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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Some implications of associated mycoflora during hydrated storage of recalcitrant seeds of Avicennia marina (Forssk.) Vierh.

Calistru, Claudia. January 2004 (has links)
Three questions are considered in the context of the possible effects of seedassociated mycoflora, typified by Fusarium moniliforme, during hydrated storage of recalcitrant seeds of the tropical species, Avicennia marina. These are: 1) whether fungal infection reduces storage lifespan; 2) whether seeds become more susceptible to fungal attack during storage and whether they posses defence mechanisms that might suppress fungal proliferation in hydrated storage (production of antifungal compounds and 13-1,3-glucanase (EC 3.2.1.39) and chitinase (EC 3.2.1.14)] and 3) whether it is possible to discriminate ultrastructurally between inherent deteriorative changes and those that are fungally-induced. 1) The data indicate unequivocally that if fungal activity is curtailed, then the hydrated storage lifespan of A. marina seeds can be considerably extended. 2) When inoculated immediately with F. moniliforme, newly harvested seeds were extremely susceptible to the adverse effects of the fungus, while seeds that had been wet-stored for 4 days showed a considerably heightened resilience to the effects of the fungus prior to inoculation. The enhanced resilience, although declining, persisted in seeds stored hydrated for up to 10 days prior to inoculation, being lost after 12 days. This finding was supported by significant increase in 13-1,3-glucanase and chitinase and in antifungal compound production during 10 days of wet storage. After 14 days of wetstorage, seeds become more susceptible to the effects of fungusthanthose in the newly harvested condition. 3) The resilience of seeds that had been stored in the short-term was associated with ultrastructural changes indicative of enhanced metabolic activity associated with the onset of germination (e.g. increase in vacuolation, well-developed mitochondria and endomembrane system [ER and Golgi bodies]). However, with sustained stress associated with wet-storage IV conditions, the seeds became increasingly badly affected by the fungus, showing some ultrastructural fungally-induced abnormalities (e.g. nuclear lobing, presence of lipid bodies and prevalence of Golgi bodies that had many associated vesicles) and a decrease in 13-1,3-glucanase and chitinase activity. It is suggested that the decreased susceptibility of A. marina seeds during short-term storage relies on the ability to create an antifungal environment prior to infection (through synthesis and accumulation of pre-formed and induced antifungal compounds and antifungal enzymes), which would also be an effective strategy during germination in the natural environment. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2004
2

Investigations into the responses of axes of recalcitrant seeds to dehydration and cryopreservation.

Wesley-Smith, James. January 2002 (has links)
Achieving long-term storage of germplasm is critical for the conservation of plant biodiversity. Seed storage practices require that degradative reactions causing ageing be limited. By reducing the water content, cytoplasmic viscosity is increased to levels that minimise deteriorative reactions. Reducing the storage temperature additionally increases the storage lifespan by further reducing the rate at which such deleterious processes occur. Two broad categories of seeds can be distinguished based on their storage behaviour. Orthodox seeds are desiccation-tolerant; generally shed in the dry state and are metabolically quiescent. Such seeds are usually stored at low water contents (e.g. 5%), and their high cytoplasmic viscosity prevents freezing damage during cooling to subzero temperatures. On the other hand, desiccation-sensitive (recalcitrant) seeds do not undergo a maturation-drying phase, they are metabolically active at shedding, and sensitive to extreme or prolonged drying. Accordingly, recalcitrant seeds cannot be stored under conventional conditions because they do not survive drying to low water contents and are damaged by sub-zero temperatures, even when dried to the lowest water content tolerated. Therefore, procedures that facilitate harmless drying and cooling to low temperatures are required to achieve long-term storage of recalcitrant germplasm. Recalcitrant seeds that are dried rapidly can attain relatively lower water contents without injury. However, these seeds are usually large and this limits the drying rates that can be achieved even under favourable conditions. Isolating embryonic axes from the rest of the seed facilitates faster drying, and a consequent reduction in the water content at which damage occurs. In axes of many species, the level of drying attained before lethal desiccation damage occurs is sufficient to limit freeZing damage during cryogenic exposure and facilitate survival in vitro. However, many others are damaged when dried to water contents that preclude freezing, and also are killed if cooled to sub-zero temperatures at higher water contents. In such instances, the window of permissible water contents leading to survival may be small or nonexistent. A basic premise explored in this thesis is that by restricting the growth of intracellular ice crystals using increasingly rapid cooling rates, the range of permissible water contents can be widened, facilitating survival of axes at higher water contents. An overview of the problems associated with the long-term storage of recalcitrant germplasm, and the rationale behind such rapid cooling approach are presented in Chapter 1 of the present thesis. Subsequent chapters report investigations on the effects of variables required to dry and cryopreserve embryonic axes with minimum damage, in keeping with this approach. Collectively, those studies aimed at establishing a robust cryopreservation procedure for the conservation of recalcitrant germplasm with broad applicability across species. The approach presently adopted entailed manipulating the water content of excised axes using rapid drying to discrete water content ranges, and also using different methods to cool axes to cryogenic temperatures at various rates. The calorimetric properties of water in axes were investigated for Camellia sinensis (L.) O. Kuntze using differential scanning calorimetry (DSC). For all species, the effect of any drying or cooling treatment tested was determined by assessing the survival of axes in vitro, which provided the most reliable indicator of cellular damage. Additionally, the effects of different treatments upon the structural and functional integrity of axes were assessed using light and electron microscopy as well as measurement of electrolyte leakage. The studies undertaken are presented in a similar sequence to that in which they took place during the course of the experimental phase of this work. These are summarised below. Partial drying plays a pivotal role in the approach developed, and microscopy has contributed towards increasing present understanding of desiccation damage. Microscopy was used to determine the effects of drying rate upon the ultrastructure of recalcitrant axes. It was necessary to find reliable protocols to prepare specimens for light and electron microscopy that did not alter the architecture of the cells in the dry state. Freeze-substitution and conventional aqueous fixation were compared in Chapter 2 using variously dried material from three species. The results obtained revealed that an unacceptably high extent of artefactual rehydration occurs during aqueous fixation, and highlight the need for anhydrous processing of dehydrated samples. Significantly, that study also revealed that many cellular events commonly associated with desiccation damage (e.g. withdrawal, tearing and/or vesiculation of the plasmalemma) are not seen in freeze-substituted preparations, and are likely artefacts of aqueous fixation. Freeze-substitution was used subsequently (Chapter 3) to assess the effects of slow drying (2 - 3 days) or rapid drying (min) upon the survival of embryonic axes of jackfruit (Artocarpus heterophyllus Lamk.) Results confirmed the beneficial effects of rapid drying, and also provided insights into ultrastructural changes and probable causes underlying cellular damage that occur during a drying/rehydration cycle. Efforts subsequently focused on determining the effect of cooling rate upon survival of recalcitrant axes at various water contents. The study on embryonic axes of recalcitrant camellia sinensis (tea; Chapter 4) tested the hypothesis that rapid cooling facilitates survival of axes at higher water content by restricting the growth of ice crystals to within harmless dimensions. The presence of sharp peaks in DSC melting thermograms was indicative of decreased survival in vitro. These peaks were attributed to the melting of ice crystals sufficiently large to be detected by DSC as well as to cause lethal damage to axes. Increasing the cooling rate from 10°C min-1 to that attained by forcibly plunging naked axes into sub-cooled nitrogen increased the upper limit of water content facilitating survival in vitro from c. 0.4 to 1.1 - 1.6 g H20 g-1 (dry mass [dmb]). Subsequent studies tested whether a similar trend occurred in other recalcitrant species cooled under similar conditions. In order to investigate further the relationship between water content, cooling rate and survival it was necessary to achieve cooling rates reproducibly, and to quantify these reliably. The plunging device required to achieve rapid cooling, and instruments required to measure the cooling rates attained, are described in Chapter 5. That study investigated the effects of cryogen type, depth of plunge and plunging velocity on the cooling rates measured by thermocouples either bare or within tissues of similar size and water content as encountered in cryopreservation experiments. This plunger was used in subsequent studies to achieve the fastest cooling conditions tested. Favourable cooling conditions were selected, and the efficacy of this procedure to cryopreserve relatively large axes was tested (Chapter 6) using embryonic axes of horse chestnut (Aesculus hippocastanum L.) Axes at water contents above c. 0.75 g g-1 could not be cooled faster than c. 60°C S-1, but cooling rates of axes below this water content increased markedly with isopentane, and to a lesser extent with subcooled nitrogen. Axes were killed when cooled at water contents above 1.0 g g-1 but survived fully (albeit abnormally) when cooled in isopentane between 1.0 and 0.75 g g-1. Complete survival and increasingly normal development was attained at water contents below 0.75 g g-1, especially if isopentane was used. The study on horse chestnut axes emphasised that water content and cooling rate are co-dependent during non-equilibrium cooling. Accordingly, that study could not determine whether survival at lower water contents increased because of the corresponding increase in cooling rates measured, or because of the higher cytoplasmic viscosity that resulted from drying. That uncertainty was addressed by the study discussed in Chapter 7, using axes of the trifoliate orange (Poncirus trifoliata [L.] RAF.) That study investigated the effect of cytoplasmic viscosity upon survival of axes cooled and warmed at different rates. Survival and normal development was high at lower water contents, and seemingly independent of cooling rate at about 0.26 g g-1. At higher water contents the range of cooling rates facilitating survival became narrower and displaced towards higher cooling rates. This study revealed two conspicuous inconsistencies that questioned the beneficial effect of rapid cooling. Firstly, the fastest cooling rates did not necessarily facilitate the highest survival. Secondly, survival of fully hydrated axes was higher when cooled under conditions that encouraged - rather than restricted - the growth of intracellular ice crystals. These inconsistencies were explored further using embryonic axes of silver maple (Acer saccharinum L.). Freeze-fracture replicas and freeze-substitution techniques provided valuable insights into the way in which ice crystals were distributed in cells cooled using different methods at rates ranging between 3.3 and 97°C S-1. Extensive intracellular freezing was common to all treatments. Unexpectedly, fully hydrated axes not only survived cryogenic exposure, but many axes developed normally when cooled using the relatively slower methods (77 and 3.3°C S-1) if warming was rapid. The most conspicuous ultrastructural difference between plunge cooling and the relatively slower methods was the exclusion of ice from many intracellular compartments in the latter. It is possible that even the fastest warming cannot prevent serious cellular damage if ice crystals form within such 'critical' compartments. It is proposed that the intracellular location of ice is a stronger determinant of survival that the size attained by ice crystals. The study of A. saccharinum also investigated the recovery of axes cooled fully hydrated either rapidly (97°C S-1) or slowly (3.3°C S-1). This facet of the study showed that cell lysis became apparent immediately after warming only where damage was most extensive. In other cells damage became apparent only after 2.5 to 6 h had elapsed, thus cautioning against inferring survival from the ultrastructural appearance of cells immediately after warming. Microscopy enabled cell repair as well as the pattern of growth of cryopreserved tissues to be appraised at the cellular, tissue and organ levels. Similar studies are required to understand further the nature of freezing damage, and how those events affect cell function. The salient trends observed in previous chapters are brought together in Chapter 9. / Thesis (Ph.D.)-University of Natal, Durban, 2002.
3

Development of strategies towards the cryopreservation of germplasm of Ekebergia capensis Sparrm. : an indigenous species that produces recalcitrant seeds.

Hajari, Elliosha. January 2011 (has links)
The conservation of germplasm of indigenous plant species is vital not only to preserve valuable genotypes, but also the diversity represented by the gene pool. A complicating factor, however, is that a considerable number of species of tropical and sub-tropical origin produce recalcitrant or otherwise non-orthodox seeds. Such seeds are hydrated and metabolically active when shed and cannot be stored under conventional conditions of low temperature and low relative humidity. This poses major problems for the longterm conservation of the genetic resources of such species. Presently, the only strategy available for the long-term conservation of species that produce recalcitrant seeds is cryopreservation. Ekebergia capensis is one such indigenous species that produces recalcitrant seeds. The aim of the present study was to develop methods for the cryopreservation of germplasm of this species. Different explant types were investigated for this purpose, viz. embryonic axes (with attached cotyledonary segments) excised from seeds, and two in vitro-derived explants, i.e. ‘broken’ buds excised from in vitro-germinated seedlings and adventitious shoots generated from intact in vitro-germinated roots. Suitable micropropagation protocols were developed for all explant types prior to any other experimentation. Before explants could be cryopreserved it was necessary to reduce their water content in order to limit damaging ice crystallisation upon cooling. All explants tolerated dehydration (by flash drying) to 0.46 – 0.39 g gˉ¹ water content (dry mass basis) with survival ranging from 100 – 80%, depending on the explant. In addition, penetrating and non-penetrating cryoprotectants were used to improve cryo-tolerance of explants. The cryoprotectants tested were sucrose, glycerol, DMSO and a combination of sucrose and glycerol. Explant survival following cryoprotection and dehydration ranged from 100 – 20%. Cryoprotected and dehydrated explants were exposed to cryogenic temperatures by cooling at different rates, since this factor is also known to affect the success of a cryopreservation protocol. The results showed that ‘broken’ buds could not tolerate cryogen exposure. This was likely to have been a consequence of the large size of explants and their originally highly hydrated condition. Adventitious shoots tolerated cryogenic exposure slightly better with 7 – 20% survival after cooling in sub-cooled nitrogen. Limited shoot production (up to 10%) was obtained when axes with attached cotyledonary segments were exposed to cryogenic temperatures. In contrast, root production from axes cooled in sub-cooled nitrogen remained high (67 – 87%). Adventitious shoots were subsequently induced on roots generated from cryopreserved axes by applying a protocol developed to generate adventitious shoots on in vitrogerminated roots. In this manner, the goal of seedling establishment from cryopreserved axes was attained. Each stage of a cryopreservation protocol imposes stresses that may limit success. To gain a better understanding of these processes the basis of damage was investigated by assessing the extracellular production of the reactive oxygen species (superoxide) at each stage of the protocol, as current thinking is that this is a primary stress or injury response. The results suggested that superoxide could not be identified as the ROS responsible for lack of onwards development during the cryopreparative stages or following cryogen exposure. The stresses imposed by the various stages of a cryopreservation protocol may affect the integrity of germplasm. Since the aim of a conservation programme is to maintain genetic (and epigenetic) integrity of stored germplasm, it is essential to ascertain whether this has been achieved. Thus, explants (axes with cotyledonary segments and adventitious shoots) were subjected to each stage of the cryopreservation protocol and the epigenetic integrity was assessed by coupled restriction enzyme digestion and random amplification of DNA. The results revealed little, if any, DNA methylation changes in response to the cryopreparative stages or following cryogen exposure. Overall, the results of this study provided a better understanding of the responses of germplasm of E. capensis to the stresses of a cryopreservation protocol and two explant types were successfully cryopreserved. Future work can be directed towards elucidating the basis of damage incurred so that more effective protocols can be developed. Assessment of the integrity of DNA will give an indication as to the suitability of developed protocols, or where changes should be made to preserve the genetic (and epigenetic) integrity of germplasm. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2011.
4

A study of some chilling responses of recalcitrant seeds of Avicennia marina (Forssk.) Vierh. and Ekebergia capensis Sparrm.

Lewis, Elisabeth Jacqueline. January 2002 (has links)
Seeds remain the most convenient and successful way for storing the genetic diversity of plant species and for producing new plants routinely for agriculture and horticulture. The importance of seed storage and the ability to predict seed longevity must therefore not be underestimated. To be successful, storage conditions must maintain seed vigour and viability and ensure that normal seedlings are subsequently established under field conditions. Seed quality is best retained when deteriorative events are minimised, which is achieved by storage of low moisture-content seeds under cool to cold, or even sub-zero, temperatures. Such conditions are employed for 'orthodox' seeds, which are desiccation tolerant and able to survive at sub-zero temperatures in the dehydrated state for extended periods. It is seeds referred to as 'recalcitrant' that cannot be dehydrated and often not stored at low temperatures because they are desiccation sensitive and may not tolerate chilling. According to almost anecdotal records chilling temperatures for such seeds are those below 15°C down to 0°C, depending on the species. The limited storage lifespan of recalcitrant seeds presents a problem even for short-term storage, and as most research on chilling sensitivity has been conducted on vegetative tissue, relatively little data exist for seeds, especially recalcitrant types. The purpose of this study was to gain an understanding of the chilling response of recalcitrant seeds, as reduced temperature could have the potential to extend, rather than curtail, storage lifespan, depending on the species. Selected physiological, biochemical and ultrastructural responses of recalcitrant seeds of Avicennia marina and Ekebergia capensis were characterised. Seeds of the two species were stored at 25, 16 and 6°C. Germination, water content (determined gravimetrically), respiration (measured as CO2 production) and leachate conductivity (tissue electrolyte leakage over time) were assessed at regular intervals. Chilling response at the subcellular level was examined using transmission electron microscopy (TEM). Changes in sugar metabolism and activities of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR) were assessed for A. marina seeds, which were severely affected by the chilling temperature of 6°C, losing viability after 1 week. In contrast, the seeds of E. capensis retained viability after 12 weeks of storage at 6°C, indicating the marked difference in chilling response between seeds of the two recalcitrant species, despite their common tropical provenance. However, when E. capensis seeds were stored at 3°C viability decreased significantly after 8 weeks, thus indicating how critically temperature must be controlled if such conditions are to be profitably employed. Ultrastructural studies revealed that in both E. capensis and A. marina seeds vacuole formation was initiated more rapidly at lower temperatures than at higher temperatures, indicating that this was a response specific to the chilling stress imposed. Once again, 'lower temperatures' differed relative to the species concerned. In the E. capensis seeds, nucleolar morphology was affected and the extent of chromatin patches in the nuclei increased as the storage temperature was reduced. Other ultrastructural findings could not be linked specifically to the chilling stress imposed on the E. capensis and A. marina seeds. Activity of the antioxidant enzymes SOD and GR was detected in the A. marina seeds. No measurable CAT activity was detected. Glutathione reductase activity increased in response to chilling stress, the rate of the increase depending upon the severity of the chilling stress imposed. Other than when the A. marina seeds were placed directly at 6°C, there were no notable increases in SOD activity. Interestingly, SOD and GR activity was not the same in the axes as in the cotyledons. Superoxide dismutase activity was found to be higher in the axes and GR activity higher in the cotyledons. It would have been beneficial to determine the extent of antioxidant enzyme activity in the E. capensis seeds as well if this had been possible. Generally, chilling of recalcitrant seeds seems to evoke a response similar to that of dehydration below a critical water content. This could lead to the conclusion that recalcitrant seeds do not possess the genetic ability to cope with dehydration or chilling stress, if it were not for the existence of recalcitrant seed species that are more chilling tolerant. / Thesis (M.Sc.)-University of Natal, Durban, 2002.
5

Aspects of post-harvest seed physiology and cryopreservation of the germplasm of three medicinal plants indigenous to Kenya and South Africa.

January 2002 (has links)
The current state of global biodiversity is one of sustained and increasing decline especially in developing countries such as South Africa, where, medicinal plants face a particular threat due the herbal medicine trade, and because in situ conservation measures have not stemmed the exploitation of these plants (Chapter 1). Furthermore, seed storage, which offers an efficient ex situ conservation technique, cannot presently be applied to many medicinal plants, either because these species produce short-lived, recalcitrant seeds, or the post-shedding behaviour of the seeds is altogether unknown. This study investigated three medicinal plant species indigenous to Kenya and South Africa: Trichilia dregeana and T. emetica, of which no population inventories exist and no wild populations were encountered locally during the course of this study; and Warburgia salutaris, one of the most highly-utilised medicinal plants in Africa, and which is currently endangered and virtually extinct in the wild in some countries such as South Africa. Aspects of post-shedding seed physiology (Chapter 2) and the responses of the germplasm of the three species to cryopreservation (Chapter 3) were studied using viability and ultrastructural assessment, with the aim of establishing methods for both short-term and the long-term preservation, via appropriate seed storage and cryopreservation, respectively. The effect of cryopreservation on genetic fidelity, a crucial aspect of germplasm conservation, was assessed by polymerase chain reaction (PCR) based random amplified polymorphic DNA (RAPDs), using W. salutaris as a case-study (Chapter 4). The seeds of all three species were found to exhibit non-orthodox behaviour. On relatively slow-drying, seeds of T. dregeana and T. emetica lost viability and ultrastructural integrity at axis water contents of 0.55 g g-l (achieved over 6 d) and 0.42 g g-l (after 3 d) respectively, while flash-drying of embryonic axes facilitated their tolerance of water contents as low as 0.16 g g-l (T. dregeana, flash-dried for 4 h) and 0.26 (T. emetica, flash-dried for 90 min). Seeds of W. salutaris were relatively more tolerant to desiccation, remaining viable at axis water contents below 0.1 g g-l when desiccated for 6 d in activated silica gel. However, excised embryonic axes flash-dried to similar water contents over 90 min lost viability and were ultrastructurally damaged beyond functionality. In terms of storability of the seeds, those of T. dregeana could be stored in the fully hydrated state for at least 5 months, provided that the quality was high and microbial contamination was curtailed at onset of storage, while those T. emetica remained in hydrated storage for about 60 d, before all seeds germinated in storage. Seeds of W salutaris, even though relatively tolerant to desiccation, were not practically storable at reduced water content, losing viability within 49 d when stored at an axis water content of 0.1 g g-l. The seeds of all three species were sensitive to chilling, suffering extensive subcellular derangement, accompanied by loss of viability, when stored at 6 °C. Thus, T. dregeana and T. emetica are typically recalcitrant, while those of W. salutaris are suggested to fit within the intermediate category of seed behaviour. For either recalcitrant or intermediate seeds, the only feasible method of long-term germpalsm preservation may be cryopreservation. Subsequent studies established that whole seeds of W. salutaris could be successfully cryopreserved following dehydration in activated silica gel. However, whole seeds of T. dregeana and T. emetica were unsuitable for cryopreservation, and excised embryonic axes were utilised. For these, in vitro germination methods, as well as cryoprotection, dehydration, freezing and thawing protocols were established. Post-thaw survival of the axes of both species was shown to depend on cryoprotection, rapid dehydration and cooling (freezing) in cryovials. Embryonic axes of T. dregeana regenerated only as callus after cryopreservation, while those of T. emetica generated into apparently normal plantlets. Thawing/rehydration in a 1:1 solution of 1 µM CaC12.2H2O and 1 mM MgC12.6H2O increased the percentage of axes surviving freezing, and that of T. emetica axes developing shoots. The effect of the extent of seed/axis development on onward growth after cryopreservation was apparent for seeds of W. salutaris and excised axes of T. emetica. The seeds of W. salutaris surviving after cryopreservation germinated into seedlings which appeared similar to those from non-cryopreserved seeds, both morphologically and in terms of growth rate. Analysis using PCR-RAPDs revealed that there were no differences in both nucleotide diversity or divergence, among populations of seedlings from seeds which had been sown fresh, or those which had either been dehydrated only, or dehydrated and cryopreserved. Thus, neither dehydration alone, nor dehydration followed by cryopreservation, was associated with any discernible genomic change. The above results are reported and discussed in detail in Chapters 2 to 4, and recommendations and future prospects outlined in Chapter 5. / Thesis (Ph.D.)-University of Natal, Durban, 2002.
6

The effect of developmental status and excision injury on the success of cryopreservation of germplasm from non-orthodox seeds.

Goveia, Meagan Jayne Theresa. January 2007 (has links)
The zygotic germplasm of plant species producing desiccation-sensitive seeds can be conserved in the long-term only by cryopreservation. Usually the embryonic axis is excised from the cotyledons and is used as the explant for cryopreservation as it is small and provides a large surface area:volume ratio. However the shoot of the axis of most species studied does not develop after excision, with the result that survival after cryopreservation is often recorded as callus production or simply explant enlargement and/or greening. Thus, besides explant size, factors such as in vitro regeneration techniques, physical injury induced upon excision and developmental status of the seed could compromise the success of cryopreservation. This study investigated the effect of the factors mentioned above, with particular attention to the developmental status of the seeds on explant in vitro development (section 3.1), response to dehydration (section 3.2) and cryopreservation of the desiccation-sensitive embryonic axes (section 3.3) of two species: Trichilia dregeana, T. emetica and embryos of a third, Strychnos gerrardii. For all three species, investigations were conducted on the embryonic axes/embryos excised from mature seeds immediately after fruit harvesting and from mature seeds stored under hydrated conditions for different periods (in order to achieve different degrees of development). In addition, preliminary studies were carried out on axes of T. dregeana to assess whether generation of reactive oxygen species (ROS) occurs in response to wounding upon axis excision (section 3.4). Excised embryonic axes of T. dregeana and T. emetica did not develop shoots in vitro unless the explants included attached cotyledonary segments. Following the development associated with short-term storage, however, the excised axes could develop shoots after complete cotyledon excision. The embryos from the (endospermous) seeds of S. gerrardii which included the paper-thin cotyledons, developed normally in vitro, with percentage germination increasing with seed storage time. For all three species, in vitro axis germination was promoted when activated charcoal was included in the germination medium, regardless of the developmental stage of the seeds. When dehydrated to approximately 0.3 g H2O g-1 dry mass (g g-1), embryonic axes from all three species failed to develop shoots even though a minimum of 50% produced roots in all cases. Hence, shoot production was shown to be more sensitive to desiccation than was root production. Furthermore, the sensitivity of the shoot apical meristem to desiccation was not ameliorated with seed storage for T. dregeana and T. emetica, but did decrease for S. gerrardii when seeds were stored for 6 – 8 weeks. The application of certain cryoprotectants did facilitate production of shoots after dehydration by a few axes of both Trichilia spp. For T. dregeana explants, combination of glycerol and sucrose allowed for 10% of the axes to retain the ability for shoot production after dehydration while for T. emetica explants, the combination of DMSO and glycerol (10 - 20% shoot production after dehydration) was best. The efficacy of the cryoprotectants was not influenced by storage period. The provision of cryoprotectants still needs to be tested for S. gerrardii. Survival of subsequent cryopreservation of T. dregeana and S. gerrardii explants was best achieved with rapid cooling in nitrogen slush, with the cooling procedure for T. emetica explants still to be optimized. The highest post-cryopreservation survival of T. dregeana axes was achieved when seeds had been stored for three months, while the seed storage period did not affect post-thaw survival of the axes of T. emetica or S. gerrardii. A small proportion of S. gerrardii explants only, produced shoots after cryopreservation, whereas the surviving embryonic axes of T. dregeana and T. emetica regenerated only as non-embryogenic callus. Although callus production is less desirable than successful seedling establishment, it has the potential for micropropagation if embryogenicity can be induced. Ultrastructural examination of the shoot apical meristem of T. dregeana after a 3-d recovery period, following excision, revealed considerable cellular derangement, although damage of individual organelles could not be resolved microscopically. Preliminary studies on T. dregeana involving a colorimetric assay using epinephrine, confirmed the generation of ROS in response to wounding associated with axis excision. Reactive oxygen species generated appeared to persist over prolonged periods rather than occurring only as a single oxidative burst. Hence, ROS production at the wound site could be the primary factor contributing to lack of shoot development. Axes immersed in the anti-oxidant, ascorbic acid (AsA) immediately after excision, showed lower ROS production and 10% shoot development when cultured in vitro, indicating that the oxidative burst coincident with, and after excision might be counteracted if immediate ROS production can be adequately quenched. Future investigations should aim to identify the specific ROS associated with wounding and optimize an anti-oxidant treatment(s) that will facilitate shoot development. Thus, the successful cryopreservation of the germplasm of the species tested, and others producing recalcitrant seeds, depends on a spectrum of species-specific factors, some still to be elucidated. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2007.
7

Some invetsigations on the responses to desiccation and exposure to cryogenic temperatures of embryonic axes of Landolphia kirkii.

Kistnasamy, Provain. 17 May 2013 (has links)
Landolphia kirkii is scrambling shrub forming an integral part of the flora along the coastal areas of north-eastern South Africa. The non-sustainable harvesting of fruit as food source, by monkeys and rural communities and the highly recalcitrant nature of their seeds threatens the continuation of the species. In addition, the ability of the plants to produce high quality rubber makes its long-term conservation highly desirable. Previously, attempts have been made to cryopreserve germplasm of L. kirkii, but no survival had been recorded at cryogenic temperatures of below -140ºC. The present study reports on the effects of rapid dehydration, chemical cryoprotectants and various cooling rates, thawing and imbibition treatments on survival of embryonic axes excised with cotyledons completely removed, as well as with 3 mm portion of each cotyledon attached, from fresh, mature, recalcitrant seeds of L. kirkii. Survival was assessed by the ability for both root and shoot development in in vitro culture, the tetrazolium test and electrolyte leakage readings. At seed shedding, embryonic axes were at the high mean water content of 2.24 g gˉ¹ (dry mass basis). All axes (with and without attached cotyledonary segments) withstood rapid (flash) drying to a water content of c. 0.28 g gˉ¹; however, the use of chemical cryoprotectants, singly or in combination, before flash-drying was lethal. Rapid cooling rates were detrimental to axes flash-dried to 0.28 g gˉ¹, with no explants showing shoot production after exposure to -196ºC and -210ºC. Ultrastructural examination revealed that decompartmentation and loss of cellular integrity were associated with viability loss after rapid cooling to cryogenic temperatures, although lipid bodies retained their morphology regardless of the thawing temperature employed. Furthermore, analysis of the lipid composition within embryos of L. kirkii revealed negligible amounts of capric and lauric acids, suggested to be the medium-chained saturated fatty acids responsible for triacylglycerol crystallisation when lipid-rich seeds are subjected to cryogenic temperatures. Hence, lipid crystallisation was not implicated in cell death following dehydration, exposure to cryogenic temperatures and subsequent thawing and rehydration. Rapid rehydration of embryonic axes of L. kirkii by direct immersion in a calcium-magnesium solution at 25ºC for 30 min (as apposed to slow rehydration on moistened filter paper or with rehydration in water) was associated with highest survival post-dehydration. Cooling at 1ºC minˉ¹ and 2ºC minˉ¹ facilitated survival of 70 and 75% respectively of axes with attached cotyledonary segments at 0.28 g gˉ¹ after exposure to - 70ºC. Viability retention of 40 and 45% were recorded when embryonic axes with attached cotyledonary segments were cooled at 14 and 15ºC minˉ¹ to temperatures below -180ºC. However, no axes excised without attached cotyledonary segments produced shoots after cryogenic exposure. The use of slow cooling rates is promising for cryopreservation of mature axes of L. kirkii, but only when excised with a portion of each cotyledon left attached. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2011.

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