**3. Genotype selection**

When the results of variety evaluations conducted in large-scale chlorosis nurseries are considered, there is little evidence of consequential decreases in VCS among cultivars during the last decade (Fig. 7). However, selecting more resistant genotypes in large screening nurseries is complicated by the large genotype x nursery (environment) interaction, especially where VCSs are used to estimate 'resistance'. Inconsistent variety responses have been attributed to environments, soil heterogeneity, and large variations in soil chemistry (Jolley et al., 1996; Fairbanks, 2000). Ferric chelate reductases and quantitative determination of iron reduction have been suggested as reliable indicators of the genetic potential for chlorosis resistance (Jolley et al., 1996; Fairbanks, 2000). Factors controlling absorption and transport of Fe are known to be located in the root and to be genetically determined (Brown et al., 1958; Brown et al., 1972). Although these measures appear to be reliable, they require specialized equipment and knowledge, limiting the number of potential genotypes that can be evaluated in a reasonable amount of time. Within years, genotypic rank correlations of VCSs (Table 2) are often highly significant across locations, suggesting reasonable reliability. This can be deceiving, however, because large-scale nurseries often have 'normal' distributions with nearly all VCSs being between 2.5 and 3.5 at each location (Fig. 8). In a field study conducted during 2007, 2008, and 2009 rank correlations were calculated among 14 genotypes that had been included each year (Table 5). These results suggest that VCSs may not be the most appropriate measures of Fe efficiency. During the same decade, micronutrient densities (primarily Fe and Zn) in both

one-third, to 40 µg g-1 in the top one-third (Fig. 3). This decrease occurred under both nil and severe Fe chlorosis and suggests that developmentally younger and older seeds respond similarly to increasing Fe-EDDHA rates. Increases in seed [Fe] occur primarily in resistant cultivars grown under harsh Fe deficiency. Susceptible cultivars show little response to

With limited Fe deficiency (Fisher, MN, 2003), both resistant and susceptible cultivars attain their genetically predetermined seed [Fe] (Fig. 3). Taken together, these results suggest that developmentally younger, intermediate, and older seed accumulate Fe at similar rates, but for different lengths of time and that cultivars and canopy positions have very similar

The response of soybean cultivar resistance to IDC to differing N rates was evaluated in a field study. Six rates of fertilizer N (0, 34, 68, 102, 136, and 170 kg ha-1) were applied to six cultivars differing in resistance to IDC (2 Fe efficient, 2 moderately Fe efficient, and 2 Fe inefficient) over a three year period. Nodulation decreased linearly in response to added N for all cultivars, regardless of their Fe efficiency characterization or yearly growing conditions. In contrast, relative foliar chlorophyll concentrations (SPAD readings) differed markedly among cultivars, but showed little consequential response to increasing nitrogen rates (NR) (Fig. 5). Plant height, seed number, grain yield, and seed [Fe] decreased linearly in response to increasing NRs for Fe-inefficient cultivars, whereas these responses in Feefficient and moderately efficient cultivars changed little as NR increased (Figs. 5 and 6). Despite these differences, the ranking of cultivars based on seed [Fe] was only slightly

When the results of variety evaluations conducted in large-scale chlorosis nurseries are considered, there is little evidence of consequential decreases in VCS among cultivars during the last decade (Fig. 7). However, selecting more resistant genotypes in large screening nurseries is complicated by the large genotype x nursery (environment) interaction, especially where VCSs are used to estimate 'resistance'. Inconsistent variety responses have been attributed to environments, soil heterogeneity, and large variations in soil chemistry (Jolley et al., 1996; Fairbanks, 2000). Ferric chelate reductases and quantitative determination of iron reduction have been suggested as reliable indicators of the genetic potential for chlorosis resistance (Jolley et al., 1996; Fairbanks, 2000). Factors controlling absorption and transport of Fe are known to be located in the root and to be genetically determined (Brown et al., 1958; Brown et al., 1972). Although these measures appear to be reliable, they require specialized equipment and knowledge, limiting the number of potential genotypes that can be evaluated in a reasonable amount of time. Within years, genotypic rank correlations of VCSs (Table 2) are often highly significant across locations, suggesting reasonable reliability. This can be deceiving, however, because large-scale nurseries often have 'normal' distributions with nearly all VCSs being between 2.5 and 3.5 at each location (Fig. 8). In a field study conducted during 2007, 2008, and 2009 rank correlations were calculated among 14 genotypes that had been included each year (Table 5). These results suggest that VCSs may not be the most appropriate measures of Fe efficiency. During the same decade, micronutrient densities (primarily Fe and Zn) in both

added Fe-EDDHA whether Fe deficiency is nil or severe.

**2.4 Nitrogen rates** 

affected by increasing NRs.

**3. Genotype selection** 

regression slopes, but different intercepts or "thresholds" (Fig. 4).

grasses and legumes have been found to be reliable and consistent across both years and locations. Research conducted on dry bean (*Phaseolus vulgaris* L.), wheat (*Triticum aestivum* L.), and rice (*Oryza sativa* L.) cultivars demonstrated that genotypes with high micronutrient densities of Fe and Zn during one year at one location will also be among the highest at another location in another year (Gregorio, 2002; Shen et al., 2002; Bouis et al., 2003; Nestle et al., 2006; Blair et al., 2009; Blair et al., 2010). Perhaps, measures of resistance to Fe deficiency in soybean should involve integrated estimates of uptake, transport, and accumulation of Fe that are manifest at maturity, such as Fe content 1000-1 seeds, seed [Fe], and/or iron removal with seed (µg Fe m-2).
