3.2. Trace and Macroelement Content of Yellow-Fleshed Cassava Roots by Genotype
Table 3 and
Table 4 show the trace and macroelement content of yellow-fleshed cassava roots by genotype across the five growing locations. The concentrations of Fe, Zn, and Mn were found to be higher than B, Cu, Ni, Al, Ti, Cr, and Pb, and implies that cassava roots could be a good source of Fe, Zn, and Mn which are highly essential micronutrients. The Fe content ranged from 2.6–4.9 mg/kg (mean 10.4 ± 5.5 mg/kg), Zn from 3.4–30.9 mg/kg (mean 11.1 ± 4.4 mg/kg), and Mn from 1.6–57.8 mg/kg (mean 14.7 ± 8.3 mg/kg). Also, B had a mean value of 2.0 ± 0.8 mg/kg, Cu 3.2 ± 1.4 mg/kg, Ni 1.3 ± 0.6 mg/kg, A1 5.8 ± 4.1 mg/kg, Ti 0.5 ± 0.5 mg/kg, Cr 0.4 ± 0.3 mg/kg, and Pb 1.3 ± 0.9 mg/kg. On the other hand, among the macroelements, K has the highest concentrations ranging from 2100–43,000 mg/kg (mean 10,637 ± 7172), Ca from 410–7600 mg/kg (mean 1604 ± 1057), Mg from 4600–8100 mg/kg (mean 1576 ± 1077), P from 450–3100 mg/kg (mean 967 ± 458), S, the lowest, from 138–700 mg/kg (mean 297 ± 94), while the element with the widest concentration range is Na, 4–4100 mg/kg (mean 167 ± 297). Chavez et al. [
27] reported the average concentrations of five trace elements (Fe, Zn, Mn, B, Cu) and six macroelements (Ca, Mg, Na, P, K, S) for 20 yellow-fleshed cassava genotypes. For Fe, the average concentration was 9.6 mg/kg, Zn 6.4 mg/kg, Mn 1.2 mg/kg, B 2.4 mg/kg, and Cu 2.2 mg/kg, which are low compared with values in this study except for boron. The concentration reported for the selected macroelements are Ca 590 mg/kg, Mg 1153 mg/kg, Na 66.4 mg/kg, P 1284 mg/kg, K 8903 mg/kg, and S 273 mg/kg. All the values fall within the range, as analyzed in this study.
The Fe and Zn content of the yellow-fleshed cassava is comparable with some raw vegetables and even higher for some genotypes [
2]. Interestingly, macro element concentrations for K, Ca, and Na in some yellow-fleshed cassava genotypes surpass values reported for popular cereals and soybean, a tested and trusted legume [
2]. Fe and Zn are essential micronutrients for children and adults. Iron helps the human body in the creation of new red blood cells, and together with Cu, boosts red blood cells, thereby preventing anaemia [
3]. Zn, on the other hand, strengthens the immune system, assists in protein synthesis, boosts gene expression, aids wound healing, among many more benefits. In Nigeria, about 75 percent of the preschool children and 67 percent of pregnant women are reported to be anemic, and 20 percent of children below five years suffer from zinc deficiency [
28]. K plays an important role in the tuberization of cassava because it uses it as a primary osmolyte to osmotically adjust and positively respond to water stress [
29]. Therefore, this accounts for its abundance in cassava roots. K is a vasodilator due to its ability to reduce strain on the cardiovascular system. Also, it maintains fluid balance in the body for efficient metabolic activities. From the results, all the yellow-fleshed cassava genotypes will convincingly combat diseases brought about by deficiencies of these essential minerals, which are very low in the white varieties commonly consumed.
Most of the yellow-fleshed genotypes had higher values of the trace elements than the three white-fleshed genotypes used as check samples (
Table 4). This observation shows that the yellow-fleshed cassava roots compared favorably with the white-fleshed cassava roots in terms of microelement content, and it confers an additional nutritional benefit to the yellow-fleshed roots. There were no significant (
p > 0.05) differences among the genotypes for Al, Ti, Cr, and Pb. It bolsters the earlier observation that genotype did not play a significant influence on these trace elements. However, they are all < 10.0 mg/100 g, unlike the Fe, Zn, and Mn that had higher values. The respective genotypes had the highest Fe (01/1442), Zn (01/1442), Mn (01/1442), B (01/1442), Cu (01/1442), Ni (01/1115), Al (01/1442), Ti (01/1442), Cr (01/1413), and Pb (01/1163) content, while the respective genotypes with the lowest values were: Fe (90/01554, 96/1089A), Zn (90/01554), Mn (94/006), B (94/006), Cu (01/1235), Ni (01/1412), Al (96/1089A), Ti (01/1368), and Pb (94/006). Genotype 01/1442 was outstanding because it showed the highest values for almost all the minerals studied. However, genotypes 90/01554 and 90/1089A were the poor ones because they had the lowest amounts of Fe and Zn, the essential micronutrient in terms of their nutritional importance. Genotype 01/1442 is a potential pipeline genotype that could be advanced for release as a high micronutrient cassava variety. It will help to address the micronutrient deficiencies in the cassava growing and consuming communities of Nigeria.
The macro element content of yellow-fleshed cassava roots by genotype shows a general increase in values as compared to white-fleshed cassava roots. There were no significant differences among the genotypes for all the elements at a 5% significance level. However, these genotypes have the highest Ca (01/1371), Mg (01/1273), Na (01/1273), K (01/1273), P (01/1331), S (01/1115), while the respective genotypes exhibit the lowest Ca (94/0006), Mg (96/1089A), Na (01/1646), K (94/0330), P (01/1235), and S (01/1206). Genotype 01/1273 showed the highest values for half of the elements under observation, while no genotype can be qualified as inferior because none has low value across all elements. Ref. [
30] reported there was no difference in concentrations of Fe and Zn in 10 cassava genotypes planted in two growing locations, namely, Nampula and Luipo. However, differences were significant at another growing location known as Umbeluzi with values for Fe (
p < 0.001) and Zn (
p = 0.0162) ranging from 8–24 mg/kg and from 8–19 mg/kg, respectively. These reported values are comparable with results in this research, although high values such as the ones reported were not recorded. This follows the same trend as observed that the growing locations have a highly significant effect (
p < 0.05) in the trace element concentration of yellow-fleshed cassava (
Table 5). Akin-Idowu et al. [
31] reported a similar trend where eight yam genotypes were planted in two growing locations (Ibadan and Onne).
There was a significant (
p < 0.05) difference between the mineral content of the samples between the growing locations. It can be observed that Mokwa, Zaria, and Ibadan growing locations have higher concentrations for all trace elements except for Onne that records the highest concentration of Cu (
Table 5). In the case of the macro elements, Ubiaja and Onne locations had higher values for all the elements except K. The variations of the mineral content further established that mineral content could fluctuate greatly depending on genetic and climatic factors, agricultural procedures, the composition of the soil, and ripeness of the harvested crops, among other factors. This applies to both macro and trace elements. However, changes in the mineral content usually occur also in the processing of raw materials, e.g., in thermal processes and material separation, which determine the available mineral content in the cassava products.
3.3. Principal Component (PC) and Cluster Analysis of the Trace and Macroelements of Yellow-Fleshed Cassava Roots Across Five Growing Locations
Table 6 showed the eigenvector of the first three axes of the Principal components (PCs) of the trace elements of yellow-fleshed cassava roots across the genotypes and growing locations. Four PCs (PC1 to 4) explained 75.2% of the data, but PC1 and PC2 accounted for above 50%, and PC1 had the highest eigenvalue of 3.5, while PC2 and PC3 had only 1.5 and 1.4, respectively. The minerals with common characteristics were loaded on each of the PCs. Thus, Fe, Mn, B, and Al were on PC1, Ti, Pb, and Mn on PC2, while Zn, Cu, and Cr were loaded on PC3. There was a positive and significant (
p < 0.05) correlation among all the minerals on PC1, a positive and significant (
p < 0.05) correlation between Mn and Pb. At the same time, both have a significant (
p < 0.05) negative correlation with Ti on PC2. On PC3, Zn and Cu correlate negatively with Cr. In a recent study, Mn and B; and K and P were reported to be positively correlated [
15]. There is a similar observation for Mn and B, but a negative correlation was observed for K and P in this study.
Furthermore, the principal component analysis for macroelements (PC1 to 3) explained the 80.3% of the data with PC1, having the highest eigenvalue of 2.0, PC2 1.7, and PC3 1.1. The macro elements contributing to PC1 are Mg, Na, K, P, and S, with K having a significant negative correlation with the others. Macro elements contributing to PC2 are Ca, Mg, and Na, all having significant positive correlations with one another. Ca, K, P, and S are loadings on PC3, having only K and P correlating significantly, while Ca and S have no significance. A similar result of negative correlation between Na and K in yellow-fleshed cassava root was reported by [
15]. Thus, the macro elements in their respective loadings are related together, which will help in breeding for suitable agronomic traits with the desired macro element(s) in the yellow-fleshed cassava genotypes under consideration. It has been established that macro elements have been absorbed more than trace elements by the stem and root crops [
19,
32], which is evident from their principal components. However, across the five growing locations, a highly significant difference is still observed for both trace and macro elements, which proves beyond a reasonable doubt that a breeder will have clarity in making decisions involving trace and macro elements as regards growing locations.
3.5. Pearson Correlation Coefficients of Trace and Macroelements of Yellow-Fleshed Cassava Roots Genotypes Across Five Growing Locations
Table 8 shows the Pearson correlation coefficients of trace elements of yellow-fleshed cassava roots genotypes across five growing locations. It can be deduced that Fe, Al, Cr, Ni, Mn, Zn, B, and Ti, have strong significant (
p < 0.001) correlations across the growing locations while Zn, Pb. and Cu have weak (
p < 0.05) correlation across the growing locations. However, some also exhibit negative correlations, which can be a basis for predicting fertilizer formulation if the need for application arises. The increased concentration of a trace element could lead to the depletion of another; thus, care is needed during a breeding program to ensure the purpose of biofortification is not forfeited. In consideration of the macroelements, there are mild to strong correlations across the growing locations, but several negative correlations are observed. S has a significant (
p < 0.05) negative correlation with K and Ca (r = −0.154, −0.119), respectively, but a significant positive (
p < 0.001) correlation with
p (r = 0.505). This correlation shows the possibility of breeding yellow-root cassava varieties with high K and
p mineral content. Also, K has a significant (
p < 0.05 and
p < 0.01) negative correlation with Mg and Na (r = −0.170 and −0.216). However, the negative correlations observed may not be detrimental in real-life situations because of research reports that micronutrients are stable across growing locations. Moreover, it implies that it is feasible to combine the high micronutrient content with high yield in breeding, unlike protein content and yield that are negatively correlated [
33].