Mineral Element Composition in Grain of Awned and Awnletted Wheat (Triticum aestivum L.) Cultivars: Tissue-Specific Iron Speciation and Phytate and Non-Phytate Ligand Ratio.

Paula Pongrac,Iztok Arčon,Hiram Castillo-Michel,Katarina Vogel-Mikuš

doi:10.3390/plants9010079

Abstract

In wheat (Triticum aestivum L.), the awns—the bristle-like structures extending from lemmas—are photosynthetically active. Compared to awned cultivars, awnletted cultivars produce more grains per unit area and per spike, resulting in significant reduction in grain size, but their mineral element composition remains unstudied. Nine awned and 11 awnletted cultivars were grown simultaneously in the field. With no difference in 1000-grain weight, a larger calcium and manganese—but smaller iron (Fe) concentrations—were found in whole grain of awned than in awnletted cultivars. Micro X-ray absorption near edge structure analysis of different tissues of frozen-hydrated grain cross-sections revealed that differences in total Fe concentration were not accompanied by differences in Fe speciation (64% of Fe existed as ferric and 36% as ferrous species) or Fe ligands (53% were phytate and 47% were non-phytate ligands). In contrast, there was a distinct tissue-specificity with pericarp containing the largest proportion (86%) of ferric species and nucellar projection (49%) the smallest. Phytate ligand was predominant in aleurone, scutellum and embryo (72%, 70%, and 56%, respectively), while nucellar projection and pericarp contained only non-phytate ligands. Assuming Fe bioavailability depends on Fe ligands, we conclude that Fe bioavailability from wheat grain is tissue specific.

Highlights

Mineral micronutrient sufficiency—a prerequisite for human well-being—can be ensured by diet diversification or consumption of mineral-dense produce [1]
We investigated the diversity in grain mineral element accumulation in 20 wheat cultivars and found that there is a link between the awn length and the Ca, Mn and Fe concentrations (Figure 1)
Because awns presumably have photosynthetic activity, awned and awnletted wheat cultivars were compared for their 1000-grain weight, mineral composition and Fe speciation and Fe local chemical environment

Summary

Introduction

Mineral micronutrient sufficiency—a prerequisite for human well-being—can be ensured by diet diversification or consumption of mineral-dense produce [1]. Mineral micronutrients are predominantly acquired from plant-based sources, in particular staple grain [2,3]. Most mineral micronutrients (manganese (Mn), iron (Fe), copper (Cu) and zinc (Zn)) in grain are tightly bound in phytate (myo-inositol hexakisphosphate), a phosphorus (P)-rich salt, which cannot be digested by mammals. This makes phytate-bound mineral elements poorly bioavailable and ineffectively exploited for normal body functions [4]. Mineral density of the cereal grain has been for a long time regarded as of minor importance compared to the crop yield [5] resulting in prevalent micronutrient deficiencies in humans [6].

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