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Typescript (photocopy). / Digitized by Kansas Correctional Industries

Thesis (MScAgric)--University of Stellenbosch, 2008. / Bibliography. Also available via the Internet.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

The aim of this study was to improve our understanding of the biology of two alien, invasive, free-floating species, the Water Fern Azollafiliculoides and the Least Duckweed Lemna minUla, which cause severe weed problems in Britain. Hence, control practices, which had been based upon anecdotal information, could be given a scientific foundation. An appraisal of the known distributions of the two species in Britain was undertaken. From an analysis of spread to date, it can be predicted that L. minUla will be present in 150 and A.filiculoides in 500, 10 x 10 km grid squares by the year 2000. Neither A. filiculoides nor L. minuta produce specialised overwintering structures and both rely heavily on ordinary, vegetative plants to overwinter. It was found that vegetative plants of both species can survive encasement in ice and laboratory studies show that they can withstand short exposure to sub-zero temperatures above approximately -4 °C. However, field evidence suggests that both species can survive much more severe temperature conditions, so both are considerably less cold-sensitive than previously suggested. L. minuta is not known to reproduce sexually in Britain. However, A. filiculoides sporulates regularly over a wide geographical range to produce numerous, viable sporocarps. These sporocarps form a 'seed' bank in the sediment and may ensure population survival because they are more freeze- and desiccation- tolerant than adult plants. Sporocarps require temperatures of not less than 10 °C and a short burst of light to germinate. L. min uta plants vary very little seasonally (although larger summer fronds can be easily confused with the common, native duckweed Lemna minor). In contrast, three forms of A. jilicuJoides can be identified; survival, mat and colonising. Competition experiments suggested the following tentative order of decreasing competitive ability; A. filiculoides > L. minuta > L. minor. This was probably a result of the more erect A filiculoides plants growing over the Lemna fronds. Not all field evidence supports this finding. Floating mats of A.filiculoides and L. minuta cause similar ecological problems because they reduce light, pH and oxygen and increase detrit~:s and probably alter nutrient loading. Laboratory studies showed that morphologically and physiologically plastic species of submerged plants, ego Elodea nuttall;;, could withstand these conditions better than less adaptable species, ego Potamogeton crispus. Four chemical control methods were tested over a range of dosages in the laboratory; diquat (as both spray and sub-surface injection), glyphosate (as spray) and terbutryn (as sub-surface application). These treatments were unsuccessful in controlling L. minuta, in contrast to anecdotal field evidence where terbutryn is considered effective. Glyphosate and diquat sprayed at 1.8 kg ai ha-1 and 1 kg ai ha-1 controlled A. jiliculoides. A. jiliculoides sporocarps were resistant to all chemical treatments.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.