Doctoral Degrees (Soil Science)

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Browsing Doctoral Degrees (Soil Science) by Author "Mavengahama, Sydney"

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    The contribution of indigenous vegetables to food security and nutrition within selected sites in South Africa
    (Stellenbosch : Stellenbosch University, 2013-12) Mavengahama, Sydney; De Clercq, W. P.; McLachlan, Milla; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: South Africa is rich in biodiversity among which are semi-domesticated vegetable species which are known as wild or indigenous vegetables. These wild indigenous vegetables have been reported to be good in nutritional qualities such as macro and micronutrients. However, there is still a high prevalence of malnutrition; especially micronutrient deficiencies among low or marginal income bracket of the population. The use of indigenous vegetables has been proposed as part of the solutions to the problems of micronutrient malnutrition among these populations. Indigenous vegetables are an important source of food in the maize based subsistence farming sector of rural South Africa. Their main role is as relish as they are used as an accompaniment for staple cereal based diets. They are also generally reported to be rich in micronutrients. Although they may be consumed in small quantities, they influence the intake of cereal staples, manage hunger and play a central role in household food security for the poorer rural groups. Mixing several indigenous vegetables species in one meal contributes to dietary diversity in terms of more vegetable types as well as in terms of choice of relish. For some very poor families indigenous vegetables are substitutes for some food crops. The seasonal occurrence of these vegetables leaves many families without a food source during the off-season. Indigenous vegetables increase agro-biodiversity at the household level. This agro-biodiversity helps in buffering against the accumulation and multiplication of pests and diseases and provides important cover for the soil. Further research on agronomic, social and economic dimensions is required to understand the roles of IV in subsistence farming systems in South Africa. The survey study revealed that indigenous vegetables were important in the diets of most rural people in the study area. They were consumed as relish although they were not being cultivated. Their method of acquisition was gathering from homesteads and the wild. These vegetables were also believed to be medicinal. The local naming of wild vegetables varied among villages in the same district such that a vegetable in one village was assigned to a different species of vegetable in another village. They were reportedly abundant during summer and there was a decrease in availability off-season leaving vulnerable people who rely on them with a food shortage. The utilisation of wild vegetables among South Africans is reported to be declining due to over reliance on introduced temperate species. Efforts to domesticate and cultivate wild vegetables could be hampered by several factors including seed dormancy and premature flowering. In this present study dormancy was observed in C. olitorius. The response of wild genotypes of C. olitorius with different seed sizes to various dry heat and hot water treatments was evaluated. Steeping seeds in boiling water (95oC) for ten seconds and soaking seeds in a hot water bath at 80oC for ten minutes resulted in the highest response to germination in this species. The study also recorded significant interactions between heat treatment and seed sizes. We concluded that C. olitorius seeds of different sizes require diverse durations of exposure to heat treatment methods to break dormancy caused by an impermeable seed coat. Cleome gynandra is another species that is consumed as a vegetable in various parts of the world including Africa. The plant is also used as a medicinal herb for the treatment of various human diseases. Among the wild vegetables, C. gynandra has been reported to flower prematurely, a phenomenon known as bolting and common in many vegetable crops. Premature flowering (bolting) can be as a response to temperature extremes and photoperiod and affects many other leafy vegetables such as lettuce (Lactuca sativa), spinach (Spinacea oleracea) and mustard rape (Brassica juncea). Bolting leads to production losses in leaf vegetable crops as they flower before they have produced an economic yield. The removal of flowers and nitrogen application resulted in significant increases in the fresh and dry weight of cleome leaves. Removal of flowers resulted in a 46% increase in fresh weight of leaves. The observed positive response of leaf yield to removal of flowers offers a possible way to deal with the problem of bolting. The continuous removal of the flowers leads to increased utilisable leaf yield. The application of incremental amounts of nitrogen top dressing results in increased leaf yield in C. gynandra. The response of selected indigenous vegetables (Corchorus olitorius and Amaranthus cruentus) to micronutrients added to the soil was compared with the response of a reference crop; Swiss chard (Beta vulgaris var. cicla). For all the levels of micronutrients applied, Swiss chard accumulated Cu, Zn and Mn in the leaves at significantly (p<0.01) higher concentrations than the wild vegetables. Variations between the vegetables in the micronutrients were greater for Zn (72–363 ppm) and Mn (97.9–285.9 ppm) for Cu (8.8–14 ppm). C. olitorius had the least capacity to concentrate Mn and Zn in the leaf, which suggested that this vegetable is a less attractive candidate for agronomic bio-fortification of these elements. However, C. olitorius accumulated Fe at a significantly higher concentration (327 ppm) in the leaves than did Amaranthus (222 ppm) or Swiss chard (295 ppm). Sulphur as a macronutrient varied little in the plant species tested. The mean S concentration in the leaves ranged from 0.26% in C. olitorius to 0.34% in Amaranthus cruentus and Swiss chard. We concluded that the different vegetables have different abilities to take up Cu and Zn in the order Swiss chard > Amaranthus > Corchorus, and that they responded to micronutrients added to the soil but only up to certain limits of supplementation. The results from this current study seem to contradict the belief that wild vegetables have the inherent ability to concentrate mineral micronutrients in their tissue. Factors such as environment, anti-nutrients, dietary diversity, plant parts, plant age, and varieties result in differences in reported nutritional composition of indigenous vegetables. Post-harvest handling, storage, cooking and preservation also alter the composition. The need to optimise protocols for each vegetable type and for different laboratories makes analysis expensive. Equipment and methods of analysis are varied and may not be comparable, making it difficult to generalise on the composition of these vegetables. The Agricultural Research Council of South Africa and other stake holders are conducting studies on some aspects of these vegetables. There are still many information gaps regarding many aspects of these vegetables which require research attention. These include; the selection and improvement of genotypes, seed biology and germination studies, agronomic (population, fertiliser, crop mixtures) studies and phyto-chemical evaluation of these important species in order to encourage the overall use of these important indigenous resources. Finally, there is need to promote their increased utilisation.