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Soil acidity and nutrient stress are the major abiotic factors affecting crop production thus contributing to food insecurity. Some plants including crop legumes may thrive in such poor conditions by regulating their secondary metabolites. In this study, we investigated how four Vigna unguiculatavarieties regulated their phenolic acid concentrations and antioxidant capacities to better adapt to acidic and nutrient-deficient grassland and savanna ecosystems. Four V. unguiculatavarieties (IT18, Batch white, Brown mix, and Dr Saunders) were grown in soils collected from four geographically distinct areas (Bergville, Ashburton, Izingolweni, Hluhluwe) in KwaZulu-Natal covering grassland and savanna ecosystems. Plants were analyzed for nutrient composition, arbuscular mycorrhizal fungi colonization percentage, and root-nodulating bacteria. Oxygen radical absorbance capacity and phenolic acid concentrations were also determined. There were differences in plant biomass, nitrogen, and phosphorus nutrition across the four V. unguiculatavarieties, with high arbuscular mycorrhizal fungi colonization of greater than 58%. V. unguiculatawas nodulated by several bacterial strains including Bacillus, Paenibacillus, Delftia, Rhizobium, and Bradyrhizobium. All four V. unguiculatavarieties had variations in phenolic acids across all soil types with vanillic acid and protocatechuic acid being the most abundant, constituting 22.59% and 17.22%, respectively, of the total phenolic acids. Principal component analysis also showed differences in phenolic acid production across the four V. unguiculatavarieties. There were variations in correlations between the phenolic acids and plant biomass, plant nutrition, soil nutrition, and arbuscular mycorrhizal fungi infection. Varieties IT18 and Batch white had relatively higher oxygen radical absorbance capacity across the four soil types and comparably higher plant biomass relative to Brown mix and Dr Saunders. Overall, the current findings demonstrated that V. unguiculatahas the potential of adapting to acidic and nutrient-stressed grassland and savanna conditions through production of phenolic acids and enhanced antioxidant capacity.

Legumes have been used to improve soil fertility however, most legume research focuses on crop and not forage legumes. Forage legumes, including Pisum sativumL., increase the nutritional value in pastures and provide high amounts of soil protein and minerals required for plant growth. We investigated the effects of varying soil composition on plant growth, symbiosis establishment, and nutrient acquisition. We also aimed to compare phenolic compound production, since phenolics are reported to play a vital role in plant defense, pollination/dispersal, and symbiosis with quorum-sensing plant growth-promoting bacteria. Using quantitative techniques, we evaluated the effect of nutrient deficiency in plant–microbe symbiosis, nutrition, and carbon costs, as well as the phenolic concentrations in P. sativum. Four distinct regional soils in KwaZulu-Natal (KZN), geographically covering grassland and savannah ecosystems, were used as growth substrates. Plants maintained their root dry weights and growth rates across the four soil types. Low pH, total cations, and high exchange acidity in Bergville soil resulted in decreased total plant dry weights. P. sativumgrown in Izingolweni soils relied more on atmospheric N fixed by endophytic/associative bacteria from the genera Cupriavidus, Paenibacillus, Cohnella,and Bacillus, while those grown in Hluhluwe soils relied on soil N. Plant associative microbes might modulate nutrient availability for plant uptake in nutrient poor grassland and savannah ecosystems. P. sativumacclimatized to changes in soil nutrient concentrations and pH in the studied ecosystems by changing N source preferences and phenolic concentrations. The acclimatization of plants is likely modulated by the presence of rhizospheric microorganisms interacting with the plants.

Vigna unguiculatais a staple food with high nutritional value and commonly consumed among many rural communities in South Africa. Its success in low nutrient soil has been attributed to the establishment of efficient symbioses with soil bacteria and fungi. We investigated how legume-microbe symbiosis affect nitrogen (N) nutrition, and growth of V. unguiculatagrown in nutrient-deficient soils of KwaZulu-Natal (KZN). Seeds of four V. unguiculataL. (Walp) varieties (IT18, Batch white, Brown mix, Dr Saunders) commonly grown by local farmers in South Africa were used in this study. The seeds were grown in nutrient-poor natural soils collected from four geographically different areas in KZN, under glasshouse conditions. From each site, homogenized soils were analyzed for soil nutrient concentration, soil chemistry, and arbuscular mycorrhizal (AM) fungal spore count. V. unguiculatawas nodulated by several bacterial strains including Bradyrhizobium, Rhizobium, Paenibacillus, and Bacillusstrains. The legume effectively fixed more than 60% of its total N from the atmosphere in all soil treatments. The V. unguiculatavarieties had symbiotic association with arbuscular mycorrhizal fungi (AMF). The AMF root colonization ranged from 58 to 100%, with Hluhluwe soil–grown plants recording the highest for all V. unguiculatavarieties. There were variations in growth kinetics, nutrient assimilation, and utilization among the four V. unguiculatavarieties grown in the soil types. All the V. unguiculatavarieties utilized both soil and atmospheric N and they maintained their total plant N concentrations despite the differences in soil nutrient properties. The current findings revealed that V. unguiculatamay adapt to nutrient-poor ecosystems by establishing symbiotic interaction with naturally occurring soil bacteria and AMF and through its ability to switch N source preferences.