Doctoral Degrees (Botany)

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Abscisic acid and other hormonal effects on growth in Spirodela.
(1969) Van Staden, Johannes.; Bornman, Chris H.
The effects of abscisic acid In particular, as well well as gibberellic acid and the cytoklnlns, 6-benzyladenine, kinetin, and 6-dlmethylal lalylamlnopurine, on the growth of Spirodela oligorrhiza were investigated. Abscisic acid effectively arrested growth permanently at concentrations down to 10¯¹ mg/I. Normal growth tended to be resumed at concentrations of 10¯² and 10¯³ mg/l between nine and twelve days after treatment. A concentration of 10[-8] mg/l, however, resulted in a significant increase in dry weight at both eight, nine and twelve days after introduction into the culture medium. It is suggested that the resumption of growth twelve days after treatment at those concentrations which inhibit growth up to nine days, was due to a possible progressive inactivation of abscisic acid resulting in a lowering of its concentration to a level that is promotive. It was furthermore found that the growth response of Spirodela in terms of dry weight production over a period of eight days is proportional to the log[10] concentration of abscisic acid. It is suggested that this curve can be used as a relatively reliable and easily performed bioassay to detect amounts of abscisic acid as low as 10[-5] μg. The assay is more reliable over the range 0.01 to 10,000 μg and appears not to be affected by gibberellin, benzyladenine and kinetin. The inhibitory effect of abscisic acid on growth in Spirodela was shown to be reversed by benzyladenine, kinetin and dimethylallalylaminopurine, although they were not equally effective in doing so. Benzyladenine at 1.0 mg/l was the most effective In overcoming growth inhibition by abscisic acid. Gibberellic acid, however, proved ineffective in reversing the inhibitory effect of abscisic acid on Spirodela oligorrhiza. The apparent Increases in growth obtained in some cases may have resulted more directly from gibberellic acid stimulation than from the Interaction of gibberel lie acid with abscisic acid.
Tissue culture studies on citrus and Welwitschia.
(1972) Button, James.; Bornman, Chris H.
Part I. IN VITRO CULTURE OF CITRUS EMBRYOS AND NUCELLAR ISOLATES Zygotic embryos of the Ellendale mandarin, a monoembryonic variety of citrus, were cultured on modified basal media of Murashige and Skoog (BM[1]), and White (BM[2]) , supplemented with various growth regulators and nutrient additives. The growth of immature embryos was greatly enhanced by the addition of 400 mg/l casein hydrolysate (CH) to the basal media. Coconut milk (CM) and malt extract (ME) enhanced growth to a lesser extent, while the addition of indoleacetic acid (IAA) and kinetin (KIN) at the concentrations used, was in no way beneficial. Nucellar isolates excised from abortive and normal Ellendale mandarin ovules eight to 20 weeks after anthesis, were cultured on BM[I] and BM[2] in the presence of various concentrations and combinations of IAA, indolebutyric acid, naph~haleneacetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), KIN, CM, benzyl adenine, 6-dimethylallylamino purine, yeast extract (YE), ME, CR, adenine (AD), adenine sulphate (AS), ascorbic acid (AA), and benzylthiazole-2-oxyacetic acid. Some of the isolates which remained alive for four months did develop callus but n~ differentiation of embryoids or other structures occurred. Unfertilized ovules from 8-12-week-old Washington Navel orange fruits provided nucellar isolates which were cultured on media similar to those upon which mandarin nucelli were unsuccessfully cultured. In the case of Navel orange nucelli however, BM[1] + 400 mg/l filter-sterilized ME, and BMl + 40 mg/l AD yielded numerous pseudobulbils which later developed embryoids. Adenine (10 mg/l) was more effective than 20 mg/l which in turn was more effective than 30 mg/l. Adenine was more effective than its equivalent amount supplemented in the sulphate form except at 10 mg/l where the two forms were equally effective. Zeatin (ZE) at 0,2 mg/l did induce some pseudobulbils and embryoids, but all these treatments were less effective than 400 mg/l ME. When transferred to BM[1] + GA[3] (1 mg/l) , embryoids developed roots and later, shoots. It was necessary to remove plantlets from the GA[3]-supplemented medium shortly after the first foliage leaves developed in order to prevent the development of weak, spindly plants . Plantlets were transferred from BM[1 ]+ GA[3] to BM[1] only, and then after careful conditioning they were planted out in soil. This appears to be the first successful attempt at inducing adventive embryogenesis 1n the nucellus of unpollinated, unfertilized citrus ovules in vitro. Part II. EMBRYO AND FREE-CELL CULTURE OF WELWITSCHIA MIRABILIS Welwitschia embryos, cultured on BMI supplemented with CR, and low levels of IAA and KIN, germinated and developed leaves but not roots. Embryos cultured on BM[I] with 5,0 and 10,0 mg/l NAA produced an abundance of friable callus from the hypocotyl root axis. This callus was used for starting suspension cultures aimed at inducing vegetative embryogenesis. A number of nutritional additives and hormones were used alone and in combination at various concentrations. Cells of numerous shapes and sizes were observed but no organogenesis was apparent in either suspension cultures or in cell colonies plated out on semi-solid agar media. A closer study of cell aggregates formed in suspensions supplemented with CM + 2,4-D revealed that internal division occurred in approximately 40 per cent of the larger cells. It is suggested that this internal division may constitute the first step in embryogenesis of Welwitsahia cells in suspension culture. It is also tempting to speculate that this process, which has been reported by other researchers, is the first step 1n embryogenesis of free cells in general. Although this attempt at inducing adventive embryogenesis in cell cultures of Welwitschia was unsuccessful, some encouraging results were obtained on potentially suitable media and possible initial stages in the organization of embryoids.
'n Plantekologiese opname van Boschberg en sy omringende gebiede met spesiale verwysing na die weidingsfaktor.
(1972) Van der Walt, Pieter Toxopeus.; Grunow, J. A.
No abstract available.
A plant ecological survey of the Umfolozi Game Reserve, Zululand.
(1972) Downing, Brian Harvey.; Bayer, A. W.
A landscape unit approach based on use of airphotos was used for investigation of Deciduous Acacia Woodland in the Reserve. The approach was useful for indicating that a quantity of as few as 531 samples could be suitable for sampling woody vegetation over the large, 493 km² area, and for showing where the samples might advantageously be distributed. Use of the approach minimised the extrapolation of community boundaries for mapping purposes. Normal association analysis of the samples revealed the nine woody consociations present and provided quantitative data on species constancy and fidelity. These data were used towards explaining low levels of homogeneity within consociations, as well as some close floristic similarities found between consociations. The successful emergence of a species to dominance in a consociation was ascribed to the presence of a particular, described soil series or rock substrate. The distribution pattern of the consociations resembled a soil-vegetation catena on the landscape. The consociations were grouped into defined physiognomic categories of Open, Closed and Riverine Woodland Associations that were distributed according to three soil associations. The effects of soil factors, fire and the biota on physiognomy, notably secondary thicket encroachment, were discussed; and the relationship between climate and phenology was mentioned. A quantitative description of the grass communities based on normal analysis of stratified sample data revealed a retrogression whereby climax grasses are being replaced by mid-seral and pioneer grasses. The retrogression was ascribed to selective grazing by an increasingly large biomass of enumerated, indigenous herbivores. Some of the management recommendations offered were based on empirically calculated estimates of the weights of dry grass required annually by the grazing animals. Copies of topography, place names, geology, land surface and vegetation maps are provided. The text is supplemented by check lists of plants and animals recorded, by eight figures, 33 tables and 54 photos.
Automatic classification and ecological profiles of South-Western Transvaal Highveld grassland.
(1973) Morris, Jeffrey William.; Villiers, T. A.
Abstract available in PDF file.
Studies on the primary phloem of Gomphocarpus physocarpus.
(1974) Botha, Christiaan Edward Johannes.; Bornman, Chris H.
Penetration of the stems and leaves of Gomphocnrpus physocarpus by the aphid, Aphis nerii was studied with light and phase microscopes. Penetration of the epidermis and ground tissue was largely intercellular, that of the phloem tissues partly intercellular and in part intracellular. Of 75 pairs of stylet tips encountered in presumably functional sieve tubes 73 were lodged in sieve tubes of the internal and adaxial phloem. Light and electron microscope investigation of the ontogeny and differentiation of the primary phloem revealed that the internal and adaxial phloem attains a more advanced stage of maturity earlier than the corresponding external and abaxial strands. Furthermore, the protoadaxial phloem sieve tubes remain functional throughout the primary stages of growth, whereas the corresponding abaxial phloem sieve plates are associated with massive deposits of definitive callose during the latter stages of primary growth. The internal and adaxial phloem was found to be capable of independent assimilate translocation in plants which had the external phloem experimentally ringbarked. Translocation in the internal and adaxial phloem of such plants was unaffected by ringbarking. The results are interpreted as indicating that a greater proportion of assimilates is translocated in the internal and adaxial phloem sieve tubes than in the external and abaxial sieve tubes. It is concluded that this governs A. nerii's preference for the internal and adaxial phloem as a prime feeding site.
Seed dormancy and germination in Protea compacta R.Br. and Leucadendron daphnoides Meisn.
(1974) Brown, Neville Arthur Claud.; Van Staden, Johannes.
The germination responses of seed to a wide variety of treatments showed that both the pericarp and the embryo contribute to the dormant condition in Protea compacta. Germination can be improved to a greater or lesser extent by scarification, high oxygen tensions, stratification and applied hormones. In Leucadendron daphnoides dormancy is mainly imposed by the pericarp which apparently acts as a barrier to oxygen diffusion to the embryo. A considerable increase in germination is brought about by scarification and high oxygen tensions applied to intact seeds. Stratification and applied hormones improve germination to a lesser extent. Leaching and light treatments fail to improve germination in both species. Attempts to characterize the major inhibitor present in seed leachates indicated that it was coumarinlike in its properties. No evidence could be found that inhibitors leached from seed were actually involved in the regulation of germination. Poor germination in both species is apparently due to the lack of germination promoters rather than to the presence of inhibitors. Germination of seed of Leucadendron daphnoides was increased by 50% chilling at 5°C and 500% by incubation in oxygen. Both these treatments brought about a four-fold increase in the level of butanol-soluble cytokinins. The latter apparently play the primary role in promoting germination. The greater effectiveness of high oxygen tensions improving germination appears to be due to the additional effect oxygen has in stimulating the production of acidic gibberellin-like substances, to a level 30 times higher than with chilling. The acidic gibberellin-like substances do not appear to have a primary role in promoting germination but their effect is additive in the presence of a threshold level of butanol-soluble cytokinins. Maximum germination does not appear to depend on phasic changes in promoter levels, but on whether an increase in the level of gibberellin-like substances coincides with an increase in the levels of butanol-soluble cytokinins. In Protea compacta the effectiveness of chilling and oxygen incubation in improving germination by approximately 50%, is apparently mediated through their effect in increasing the levels of butanol-soluble cytokinins. The latter appear to play the primary role in promoting maximum germination, which apparently does not depend on a phasic change in promoter levels.
Ultrastructure of the germinating Welwitschia mirabilis seed.
(1975) Butler, Valerie.; Bornman, Chris H.
The structure and chemical composition of quiescent Welwitschia mirabilis (Hooker fil.) seeds and resultant changes occurring during the first 7 days of germination were studied. Research was focussed on the megagametophyte and its interrelationship with the non-vascularised outgrowth produced by the embryo. This outgrowth was given the name "feeder" by Bower in 1881 who postulated that it functions as an absorptive organ. However the possibility existed that it merely fulfilled the mechanical role of anchorage. Following hydration activation of embryonic collar cells precedes that of gametophyte cells whose rate of activation is governed by relative distance from the embryo. This sequence of activation is suggestive of a stimulatory factor diffusing from the embryo into the gametophyte. Starch, protein and lipid reserves in the collar and developing feeder are consumed within 36 to 48 h. As a consequence the rapidly developing seedling is probably largely dependent on nutrient material in the gametophyte until the plumule emerges, after approximately 5 to 6 days germination. Ventral feeder cells in contact with gametophyte tissue apparently act as transfer cells, developing numerous small wall projections invested with plasmalemma which result in a much greater absorptive surface area. The large numbers of mitochondria occurring in these cells might suggest active uptake of nutrients. At the 3- to 4-day-stage the feeder and gametophyte adhere firmly. While this adherance probably facilitates translocation of nutrients it could also have the secondary function of anchoring the feeder in the gametophyte, thus providing the emerging plumule with a firm base. The apparent root cap origin of ventral feeder cells might explain the positive geotropism of the feeder, as recent work inter alia by Wilkins and Wain (1975) has shown that root cap cells may be geoperceptive. Cytochemical methods used at the light and electron microscope level suggest that reserve material within protein bodies of the embryo and gametophyte might exist as a proteincarbohydrate complex and that globoid origin might be cytoplasmic. The immediate digestion of protein body reserves in the embryo and gametophyte interface zone argues the presence of pre-existing hydrolytic enzymes laid down within the protein bodies prior to quiescence. However the enzymes responsible for reserve breakdown in deep gametophyte tissue seem to be synthesised de nova. Protein hydrolysis precedes lipid digestion which possibly indicates that some of the resulting free amino acids might be used in the de nova synthesis of lipases. Lipid bodies, microbodies, mitochondria and amyloplasts encircled with ER seem to form a complex. Fatty acids resulting from lipase action in the lipid bodies (Ching 1968) are probably converted by microbodies (glyoxysomes) to succunate (Breidenbach and Beevers 1967) which is converted to sucrose by the action of mitochondria (Cooper and Beevers 1969a, b). Excess sucrose is probably converted to starch and stored in the amyloplasts. In 5 days the mean dry mass of the gametophyte decreases by approximately 47% during which time the total amount of lipid decreases by 76.5% and protein by 14%. Although some of the hydrolysed fatty acids and amino acids are no doubt utilised in the gametophyte it is suggested that the majority of fatty acids are probably converted to sugars· which, together with free amino acids (and possibly simple peptides) are transported to and absorbed by the embryo via the feeder whence they are utilised for seedling growth.
Cytokinins in Lupinis albus L.
(1978) Davey, Joanne Edith.; Van Staden, Johannes.
A review of the literature pertaining to the sites of synthesis, distribution and metabolism of cytokinins, established that there was a need to investigate the quantitative and qualitative changes in cytokinin levels during the growth cycle of a whole plant. An annual legume, Lupinus albus L. (the white lupin), was selected as the experimental plant for such a study. The levels of cytokinin activity in the root exudate, leaves and terminal (primary) apices of mature plants were examined between 5 and 17 weeks after planting. The results indicated that the total level of activity in the root exudate of a single plant is much greater than that which accumulates in the leaves and apices up to the time of flowering. This was considered to be indicative of rapid utilization of cytokinins in the actively growing shoot. It was also established that, after flowering, the level of activity in the terminal apices increases markedly, while that in the root exudate decreases. The latter finding was examined more closely in the next experiment, during the course of which cytokinin levels in the root exudate, leaves, fruits and fruit sap were determined at 2 weekly intervals after flowering. As in the previous experiment, the results showed that, although there may be a slight increase in the amount of activity in the root exudate, this activity was insufficiently high to account for the levels of cytokinin which accumulate in the fruiting apices. Cytokinin activity was however detected in the sap passing into the fruits (largely phloem sap), thereby indicating that at least some of the cytokinins detected in both pod wall and seed extracts could have originated in other parts of the plant. Embryonic development was followed at the gross morphological, anatomical and ultrastructural levels, and estimates were made of the cytokinin levels in the embryos and adjacent tissues. The results indicated that the cytokinin levels in the flowers, and young fruits, were low, but gradually increased with time. At 2 weeks after anthesis the level of activity in the pod wall was higher than that in the individual seeds. By 6 weeks, however, the activity in each seed was in excess of that recorded in the pod wall. Decreasing levels of activity in the seeds, and increasing levels in the pod wall, reversed this trend at later developmental stages. The level of activity recorded in the maturing embryo was low, while in the adjacent tissues it was high. This was especially noticeable at 6 weeks after anthesis when the cytokinin activity in the endosperm reached a high level. It would appear that these cytokinins facilitate nutrient mobilization to the tissues surrounding the embryo, and that they are possibly utilized in order to promote certain growth processes taking place in the embryonic cells during development. The cytokinin activity in the mature embryo was low, but increased slightly in the cotyledons, radicles and young shoots during germination and seedling establishment. The cytokinins present in all white lupin extracts had chromatographic properties, on paper and/or Sephadex LH-20, similar to the naturally occurring cytokinins zeatin and zeatin riboside and their glucosylated or phosphorylated derivatives. Mass spectrometric-analysis of the compound co-eluting with zeatin showed that this cytokinin was present in white lupin extracts, while analysis of the peak co-eluting with zeatin riboside indicated that the cytokinin at this elution volume was an adenine derivative. The possible presence of dihydroderivatives of these cytokinins was also indicated. The results did not exclude the possible presence of other cytokinin-like compounds. The significance of qualitative as well as quantitative changes in cytokinin activity were assessed. It is concluded thatcytokinins do not accumulate at excessively high levels in actively growing tissues, unless these tissues function as a transitory store for nutrients, as for example the endosperm, testa and pod wall. Cytokinins are probably involved in the control of a number of growth processes of which nutrient mobilization may be of considerable importance to the plant.
Some aspects of phosphorus cycling in Midmar Dam.
(1980) Twinch, Allan John.; Breen, Charles Mackie.
No abstract available.
The biochemical and cytokinin changes in the developing and germinating seeds of Podocarpus henkelli stapf.
(1981) Dodd, Malcolm Caulton.; Van Staden, Johannes.
A review of the literature revealed that there is a lack of depth in our knowledge of gymnospermous seeds with regard to the development and germination processes. The phytohormones, particularly the cytokinins have been implicated in these processes. The seeds of Podocarpus henkelii were thus selected as experimental material for studying the biochemical and cytokinin-like changes associated with development and germination. The development of these seeds was also followed at the ultrastructural level. These studies revealed that cellular detail within the female gametophyte only began .to form in December (early summer), approximately six weeks after fertilization had taken place. At this time some reserve protein was evident and the embryo sporophyte consisted of only a few pro-suspensor and pro-embryo cells. Concurrently, the cytokinin levels were fairly high in the female gametophyte but low in the epimatium. In both seed components two cytokinin- like compounds predominated which co-chromatographed with the free base cytokinin zeatin and its ribonucleoside. The second sample was taken in late January (mid-summer) by which time the embryo sporophyte had developed rapidly into a readily distinguishable seed component. The cellular detail indicated that much cell division had recently taken place and that the cells were currently increasing in size and accumulating starch and lipid. In the female gametophyte the soluble sugars were at the maximum level recorded during these experiments and the level of starch was increasing. The extractable cytokinin content of the seed was high at this time, particularly in the embryo sporophyte. In all three seed components cytokinin-like compounds which co-chromatographed with zeatin and ribosylzeatin were present. These high levels of cytokinin coincided with the rapid increase in both fresh and dry mass of the embryo sporophyte and female gametophyte. Ultrastructural studies of the third sample collected in mid-March (early autumn) showed that cellular changes were associated mainly with increases in cell size and the accumulation of food reserves, particularly starch. The cytokinin levels had decreased in all three seed components at this time. There was an increase in the cytokinin which co-chromatographed with glucosylzeatin in the female gametophyte. The seeds matured in late April (autumn) and had the unusual features of not drying out during maturation. Fresh seeds collected from the ground had a moisture content of ca. 62 per cent. The main food reserve was starch with relatively small amounts of protein and lipid also present. The seeds of Podocarpus henkelii germinated readily after scarification in the absence of water provided that their moisture content remained ca. 60 per cent. Seeds in which the moisture content had fallen below ca. 54 per cent required additional water for germination. The moisture content of the seeds fell rapidly under natural conditions and viability was lost below a moisture content of ca. 34 per cent. Unscarified seeds of 52 per cent moisture content placed under moist conditions at a constant 25°C took 23 weeks to achieve 68 per cent germination. These experiments showed that although the epimatium limited water uptake by the seeds it did not prevent moisture loss to the atmosphere. This appears to be the main factor contributing to the seed's inefficiency as a propagule. A small degree of after-ripening was recorded with the embryo sporophyte increasing in size with storage. This appeared to contribute to the increased rate of germination of the scarified seeds. An interesting feature of the seeds of Podocarpus henkelii is that they have the ability to fix atmospheric carbon, which is subsequently translocated from the epimatium to the female gametophyte and embryo sporophyte. The mature seeds were stored at 4°C for six weeks during which time little change had occurred at the ultrastructural level. Protein vacuoles in the embryo sporophyte had disappeared and in all three seed components cytokinin levels were low. Three days after scarification and the start of incubation, little change in cellular detail was apparent as limited rehydration was necessary due to the high moisture content. The cytokinin levels in the embryo sporophyte and epimatium had increased, whilst the levels in the female gametophyte had decreased at this time. In the embryo sporophyte lipid mobilization had commenced with these reserves apparently being metabolized within vacuoles. The rate of respiration measured in terms of increases in CO[2] evolution, increased 60 hours after the start of the incubation period, just 12 hours before ten per cent germination was recorded. Germination was accompanied by a large increase in the levels of cytokinins in the female gametophyte and embryo sporophyte. The cytokinins detected co-chromatographed with the free base cytokinin zeatin and its riboside, ribosylzeatin. Concurrently, marked ultrastructural changes were recorded with increases in the amounts of dictyosomes, endoplasmic reticulum and the formation of polyribosomes, all of which are indicative of increased metabolic activity. Similar increases in the female gametophyte were ofa lower order and occurred only after nine days of incubation. By this time the levels of cytokinins had decreased considerably. After 12 days of incubation 65 per cent of the seeds had germinated. As much of the food reserves in the female gametophyte and embryo sporophyte remained, it is suggested that these reserves are utilized for subsequent seedling establishment rather than for germination. The actual role that cytokinins play in the development and germination of these seeds is not clear. High levels of this phytohormone coincide with periods of food deposition and mobilization suggesting that they play an important part in these processes. The results of the biochemical, cytokinin and ultrastructural studies are discussed in relation to the developmental and germination processes and are compared to the data of other seeds.
Dormancy and germination of the manketti nut, Ricinodendron rautanenii.
(1982) Keegan, Arthur Brian.; Van Staden, Johannes.
The distribution of Ricinodendron rautanenii trees is confined to a fairly distinct band across southern Africa. This stretches from South West Africa in the west, through Botswana and Zimbabwe, to Mozambique in the east. These plants are a potential source of both timber and food. In this respect, the fruits and seeds of this species are highly nutritious and the latter, by virtue of their high lipid content, represent an excellent source of oils for both domestic and industrial uses. For these reasons consideration is being given to cultivating this species on a commercial scale. One problem, however, is that the seeds of these plants are dormant and in the first part of this study an attempt was made to establish the cause of this dormancy and how it could be overcome. A thorough investigation of all the possible causes of dormancy revealed that ethylene was the only naturally occurring stimulus that could relieve this condition. Exogenously applied gibberellin (GA3 ) and ethrel were found to be equally as effective as ethylene, but such treatments cannot be regarded as natural. It was also found that all the dormancy breaking treatments were only effective once the endocarp had been removed. This indicated that R. rautanenii seeds had a combined coat imposed-physiological dormancy where ethylene was only able to stimulate germination once the endocarp had been removed. Scarification treatments showed that the endocarp most probably had its effect by restricting embryo enlargement since this structure did not inhibit water uptake or gaseous exchange. Once the ger- mination requirements of these seeds had been established a more detailed investigation was carried out to determine their general sensitivity to ethylene, as well as the actual role this gas played in breaking dormancy. Ricinodendron rautanenii seeds were found to exhibit a high degree of sensitivity to ethylene. The threshold concentration at which a response was obtained was approximately 10¯³ microlitres per litre and this is the lowest yet recorded for any species prior to any additional seed treatments. At concentrations above this, the response was saturated indicating that the seeds are well adapted to the ethylene concentrations most likely to occur in the field. In addition to this, manketti seeds also responded to ethylene after only very brief exposures to the gas and optimum germination was recorded after 30 minutes incubation in an ethylene saturated atmosphere. The temperature range over which an optimum response to this phytohormone was obtained was found to be between 25 and 35°C. One of the most striking features regarding the sensitivity of these seeds was the apparent ability of dry and partially imbibed seeds to perceive the dormancy breaking stimulus. Furthermore, once treated, the seeds retained the dormancy breaking effect of ethylene even when subjected to almost complete re-dehydration. In this instance, 50 per cent germination was recorded for ethylene treated seeds which had lost approximately 97 per cent of their moisture content between dormancy breaking and re-incubation. It was thus concluded that, not only could Ricinodendron rautanenii seeds respond to very low ethylene concentrations but could probably also retain the effects of this gas during adverse environmental conditions. The effects of imbibition and dormancy breaking were followed separately at the ultrastructural and biochemical level. The ultrastructure of dry embryonic axes of these seeds was characterized by massive stores of food reserves in the form of lipid and protein. Upon imbibition the number and size of spherosomes decreased and protein and globoid hydrolysis was clearly evident. Polysomes and microbodies (including mitochondria) were also visible prior to dormancy breaking but there was no evidence of any endoplasmic reticulum or dictyosomes. Imbibition also resulted in the expans ion of the nuclei and there were indications of an increase in the granular content of the associated nucleoli. The number of nucleolar vacuoles, however, remained unchanged. These features indicated that nuclear activity had commenced albeit limited. The ultrastructure ·of untreated seeds which were maintained in the imbibed state for an extended period of time (six days) was also examined. Cells of the embryonic axes of these seeds showed no further changes with regard to their nuclei and protein hydrolysis appeared to have ceased. At this time spherosomes resembled those in freshly imbibed tissue in terms of their size and numbers, suggesting that the lipid reserves had been resynthesized. No immediate ultrastructural changes were observed after ethylene treatments. However, 24 and 48 hours after dormancy breaking further expansion of the nuclei was noted. At the same time the nucleolar vacuoles disappeared and the granular content of this region increased markedly. This suggested that an increase in the synthesis of various RNA fractions was taking place. Vigorous protein hydrolysis was also observed after the ethylene treatment whereas spherosome numbers increased. Three days after the dormancy breaking treatment, the first signs of germination were visible. Externally this was characterized by a splitting of the testa in the region of the radicle. At this time, endoplasmic reticulum and dictyosomes were still not visible but from this point onwards the ultrastructural changes observed were typical of those recorded during the germination of other species. Thus, no single ultrastructural feature could be associated with the breaking of dormancy and the most notable changes which occurred during this period took place in the nucleus. Biochemical changes occurring during imbibition resulted in an overall decrease in the levels of extractable food reserves present in the embryonic axes. During this period, lipid levels were found to decrease by 44 per cent, protein levels by 12 per cent, sucrose levels by 68 per cent and glucose, fructose and starch levels by 100 per cent. These levels were found to return to their original values when seeds were incubated under moist conditions in the absence of ethylene for extended periods of time. Ethylene treatments, on the other hand caused a further, marked decrease in sucrose levels, whereas protein and lipid levels increased. Hydrolysis of the endosperm reserves commenced three days after the application of ethylene and this was characterized by a decrease in lipid levels and an overall increase in soluble carbohydrates. The timing of this event suggested that the endosperm was not involved in the actual process of dormancy breaking. The importance of protein synthesis in dormancy breaking was also investigated. It was found that seeds incubated with a protein synthesis inhibitor, cycloheximide, failed to germinate, confirming the view that protein synthesis is an essential pre-requisite for germination. Inhibition of RNA synthesis with actinomycin D, on the other hand, did not prevent germination. This suggested that the materials necessary for early protein synthesis were already present in the dry seeds. Actual measurements of protein synthesis showed that this process took place in the embryonic axis, cotyledons and endosperm of seeds imbibed for as little as two hours. Protein synthetic abilities increased considerably in most instances after 48 hours imbibition but then decreased upon application of ethylene. At the same time, however, a marked increase in the uptake of 14C-Ieucine was noted in ethylene treated axes. This may indirectly reflect an effect of ethylene on membrane permeability. Protein synthesis in cycloheximide and actinomycin D treated embryonic axes was also measured. No consistent trends were evident but it was found that after ethylene treatments, protein synthesis was generally lowest in those seeds which were destined to germinate. In addition, these seeds also exhibited the greatest uptake of [14]C-Ieucine. Ricinodendron rautanenii seeds incubated with compounds known to stimulate the pentose phosphate pathway failed to germinate. This indicated that dormancy in this species was probably not the result of a block in alternate respiration. The possible involvement of endogenous phytohormones in the overall process of dormancy breaking was also in- vestigated. In this regard, the role of gibberellic acid appeared to be enigmatic. This is based on the observation that applied gibberellins could stimulate germination whereas inhibitors of endogenous gibberellin synthesis applied to ethylene treated seeds had no effect. It was concluded from this that the effects of ethylene are not mediated via an enhancement of endogenous gibberellin synthesis. A preliminary investigation carried out on the endogenous cytokinins showed that this hormone was absent from dry and imbibed seeds. A transient increase in zeatin levels was observed 24 hours after the ethylene treatment. A similar transient increase was noted in non-induced seeds maintained under moist conditions for six days. In this latter instance, however, the peak co-chromatographed with the biologically less active cytokinin, zeatin glucoside. A basal level of endogenous ethylene production was recorded in all imbibed Ricinodendron rautanenii seeds. Ethrel, ethylene and gibberellin treatments caused an initial, transient increase in this ethylene production after which no further significant changes were recorded. It is suggested that dormancy breaking in this species is not related to enhanced endogenous ethylene synthesis. The results of the biochemical and ultrastructural studies are discussed in relation to what is known regarding features associated with dormancy and its removal and on the known effects of ethylene on seed tissues.
The use of growth kinetics in the development of a predictive model for the growth of Eichhornia crassipes (Mart.) Solms in the field.
(1982) Musil, Charles Frank.; Breen, Charles Mackie.
Abstract available in PDF file.
The plant ecology of seasonally flooded areas of the Pongolo River Floodplain, with particular reference to Cynodon dactylon (L.) Pers.
(1982) Furness, Hilton Dalton.; Breen, Charles Mackie.
The impounding of the waters of the Pongolo river, upstream of its floodplain on the Mocambique coastal plain, may adversely affect the functioning of the floodplain system. A multidisciplinary study of the functioning of the floodplain was initiated to provide a basis for the development of a management strategy for the floodplain. The study reported in this dissertation considered the flood dependence and functioning of the vegetation of the seasonally inundated area. The vegetation was mapped and the communities ordinated, according to the Braun-Blanquet technique, in relation to their positions relative to high flood level (HFL) and the level of the water after flood subsidence (i.e. maximum retention level, MRL). Community distribution was shown to be strictly determined by both the height of the floods and by the MRL. It was concluded that periodic floods are essential for the maintenance of the communities. The Cynodon dactylon (L.) Pers. Community, which forms extensive meadows in the zone of periodic inundation, was studied in detail. As the water level receded during winter, productivity was high (up to 23 kg ha¯¹ d¯¹1 dry mass) and a palatable sward was produced. This is heavily grazed by domestic stock, but as the soil dries out and water stress becomes significant, production decreases, C. dactylon becomes less palatable, and grazing shifts to newly exposed areas. The shift in grazing allows the build up of a large standing crop of both grazeable and ungrazeable (below ground and stolons) material. At the time of inundation by the next floods c. 910 kg ha¯¹ of dry mass, c. 17 kg ha¯¹ nitrogen and c. 2 kg ha¯¹ phosphorus have been removed by grazers. It is concluded that this production, which is flood dependent, forms an important supplement to stock grazing during winter. It is suggested that this source of grazing could be stimulated by irrigation during winter. Cynodon dactylon is shown to decompose rapidly during inundation, losing half of its mass and nutrients in c . 28 days. It therefore represents a major energy and nutrient input during the aquatic phase. The extent to which it is grazed during submergence is unknown. The nutrient input is derived ultimately from the soils of the inundated areas and, since nutrients are being removed by both terrestrial grazers and flushing, continued production is reliant upon the annual sediment load reaching the floodplain. Most of the sediment load will now be deposited in the impoundment, and fertilization may be necessary to maintain productivity. The response of C. dactylon to the seasonal fluctuations in water level are used to formulate proposals for water release from the dam. These include proposals for the short-term, i.e. until the demand for irrigation water conflicts with the requirements of the floodplain, and for the long-term, when less water will be available for the floodplain
Tissue culture studies on Peperomia clusifolia Hook. and Strongylodon macrobotrys A. Gray.
(1982) Peters, Deborah.; Brown, Neville Arthur Claud.
Tissue culture studies were carried out on two ornamental plant species, Strongylodon macrobotrys A. Gray and Peperomia clusifolia Hook. Successful in vitro regeneration of plantlets was achieved in the latter species, using leaf and stem explants. The basal medium of Murashige and Skoog (1962), in combination with various levels of NAA and K, was utilised. Strongylodon proved refractory to both establishment of a thriving callus culture and in vitro formation of roots and shoots. Several media were utilised, Miller's (1963) medium proving the most successful for the production of callus. Different combinations of the growth regulators NAA, IBA, BA and K were used to determine optimum levels of these substances for callus production. Root/shoot induction studies were carried out using the basal medium of Miller (1963) plus various concentrations of IBA in combination with K or BA. Alternatively,the basal medium was used without added growth regulators. Internodes, nodal segments, leaves, pulvini, flower parts and seeds were used in the study. No plantlets were obtained from Strongylodon explants.
Transport and metabolism of 8(14C)t-Zeatin.
(1982) Hutton, Margaret Joan.; Van Staden, Johannes.
A review of the literature established that there were areas of cytokinin transport which needed further investigation, in order to determine the function and/or mode of action of cytokinins during certain stages of plant development. Radioactive zeatin was applied to plant systems suitable for determining more about specific problems of cytokinin transport. The metabolism of the radioactive zeatin was monitored in relation to transport. The metabolism in, and possible export of, radioactive zeatin out of immature, mature and senescing Ginkgo biloba leaves was monitored using explants. The results showed that approximately the same percentage radioactivity was exported from the leaves at all three stages of their development. This indicates that these deciduous leaves could potentially export cytokinins, but the results were not regarded as being significant. Cytokinins would not have been expected to be transported out of expanding leaves, which rapidly utilize cytokinins, and in comparison greater cytokinin export would have been expected to occur from senescing leaves, but this did not occur. The resuIts could indicate that cytokinin glucosides in deciduous leaves are primarily inactivation products rather than storage compounds. The metabolism and transport of radioactive zeatin, applied to the leaves of Citrus sinesis trees, was monitored during the flush of new growth following a dormant period. Some of the radioactive zeatin applied to these leaves appeared to be utilized in the new shoot growth. This could imply that accumulated cytokinin glucosides in these evergreen leaves are exported out of the leaves and reutilized, and thus fulfil a storage function; although the extent to which export occurred as opposed to interconversion and/or catabolic metabolism in the leaves could not be determined. The distribution of radioactive zeatin applied to the xylem and phloem of Phaseolus vulgaris plants at three stages of development was also monitored. There appeared to be preferential transport of cytokinins in the transpiration stream, that is, applied to the xylem. The vegetative apices, buds and flowers appeared to be the major sinks for radioactive compounds in the vegetative and flowering plants, and the leaves were the most important sink in the fruiting plants. Radioactive zeatin in the phloem appeared to move passively with the assimilate stream. The transport of radioactive zeatin applied to the primary leaves appeared to confirm that cytokinins in the phloem were transported along with the assimilate stream. The results also indicated that the accumulated cytokinin glucosides in these annual leaves were primarily inactivation products, involved in regulating leaf metabolism, although some re-utilization of these cytokinins could potentially take place. could have reflected normal cytokinin transport occurring in the tissues. The main metabolic pathway, oxidation, did not, however, appear to be the main metabolic pathway in the plant tissues and may have been the result of the exogenous application of zeatin. The fact that metabolism did not appear to be the normal metabolism of the tissues, could potentially have affected transport.
Physiological aspects of shoot growth regulation in juvenile and adult Citrus sinensis (L.) Osbeck.
(1982) Hendry, Nigel S.; Van Staden, Johannes.
As the requirements for more efficient and economical crop production emerge, so the need for a better understanding of plant growth and development is felt. It is especially the need to understand means by which man may influence growth and development in order to achieve greater productivity, that requires attention. Observation and analysis of plant growth and the concomitant study of plant components, organs and chemicals, provides one of the research oriented approaches to gaining an understanding of the mechanism of growth control. Ultimately, the extent of reproductive growth (fruiting) or the bearing capacity in all plants is governed by the volume and area of vegetative growth and the supply of substances required for growth produced by those vegetative organs. Productivity then, is governed by the efficiency of conversion of solar energy to chemical energy and by the relative ability to store this energy. Plant shoots play a fundamental role in both assimilation and storage of carbohydrates. Given the same growing conditions, similar plants sometimes exhibit different rates of growth. For example, ontogenetically juvenile plants exhibit differences in form and vigour in comparison with ontogenetically adult plants. Although the genotype of such adult and juvenile plants may vary only in expression and not in constitution, there does appear to be some property in juvenile plants which induces the characteristic enhanced vegetative vigour. It is this property, amongst many others, which needs to be better understood, for it is a potential means of gaining greater yields from plants without alterations being made to the environment. A high degree of organisation of growth processes must exist to account for the orderly development of plants. A few organic compounds that influence certain biochemical and physiological reactions have been isolated from the As the requirements for more efficient and economical crop production emerge, so the need for a better understanding of plant growth and development is felt. It is especially the need to understand means by which man may influence growth and development in order to achieve greater productivity, that requires attention. Observation and analysis of plant growth and the concomitant study of plant components, organs and chemicals, provides one of the research oriented approaches to gaining an understanding of the mechanism of growth control. Ultimately, the extent of reproductive growth (fruiting) or the bearing capacity in all plants is governed by the volume and area of vegetative growth and the supply of substances required for growth produced by those vegetative organs. Productivity then, is governed by the efficiency of conversion of solar energy to chemical energy and by the relative ability to store this energy. Plant shoots playa fundamental role in both assimilation and storage of carbohydrates. Given the same growing conditions, similar plants sometimes exhibit different rates of growth. For example, ontogenetically juvenile plants exhibit differences in form and vigour in comparison with ontogenetically adult plants. Although the genotype of such adult and juvenile plants may vary only in expression and not in constitution, there does appear to be some property in juvenile plants which induces the characteristic enhanced vegetative vigour. It is this property, amongst many others, which needs to be better understood, for it is a potential means of gaining greater yields from plants without alterations being made to the environment. A high degree of organisation of growth processes must exist to account for the orderly development of plants. A few organic compounds that influence certain biochemical and physiological reactions have been isolated from the wide array of organic and inorganic substances found in plant extracts. A growing body of evidence suggests that such growth regulatory compounds are ubiquitous in plants; are active at exceedingly low concentrations and are under genetic control. Plant hormones are almost certainly involved in directing or regulating plant growth and development. The subject of the present research concerns the characterization of differences in shoot growth of juvenile and adult phases of a commercial Citrus cultivar and an investigation of the possible role which plant hormones may play in such differences. In add i tion to a comprehensive literature survey, three separate sets of experiments were conducted. In the first, an analysis of the components and parameters of the difference in vegetative shoot growth of adult and juvenile scions was carried out. Anatomical and ultrastructural comparisons were also made. In the second set of experiments the endogenous levels of gibberellins, cytokini ns and inhibitors of buds and other tissues, which might playa role in the regulation of shoot flush growth, were examined. Specific extractions were performed and appropriate bioassay techniques were employed for the three hormone classes investigated. In the final set of experiments the effects of synthetic plant growth regulators on shoot flush growth were monitored. The growth regulators were applied by means of trunk injections or as droplets on shoot terminal buds. These experiments were planned and executed with the above mentioned concepts and ideals in mind. The objective of the project was to contribute in some measure to the knowledge which will allow better use to be made of available natural resources.
A revision of the genus Scleria Bergius (Cyperaceae) in Southern Africa.
(1983) Franklin, Esme Frances.; Gordon-Gray, Kathleen Dixon.
The genus Scleria Bergius (Cyperaceae) in Southern Africa is critically examined, and the generic limits reviewed The taxonomic position of the genus in the family is examined. Diagnosis of the tribe Sclerieae is altered to circumscribe Scleria as the only genus, and diagnoses of the tribes Bisboeckelereae and Sclerieae are made. Infrageneric limits are re-assessed and two subgenera, Scleria and Hypoporum recognised, the relationship of which is postulated as co-lateral, not filial. Evidence is presented that ecological specialisation in subgenus Hypoporum has resulted in taxa which are adapted to open, seasonally dry, temperate habitats, whereas ecological specialisation in subgenus Scleria has given rise to taxa which are adapted to shady, wet, tropical and subtropical habitats. Taxa in subgenus Hypoporum are slender, usually narrow-leaved annuals, or perennials with annual aerial parts, that is, they have evolved drought/cold escape mechanisms, the annuals by completion of the life cycle in a season, the perennials by withdrawal of food reserves into a protected, subterranean perennating organ and sometimes also into enlarged culm-bases. Taxa in subgenus Scleria are more-or-less robust, usually broad-leaved perennials, or, less often, annuals. With few exceptions the plants are evergreen and do not manifest drought/cold escape mechanisms. The annual species occupy tropical habitats in areas where seasonal drought may be experienced and it is suggested that they have acquired the annual habit as a drought-escape mechanism. The only perennial species in subgenus Scleria in Southern Africa which has annual aerial parts, has evolved additional storage regioni in the swollen culm-bases. This species, S. transvaalensis occurs at higher, more temperate altitudes than other species in the subgenus. The fundamental branching pattern of the inflorescence of all species examined has been shown to be the same. It is postulated that the pattern is modified in two ways, namely, by progressive contraction of all or most ramuli leading to the "glomerate-spicate" type of inflorescence characteristic of subgenus Hypoporum, in which the bracts are reduced, glumiform structures, and, by progressive contraction of some ramuli and progressive elongation of others leading to the "interrupted-paniculate" type of inflorescence characteristic of subgenus Scleria, in which the bracts are foliaceous. It is suggested that the branched glomerate-spicate type of inflorescence is less specialised than the simply glomerate-spicate type, and that in the line with interrupted-paniculate inflorescences, the greater the degree of elongation and the greater the number of elongated ramuli, the more highly specialised the inflorescence. Evidence is put forward that the spikelet of Scleria is a monopodial, that is, racemosely-branched structure, and suggestions that it may be sympodial, refuted. It is postulated that unisexual spikelets in Scleria have been derived by reduction from bisexual (androgynaeceous) spikelets. Unisexual female spikelets are unknown in subgenus Hypoporum which has androgynaeceous spikelets and unisexual male spikelets. It is suggested that the higher the ratio of androgynaeceous to male spikelets in the inflorescence, the less specialised the inflorescence. Unisexual male and functionally unisexual female spikelets occur in subgenus Scleria and, rarely, androgynaeceous spikelets. It is suggested that species which consistently produce some androgynaeceous spikelets are more primitive than those which consistently lack them, and that species whose functionally female spikelets consistently lack any vestigial male parts are more advanced than those which have male rudiments. The hypogynium or "disc" which is present on some achenes is considered to be a new modification of the stipe of the achene and not a vestigial structure, therefore it is postulated that the type of achene found in subgenus Hypoporum which has a trigonous stipe lacking any distal elaboration as an hypogynium is primitive, and that the type of achene found in subgenus Scleria which has an obpyramidal stipe elaborated distally as an hypogynium is derivative. It is suggested that development of the hypogynium has attained its most specialised level in one group of species in which this structure serves as a flotation device. Diagnoses of sections in subgenus Scleria are based partly on differences in morphology of the hypogynium. It has been demonstrated that the cell-walls of the pericarp are silicified, that the process of silification is progressive proceeding from the apex towards the base of the fruit, and that abscission of the fruit takes place when silicification is complete and the vascular supply is severed. The achenes of subgenus Hypoporum have all cells silicified; those of subgenus Scleria have all cells except those of the free flange(s) of the hypogynium silicified. Scanning electron microscopy has revealed details of surface ornamentation of the achenes not previously known, which provide additional diagnostic characters at species level. Attemptsto germinate achenes of Scleria have been unsuccessful : the conditions required, physical and physiological, are not understood. Analysis of anatomical evidence, in particular those characters seen in transverse sections of laminas and culms, confirms that there has been specialisation along two divergent pathways; one which has led to successful occupation of relatively dry, temperate habitats (subgenus Hypoporum), and the other to successful occupation of shaded damp, and open aquatic, subtropical and tropical habitats (subgenus Scleria). One section, Hypoporum, is recognised in subgenus Hypoporum, pending survey of the subgenus on a world basis. Four sections are recognised in subgenus Scleria, namely, Scleria, Acriulus, Schizolepis and Ophryoscleria. Section Scleria may comprise several natural groups, the delimitations of which can not be attempted until a world survey has been made. A map showing world distribution of the genus, and regional distribution maps of Southern African species are provided, also a Table showing the total distribution range of species recorded from Southern Africa. Generic, subgeneric, sectional and species descriptions are provided. Two new species are described. Keys to the Southern African species for use in the herbarium, in the field, and one based on anatomical characters of the laminas are presented. Original descriptions and photographs of type specimens of taxa represented in Southern Africa are included as appendices.
Cytokinins in Ecklonia maxima and the effect of seaweed concentrate on plant growth.
(1984) Featonby-Smith, Bryan Charles.; Van Staden, Johannes.
The endogenous cytokinin levels in the brown alga Ecklonia maxima (OSBECK) PAPENF., and the effect of applications of the seaweed concentrate (Kelpak 66) prepared from this alga, on the growth and yield of various plants was investigated. Tentative identification of the cytokinins present in Ecklonia maxima using High Performance Liquid Chromatography revealed the presence of cis and trans-ribosylzeatin, trans-zeatin, dihydrozeatin and isopentenyladenosine. Seasonal and lunar variations in the endogenous cytokinin levels in fresh and processed Ecklonia maxima material were investigated. Lamina, stipe and holdfast regions of one, two and three metre plants harvested from February 1981 until January 1982 together with samples of processed material from the normal production run, collected over the same period were used in this investigation. Analysis revealed both qualitative and quantitative changes in the cytokinin levels which were closely correlated to the seasonal patterns of growth of Ecklonia maxima. During summer zeatin, ribosylzeatin and their dihydroderivatives were responsible for most of the detected activity. The cytokinin glucosides increased above the levels of free cytokinins during winter. The lunar cycle study of material harvested on a daily basis during April - May 1983 revealed marked fluctuations in the cytokinin levels in the various tissues of two metre plants which were closely correlated with the phases of the moon. Greenhouse trials were conducted to determine the effects of the commercially available seaweed concentrate (Kelpak 66) on the growth of Lycopersicon esculentum MILL. plants in nematode infested soil. Kelpak 66 at a dilution of 1 : 500 improved the growth of treated plants significantly, irrespective of whether it was applied as a foliar spray at regular intervals, or whether the soil in which the plants were grown was flushed once with the diluted seaweed concentrate. Root growth was significantly improved whenever the seaweed concentrate was applied. Associated with this improved root growth was a reduction in the infestation of Meloidogyne incognita (KOIFORD and WHITE) CHITWOOD. Finally, the effect of seaweed concentrate and fertilizer applications on the growth and endogenous cytokinin content of Beta vulgaris L. and Phaseolus vulgaris L. plants was investigated. Seaweed concentrate at a dilution of 1 : 500 applied as a foliar spray improved the growth of treated plants significantly, irrespective of whether it was applied on its own or together with a chemical fertilizer. Root growth and the endogenous cytokinin content of these roots increased with seaweed concentrate application. Increases were also detected in the cytokinin content of fruits of Phaseolus vulgaris plants treated with seaweed concentrate. Associated with this increase in the cytokinin content was an increase in the dry mass of the fruit from treated plants. The significance of these findings and the possible relationship between the endogenous cytokinins present in Ecklonia maxima and the effect of the seaweed concentrate on plant growth is discussed.
The role of Potamogeton crispus L. in the Pongolo River floodplain ecosystem.
(1984) Rogers, Kevin Hugh.; Breen, Charles Mackie.
1. Aquatic macrophytes generally dominate energy and nutrient transfers in wetland ecosystems. They provide a diverse habitat for other organisms but each species or life-form, has a different role in determining ecosystem structure and function. A multidisciplinary research project on the Pongolo River Floodplain, Zululand, South Africa, provided a good opportunity to study the role of a submerged species, Potamogeton crispus L. in a wetland ecosystem. 2. The role of a plant species in ecosystem structure and functioning is contingent primarily upon its adaptations to the interactive biotic and abiotic components of the multifactorial environment. This role is manifest in the contribution the species makes both to the sum total of transfers of resources within the ecosystem and to the nature of the physical and chemical environment. These contributions can be to both the conservative energy-matter transfers of the ecosystem and to the information network which regulates ecosystem functioning. 3. This study set out, using Tete pan as the main field site, to: i) Characterise the major physical, chemical and biotic variables of the floodplain ecosystem which are most likely to affect the acquisition of resources by P. crispus. ii) Determine the way the species influences the environment. iii) Assess the species adaptations to environmental characteristics. iv) Assess transfers of resources from P. crispus to other ecosystem components, the factors affecting them and the contributions they make to ecosystem structure and functioning. v) Outline the implications of the findings for management of the ecosystem which is threatened by the construction of an upstream dam. Annual flooding of the Pongolo river and resultant poor light climate and fluctuating water levels create an unfavourable habitat for submerged plant growth in Tete pan during summer. During autumn through spring, however, stable water levels and good light penetration are favourable for plant growth. The duration of favourable and unfavourable periods varies annually, and on a longer time scale periodic droughts cause the pan to dry up completely. Nutrient concentrations and salinity of the water were favourable for freshwater plant growth during the study. 5. Potamoget on crispus on the Pongolo r.iver floodplain is a winter producing annual which reproduces both sexually (achenes) and asexually (turions). Winter growth is initiated by turion germination each year and achenes ensure survival after drought. If floods continue into autumn young plants remain dormant until the light climate improves. Individual plants have a life span of 4 - 5 months but staggered germination of turions from April to June ensured that the population was present for up to 8 months. Pot amogeton crispus was classified as a competitiveruderal with multiple regenerative strategies. This life history enables the species to exploit the favourable period between floods and survive both summer flooding of unpredictable duration and intermittent droughts. 6. The major source of nutrfents for P. crispus is the sediment pool and plant growth brings large amounts of nitrogen and phosphorus into circulation each year. The rapid completion of the life cycle ensures that in most years Nand P released to the water during decay are taken up by other components of the ecosystem before the next summer floods. Thus, despite a potential for flushing of nutrient rich water by floods, pans such as Tete experienced a net gain in nutrients between flood periods during the study period. 7. Potamogeton crispus was the major primary producer in Tete pan during this study and although production was seasonal it was high (126 gm¯³ in comparison to other species reported in the literature. The presence of an abundant and nutritious food source in the form of P. crispus turions, attracts a large and diverse waterfowl population to the floodplain each winter. During feeding the waterfowl uproot P. crispus plants and remove almost 90% of the turions. This represents a potentially unstable grazing system as the birds utilize the "residual growth potential" of the plant population. The hypothesis was formulated that stability of the grazing system and maintenance of P. crispus reproduction may be achieved by production of small turions which are energetically unrewarding for the waterfowl but form viable propagules. Overall, transfer of nutrients and organic matter to this grazing food chain represents a small (<10 % ) proportion of P. crispus production. 9. The growth of P. crispus in Tete pan provides a large surface area for epiphyton growth which increases in density and species diversity as plants age. Experiments showed that the host plant epidermis was invaded by necrotrophic bacteria but grazing of epiphyton by snails retarded this process in young leaves. However, as leaves aged the action of the bacteria softened (conditioned) the host tissues and promoted consumption of the leaves by snails. The co-existence of epiphyton and host plant is therefore achieved through a complex set of interacting attributes of the epiphyton community, the host plant and grazing animals. Comparisons with previously published hypotheses of the macrophyte-host relationship indicated that attempts to find a single controlling factor may be misleading and that adaptation for co-existence should be sought in a number of interacting variables. 10. Since conditioning of P. crispus by epiphytes and consumption of plants by snails follows a succession which reaches a climax during host senescence, it was important to develop a method of studying decomposition which included these processes. Experiments showed that both snails and bacterial conditioning had marked effects on the rates and pathways of dry matter and nutrient transfer during decomposition and that no single technique (eg. litter bags) will permit a thorough understanding of decomposition processes. Conservative transfers of organic matter, nitrogen and phosphorus in Tete pan are dominated by the detrital food web with only a small proportion going to the grazing food chain. The consumption of P. crispus and its epiphyton by snails in winter and spring provides the major pathway of allochthonous production to the benthic detrital aggregate and detrital food web. These transfers are separated in both time and space from the allochthonous inputs of C. dacty lon detritus which occur as a result of decay in the flooded pan margins during summer. The major input of phosphorus to the pans appears to occur by deposition of silt during floods but fixation of di-nitrogen by the epiphyton and in the detrital aggregate greatly exceed flood related inputs of nitrogen. 12. The continued functioning of the detritus dominated foodweb of the floodplain pans is dependent upon a sequence of resource transfers which are dominated, during periods between floods, by P. crispus and its epiphyton. The processes which regulate these conservative transfers show characteristics typical of the information network of cybernetic ecosystem functioning and hence it was hypothesised that the Pongolo floodplain ecosystem has the potential to buffer changes in resource input which will occur now that a dam has been constructed upstream. While the hydrological regime of the Pongolo river forms the primary determinant of the floodplain ecosystem, structure and function management of the system must also account for the "managerial" role of animals and temperature which initiate and rate regulate major ecosystem processes.

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