1. Introduction
Pigeonpea (
Cajanus cajan) is a drought-tolerant food legume which is cultivated widely in tropical countries. The grain of pigeonpea sustains the livelihoods of rural communities as a source of affordable protein in many parts of Africa and Asia [
1,
2]. Moreover, it plays a vital role in food security and income generation through trading in both formal and informal markets. In southern Africa, it is often consumed as fresh green bean or boiled dry grain [
3]. The main pigeonpea-producing countries in southern Africa include Malawi, Mozambique and Tanzania. In South Africa, it is cultivated as a minor crop largely in the eastern coastal region of the country.
Pigeonpea grain contains abundant levels of carbohydrates, minerals and proteins [
2]. It possesses about 20–26% protein, 65% carbohydrates and 2% fats [
4,
5]. It can be used as a supplement to cereal-based diets that are deficient in protein, vitamin B and beta-carotenes [
6]. The range of minerals in the grain of improved pigeonpea genotypes includes calcium (Ca) (130.00 mg/100 g), potassium (K) (1329.00 mg/100 g), iron (Fe) (5.23 mg/100 g) and zinc (Zn) (2.76 mg/100 g) as well as thiamine (0.64 mg/100 g) and niacin (2.96 mg/100 g) [
7]. Medicinally, it is used to treat measles, hepatitis, diabetes and liver dysfunction [
8,
9]. In addition, pigeonpea possesses considerable amounts of natural antimicrobial compounds such as tannins, flavonoids and alkaloids and a broad spectrum of phytochemical, anticarcinogenic, anti-inflammatory and antidiabetic properties [
10,
11,
12,
13]. Grain extracts appeared to reduce red blood cell sickling, suggesting the potential to benefit people with sickle cell anaemia [
13,
14].
In many parts of Africa where poor rural communities typically depend on starch-based diets leading to nutrient deficiencies such as anaemia, hypocalcaemia as well as kwashiorkor, pigeonpea is an affordable source of good-quality protein [
15,
16]. Biofortification and dietary diversification with nutrient-rich legumes such as pigeonpea can reduce nutrient deficiencies. However, previous research efforts placed little emphasis on the nutrient profiles of the pigeonpea [
17,
18,
19]. Likely, this was due to inadequate funding. Therefore, the objective of this study was to determine the diversity in grain mineral and protein content among pigeonpea landraces that are available in the pool maintained at the University of Venda with a view to exploit their potential in human diet in future.
4. Discussion
The genotypic variability observed for the various minerals including Ca, Cu, Mg, Mn, P and Zn indicated the potential for exploiting the landraces in pigeonpea genetic improvement programs that are aimed at the optimization of the grain of such minerals. Similar findings of genetic variation in nutritional characteristics were previously reported in improved and vegetable pigeonpea varieties [
22,
23]. In a similar study involving common bean, differences in individual minerals and protein were attributed to fertilizer applications and edaphic factors [
24]. For instance, the grain content of Zn and Ca was due to soil and/or foliar applications of chemical fertilizers containing these minerals [
25,
26]. Nonetheless, the pigeonpea genotypes in this study were grown without the use of fertilizers. In the current study, genotype ‘G-03’ attained the highest concentrations for most mineral elements, thus demonstrating its superiority and potential for exploitation in future pigeonpea programs for enhancing grain nutrients.
The grain mineral elements in these pigeonpea landraces are essential for the optimal functioning of the immune system [
23,
27]. For example, calcium is important for normal heartbeat and muscle functioning, bone health as well as neurotransmission [
28]. The highest Ca content observed in genotype ‘G-03’ (2103.43 mg/kg) was double that in similar studies [
29]. Moreover, the Ca content which was observed in this study for ‘G-03’ was higher than in common bean [
24]. The high levels (>15,000 mg/kg) of K accumulated by pigeonpea landraces such as ‘G-13’, ‘G-06’ and ‘G-09’ in this study also suggested the potential to reduce the risk of cardiovascular diseases and diabetes when included in the diet [
30]. The presence of K in the body is also associated with an increase in iron utilization and hypertension control [
31]. In comparison with other leguminous species, the P content in the pigeonpea landraces was higher than in both lentil (
Lens culinaris) (299.45 mg/100 g) and garden pea (
Pisum sativum) (352.79 mg/100 g). On the other hand, the Mg content observed in this study was almost two-fold that observed in a similar study in Nigeria (84.31 mg/100 g), suggesting that the accumulation of this mineral depends on both genetic and environmental factors since distinct genotypes were used in the studies [
32]. Mg acts as a cofactor for more than 300 enzymes, thus regulating multiple fundamental functions such as glycaemic control, type 2 diabetes, myocardial contractions and osteoporosis [
33,
34].
Among the micronutrients in this study, iron was the most abundant microelement (ranging between 32.99 and 66.05 mg/kg), and was similar to the content observed in chickpea (48.6–55.6 mg/kg), but was significantly lower than in lentil (75.6–100 mg/kg) [
35,
36]. The landraces ‘G-11’ and ‘G-13’, which showed relatively high grain Fe content, could be used in biofortification aimed at fighting anaemia in vulnerable groups such as women and children. Some of the pigeonpea landraces also contained significantly higher Zn than chickpea (21.1–28.3 mg/kg). Zn is essential for different biological functions such as wound healing, protection from oxidative damage by reactive oxygen species and prevention of both pancreatic and prostate cancer [
37]. The two landraces (‘G-13’ and ‘G-09’) which contained significantly high Cu and Zn could also be useful in mineral biofortification in pigeonpea. Various studies reported successful improvements in micronutrients such as Fe and Zn in legumes including pigeonpea through biofortification [
38,
39,
40,
41]. The highest grain protein content (23.50%), which was attained by the landrace ‘G10’, was comparable but lower than the variability (23.35–29.50%) which was observed among 600 pigeonpea genotypes from a regional genebank in India [
42]. Nonetheless, this study identified useful landraces that contain a good balance of protein and mineral content; hence, they can be use as donor parental materials in pigeonpea breeding programs aimed at enhancing grain protein content to benefit end-users, particularly in poor rural communities that are prone to malnutrition.
Correlation analysis is critical for detecting the relationship between nutritional features that can be used to determine effective breeding strategies [
43]. In this regard, positively correlated mineral elements could make concomitant biofortification with multiple minerals easy. However, significant negative correlations were observed between grain protein and Fe and Mn. Nonetheless, the improvement of Ca content in pigeonpea is favoured by Mg selection since a significant positive correlation between Ca and Mg was evident among the landraces. Various studies reported that Fe and Mn are cofactors in several enzymes (such as arginase, glutamine synthetase, Mn superoxide dismutase and pyruvate carboxylase), thus suggesting that, perhaps, deficiencies in these minerals could diminish grain protein content [
44,
45]. In contrast, excess Mn was reported to decrease some proteins associated with signalling pathways as well as negatively affect the utilization of other minerals such as Ca and Fe and eventually cause oxidative stress [
45].
Principal component analysis (PCA) is an important tool for determining the significance of various traits and genotypes based on their contribution to the overall variation. Mineral elements such as Ca, Mg, Mn and P could be considered = the most informative for evaluating genetic diversity for the grain and protein content among the pigeonpea landraces due to their strong association with the first principal component (PC1). In this regard, the landraces such as ‘G-03’, ‘G-04’ and ‘G-05’ which were strongly associated with Ca, P, Mg and Mn could be considered for introgressing genes that control the accumulation of these nutrient elements. Similar studies involving PCA showed that pigeonpea and lentil were characterized by high calcium and protein contents, respectively [
35,
42]. The six landraces (‘G-03’, ‘G-04’, ‘G-10’, ‘G-01’, ‘G-09’ and ‘G-14’) which were positioned far from the origin of the biplot indicated that they were the most distinct and probably possessed unique genes associated with the grain traits.
The cluster analysis partitioned the landraces into three major groups, revealing considerable intra-cluster variability. Two landraces (‘G-04’ and ‘G-09’), which were grouped as singletons, were among the most divergent for the traits. Similar studies in pigeonpea also revealed high genetic diversity based on cluster analysis [
42]. The pigeonpea genotypes that were grouped together may not be recommended for hybridization as they could be related. However, molecular tools can be used to validate the genetic similarity of the landraces with regard to the grain mineral and protein content. This is partly because phenotypic expression is influenced by the environment and provides a proxy indicator of genetic variation, but molecular markers are more reliable.