Masters Degrees (Botany)

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Phytosociology of the Namib Desert Park, South West Africa.
(1976) Robinson, Ernest Richard.; Breen, Charles Mackie.
The geology, topography, soils and climate of the Namib Desert Park south of 23 S. Lat. are described briefly. The vegetation was investigated using the techniques of the Zurich - Montpellier school of phytosociology. Floristic and other site data were collected from 472 sites in the standard Braun - Blanquet manner and the techniques are described in some detail. These data are presented by means of phytosociological tables and detailed descriptions of each community. A total of 34 noda, communities and sub-communities, were delimited in the study area and the floristic and ecological relationships of these are discussed. Only a few of the communities can be assigned to syntaxa from other parts of the world or southern Africa because most of the Namib Desert communities seemed too different to be compared to associations from North Africa, the Middle East, South America or Australia on more than a superficial, structural basis. Very few data from other arid or semi-arid areas in southern Africa have received formal phytosociological treatment and there is therefore insufficient material to draw meaningful comparisons. The "foam structure" described by Volk & Geyger (1970) was found to be widely distributed in soils of the plains of the Namib Desert, and it was shown to have a profound influence on water penetration. The effects on vegetation development are discussed. The study generated a number of questions about the vegetation and ecosystems of the Namib Desert and some recommendations are made concerning future synecological and autecological studies. A list of species and synonyms of the names of all higher plants recorded in the study area are given in Appendix I. It is concluded that the Braun - Blanquet method is efficient in terms of time required to collect data which can be used for a number of purposes, but that a classification of vegetation ' should be followed up by autecological and detailed synecological studies of species (particularly those which characterize communities) and individual communities to determine the controlling factors more precisely and to enable more accurate predictions concerning the effects of management programmes to be made.
A contribution to knowledge of the genus Hypoxis L. (Hypoxidaceae) in Natal, South Africa.
(1976) Wood, Susan Elizabeth.; Gordon-Gray, Kathleen Dixon.
Abstract available in PDF.
On the vacuolar system in maize roots.
(1979) Lamb, Jennifer Margaret.; Berjak, Patricia.
Root-cap cells of Zea mays L proliferate by division in the cap meristem, and subsequently differentiate and mature as they move towards the periphery of the cap, where ' they undergo autolysis and are sloughed. Vacuolar ontogeny has been shown to be complex, several different mechanisms operating not only within the root cap tissue, but within the single cells. Vacuolar initials (provacuoles) are formed in the meristem by the pinching off of single- or doublemembrane bound vesiculations of the E.R. In some instances large vacuoles appear to be formed in the mature region of the cap through the sequestering of large organelle-free regions of cytoplasm by vesicles and small cisternae, thought to be of E.R. origin. Further development of provacuoles comprises their expansion and extensive fusion, this process culminating in the formation, in a mature cell, of just one large vacuole. The vacuoles of the mature region are autophagically active, engulfing all types of cytoplasmic organelle which are subsequently lysed; these vacuoles show a positive cytochemical reaction for acid-phosphatase, further indicating that they are lysosomal in nature. The dictyosomes of the late mature cells are hypersecretory and autoradiographic and cytochemical evidence indicates that the vesicles contain an accumulation of polysaccharide. These vesicles appear to follow two secretory pathways; firstly fusion with the plasmalemma, with secretion of their content into the extra-protoplasmic space where it accumulates, finally penetrating the cell wall and middle-lamella and forming viscous polysaccharide slime on the exterior of the cap. Secondly, these vesicles appear to be engulfed by and broken down within the vacuoles. At this stage the vacuole expands considerably, and it has been postulated (Berjak and Villiers, pers. corom.) that hydrolysis of the dictyosomally-derived polysaccharide within the vacuole to monosaccharide units results in osmotic. changes leading to an influx of water into the vacuole, and its consequent expansion. Autoradiographic, cytochemical and chromatographic evidence is not inconsistent with an accumulation of monosaccharide units being at least partially responsible for the osmotic uptake of water into the swelling vacuole. Finally, the vacuolar membrane becomes discontinuous, allowing hydrolytic enzymes p~esumably contained within the vacuole to come into contact with the cytoplasm, which consequently undergoes autolysis. At this stage the cell is sloughed from the cap.
A taxonomic revision of the tribe Desmodieae (Leguminosae - Papilionoideae)
(1984) Schrire, Brian David.; Hennessy, Esme F.
The tribe Desmodieae has a pantropical distribution and is one of the most advanced tribes in the subfamily Papilionoideae. Despite this fact, the tribe was already present in the fossil record of the early Tertiary and so the Papilionoideae appear to be much older than is currently accepted. With its greatest centres of development in tropical Asia and America, Africa is relatively poorly endowed and only four genera comprising sixteen species occur in the Flora of southern Africa. Many of these species are widespread in the old World tropics and the few African endemics appear to be closely related to them. Desmodium is the largest genus with the bulk of species belonging to the Asian subgenus Sagotia. Of the two sections of Sagotia represented in Africa, section Nicolsonia is clearly more derived than section Heteroloma, showing many characters intermediate between Heteroloma and the considerably more derived genera Alysicarpus and Lespedeza. Other Desmodium species either of the Old or New World have often become naturalised as weeds in Africa. Pseudarthria is a genus derived by an aneuploid increase of the basic x = 11 for the tribe rather than an aneuploid decrease as in the case of Alysicarpus and Lespedeza. A re-count of the chromosome number of P. hookeri (2n = 26) matches the count obtained in another species of Pseudarthria for the first time. Flowering strategy and flower structure in the tribe are the result of intense coevolution with the ecological requirements of skilled bee pollinators and although many bees can trip Desmodieae flowers, only a relatively few high energy demanding, traplining bees are able to bring about effective pollination. The overall breeding system represents a fine balance between the need for selfing and the need for outcrossing in order to produce enough seed of sufficient quality to survive in unstable environments.
A comparative study of the plant ecology of three estuaries : Mgeni, Mhlanga and Mdloti.
(1986) Raiman, Feisal.; Ward, C. J.
The vegetation of the Mgeni Estuary, Mhlanga Estuary and Mdloti Estuary was analysed according to the Braun-Blanquet phytosociological method using quadrats of 4m2 and 25m2 in area placed subjectively and the Point-Centred Quarter method with points chosen whilst traversing. A classification of plant communities is given. These are described floristically and related to habitat variables. Indicator species of the climatic climax of coast forest occur within the study area. The absence of coast forest is ascribed to low altitude, tidal inundation and the consequent effect of salinity, basal inundation resulting in a high water-table and the influence of man. The floodplain of the Mgeni Estuary is dominated by mangrove vegetation comprising mainly Bruguiera gymnorrhiza and Avicennia marina whereas vegetation established on Athlone Island is dominated by mesophytic thicket comprising mainly Schinus terebinthifolius, Lantana camara, Chromolaena odorata and Cardiospermum grandiflorum. The floodplain of the Mhlanga Estuary is dominated by Phragmites australis. The shores of the Mdloti Estuary are dominated by Barringtonia racemosa, Phragmites australis and Echinochloa pyramidalis. Major differences in vegetation patterns of the three systems are related primarily to the differences in the open nature of the river mouths. This is controlled mainly by river flow and longshore drift. Differences in vegetation patterns within an estuary are dependent on differences in tolerances to salinity, basal inundation and shade, together with variations in altitude and edaphic factors and competition between species. Generally soils of Mgeni Estuary had higher contents of small sized fractions, bulk densities, reserve acidities, organic matter, salts and exchangeable bases and lower pH than soils at Mhlanga Estuary and Mdloti Estuary. Differences exist between mangrove and non-mangrove soils at Mgeni Estuary and differences between the non- mangrove soils at the three study sites. Major differences in soil characteristics are as a result of differences in tidal inundation, geogenetic parameters and biotic factors. Information on topography, hydrology, geology, climatic factors, biotic factors and historical background of the area is given. A check-list of vascular plants is included. The work is illustrated by 44 figures.
Lipid peroxidation and ageing in seeds of cabbage and soya bean.
(1986) Hailstones, Milson Donald.; Smith, Michael Trevor.; Campbell, Glen Kerry.
It has been suggested that lipid peroxidation is involved in the loss of seed vigour, although many attempts to examine the relationships between lipid peroxidation and seed vigour have proved equivocal. Studies were undertaken on seed lots of cabbage and soya bean to find evidence for peroxidation by the analysis of i) total and polar fatty acid levels; ii) lipid hydroperoxides; iii) volatile products produced on heating dry seeds; and iv) volatile products produced on imbibition. The loss of polyunsaturated fatty acids (PUFAs) detected in the dry seeds was clearly related to germinability in both soya bean and cabbage seeds. Furthermore, an increase in hydroperoxides was observed in both seed types. Although the relationship of the level of hydroperoxides to germinability was less clear than for the decline in the level of PUFAs, these results suggested that the loss of PUFAs was possibly due to evidence the peroxidation of the seed obtained from the heating lipid, indirect of the seeds suggesting that hydroperoxide breakdown may be necessary in order that the changes in PUFAs become apparent. In contrast to the poor relationship observed between germinability and hydroperoxide level, a marked relationship between hydroperoxide level and seed moisture content was observed in the cabbage seeds. This may be significant with regard to the observed relationship between storability and seed moisture content, although no such relationship was seen in the soya beans. Certain volatile compounds derived from dry heated seeds were related to seed vigour in both seed types and evidence suggests that the lipid hydroperoxides were the source of these compounds. Although the total volatiles counts evolved from imbibing cabbage seeds showed no quantitative relationship to seed vigour, one peak was noted which was clearly associated with the vigour of these seeds. The variability in the volatiles evolved from soya beans on imbibition, however, precluded the detection of any possible relationship between these and seed vigour. In both seed types, results suggest that the volatiles derived on imbibition were of a different source to those derived on heating. A marked increase in the level of hydroperoxides was observed in whole cabbage seeds and soya bean axes of low vigour over the first hour of imbibition. This may suggest that an exacerbation of damage on imbibition was associated with low vigour seeds. In contrast to this, in the seeds of high vigour, the increase in hydroperoxide levels was markedly less or rapidly reduced, suggesting the possible activity of repair mechanisms. Ferrous ions were shown to invigorate both seed types, particularly cabbage seeds. It is suggested that the invigorating effect of these compounds was due to the facilitation of repair, including hydroperoxide breakdown and the quenching of any free radicals.
Ultrastructural features of the leaf blade epidermis and squamulae intravaginales of the marine angiosperm Halophila Ovalis (R.Br.) Hook.f.
(1986) Naidoo, Yougasphree.; Barnabas, Alban D.
No abstract available.
Biological nitrogen fixation (acetylene reduction) associated with blue-green algal communities in the Mgeni estuary mangrove swamp.
(1987) Mann, Fiona Denise.; Lambert, G.; Steinke, Trevor Desmond.
Nitrogen fixation of blue-green algae associated with Avicennia marina (Forssk.) Vierh. pneumatophores and wet and dry surface sediments were investigated in the Mgeni Estuary mangrove swamp by means of the acetylene reduction technique. Optimum partial pressures of acetylene ranged from 0,15 to 0,2 atm. for the different habitats. A lag phase of 3 h was observed in all habitats, followed by a period of linear ethylene production of 42 h for the pneumatophores and 72 h for the wet and dry mat areas. An assay period of 24 h was employed in all habitats. Laboratory studies revealed percentage moisture and temperature to be the prime factors influencing ARA (acetylene reduction activity) in all habitats and rates were highest under submerged conditions and at 22˚C. Short-term variations in ARA and salinity in the wet and dry mat areas, measured at 6 day intervals, were also related to percentage moisture. High concentrations of inorganic nitrogen (between 1 and 5 mg 1-1) significantly depressed ARA in all habitats. Increases in ARA occurred with increase in light intensity up to 40 μE m -2 s-1, with negligible dark rates being recorded in the wet and dry mat areas. Significant dark rates of ARA and stimulation of ARA by sucrose in association with the pneumatophores indicated that bacteria may also be contributing to ARA in this habitat. No organic carbon stimulation was noted in the other sites. Salinity had little effect on ARA over the range generally experienced in each habitat. Field studies revealed a marked seasonal variation in ARA, with summer maxima of 78, 678 and 341 nmol C2H4 cm-2 24 h-1 associated with the pneumatophore, wet and dry mat areas respectively. This coincided with maximum nitrogen-fixing blue-green algal numbers, temperature, light intensity and day length. No seasonal variations in organic carbon, inorganic nitrogen, salinity, percentage moisture or bacterial numbers were apparent. Rates of bacterial ARA associated with decomposing litter of A. marina were highest under exposed conditions and reached a maximum of 25 935 nmol C2H4 g dry wt -1 24 h -1 after 3 weeks. Maximum rates of ARA under submerged conditions of 5394 nmol C2H4 g dry wt -1 24 h -1were reached after 4 weeks of decomposition. An increase in percentage nitrogen occurred during decomposition and was greatest under submerged conditions. Rates of decomposition were highest under exposed conditions. It was estimated that nitrogen fixation by blue-green algal communities supplies 23,8% of the annual nitrogen requirements of the mangrove swamp.
A taxonomic review of the Natal members of the genus Oxalis L.
(1988) Moodley, Renee Ansuria.; Hennessy, Esme F.
No abstract available.
The effect of H2O2 on the photosynthetic biochemistry of Pisum sativum.
(1990) Ford, Lesley.; Pammenter, Norman William.; Amory, Alan M.
To study the short-term effect of hydrogen peroxide on plant metabolism aminotriazole (2 mM), a catalase inhibitor was applied through the transpiration stream of pea seedlings and the gas exchange characteristics, ascorbate peroxidase, glutathione reductase and catalase activities, and the levels of hydrogen peroxide, glutathione and formate were determined. CO2 assimilation rates were inhibited after the addition of aminotriazole: photorespiratory conditions (high 02 and low CO2 concentrations, high light intensities) exascerbated the inhibition. CO2 response curves showed that aminotriazole reduced both the RuBP regeneration rate and the efficiency of the carboxylation reaction of Rubisco. The inhibition of the CO2 assimilation rate during the first 100 minutes after the addition of aminotriazole was alleviated by feeding PGA through the transpiration stream. Catalase activity was completely inhibited within 200 minutes of aminotriazole application, but there was no concomitant increase in the hydrogen peroxide concentration, suggesting that hydrogen peroxide is metabolised by an alternative mechanism. The treatment had no effect on ascorbate peroxidase and glutathione reductase activities or total glutathione pool size, but greatly increased formate levels. These results suggest that hydrogen peroxide is metabolised by reacting with glyoxylate to form formate and CO2, The increased production of formate may reduce the flow of carbon through the normal photorespiratory pathway and may be used anapleurotically as a precursor of products of l-C metabolism other than serine. This would prevent the return of photorespiratory carbon to the BensonCalvin cycle in the form of 3-PGA. It is proposed that when RuBP regeneration levels are low high formate levels may be an effector of Rubisco, and competitively inhibit the binding of CO2 and 02. These results suggest that under photorespiratory conditions the uninterrupted flow of carbon through the photorespiratory pathway is fundamental to the functioning of photosynthetic metabolism.
The validity of currently recognised sectional limits within Combretum Loefling, subgenus Combretum (Combretaceae) in Southern Africa.
(1990) Rodman, Sharon.; Hennessy, Esme F.
Taxonomic confusion associated with Combretum Loefling, subgenus Combretum (Combretaceae) resulted in this investigation into the validity of sectional limits within the subgenus in southern Africa. Eight sections, together with their species and infraspecific taxa, totalling 29, are currently recognised in this region. Light and scanning electron microscopy on herbarium samples representing the taxa concerned were used to observe a final total of 171 selected characters. Data from individual specimens were collected numerically and processed to produce phenograms using Rohlf's NTSYS-pc program; also to produce descriptions in natural language format. A data processing program, TAXON, was specially designed and implemented to facilitate the desired processing and output. With the possible exceptions of Macrostigmatea and Ciliatipetala which require further investigation before certainty is reached, currently recognised sections were found to be valid and the limits between them acceptable. The main sources of confusion in this subgenus were found to lie in certain currently recognised species and their infraspecific taxa. These taxa were observed not to be distinct. The status of non-distinct species has been changed whenever clear evidence exists in favour of these changes. Other changes to species and infraspecific taxa have been recommended, but only to be implemented if supportive evidence is found after further investigations. Changes of status have been made in section Angustimarginata Engl. & Diels, where C. caffrum (Eckl. & Zeyh.) Kuntze and C. woodii Dummer have become synonyms of C. erythrophyllum (Burch.) Sond., and C. vendae Van Wyk has become a subspecies of C. erythrophyllum (Burch.) Sond., i.e. ssp. vendae (Van Wyk) Rodman comb. & stat. nov. (ined.); and in section Spathulipetala Engl. & Diels, where C. mkuzense Carr & Retief ,(section Macrostigmatea Engl. & Diels) has become a synonym of C. zeyheri Sond. C. collinum ssp. gazense (Swynn. & Bak.F.) Okafor, section Metallicum Fresen., has been changed to C. collinum ssp. coriaceum (Schinz) Rodman (ined.) in accordance with international botanical nomenclature.
Investigation of the processes involved during the photoinhibition of Zea mays L. seedlings.
(1990) Ripley, Bradford Sherman.; Amory, Alan M.; Pammenter, Norman William.
It has been proposed that the protective systems (photorespiration, the anti-oxidant system and non-radiative energy dissipation) alleviate or reduce photoinhibitory damage under high light conditions. To investigate the role of these mechanisms in C4 photosynthetic species, nine day old Zea mays seedlings were photoinhibited (30 minutes of 2500 J,Lmol m-2 s-1 PPFD) in the presence of various concentrations of 02 or CO2; or by photoinhibiting leaves in N2 after they had been fed glycolate or phosphoglycerate via the transpiration stream. The extent of the photoinhibition and the subsequent recovery from the photoinhibitory treatments was monitored with both CO2 gas exchange and chlorophyll fluorometry. Photoinhibitory treatments resulted in both a decrease in the rate of CO2 fixation and an interruption of PSII electron transport. CO2 response curves were used to monitor the efficiency of the carboxylation processes and the level of carbon metabolism substrate cycling during recovery following photoinhibitory treatments. Both were decreased by the treatment and recovered once leaves were returned to normal conditions. Low concentrations of 02 (2%) markedly reduced the extent of the photoinhibition. This protection could not be accounted for by photorespiration, which would be inoperative at such a low 02 concentration. Leaves fed glycolate exhibited enhanced photoinhibtion. It is also unlikely that the anti-oxidant system (Mehler reaction and associated glutathione and ascorbate reactions) could utilize sufficient reductant at such low 02 concentrations to produce the observed protection. Leaves inhibited in the presence of 02 had decreased maximum fluorescence yields (Fm) and little altered initial fluorescence yields (F0)' resulting in decreased PSlI efficiency (Fv/Fm)' Photoinhibition resulted in a small increase in the slow relaxing component (60 minute) of non-radiative energy dissipation. This component became more predominant as the 02 concentration was increased. The rate constant for photochemistry was also decreased by the inhibitory treatment. Leaves supplied with CO2 at a concentration above 50 J,Lmol mol-1 exhibited little photoinihibition suggesting that the protection was not due to a quantitative utilization of energy. PGA, fed via the transpiration strea~ enhanced the photoinhibition, suggesting that more than just the Benson-Calvin cycle is required to protect C4 plants from photoinhibition. At CO2 concentrations below this, the Fv/FID ratio was decreased due to large increases in the F0 values. Fm was little altered. These changes are characteristic of a decrease in the rate constant for photochemistry. The rate constant for non-radiative energy dissipation was little altered by the photoinhibition. The protection observed in the presence of either CO2 or 02 was not due to a quantitative utilization of energy and the different responses of F0' Fm and the rate constants KD and Kp, suggest that different mechanisms were operative in the presence or absence of oxygen.
The inter-relations among xylem anatomy, hydraulic conductivity and leaf water status in five sub-tropical tree species.
(1991) Sherwin, Heather Wendy.; Pammenter, Norman William.
The wood anatomy, hydraulic properties and leaf water status of five sub-tropical plant species were studied. The specimens studied were growing in a private, irrigated garden. Consequently, any differences in xylem anatomy would be a result of phylogenetic and not environmental factors. Podocarpus latifolius, being a gymnosperm, had only narrow, short tracheids as the conducting conduits. The size of the vessels of the four angiosperms increased in the following order: Tecomaria capensis, followed by Cinnamomum camphora, Trichilia dregeana and finally Barringtonia racemosa had the widest vessels. T. capensis and T. dregeana had the shortest vessel lengths. Those of C. camphora were slightly longer and the vessels of B. racemosa were the longest. Measurements of hydraulic conductance (Kh), twig specific conductivity (TSC) and xylem specific conductivity (XSC) followed similar trends to those of conduit diameters. The measurements of leaf specific conductivity (LSC) , however, did not follow these trends. T. dregeana, which had a far higher Kh than P. latifolius, did not have a significantly different LSC. This is because the twigs of T. dregeana supported a far greater leaf area than did the twigs of P. latifolius. There was also no significant difference in LSC among T. capensis, C. camphora and B. racemosa, although their LSC's were all significantly higher than those of P. latifolius and T. dregeana. The consequence of efficient xylem anatomy thus seems to be, not only a greater supply of water to the leaf but also, and perhaps more importantly, it allows a greater leaf area to be produced. The length of the vessels was also shown to have a large effect on the hydraulic conductivity of the twigs. The Kh values measured on excised twigs were found to range between 40% and 87% of the Kh calculated using the Hagen- Poiseuille equation. Conduit size distributions were also found to be important in calculating the Kh. The more efficent xylem anatomy of B. racemosa resulted in little decrease in plant water potential even with large increases in transpiration. P. latifolius on the other hand showed a considerable decrease in leaf water potential with just a slight increase in transpiration. The other three species showed decreases in leaf water potential inter-mediate to these two extremes. The inferred root-to-leaf condutivity, shown by the inverse of the slopes of the water potential versus transpiration curves, were lower than the LSC measurements taken on excised twigs in the laboratory. The difference between the inferred and the measured LSC's could give an indication of resistances such as those within the root and at the soil-root interface.
Effects of nitrogen nutrition on salt stressed Nicotiana tabacum var. Samsum in vitro.
(1992) Sweby, Deborah Lee.; Watt, Maria Paula Mousaco Deoliveira.; Huckett, Barbara Isobel.
The responses of Nicotiana tabacum L. var. Samsun to alterations in the nitrogen (N) supply under saline conditions in vitro were monitored. The aim was to test the hypothesis that nitrate-nitrogen supplementation to salt stressed plants alleviates the deleterious effects of salt on plant growth. Due to its capacity to be maintained under stringent environmental conditions, in vitro shoot cultures were chosen as the system of study. Nicotiana tabacum plantlets regenerated from callus in vitro were excised and rooted on solid MS culture medium containing a range of concentrations of NaCI (0 - 180 mM) and N (0 - 120 mM, as NO3--N, NH4+-N or a combination). A variety of parameters of root and shoot growth, nutrient utilisation and nitrogen metabolism were assessed over a 35 d period. Plant growth on 40 mM NO3--N + 20 mM NH4+-N (standard MS nutrients) was inhibited by the presence of salt, with root growth being more adversely affected by salt than stem growth. Root emergence was delayed from 6 d (0 mM NaCI) to 15 d (180 mM NaCI). Similar suppression of growth for all parameters, except root mass and leaf chlorophyll content, was observed when NaCI was replaced with mannitol at equivalent osmolalities. Root mass and leaf chlorophyll were significantly improved in plantlets supplied with mannitol. The time of root emergence was unaffected by mannitol supply, with all roots emerging after 10 d in culture. Plantlet growth on NH4+-N only (0 - 60 mM) was severely inhibited, even in the absence of NaCI, and was inferior to growth on NO3--N. Nitrate additions to salt stressed plantlets could not match growth in control (0 mM NaCI) plantlets. When plantlets were cultured on NO3--N only (0 mM, 30 mM, 60 mM, 120 mM), the increase in nitrate supply up to 60 mM resulted in a small improvement in growth on 90 mM NaCI, but had almost no effect on growth at 180 mM NaCl. A nitrate supply of 120 mM led to growth inhibition in all parameters, even in the absence of NaCl. Plantlet growth on isosmotic concentrations of mannitol in the presence of 0 - 120 mM NO3--N essentially mimicked that of NaCI, except for leaf chlorophyll content which was improved on mannitol at all NO3-·N levels. Nitrate uptake (measured as depletion from growth medium) by plantlets grown on 0 - 180 mM NaCI was positively correlated to availability of nitrate but negatively correlated to NaCI supply. Similar results were obtained for a mannitol supply except nitrate uptake was enhanced significantly on mannitol compared to NaCl. Sodium and chloride uptake appeared unaffected by nitrate concentration. Leaf protein content responded favourably to an increase in the NO3--N supply up to 60 mM and, in particular, appeared to be stimulated in the presence of 180 mM NaCl. Nitrate reductase (NR) activity was found to be inhibited drastically by salt and NO3--N supplementation to the salt medium had no effect on enzyme activity. A reduction in leaf total RNA content was recorded with an increase in NaCI concentration from 0 - 180 mM. A positive response to an increase in the NO3--N supply from 30 mM to 60 mM was detected in the presence of NaCl. Attempts were made to assess the levels of mRNA for NR in response to the various NaCl and N regimes. The plasmid pBMCI02010 containing a NR cDNA insert was isolated and purified and used in both radioactive and non-radioactive RNA slot blot hybridisation procedures. However, due to problems of non-specific binding of the probe, no quantification of the levels of NR mRNA in response to the various treatments could be made. Nitrate supplementation to plantlets of Nicotiana tabacum growing in vitro did not appear to ameliorate the effects of salinity stress, such that growth of plantlets in the presence of NaCI was always inferior to that in the absence of NaCl. As a large portion of growth inhibition was found in this study to be a result of osmotic rather than ionic effects of salt, it is questioned whether a nitrate supply would have an ameliorating effect on plant growth under field conditions.
The effect of sun and shade on the leaves of four coastal tree species.
(1992) Kemp, Lynley Claire.; Pammenter, Norman William.
Mimusops caffra, Euclea netetensis, Olea woodiana and Peddiea africana are tree species associated with different successional stages in a coastal dune forest. Saplings of these tree species were established in four different light intensities. These were full sun, 40% shade, 70% shade and 90% shade. The hypothesis proposed that the species from different successional stages are preadapted for a particular light environment and are disadvantaged in other light environments. Growth, morphological, biochemical and physiological aspects of the four species in the four light environments were determined. Growth rates showed no consistent pattern with respect to light intensity. However, most species, irrespective of their successional status, had the best growth response in either 40% or 70% shade treatments. All the species showed typical sun and shade responses for morphological, anatomical and some biochemical characteristics. Photosynthetic responses were complex and showed no relationship between the successional status of the species and the light conditions in which they were grown. There appears to be very little relationship between the growth responses, the measured biochemical and morphological aspects, assimilation rates and the successional status of the species. Light intensity is therefore not the sole driving force of forest succession but one of the many factors that contribute to the overall process.
Regulation of nitrate reductase during in vitro differentiation of nicotiana tabacum L. var. samsun.
(1993) Roberts, Michael Austin.; Cresswell, Christopher F.; Huckett, Barbara Isobel.; Watt, Maria Paula Mousaco Deoliveira.
The commencement of in vitro differentiation is mediated by genetic changes that result in selective expression of genes and a shift in metabolism. The role of nitrate reductase, a key enzyme of nitrate assimilation, during differentiation was examined in this study using an in vitro Nicotiana tabacum (tobacco) callus culture system. In particular, the effects of nitrogen and light/dark regimes on callus differentiation and nitrate reductase were investigated. Methodology required for the analysis of nitrate reductase regulation during in vitro tobacco callus differentiation was established. Optimised in vivo, in situ and in vitro nitrate reductase assays yielded similar values and patterns during tobacco callus culture development, and the in vivo assay was selected for nitrate reductase activity measurement during subsequent experiments. Western blot analysis of tobacco callus acetone-extracted protein after sodium dodecyl sulfate-polyacrylamide gel electrophoresis using a spinach polyclonal nitrate reductase antibody yielded major bands at 71 and 48 kD, with numerous minor bands. Extraction of callus protein in the presence of various protectants did not prevent cleavage of putative nitrate reductase polypeptide. Slot blot detection of nitrate reductase mRNA using a [32p]- labelled nitrate reductase cDNA probe isolated from the plasmid pBMC102010 was not possible due to non-specific binding to nitrocellulose filters. Northern blotting of RNA fractionated by agarose gel electrophoresis using a [32p]-labelled nitrate reductase cDNA probe identified a single mRNA species at 3.5 kb, the expected size of tobacco nitrate reductase mRNA. In vitro tobacco callus differentiation on 60 or 120 mM nitrogen regimes and under light/dark (16/8 h), continuous dark or continuous light treatments were comparable in terms of fresh weight, protein and nitrate uptake. Higher levels of in vivo nitrate reductase activity were observed prior to visible shoot primordia in all treatments, suggesting that the developmental status of callus mediated the regulation of nitrate reductase. Putative nitrate reductase protein levels were not correlated with in vivo nitrate reductase activity during initial stages of tobacco callus differentiation under various light treatments; nitrate reductase mRNA levels could not be ascertained. These results suggested that post-translational control mechanisms were involved in nitrate reductase regulation during in vitro tobacco callus differentiation.
Biochemical and biophysical indicators of chilling tolerance in some species of Eucalyptus.
(1993) Stewart, Gwethlyn Meriel.; Amory, Alan M.
Although Eucalyptus species are indigenous to Australia, they have been successfully cultivated in South Africa, where they are mainly used in the mining and paper industries. With the explosion in these industries it has been necessary to increase Eucalyptus plantations, often into areas which experience frost and chilling temperatures. To combat this, high yielding Eucalyptus species able to cope with these conditions would be desirable. The ability to rank species according to their chilling tolerance will enhance decisions as to the suitability of species for use in the field. To this aim, two biochemical and two biophysical parameters were chosen to investigate and characterise Eucalyptus nitens, E. smithii, E. macarthurii, E. grandis and E. grandis x nitens (GNI026). Ranking of these species in terms of chilling tolerance did not appear possible using the data from the biochemical parameters (proline concentration and glutathione reductase activity), but the biophysical parameters (fluorescence characteristics and onset of temperature of melt) gave results comparable to those obtained in field trials using these species. Fluorescence characteristics were particularly useful in assessing the chilling tolerance of the species in question. Fluorescence is the recommended technique for further studies as it is relatively inexpensive, rapid, does not require destructive sampling and can be used in both the laboratory and field.
A revision of the genus Ledebouria Roth (Hyacinthaceae) in South Africa.
(1993) Venter, Stephanus.; Edwards, Trevor John.
Members of the genus Ledebouria Roth (Hyacinthaceae), which occur in South Africa, are revised. This genus occurs throughout Africa, India and Madagascar. 33 Species are recognized and placed into nine provisional infrageneric groups. A multidisciplinary approach was used in an attempt to provide natural groupings. The following characters were analysed; morphology, micromorphology, palynology and caryology. Aspects of ovary structure and leaf micromorphology proved especially useful in the synthesis of infrageneric and specific concepts. Keys, descriptions, illustrations, distributional, ecological, medicinal and toxicological data are provided. This study is based on plants in their natural habitat, cultivated specimens and representative herbarium specimens from herbaria in South Africa and in Europe.
Production and physiological responses of Italian ryegrass and white clover grown in mono cultures and mixed stands.
(1993) Gerber, Liesel.; Pammenter, Norman William.
The aim of this study was to test the hypothesis that ryegrass and clover, when grown under optimal conditions in mixed stands, interact in response to available light energy. with other resources non-limiting, pasture yield is determined by the efficiency with which solar radiation is intercepted by canopies and converted into dry matter. The clover-ryegrass interaction under varying light regimes, as experienced in the canopies of these pastures, was studied by investigating the growth and production of ryegrass and clover in relation to light harvesting abilities and photosynthetic utilization of intercepted light. Pasture canopy structure and growth were studied under a four-weekly clipping treatment. The interception of photon flux density (PFD) in the pasture canopy was monitored diurnally and seasonally in mixed and mono cultures. Light use efficiency (C02 fixed/ unit absorbed PFD) as well as photochemical efficiency of photosystem II (Fv/Fm) were studied by monitoring CO2 assimilation rates and chlorophyll fluorescence respectively. The results obtained from this study indicated that interaction did occur between ryegrass and clover, cultivated in mixed pastures. The mixture was capable of more efficient light interception than the mono cultures, which resulted in higher productivity. Light interception abilities, as manifested in canopy architecture, and not physiological utilization of light energy, were found to govern the interaction between the two pasture components.
The development of regeneration and transformation systems for Eucalyptus spp.
(1994) Hope, Belinda Anne.; Watt, Maria Paula Mousaco Deoliveira.; MacRae, Sharmane.
In South Africa, Eucalyptus breeding programmes are aimed at the selection of fastgrowing varieties, with appropriate wood characteristics and/or resistance to pests and diseases. However, the slow growth rate, long generation time and heterozygosity of trees make this a difficult task. Such problems may be overcome by the adoption of a biotechnological approach for plant propagation and modification. Towards this end, the aims of this investigation were to establish protocols for the micropropagation of Eucalyptus grandis and for the Agrobacterium-mediated transformation and subsequent plant regeneration of this important species. The usefulness of transformed cells and/or tissues is dependent upon the availability of methods for their regeneration into plants. Consequently, methods for plant regeneration via indirect organogenesis from leaf discs and cell suspension cultures were investigated. Organogenic calli were produced from leaf explants on MS medium with 16 mg.1-1 &l:em•calcltrate, 20 g.I-I sucrose, 1mg.I-I NAA and 0.05 mg.1--<1 BA. Shoots were induced on MS medium containing 1 mg.1-1 ZEA and 0.2 mg.1-1 IAA, and subsequently rooted on medium containing MS nutrients supplemented with 1 mg.1- 1 IAA. Cell suspension cultures were established but not regenerated via indirect organogenesis. Additionally, various media were investigated in order to obtain somatic embryos from cell suspension cultures. The MS media supplemented with 30 g.1-1 sucrose, 12 mg.1- 1 ABA and/or 40 g.1-1 PEG were found to be most suitable, resulting in the production of embryoids; germination results are not available at this stage. In order to establish methods for the transformation of both leaf discs and cell suspension cultures of Eucalyptus, a triparental mating was performed between Escherichia coli pnT119 (donor), A. tumefaciens LBA4404 (recipient), and E. coli HBI0l::pRK2013 (helper), resulting in the transconjugant LBA4404 (pnT119); insertion of the pJIT119 plasmid was demonstrated using agarose gel electrophoresis. The transconjugant CS8C1 (pMP90) (pJIT119) was also used. Protocols for the transformation of both leaf discs and cell suspension cultures were established, and resulted in the production of putatively transformed calli which were GUS positive and with stood selection on kanamycin (50 Ilg.mr1) and/or sulfadiazine (50 Ilg.mr1). Also, Southern blotting analysis indicated that the gene transfer process was successful. Due to difficulties in the regeneration of plants from transformed calli transgenic plants were not obtained. Future research strategies and applications of the developed protocols to Eucalyptus breeding programmes are discussed.

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