods The composition and Zn speciation of Cambodian rice grain is analyzed using synchrotron-based microprobe X-ray fluorescence (µ-XRF) and extended X-ray absorption fine-structure spectroscopy (EXAFS). Results Zn levels in rice grain ranged between 15–30 mg kg-1 and were not correlated to Zn availability in soils. 72%-90% of Zn in rice grains is present as Zn-phytate, generally not bioavailable, while smaller quantities of Zn are bound as labile nicotianamine complexes, Zn minerals like ZnCO3 or thiols, using EXAFS method based on coordination numbers for Zn binding to oxygen and sulfur. Conclusion Zn speciation in rice grain is affected by nutrient limitation more than previously recognized. The Zn phytate concentration in rice grain was highest for rice produced in Zn-deficient soils, consistent with increased phytate production under nutrient limitation. Phytates are generally not bioavailable, so low soil Zn fertility may not only impact grain yields, but also decrease the fraction of bioavailable Zn in grain consumers. The reduced accessibility of human zinc due to low soil nutrient levels and its exacerbation of hidden hunger may require widespread global attention.