Genetic variation of bioavailable iron and zinc in grain of a maize population
Introduction
More than one-third of the world's population is afflicted by iron (Fe) and zinc (Zn) deficiencies, these ranking fifth and sixth among the ten most important risk causes of illness and disease in low-income countries (WHO, 2002). Among strategies for enhancing iron and zinc levels in cereal grains, plant breeding strategy (biofortification) appears to be the most sustainable and cost-effective approach (e.g. Cakmak, 2008, Graham et al., 1999, Welch and Graham, 2002). However, there are inherently low iron and zinc concentrations in cereal grains whose utilisation in the human digestive tract was additionally limited by substances that influence the bioavailability of these nutrients.
Bioavailability can be defined as the proportion of the total amount of mineral element that is potentially absorbable in a metabolically active form (House, 1999). Unrefined cereals contain high levels of phytate (myo-inositol hexaphosphate), known to be a powerful inhibitor of iron and zinc absorption in both adults (Egli et al., 2004, Mendoza et al., 1998) and children (Davidsson et al., 2004). Lönnerdal (2002) demonstrated that any reduction in dietary phytate can have a positive effect on zinc absorption. The same can be true also for iron (Mendoza et al., 1998). Thus, the negative effect of phytate on iron and zinc absorption follows a dose dependent response, and the phytate/iron or phytate/zinc molar ratio of a diet may be used to predict the proportion of absorbable dietary iron and zinc. Gibson (2006) has stated that particularly high phytate/zinc molar ratios are in diets of children from Malawi, Kenya, Mexico and Guatemala who consume only unfermented maize products.
There are low-phytate strains in maize (Raboy et al., 2000), but they express some less favourable agronomic features, e.g. reduction of seedling vigour (Bänziger and Long, 2000). Phytate also has some beneficial effects such as inhibition of different types of cancers (Somasundar et al., 2005, Vucenik and Shamsuddin, 2003). Therefore, biofortification programmes usually do not include decreasing phytate as a breeding objective beyond naturally occurring variability (Ortiz-Monasterio et al., 2007 for HarvestPlus programme).
The total phosphorus (P) in maize grain could be an indicator of phytate since 80% of the P is in the form of phytate (Raboy, 1997). Hence, identifying the relation between phosphorus and iron as well as between phosphorus and zinc concentrations in standard maize strains could be a way of increasing iron and zinc bioavailability in maize grain. There are several published studies about genetic variation of iron and zinc in grain of maize (Ortiz-Monasterio et al., 2007 for a review), but a thorough quantitative-genetic analysis for these traits is still lacking. Furthermore, there was no attempt to include bioavailability quantitatively to biofortification studies of iron and zinc. The objectives of this study were (1) to estimate genetic variation of iron and zinc in a maize population including P/Fe and P/Zn molar ratios as quantitatively inherited traits; (2) to determine relations between yield, phosphorus, iron, zinc, P/Fe and P/Zn ratios; and (3) to define the implications of those on biofortification programmes in maize.
Section snippets
Materials
As part of a biofortification project in maize at the Agricultural institute Osijek, Croatia, two temperate maize elite dent inbred lines having significantly different micronutrient concentrations according to our previous studies (Brkić et al., 2003, Brkić et al., 2004) were crossed in order to commence genetic studies. The two inbred lines (B84 and Os6-2) belong to opposite gene pools of U.S. Corn Belt germplasm: the line B84 is a well known BSSS line, while OS6-2 is related to the line C103
Results
There were significant differences between the means at two environments for all traits except iron (Table 1). Generally, there were higher yields, phosphorus and iron concentrations, as well as P/Fe and P/Zn molar ratios in 2006. Only zinc concentrations were significantly lower for 4.86 mg/kg in 2006 compared to 2005. Means of individual F4 lines varied significantly for all six traits, differing in 6.24 t/ha for yield, 1011 mg/kg for phosphorus, 16.95 mg/kg for iron, 12.21 mg/kg for zinc,
Discussion
Quantities of minerals in grain are influenced by numerous complex factors including genotype, soil properties, environmental conditions and nutrient interactions (House, 1999). Our study demonstrated that mineral concentrations of grain were affected by environmental conditions and year to some extent. However, if there were no adverse soil chemical properties (Cakmak, 2008 for review) or extreme weather conditions like in our study, differences among genotypes in absorbing sufficient amounts
Acknowledgments
The authors thank I. Kadar and J. Koncz at the Laboratory of the Research Institute for Soil Science and Agricultural Chemistry of Hungarian Academy of Science and Arts in Budapest, Hungary for ICP-OES analyses.
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