1. Introduction
In many parts of southern Africa, rural communities rely significantly on indigenous leafy vegetables as sources of affordable nutritious food in their daily meals that are often starch-based. Frequently, the leafy vegetables are often harvested from the wild during the summer (rain) season [
1,
2]. For instance, amaranth (
Amaranthus cruentus), jute mallow (
Corchorus olitorius) and spider plant (
Cleome gynandra) are traditionally collected from the wild in South Africa and Zimbabwe, but cowpea leaf (
Vigna anguiculata) and pumpkin leaf (
Cucurbita pepo) are cultivated in the field, and the leaves are harvested during the summer (or cropping) season [
3,
4]. Each of these leafy vegetable types is often diverse in horticultural traits. For instance, the cultivated cowpea consists of both prostrate (indeterminate) and erect (determinate) types which differ from each other in many traits including duration to maturity and taste (or flavor perceived). The prostrate types are generally late maturing and produce relatively high dry matter per unit area [
5,
6,
7]. In some areas in southern Africa, for instance in Zambia and Zimbabwe, pumpkin leaf is one of the most popular leafy vegetables during the rainy season [
8]. In addition, the bioactive compounds (molecules) in these leafy vegetables vary widely in both quality and quantity depending on the genetic and environmental factors [
9,
10,
11,
12].
The leaves of these vegetables possess health-promoting phytonutrients and bioactive compounds such as antioxidants, flavonoids, minerals and vitamins [
13,
14]. For instance, the leaves contain phenolic compounds which have redox properties responsible for antioxidant activity [
15]. Both phenolic and flavonoid compounds are secondary metabolites in the plant which contain an aromatic ring bearing at least one hydroxyl group, thus rendering them good electron donors in antioxidant activities [
16,
17,
18,
19]. In addition, some of the leafy vegetables possess sulfur- and nitrogen-containing secondary metabolites (glucosinolates) that confer a bitter taste and vary in different plant organs [
20]. Moreover, some of the compounds can induce the synthesis of endogenous antioxidant molecules in cells of biological systems and exhibit free radical inhibition, oxygen scavenging and peroxide decomposition [
20,
21]. In some cases, the levels of nutrients in the common leafy vegetables are similar to those in exotic vegetables such as Swiss chard (
Beta vulgaris L. var.
cicla) [
22]. Therefore, this study examined the comparative variation in the occurrence of leaf flavonoids, total phenolics and minerals in common leafy vegetables that are largely available to rural communities in southern Africa.
4. Discussion
The intensity of both micro- and macro-elements in the four common leafy vegetables was comparable with those reported in similar studies [
30]. The observed differences in selected specific minerals were likely due to the environmental, ontogenic and genetic factors. For instance, the application of calcium ammonium nitrate fertilizer enhanced leaf Ca content, but farmyard manure decreased Fe content in spider plant [
31]. In another study involving parsley (
Petroselinum crispum), baby greens accumulated more Ca and Mg than microgreens [
32].
The superiority of micro-element content in the Swiss chard leaves relative to the common leafy vegetables which was observed in this study was consistent with findings from other similar studies [
33]. However, in this study, the common leafy vegetables showed superiority in macro-element content suggesting that consumers can derive optimum benefits from combining (or including) the various types of leafy vegetables. It should be emphasized that the threshold quantities of individual nutrient elements that are necessary for human consumption per designated period are more important than the mere presence of the elements in the leaves in the vegetables. Studies have also elucidated the necessity of balancing the nutrient elements in diets and their role particularly in conferring strong bones (teeth), transmitting nerve impulses and forming integral constituents of different hormones [
34]. Microelements also participate in the formation of erythrocyte cells while macro-minerals, such as Ca and Mg, have a high potential to control blood pressure and the immune system [
34].
The results also showed that no single leafy vegetable type contained all the desirable minerals, flavonoids and phenolic compounds suggesting that perhaps human intervention, particularly through genetic enhancement, can develop novel genotypes of the leafy vegetables that possess preferred combinations of the nutrients. One of the possible reasons for the observed absence of specific mineral elements or bioactive compounds in some of the leafy vegetables could be attributed to the existence of inverse relationships between them in the vegetable species. Nonetheless, such inverse relationships can be broken through plant breeding techniques such as mutation and probably the CRSPR technology [
35,
36].
The results also revealed the availability of a wide range of quercetin-related compounds in all the leafy vegetables particularly in spider plant. These findings agreed with the results that were reported for a similar study [
37]. The role of quercetin as an antioxidant was documented widely as a potent antioxidant flavonoid (or more specifically, a flavanol) which occurs also in onion, broccoli and citrus fruits and is protective against tissue injury caused by drug toxicities [
38,
39,
40,
41]. However, it is unclear if this desirable group of nutrients can be introgressed into the other leafy vegetables without diminishing the already existing beneficial nutrient elements in such vegetables. Another drawback in the study was that fresh leaf samples were processed for the analysis such that the effects (on the quality and levels of these nutrients) of homemade processing of the vegetables for long term preservation remain unclear. Previously, contrasting findings were reported with regard to this vegetable home processing [
42,
43,
44]. Perhaps future investigations could focus on this aspect. Similarly, the effects of fertilizing vegetable plants with chemicals or organic manure could be of interest to researchers and users. In addition, the absence of some of the flavonoids that were detected in similar studies in spider plant might have been due to the small number of standards that were used in the current study. Moreover, the alteration of regulatory networks in plants due to UV-B radiation was observed previously [
45].
The positive significant correlations between individual minerals (for instance, Zn and Al) indicated that there was a relationship between the pair, and likely selection for increased content of either mineral might inadvertently result in elevating the content of the other one. However, such a conclusion will need to be approached with caution since several species of leafy vegetables were involved in this study. The relationships between the same pairs of candidate minerals may be relatively weaker in a specific leafy vegetable species but masked by a combined correlation analysis of parameters among all the species. Therefore, in future, separate correlation studies between the different parameters involving many plants within each leaf, vegetable species will be merited and might produce unequivocal conclusions regarding the relationships.
While these results indicated the relative superiority of the leafy vegetables for specific minerals and bio-compounds, it appears that future studies need to consider the ontogenic stages probably by bulking leaf samples harvested at different growth stages prior to the analysis. At the juvenile phase, the concentration of bio-compounds may increase partly due to a high uptake rate of the young roots as well as the high relative growth rate of the plants. Moreover, such studies can also benefit from the inclusion of a wide range of internal standards to enhance the detection of a broad spectrum of bio-compounds. Nonetheless, in this study, the important bio-compounds, including eight quercetin-related derivatives, were detected in most of the leafy vegetable species. In conclusion, the common leafy vegetables were comparatively rich in macro-elements. The findings of this study were significant for end-users in the common leafy vegetable value chain. The empirical evidence presented in this paper supported the need to widen the range of leafy vegetables to optimize the spectrum of beneficial nutrients. The spider plants possessed a superior spectrum of desirable nutrient elements suggesting that it can be recommended highly to consumers.