Sweetpotato (Ipomoea batatas) is a major staple food in Africa and the rest of the world where they are discovered to be a good source of carbohydrates, vitamin A, vitamin C and protein. The maximum production potential of the crop is being hampered by severe drought which ravages most parts of Africa. The main aim of this project therefore is to screen collected accessions of sweetpotatofor drought tolerance in a quick screening method with a view to identify cultivars that can perform well under water stress conditions. Fifty selected sweetpotato accessions consisting of cultivars and breeding lines collected from the ARC-VOPI gene bank were planted for drought screening in the glass house for 6 weeks during which water was withheld to induce stress. Observations were made on number of dead plants and days to wilting point, the results were analyzed and 12 best performing cultivars were selected for field trials. The field trial was carried out in Lwamondo, Thohoyandou for 6months under rain-fed conditions. The experiment was conducted in a complete randomized block design with 6 replicates.Yield data and growth parameters were collected every 8 weeks during the trial period and the data collected was analyzed using ANOVA. The best performing cultivars were Zapallo, Tacna, Ejumula, 2004-9-2 and Ndou. They were therefore recommended for further evaluation in other drought prone areas of the country. / Agriculture and  Animal Health / M. Sc. (Agriculture)

Increases in temperature, altered precipitation patterns, and the occurrence and severity of extreme climatic events have been important characteristics of the climate change observed to date. This has had many and diverse impacts upon the living world, with one recent observation being a global reduction in the net primary production of all terrestrial vegetation. Increases in temperature and the frequency of extreme events are predicted to continue throughout the 21st century, and can be expected to have far reaching effects on global terrestrial ecosystems. Increases in temperature and drought occurrence could fundamentally impact upon the growth rates, species composition and biogeography of forests in many regions of the world, with many studies indicating that this process is already underway. European beech, Fagus sylvatica, is one of Europe’s most widespread and significant broadleaved tree species, forming an important and frequently dominant component of around 17 million hectares of forest. However, the species is also considered to be drought sensitive. Thus, much research interest has focused on eliciting the details of its physiological response to increased water stress, whilst dendroecological studies have attempted to identify sites and regions where reductions in growth might be found. A significant knowledge gap exists regarding a multi-regional, range-wide view of growth trends, growth variability, climate sensitivity, and drought response for the species. Predicting the potential effects of climate change on competition and species composition in mixed species forests remains an important challenge. In order to address this knowledge gap, a multi-regional tree-ring network was constructed comprising of 46 sites in a latitudinal transect across the species’ Western European range. This consisted of 2719 tree cores taken from 1398 individual trees, which were used to construct tree-ring chronologies for each site in the network. As a first step in a multi-regional assessment for F. sylvatica, a combination of the tree-ring chronologies and environmental data derived from a large scale gridded climate dataset were used in a multivariate analysis. Sites in the latitudinal transect were partitioned into geographically meaningful regions for further analysis. The resulting regions were then studied using climate-growth analysis, pointer year analysis of drought years, analysis of growth trends and growth variability, in order to examine regional variation in the response of the species to climate. Furthermore, a combination of long-term monitoring data from one specific site was combined with tree-ring sampling of multiple cohorts of F. sylvatica and one co-dominant competitor, Quercus petraea, to study the effects of an extreme drought event in 1976 on mortality and subsequent recovery. Key results of the multi-regional analysis are that large scale growth reductions are not evident in even the most southerly and driest portions of the species’ range. Radial growth is increasing, both in the north and in the core of the species’ range, with southern range edge forests maintaining stable growth. However, the variability of growth from year to year is increasing for all regions, indicative of growing stress. Crucially, the southern range edge, which previous studies had identified as an ‘at risk’ region, was shown to be more robust than expected. Climate sensitivity and drought impacts were low for this region. Instead, forests in the core of the species range, both in continental Europe and in the south of the UK, were identified as having the highest climate sensitivity, highest drought impacts, and experiencing periodic reductions in growth as a result. Northern range edge forests showed little sign of being affected by drought, instead having low climate sensitivity and strongly increasing growth trends. Extreme drought was found to affect species differently: the dominant species (F. sylvatica) failed to recover pre-drought levels of growth, whilst a transient effect of competitive release occurred for the co-dominant species (Q. petraea). There was also a long term effect on the relative abundance of the two species within the woodland, due to differences in the levels of drought induced mortality experienced by the species. This shows that in the case of extreme climatic events where thresholds in the ability of species to tolerate water stress are breached, the effects of drought can be rapid and long lasting. Drought impacts can cascade beyond that experienced by the most drought sensitive species, due to changes in competitive interactions between species in mixed species forests. The implications of this work suggest opportunities, risks and strengths for F. sylvatica. In the northern portion of the species’ range, predicted increases in productivity are confirmed by recent growth trends, indicating a good outlook for the species. At the southern range edge, F. sylvatica forests exist either in locations where precipitation is high or locations where local environmental conditions buffer them from an inhospitable regional climate. These factors result in southern range edge forests which are highly resilient to the effects of increasing climate stress. It is instead in the core of the species’ range where the most sensitive forests are found. The effects of extreme drought on a range core forest demonstrated here provide a cautionary note: where drought stress tolerance thresholds are breached, rapid and long lasting effects on growth and mortality can occur, even in regions where drought has not previously been considered to pose a strong risk to the species.

Sweetpotato (Ipomoea batatas) is a major staple food in Africa and the rest of the world where they are discovered to be a good source of carbohydrates, vitamin A, vitamin C and protein. The maximum production potential of the crop is being hampered by severe drought which ravages most parts of Africa. The main aim of this project therefore is to screen collected accessions of sweetpotatofor drought tolerance in a quick screening method with a view to identify cultivars that can perform well under water stress conditions. Fifty selected sweetpotato accessions consisting of cultivars and breeding lines collected from the ARC-VOPI gene bank were planted for drought screening in the glass house for 6 weeks during which water was withheld to induce stress. Observations were made on number of dead plants and days to wilting point, the results were analyzed and 12 best performing cultivars were selected for field trials. The field trial was carried out in Lwamondo, Thohoyandou for 6months under rain-fed conditions. The experiment was conducted in a complete randomized block design with 6 replicates.Yield data and growth parameters were collected every 8 weeks during the trial period and the data collected was analyzed using ANOVA. The best performing cultivars were Zapallo, Tacna, Ejumula, 2004-9-2 and Ndou. They were therefore recommended for further evaluation in other drought prone areas of the country. / Agriculture and  Animal Health / M. Sc. (Agriculture)

Stadtbäume unterliegen besonders häufigem und intensivem Trockenstress. Der bereits stattfindende Klimawandel führt zudem zu einer verschärften Wirkung der Klimafaktoren auf das Pflanzenwachstum. Demnach ist die Wahl standortgerechter und an urbane Bedingungen angepasster Gehölze von besonderer Bedeutung. Nicht nur durch das Stadtklima und den Einfluss des Klimawandels, sondern auch zwischen Rodung und Pflanzung und insbesondere nach dem Auspflanzen an den urbanen Standort sind junge Gehölze oft erheblichem Trockenstress ausgesetzt, wodurch sich das Ausfallrisiko erhöht. Dem könnte durch die frühzeitige Anpassung von Baumschulware an Trockenheit vorgebeugt werden. Das Ziel der vorliegenden Arbeit war daher, die Möglichkeit der Erhöhung der Trockenheitstoleranz verschiedener Baumarten durch die Anwendung eines bestimmten Bewässerungsverfahrens während der Anzucht zu untersuchen. Zudem sollte die Trockenheitstoleranz der Arten in der juvenilen Phase eingeschätzt werden. Im Freilandversuch wurden dafür fünf Baumarten bei drei Bewässerungsstrategien, optimale Bewässerung (V1), geringe Bewässerung (V2) und zeitweise Bewässerung (V3), in den Jahren 2012 und 2013 kultiviert und in der anschließenden Vegetationsperiode 2014 Trockenheit ausgesetzt. Neben Zuwachsanalysen (Höhe, Stamm- und Wurzelhalsdurchmesser, Trockenmasse, Spross-Wurzel-Verhältnis) und Wassergehaltsuntersuchungen wurden Blattwasserpotential- und Chlorophyllfluoreszenzmessungen sowie Gefäß-analysen in den Jahren 2013 und 2014 durchgeführt. Somit konnten die artspezifische und bewässerungsabhängige Entwicklung während der Anzucht nachvollzogen und daraus die Stressbelastung sowie Trockenheitsreaktionen bzw. Strategien im Umgang mit Wassermangel und mögliche bewässerungsbedingte Anpassungen an Trockenheit abgeleitet werden. Die Untersuchungsergebnisse von Sorbus aucuparia lassen auf eine gewisse Angepasstheit an zeitweise auftretende bzw. moderate Trockenheit in der juvenilen Phase schließen. Die während kurzfristiger Trockenheit entstandenen Wachstumsverluste konnte S. aucuparia durch Wachstumsschübe infolge von Wassereinträgen kompensieren. Hingegen zeichnet sich anhand der Wachstumsanalysen und Blattwasserpotentialmessungen eine geringe Toleranz gegenüber stärkerem bzw. langanhaltendem Wassermangel in der Vegetationsperiode ab. Das vergleichsweise gute Wachstum von SaV1 und SaV3 unter Trockenheit im Jahr 2014 steht vor allem in Verbindung mit dem durch vormals bessere Wasserversorgung höheren Wasserspeicher der Pflanzen. Infolge der verstärkten Bildung kleinerer und der geringeren Bildung großer Gefäße bei SaV3 steigt die Sicherheit vor Embolien. Um eine langfristig bessere Angepasstheit und gleichzeitig gute Wachstumsergebnisse zu erzielen, scheint für S. aucuparia eine Konditionierung durch zeitweise Bewässerung zur Anpassung an Trockenheit sinnvoll zu sein. Für Acer campestre lassen sich anhand der Wachstumsanalysen und Chlorophyllfluoreszenzmessungen eine geringe Empfindlichkeit gegenüber kurzfristiger bzw. zeitweiser Trockenheit und eine mittlere bis hohe Empfindlichkeit gegenüber langfristig mangelnder Wasserversorgung in der juvenilen Phase ableiten. Die Wachstumsergebnisse unter Trockenheit im Jahr 2014 deuten auf eine bessere Angepasstheit vormals optimal und zeitweise bewässerter Pflanzen (AcV1 und AcV3) hin. Eine langfristig bessere Angepasstheit an Trockenheit kann bei A. campestre jedoch vermutlich durch eine geringe Bewässerung erreicht werden, auch wenn mit schwachen Jahreszuwächsen zu rechnen ist. Dies leitet sich aus der geringeren Gefahr von Embolien aufgrund der stärkeren Bildung englumiger Gefäße, dem an Trockenheit angepassten geringeren Spross-Wurzel-Verhältnisses sowie der Aufrechterhaltung vergleichsweise hoher Pflanzenwassergehalte bei wiederholter Trockenheit bei vormals gering bewässerten A. campestre ab. Sowohl die Wachstumsanalysen als auch die physiologischen Untersuchungen weisen auf die hohe Trockenheitsempfindlichkeit von juvenilen Tilia platyphyllos hin. Die Gefäßanalysen bestätigen dieses Ergebnis. So erfolgte während Trockenheit weitestgehend keine Reduzierung der Gefäßgrößen. In Verbindung mit den Ergebnissen der Trockenmasseuntersuchungen lässt sich auf eine gewisse Angepasstheit von T. platyphyllos an wiederkehrende Trockenperioden bzw. vereinzelte Trockenjahre schließen. Vormals optimal bewässerte Pflanzen (TpV1) konnten einjährige Trockenheit im Jahr 2014 durch Nutzung ihres Wasserspeichers kompensieren und beste Wachstumsergebnisse bei gleichzeitig geringen Blattwasserdefiziten erzielen. Die vergleichsweise geringe Stressbelastung von TpV1 im Jahr 2014 spiegelt sich auch in den Chlorophyllfluoreszenzmessungen wider. Vormals zeitweise bewässerte Pflanzen (TpV3) wiesen zwar unter Trockenheit im Jahr 2014 keine bessere Wuchsleistung als TpV1 auf, jedoch deuten die Ergebnisse der Blattwasserpotentialuntersuchungen auf osmotische Anpassungsvorgänge sowie eine angepasste Stomataregulation hin. Die Gefäßanalysen belegen geringfügige trockenheitsbedingte Anpassungen von TpV3. Demnach kann bei T. platyphyllos sowohl durch optimale als auch durch zeitweise Bewässerung während der Anzucht eine gewisse Angepasstheit an spätere Trockenereignisse erreicht werden. Styphnolobium japonicum zeichnet sich durch eine sehr hohe Trockenheitstoleranz gegenüber zeitweiser und eine mittlere bis hohe Trockenheitstoleranz gegenüber langer und mehrjähriger Trockenheit in der juvenilen Phase aus. Dies spiegelt sich in den Wachstumsanalysen und in den physiologischen Untersuchungen wider. Die Ausbildung eines kräftigen Herzwurzelsystems mit tiefreichenden Senkerwurzeln ermöglicht es S. japonicum große Bodenwasserreserven zu erschließen. Die Wurzelanatomie, die Anatomie der Blätter und des Stammes sowie das physiologische Verhalten (geringe Absenkung der Mittags-Blattwasserpotentiale) zielen auf eine Strategie der Trockenstressvermeidung. Durch die geringere Bildung großlumiger und verstärkte Bildung kleinlumiger Gefäße während Trockenheit wird zudem die Sicherheit vor Embolien erhöht. Dadurch waren vormals gering und zeitweise bewässerte Pflanzen (SjV2, SjV3) auch an Trockenheit im Jahr 2014 besser angepasst. Die guten Wachstumsergebnisse lassen in Verbindung mit den Blattwasserpotential- und Wassergehaltsmessungen auch eine verbesserte Wassernutzungseffizienz, eine verstärkte Osmoregulation sowie eine Anpassung der Stomataregulation vermuten. Die Ergebnisse deuten darauf hin, dass eine langfristig bessere Angepasstheit an Trockenheit und gute Wachstumsergebnisse durch eine zeitweise Bewässerung von S. japonicum erreicht werden kann. Populus nigra 'Italica' folgt ebenfalls der Strategie der Stressvermeidung mit geringen Blattwasserpotentialabsenkungen. Durch schnelle Ausbreitung des Wurzelsystems können größere Wasservorräte im Boden erschlossen werden. Die Entwicklung einer höheren Anzahl kleiner und einer geringeren Anzahl großer Gefäße während Trockenheit senkt die Kavitationsgefahr. Die Ergebnisse der Wachstumsanalysen der juvenilen P. nigra 'Italica' deuten unter Berücksichtigung der Blattwasserpotential- und Chlorophyllfluoreszenzmessungen auf eine mittlere Trockenheitstoleranz gegenüber langanhaltendem Wassermangel in der Vegetationsperiode und eine höhere Trockenheitstoleranz gegenüber zeitweiser Trockenheit hin. Es wurde ersichtlich, dass P. nigra 'Italica' in Abhängigkeit von Intensität, Dauer und Häufigkeit der Trockenperioden unterschiedliche Anpassungsmechanismen aufweist. / Due to their specific location urban trees are typically subjected to heavy and intense drought stress. The already apparent negative effects of climate change enhance the effect of climate factors on plant growth. Therefore, choosing trees well-adapted to urban conditions and suitable for the respective location is of especial importance. Aside from city climate and the influence of climate change, young trees suffer considerably from drought stress as a result of clearing and planting, especially after planting out, because of which the risk of failure increases. This could be prevented by an early adjustment of nursery plants to drought, in order to obtain long-lived, vital and robust trees. Thus the goal of this work is to examine options of increasing drought tolerance in several tree species by the use of particular irrigation practices during cultivation. Additionally, there will be an assessment of the different species' drought tolerance during their juvenile phase. In 2012 and 2013 five tree species were cultivated under different irrigation strategies in a field experiment, with well (V1), with low (V2) and with occasional watering (3). Over the course of the subsequent vegetation period in 2014 they were exposed to drought. During 2013 and 2014, apart from analyses of growth (height, stem and root collar diameter, dry weight, shoot- root-ratio) and water content as well as leaf water potential, chlorophyll fluorescence and vessel parameters were measured. This enabled tracking the species-specific and irrigation-dependent development during cultivation and allowed conclusions on stress level and drought reactions or strategies to cope with water shortage. As well as possible irrigation-induced adaptations towards drought could be identified. The results of Sorbus aucuparia suggest a certain degree of adjustment to periodic or moderate drought during the juvenile phase. Loss of growth caused by short-term droughts could eventually be compensated by growth spurts after irrigation or rainfall. However, growth analyses and the data on leaf water potential indicate low tolerance against longer and more severe periods of water shortage during the growing season, especially when coinciding with atmospheric drought. Sorbus aucuparia were well-adapted to drought in 2014 both when they were formerly well irrigated (SaV1) as well as occasionally irrigated (SaV3). Their comparatively good growth is related to their larger plant water storage caused by early on better water supply. In order to achieve both long-term adaptation as well as satisfying growing rates, occasional irrigation seems reasonable for allowing adjustment to drought through conditioning for Sorbus aucuparia. The formation of narrower vessels decreases the risk of embolism. The findings are also indicative of a higher water use efficiency of formerly occasionally irrigated Sorbus aucuparia. Based on growth and chlorophyll fluorescence analyses Acer campestre seems to be less sensitive to short-term and occasional drought, but medium to highly sensitive to long-term water shortage during the juvenile phase. Growth during the dry conditions in 2014 indicates a better adjustment of formerly well and occasionally irrigated Acer campestre (AcV1 and AcV3). Probably a better adjustment in the long term can be achieved by early low irrigation, even if lower overall growth per year has to be expected. This is due to the formation of more narrow vessels that prevent embolism, and the low shoot-root-ratio well adapted to drought, as well as by keeping up a relatively high water content in recurring drought. Both growth and physiological investigation suggest a high susceptibility to drought in juvenile Tilia platyphyllos in severe long-term water shortage during the growing season and in perennial drought. Vessel analyses confirm this result. Thus no reduction of vessel size occurred during drought. Only formerly occasionally (TpV3) and low irrigated plants (TpV2) show minor adaptation in vessel size. Combining with the results of the dry-weight analysis it can be suggested a certain adjustment of Tilia platyphyllos to recurring periods of drought and singular dry years. Formerly well irrigated plants (TpV1) were best at withstanding the 2014 one-year drought by using their water storage and showed better growth with simultaneously low leaf water deficits. The comparatively low stress level of TpV1 is also reflected by chlorophyll fluorescence measurements. Despite not showing higher growth rates during the 2014 drought, analyses of leaf water potential of formerly occasionally watered plants (TpV3) point to osmotic adjustment as well as adaption of stoma regulation. The vessel analyses confirm these findings. Therefore, Tilia platyphyllos seems to benefit from both ideal as well as occasional irrigation (TpV1 and TpV3) during cultivation to adjust to subsequent droughts. However, a conclusive recommendation on watering Tilia platyphyllos does not seem possible at this point. Styphnolobium japonicum is characterized by very high drought tolerance towards occasional drought events and by medium to high tolerance towards long-term and perennial drought events during their juvenile phase. This is reflected by investigations of growth and physiology. The early development of a sturdy heart-root-system and deep sinker roots allows the acquisition of bigger soil water reservoirs. Not only root, but also leaf and trunk anatomy, as well as physiological reactions (slight reduction of midday leaf water potential) are main strategies to avoid drought stress. Here as well the protection from embolism is achieved by developing only medium to smaller vessels during drought. Therefore, formerly low and occasionally irrigated plants (SjV2 and SjV3) were even better adapted to the drought in 2014. Furthermore, together with analyses of leaf water potential and water content, the high growth rates suggest an improved water use efficiency, but also intensified osmoregulation as well as an adjustment of stoma regulation. The findings point to better long-term drought adaptation as well as satisfying growing rates using the occasional irrigation strategy for Styphnolobium japonicum. Populus nigra 'Italica' also follows a stress avoidance strategy by slightly lowering its leaf water potential. Its rapidly spreading root system can obtain access to bigger soil water resources. Developing a high number of small vessels and a rather low number of big ones decreases the risk of cavitation. Considering the leaf water potential and chlorophyll fluorescence measurements, the results of the growth analysis of Populus nigra 'Italica' indicate a medium drought tolerance towards long-term water shortage during the growing season and a higher tolerance towards occasional drought events. Yet a conclusive assessment of drought tolerance and improved adaptation to drought by the usage of a certain irrigation strategy, especially when it comes to perennial water shortage, is not possible for Populus nigra 'Italica' given the present data. However, it became apparent, that Populus nigra 'Italica' shows different adjustment mechanisms that are subject to intensity, duration and frequency of dry periods.

Thesis (M. Sc. (Agricultural Economics)) -- University of Limpopo, 2021 / Agriculture is not only the backbone, but also an important sector of the South African economy. It provides food and employment to a majority of people in the country, especially in the rural areas. Smallholder farmers play an important role in livelihood creation and the alleviation of poverty among the population of the Limpopo Province. However, despite their significant contribution, smallholder farmers’ production is still low. Climate change has bought increasing frequencies and severity of drought conditions and uncertainties in the length and quality-growing season. Drought threatens the production of maize as a staple food and without measures to counter climate change, food security will be a major problem in South Africa. This study therefore examined factors determining the adoption of drought tolerant maize among smallholder farmers in the Lepelle-Nkumpi Municipality. Primary data was collected using semi-structured questionnaires to achieve the objectives of the study. Multistage sampling was used for the study because larger clusters were subdivided into smaller and more targeted groupings for surveying. Descriptive Statistics and the Binary Probit Model were used to analyse the data. The results of the Probit Regression analysis indicated that farm size, hired labour and maize produced per hectare had positive significant influence on the probability of farmers adopting drought tolerant maize varieties. Farm size and maize produced per hectare were statistically significant at 1% and hired labour was statistically significant at 5%. Based on the sample of this study,74% of the households grew non-drought tolerant maize varieties, while 26% of the smallholder farmers grew drought tolerant maize varieties. The results indicate that 24,4% of the farmers were not affected by any constraints in terms of their adoption of drought tolerant maize varieties whilst 76,6% said they are affected by those constraints in Lepelle-Nkumpi Municipality. Based on the study’s findings, it is recommended that extension officers should make it a priority to provide smallholder farmers with timely and accurate information. Extension officers should effectively disseminate information about the adoption of drought tolerant maize through a combination of different pathways.

Typescript (photocopy). / Digitized by Kansas Correctional Industries

Call number: LD2668 .T4 1964 M43 / Master of Science

Call number: LD2668 .T4 1978 S563 / Master of Science

Call number: LD2668 .T4 1985 M378 / Master of Science

Doctor of Philosophy / Department of Biology / Walter K. Dodds / Streams exist in a state of dynamic equilibrium with frequent floods and drought. The frequency and intensity of stream disturbances are projected to increase with greater water withdrawal for agriculture and biofuel production, watershed development, and altered climate. Changes in the hydrologic regime may alter stream ecosystems. I studied how stream communities return after disturbances and how nutrients, consumers, and substrata heterogeneity influence recovery trajectories. Large consumers were excluded from pools following a severe drought to assess how community structure and function returned in their absence. Large consumers reduced algal biomass, primary productivity, and nutrient uptake rates, and delayed macroinvertebrate recolonization. However, grazer effects were temporary and their influence weakened after five weeks. In a second experiment, I assessed the relative influence of grazer density and nutrient loadings on algal recovery from flood. Nutrients had a stronger effect on recovery than grazers, but the strength of each varied temporally. Grazer control decreased and nutrient control increased over time. A third experiment addressed the physical properties of stream substrata on algal development. The relationship among algal accumulation and substrata surface topography was assessed by growing algae on substrata with varying orientation and roughness. Total algal biomass decreased on surfaces with angles > 45 degrees, and peaked at an intermediate roughness (pit depth of [similar to]17 [Mu]m). Rougher surfaces collected more tightly attached (grazer resistant) forms and less loosely attached (grazer susceptible) forms. Individual algal forms responded differently to grazing pressure, nutrient availability, and surface features. I developed a method using Fourier-transform infrared microspectroscopy to measure single-cell physiological responses in benthic algae. Nutrients and consumers were strong regulators of ecosystem succession following disturbance, but nutrient influence was stronger. The influence of nutrients and consumers were context dependent, and changed over the course of recovery. Rougher surfaces increase algal growth and shifted algal assemblages to more grazer resistant forms, which may decrease the influence of large consumers on stream function. Altering the severity and frequency of disturbances can change the trajectory of stream recovery and ultimately change community composition and stream metabolic activity, which may alter ecosystem services such as water purification and recreation.