Effects of pruning and fertilizer on growth, phytochemistry and biological activity of Sutherlandia frutescens (L.) R.Br.

Abstract

Sutherlandia frutescens (L.) R.Br. (Fabaceae), commonly known as cancer bush, is a herb with a long history of traditional use by a variety of cultures. The plant mainly grows in the dry parts of southern Africa, mostly in the Western and Eastern Cape as well as the neighbouring countries like Lesotho, Botswana and Namibia. Cancer bush is traditionally used for the treatment of external wounds, internally for fevers, stomach problems, cancer, diabetes, influenza, HIV, depression, eye problems, TB, colds and asthma. The plant is famously known for the treatment of Type 2 diabetes, cancer and HIV. However, these claims remain inconclusive. Recent studies have shown S. frutescens to have antidiabetic, anti-HIV, anti-inflammatory, antibacterial, analgesic, anti-stress, anticonvulsant, antiproliferative and antithrombotic activities. Phytochemical investigations of S. frutescens leaves detected the presence of high levels of free amino acids and non-protein amino acids namely: canavanine, gamma aminobutyric acid (GABA) and pinitol. The presence of these compounds has been reported to be responsible for its reputed effectiveness in a wide range of illnesses. In view of its importance as a multipurpose medicinal crop, it is important to bring this plant under cultivation and determine agronomic requirements for its successful cultivation. Several factors can be investigated to enhance the growth and increase the level of active ingredients. The current study was aimed at evaluating the effect of pruning and fertilizer levels on the growth, phytochemistry and biological activity of Sutherlandia frutescens. Seeds were sown in seedling trays to produce seedlings. One-month-old seedlings were then transplanted on a prepared field. The study trial was carried out at the Agricultural Research Council-Vegetable and Ornamental Plants Research Station (ARC-VOP). The experiment was conducted in a randomized complete block design (RCBD) with three pruning levels, three fertilizer levels and four replicates. There were 9 treatments, namely; no pruning (P0), tip-pruning (P1) and heading back (P2) in combination with levels of fertilizers as follows: 200 kg/ha NPK (F1), 100 kg/ha NPK (F2) and 0 kg/ha NPK (F0). Plants without treatments were considered as controls. Growth parameters taken were plant height, stem diameter, chlorophyll and Leaf Area Index (LAI). Dried leaf samples were analysed for the presence of secondary metabolites and antidiabetic activities. There was no significant interaction effect between pruning and fertilizer levels amongst all parameters measured in this study. Pruning treatments had a significant effect on the LAI at week one and week two but did not affect the plant height, stem diameter and chlorophyll content. Different levels of fertilizers had a significant effect on the LAI, where 100 kg/ha NPK significantly increased LAI at week one and week two. In an investigation which was conducted from January to May 2015, plants showed yellowing, stunting, and high levels of infestation expressed as extensive galling on the roots which led to the nematode infestation study. Nematodes were extracted from the roots of a healthy living, a wilted and a dead plant, as well as from the rhizosphere soil. A small population of Scutellonema, Pratylenchus, Helicotylenchus and Tylenchorhynchus were identified. Examination of the root of an infected plant revealed the presence of root-knot nematodes (Meloidogyne javanica) in large numbers. Juveniles, eggs and females were isolated, and the species were identified on the basis of morphological characteristics. Symptoms usually associated with root-knot nematodes were observed on the roots of the wilted plant and the soil in which the plant was growing. Extracts from all nine treatments showed stronger activity against α-glucosidase than the positive control acarbose. The highest α-glucosidase inhibitory activity was demonstrated by the treatment with no pruning (P0) while the heading back (P2) treatment exhibited the lowest inhibitory activity. Fertilizer levels at 200 kg/ha (F1) NPK resulted in a significantly higher α-glucosidase inhibitory activity compared to other fertilizer treatments. The presence of secondary metabolites (including total phenolics and flavonoids) was determined qualitatively. The total phenolic content was determined using the Folin-Ciocalteu method and flavonoids were determined using the vanillin HCL assay. The study showed that pruning and fertilizer increased the production of secondary metabolites in S. frutescens as compared to the control. Fertilizer at 200 kg/ha NPK (F1) did seem to improve phenolics and flavonoids with pruning but phenolic levels were actually quite low when treatment was P0F1. Total phenolics and flavonoids were significantly increased by the heading back (P2) treatment and decreased in the treatment with no pruning (P0). The application of fertilizer at 200 kg/ha NPK (F1) improved the production of secondary metabolites, and reduced production of secondary metabolites was recorded in plants that received no fertilizer. There was no direct correlation between the level of phytochemicals and the antidiabetic activity recorded. This study examined the effects of different levels of pruning and fertilizers on the growth of S. frutescens. The results showed that there was no significant difference. At this stage, no positive recommendations can be made for cultivating S. frutescens. Plant extracts showed good antidiabetic activities in response to different pruning and fertilizer treatments. This was further seen as an increase in the production for secondary metabolites. However, further investigation of plant cultivation practices and further screening for bioactivities is required. S. frutescens may offer a new source of drugs for diabetes mellitus and other related diseases.