The effects of selected soil physical properties on the soil water balance, soil water redistribution and pyrrolizidine alkaloids loads in two diferent textured soils in the nieuwoudtville rooibos tea production area

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Stellenbosch : Stellenbosch University
ENGLISH ABSTRACT: Rooibos (Aspalathus linearis) is an endemic crop of South Africa produced only in the south western parts of the country. This includes the Northern Cape where the production was reported to have declined over past five years. The first aim of this study was to look at the soil water dynamics in relation to the Normalized Difference Vegetative Index (NDVI) on Rooibos plantation around Nieuwoudtville area in the Northern Cape. Pyrrolizidine alkaloids (PA’s) contamination on Rooibos has been reported from several markets around the globe and pose a health risk towards consumers. Hence the second aim of the study was to investigate the PA’s in Rooibos plantation. Field trials were conducted at Rogland, Meulsteenvlei, Oorlogskloof and Klein Blomfontein farms near Nieuwoudtville while a pot trial was conducted at Vaalharts Research Station near Jan Kempdorp in the Northern Cape Province. Soil water content (SWC) was monitored at an hourly basis throughout the growing season (October 2017 to February 2019) using ECH2O sensors in the field. At the end of 2018/19 growing season, soil water balance and NDVI of selected sites around Nieuwoudtville were determined. The total rainfall received at Rogland, Meulsteenvlei, Oorlogskloof and Klein Blomfontein was 373, 495, 413 and 391 mm, respectively. The cumulative evapotranspiration ( surrounded the Rooibos plants and known to contain PA’s. Further investigation was conducted in pots to evaluate PA’s uptake by Rooibos plant. Soil collected from Rogland was treated with three different weeds (Chrysocoma oblingifolia [0.225 ppm total PA’s], Othonna coronopifolia [0.377 ppm total PA’s] and Raphanus rhaphnistrum [46.008 ppm total PA’s]). Weeds were applied at a rate of 1% of soil volume and replicated 6 times. The pots were planted with Rooibos seeds obtained from Oorlogskloof on the 25th of July 2018 and the experiment continued up until the 1st of February 2019. The total weed population density between the study sites was not significantly different. However, the significant difference between the study sites was only found on Arctotheca calendula, Chrysocoma oblongifolia, Cleretum papulosum, Cynodon dactylon, Ehrharta longiflora, Juncus capensis, Senecio arenarius, and Ursinia weeds species. The dominant weeds species found to contain PA’s at the study sites were Arctotheca calendula, Chrysocoma oblongifolia, Othonna coronopifolia and Raphanus rhaphnistrum with an average total PA’s of 5, 75, 2 817 and 15 330 μ, respectively. The mean total PA’s concentration in Rooibos plantation at the study sites was 7.2 ppm (Klein Blomfontein). 15.5 ppm (Meulsteenvlei), 16 ppm (Rogland) and 43.2 ppm (Oorlogskloof). The higher total mean PA’s concentration at Oorlogskloof can be due to the high density of Chrysocoma oblongifolia as compared to the other study sites. The Chrysocoma oblongifolia could have released PA’s in soil during decomposition which could have let to the uptake of PA’s by Rooibos plants. Furthermore, this study confirm that Rooibos itself cannot produce PA’s but rather, it absorbs PA’s from the soil. The lateral transfer of PA’s from weeds to Rooibos was also found to be inconclusive.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Thesis (MScAgric)--Stellenbosch University, 2021.
Rooibos (Aspalathus linearis) -- South Africa, Soil water content, Weeds -- Identification, Soil conservation, Rooibos tea -- Effect of soil moisture on, Rooibos plant -- Effect of climatic changes on, UCTD