Growth, yield and chemical composition of Pelargonium sidoides DC. in response to nitrogen and soil water management

Abstract

Pelargonium sidoides DC. is one of many medicinal plant species that are harvested in the wild. The current trade in these medicinal plants has negatively affected their distribution in the wild due to unsustainable harvesting. The demand for medicinal plants is unlikely to decrease, but sustainability of the supply is questionable. This is because commercial exploitation threatens to deplete their populations, resulting in many species being considered vulnerable to extinction and being lost from their natural habitat. Increased demand, which is already too high to be met by sustainable harvesting, and price increases presents potential opportunities for cultivating indigenous medicinal plants at a commercial scale. A need for basic information on how to grow the plants and other related matters necessitates that field trials be conducted, before farmers could be expected to venture into cultivating medicinal plants and this study aimed at developing such information. The specific objectives of the study were to investigate 1) the physiological and morphological, 2) the yield components and 3) chemical composition of P. sidoides in response to soil water and nitrogen levels. The study was conducted under a rainshelter as a randomized complete block design with three irrigation levels, four nitrogen levels and three replicates. The irrigation treatments were 30% allowable depletion level (ADL) (well watered treatment), 50% ADL (moderately stressed treatment) and 70% ADL (severely stressed treatment), while the nitrogen levels were 0, 50, 100 and 150 kg · N · ha-1. Dried root samples were analyzed for the presence of two standard compounds, scopoletin and esculin, using thin layer chromatography (TLC) and for metabolite profiling using the nuclear magnetic resonance technique (NMR). Nitrogen and water level had no significant interaction effect on all measured parameters. Water stress significantly reduced stomatal conductance, while nitrogen had no significant effect on it. The well watered control had a significantly higher leaf area index, plant height and leaf area compared to the water stressed treatments. Nitrogen had a significant effect on the number of leaves, where 100 kg · N · ha-1 had a significantly higher number of leaves compared to other nitrogen treatments. The well watered treatment again had a significantly higher total biomass, fresh and dry root yield; and nitrogen use efficiency compared to the water stressed treatments. The water use efficiency was significantly decreased in the well watered treatment. Water stress significantly increased nitrogen content and chlorophyll content of P. sidoides plants and leaves, respectively. Nitrogen levels of 50 and 100 kg · N · ha-1 resulted in a significantly higher total biomass compared to the control. TLC analysis showed the presence of the two standard compounds in all treatment samples analyzed. The orthogonal partial least square discriminatory analysis (OPLS-DA), which was performed on the NMR spectral data, showed separation between the irrigation treatments, resulting in two clusters representing the well watered treatment and the water stressed treatments. Asparagine, arginine, sucrose, xylose, glucose and citric acid were found to be the compounds associated with the separation. There was no separation of the samples regarding the nitrogen treatments which is indicative of the small effect of nitrogen on the metabolite content of the treatments. The results from this study showed a relationship between physiological, morphological and yield response as well as chemical composition of P. sidoides. The observed stomatal closure under water stress conditions, due to low turgor pressure in the guard cells, had a significant negative effect on leaf area, leaf area index, total biomass and root yield. Also the increases in total biomass and root yield under well watered conditions could be attributed to the increased primary metabolite content, under such conditions. The recommendation from the study is that P. sidoides plants should be grown under well watered conditions for a year or two to increase growth and root yield. Thereafter plants can be exposed to water stress in the second season, which is expected to increase the concentration of important secondary metabolites.