Master of Science / Department of Plant Pathology / Karen A. Garrett / While plants are typically exposed to multiple stressors in the field, studies of genome-wide gene expression and phytohormone responses in wild plant species exposed to multiple stressors are rare. Our objectives were to determine the effects of drought and rust stress on gene expression in Andropogon gerardii, the dominant grass in tallgrass prairie, and associated levels of phytohormone production. In a factorial design, plants experiencing drought or non-drought conditions were either inoculated with the rust pathogen Puccinia andropogonis or not inoculated. Gene expression was evaluated with maize microarrays. Drought-stressed plants significantly decreased expression of genes associated with photosynthesis and the hypersensitive response, while expression of genes associated with chaperones and heat-shock proteins increased. No significant differences in gene expression in response to the rust treatment were detected using a mixed model analysis of variance and false discovery rate protection, probably because of the low infection rate. Phytohormone production increased when both stresses were present. The rust treatment significantly increased benzoic acid (BA) production in the presence of drought, while the drought treatment alone significantly increased salicylic acid (SA) production. Leaf tips usually had higher levels of all phytohormones in all treatments and the leaf section evaluated had a larger effect on phytohormone level than did the treatments applied.

>Magister Scientiae - MSc / Drought stress is a major contributor to reduced soybean crop yield and quality, this can however be mitigated by the plant’s antioxidant defence mechanisms. One group of antioxidant enzymes that are active in these defence mechanisms are glutathione peroxidases (GPXs). GPXs are antioxidant proteins which are able to reduce H2O2, a toxic reactive oxygen species which accumulates under stress conditions. This study aims at isolating the protein encoded by Glyma01g42840 and determining if it has Phospholipid hydroperoxidase glutathione peroxidase (PHGPX) and/or Thioredoxin dependent peroxidase (TRX-PX) activity as well as assaying the effect of Drought stress on the expression of this putative GPX . This will be accomplished by molecular cloning, sequencing as well as the expression of the isolated protein to assay it enzymatic activity. It was found that the enzyme encoded by Glyma01g42840 is able to use glutathione and thioredoxin as electron donors for the detoxification peroxides, however enzymatic activity is more efficient when using glutathione as an electron donor. In conclusion it was found that glyma01g42840 encodes an enzyme which is able to utilise more than one electron donor and as glutathione produces the greatest amount of enzymatic activity it can be said that glyma01g42840 encodes a GPX.

Magister Artium (Development Studies) - MA(DVS) / With changes in climatic conditions being increasingly noticed around the world in the past 2 decades, drought has been identified as one of the main causes of food insecurity. Given the inseparable nature of food security and poverty, it has been impossible for Africa to attain the poverty reduction Millennium Development Goal as most of the continent still suffers food insecurity. Impacts of the drought described as one of the worst droughts which occurred in West an East Africa between 2009-2011, are still felt in some parts of Kenya, Ethiopia, Somalia, Djibouti and parts of Northern Tanzania close to Kenya. With almost 70% of sub- Saharan Africa’s population involved in agriculture, the occurrence of drought not only causes failure in food production but also negatively impacts food security and increases poverty. Understanding the impacts of drought on household food security is important for reducing household’s risk of poverty. Following this the study proposes policies for reducing the impact of drought on food security and other policies that can ensure complete eradication of food insecurity.

Changing climate in combination with growing world populations mean that there is growing need for plants to be grown on land that is currently considered marginal for agriculture. Sorghum is a C4 plant that serves as an important food crop in Africa and India. It is also known to be highly drought tolerant but the mechanisms responsible for this tolerance are unclear. The overall aim of this study was to understand the drought tolerance mechanisms that enable the plant to maintain leaf function for a long time during water deficit. In Chapter 2 of this thesis, I studied the underlying physiological mechanisms for tolerating drought in two sorghum varieties with differing degrees of drought tolerance compared to a closely related species, Zea mays. During progressive drought, the more tolerant sorghum variety Samsorg 17 maintained net CO2 assimilation and photochemistry longest relative to the less tolerant Samsorg 40 and Zea mays. Differences were also seen in stomatal aperture, stomatal density, total chlorophyll content, chl a:b and A/Ci curve responses with maize more affected than the sorghum varieties. In Chapter 3, I identified novel drought tolerance mechanisms in the sorghum varieties. The less tolerant Samsorg 40 lost PsbA (D1) and Rubisco proteins and reengineered its photosynthetic apparatus to accumulate amino acids and sugars in order to maximise survival under drought. Samsorg 17 maintained photosynthetic proteins notably PsbA (D1) and Rubisco and accumulated high constitutive sugar content allowing for the maintenance of transpiration and photosynthesis. The two sorghum varieties had strikingly contrasting approaches of tolerating drought as demonstrated in Chapter 3. In Chapter 4, the aim was to characterise biochemical and metabolic changes that occur in response to drought. In particular, to identify sugars that are accumulated constitutively in Samsorg 17 and nitrogen sinks for lost N in Samsorg 40. My findings indicated a contrasting response in terms of sugar content in Samsorg 17 but support for amino acids as N sinks in Samsorg 40 as reported earlier. Sugars, sugar alcohols, lipids, organic acids, heat shock proteins and dehydrins were generally higher or more induced in Samsorg 17 relative to Samsorg 40. Samsorg 40 rather made amino acids. The implications of my findings and future work arising from this study were discussed in detail in the final chapter. In conclusion, in this thesis, it was demonstrated that closely related plants can have mechanistically different physiological and biochemical mechanisms for responding to drought.

Climate change is a major threat to global biodiversity, and will alter species ranges, plant and animal phenology, and species interactions within ecosystems. Climate mediates plant-insect interactions, and consequently has the potential for positive or negative effects on biological control systems. Observational evidence suggests that a recent dramatic reduction in the density of diffuse knapweed, Centaurea diffusa Lamarck, in sites in British Columbia, Canada is attributed to the biological agent Larinus minutus Gyllenhal (Coleoptera: Curculionidae). This decline took place over several years of late spring and summer drought which suggested an association between dry conditions and successful biological control. To explore this, I conducted field experiments using rain shelters and watering treatments to assess the effectiveness of plant attack by L. minutus under moist and dry conditions. I found that L. minutus reduced seed production regardless of moisture conditions, with a trend towards greater seed reduction under dry conditions. Two or more species of insects in the same genera have been introduced in some weed biocontrol programs. If the species are ecological equivalents and compete the introduction of both species may be detrimental and reduce their impacts. If however the species vary in their distributions, the introduction of congeners may be advantageous. I review the following cases of species pair releases: the beetles Chrysolina quadrigemina (Suffrian) and Chrysolina hyperici (Forster) for St. John’s wort (Hypericum perforatum L.); the gallflies Urophora affinis Frfld. and Urophora quadrifasciata (Meig.) for Centaurea species; the weevils Neochetina bruchi Hustache and Neochetina eichhorniae Warner for water hyacinth (Eichhornia crassipes (Mart.) Solms.) and the beetles Galerucella pusilla Duftschmidt and Galerucella calmariensis L. for purple loosestrife (Lythrum salicaria L.). I found that congeneric agents can offer complementary control of target weeds in slightly different habitats. Given the increasing focus on possible non-target effects of biological control introductions, I recommend that greater care be taken to avoid mixed species introductions and that judicious use be made of controlled field experimentation to determine species impacts. Molecular studies of species before introduction could help prevent the accidental introduction of multiple species. / Science, Faculty of / Zoology, Department of / Graduate

Widespread, high levels of tree mortality, termed forest die-off, associated with drought and rising temperatures, are disrupting forests worldwide. Drought will likely become more frequent with climate change, but even without more frequent drought, higher temperatures can exacerbate tree water stress. The temperature sensitivity of drought-induced mortality of tree species has been evaluated experimentally for only single-step changes in temperature (ambient compared to ambient + increase) rather than as a response surface (multiple levels of temperature increase), which constrains our ability to relate changes in the driver with the biological response. Here we show that time-to-mortality during drought for seedlings of two western United States tree species, Pinus edulis (Engelm.) and Pinus ponderosa (Douglas ex C. Lawson), declined in continuous proportion with increasing temperature spanning a 7.7 degrees C increase. Although P. edulis outlived P. ponderosa at all temperatures, both species had similar relative declines in time-to-mortality as temperature increased (5.2% per degrees C for P. edulis; 5.8% per. C for P. ponderosa). When combined with the non-linear frequency distribution of drought duration-many more short droughts than long droughts-these findings point to a progressive increase in mortality events with global change due to warming alone and independent of additional changes in future drought frequency distributions. As such, dire future forest recruitment patterns are projected assuming the calculated 7-9 seedling mortality events per species by 2100 under business-as-usual warming occur, congruent with additional vulnerability predicted for adult trees from stressors like pathogens and pests. Our progressive projection for increased mortality events was driven primarily by the non-linear shape of the drought duration frequency distribution, a common climate feature of drought-affected regions. These results illustrate profound benefits for reducing emissions of carbon to the atmosphere from anthropogenic sources and slowing warming as rapidly as possible to maximize forest persistence.

Les forêts du monde entier sont menacées de mortalités importantes lors de sécheresses intenses liés au changement climatique. Les conifères en particulier semblent extrêmement vulnérables à la mort par dysfonctionnement hydraulique de leur système vasculaire ou embolie. Le principal objectif de cette thèse est d’étudier la résistance à l’embolie des conifères dans un cadre évolutif. Premièrement, nous avons mis en évidence que la résistance à l’embolie varie d’un facteur neuf sur plus de 250 espèces parmi les 7 familles de conifères, atteignant un nouveau record du monde avec Callitris tuberculata (P50 = -18.8 MPa). Nous avons montré le lien évolutif entre cette résistance et l’anatomie des ponctuations aréolées. En combinant cette base de données unique avec une phylogénie calibrée de plus de 300 espèces, nous avons retracé la diversification des conifères et l’évolution de leur résistance à l’embolie. Nous avons découvert que plusieurs lignées de conifères ont brusquement changé de dynamiques évolutives, avec l’accélération de la spéciation et de l’évolution de résistance à l’embolie. En outre, les conifères plus résistants se sont diversifié plus rapidement, notamment les genres Cupressus, Juniperus et Callitris (Cupressaceae). La diversification de ces derniers s’est accélérée avec l’aridification de l’Australie sur les derniers 30 Millions d’années. Nous montrons que leur xylème a été façonné par la sécheresse, devenant plus résistant à l’embolie mais surtout sans compromettre l’efficience du transport de l’eau ou augmenter son coût de construction. Cette thèse élargit notre compréhension de l’évolution des plantes vasculaire face aux sécheresses intenses. / Forests worldwide are at increased risk of widespread mortality due to intense drought under current and future climate change. In particular, conifer species seem extremely vulnerable to mortality due to hydraulic failure or embolism. The main objective of this thesis was to examine conifer resistance to embolism in an evolutionary framework. Firstly, we uncovered 9-fold variation in resistance to embolism across 250 species from the 7 conifer families, culminating in a new world record in Callitris tuberculata (P50 = -18.8 MPa). We demonstrated the evolutionary relationship between increased embolism resistance and the anatomy of bordered pits. By combining this unprecedented physiological dataset with a time-calibrated phylogeny of over 300 species, we retraced conifer diversification and the evolution of embolism resistance. We discovered multiple evolutionary dynamics with several conifer lineages shifting to higher rates of speciation and trait evolution. We found that conifers with high drought resistance diversified more rapidly, especially crown groups of Cupressaceae composed of the Cupressus-Juniperus clade and the Callitris clade. Within this last group, diversification rates increased over the course of the aridification of Australia over the last 30 million years. We show how their xylem has been shaped by drought, becoming more resistant to embolism, but crucially we found no trade-off with water transport efficiency or construction costs. This work greatly expands our understanding of how vascular plants have evolved to cope with extreme drought.

Avec le changement climatique, l'agriculture va devoir s'adapter à des sécheresses de plus en plus fréquentes et sévères. C'est le secteur le plus consommateur d'eau et celui à qui le plus d'efforts dans la réduction des prélèvements va être demandé. Les mécanismes d'allocation de l'eau existants (prix, marché, dispositifs de rationnement...) négligent souvent la forte variabilité de la disponibilité de la ressource. Or, la façon dont est partagée l'eau en cas de rareté influence le risque subi par les agriculteurs. Afin de réduire le coûts pour les agriculteurs et la société du risque sécheresse, il convient d'améliorer les outils de partage de l'eau et d'y intégrer des mécanismes de gestion du risque. C'est la question qui est étudiée dans cette thèse. Cette réflexion a été menée dans deux contextes très différents: la France, où le partage de l'eau est administré par les représentants de l'État en cas de sécheresse; l'Australie où des systèmes de marchés de l'eau ont été mis en place pour permettre les échanges d'eau entre agriculteurs. La thèse est composée de 3 articles (chapitres 2,3 et 5 en anglais) et d'éléments de cadrage (chapitres 1 et 4 en français). Le chapitre 1 présente la gestion de l'eau agricole en France. Les chapitres 2 et 3 étudient les interactions entre les règles utilisées pour le rationnement de l'eau d'irrigation en cas de sécheresse en France et les stratégies d'auto-assurance des agriculteurs. Nous étudions notamment le cas du stockage d'eau au niveau de l'exploitation, outil de gestion du risque de plus en plus mobilisé par les agriculteurs français. Le chapitre 2 modélise une situation où des agents ont la possibilité de substituer l'usage d'une ressource coûteuse mais sécurisée à l'usage de la ressource commune risquée. Nous déterminons la règle de partage optimale sous deux hypothèses distinctes: i) le planificateur définit simultanément la règle de partage et les choix de diversifications de façon à maximiser le bien être social; ii) le décideur utilise la règle de partage comme un instrument incitant les usagers à faire des choix de diversification optimaux d'un point de vue social. Le chapitre 3 est une expérience de laboratoire visant à comparer différentes règles de partage d'une ressource commune de taille incertaine quand les sujets peuvent choisir entre l'utilisation de cette ressource commune ou l'investissement dans une ressource sécurisée. Les règles sont évaluées sur leur aptitude à faciliter la coordination des agents autour du niveau optimal d'investissement dans la ressource sécurisée. La discussion des résultats de ces deux articles permet de proposer des recommandations aux administrations en charge de la mise en place des règles de partage de la ressource en cas de sécheresse, notamment dans le cadre de la réforme en cours en France (création des Organismes Uniques). Le chapitre 4 présente le fonctionnement des marchés de l'eau et le système de gestion des sécheresses en Australie. Le chapitre 5 étudie les gains de la mise en place de droits d'eau avec différents niveaux de sécurité, à l'aide d'une expérience de laboratoire capturant l'essence des marchés de l'eau australiens. Nous trouvons qu'un système avec deux niveaux de sécurité améliore l'efficacité allocative et le partage du risque mais seulement quand les coûts de transaction sont plus élevés sur le marché des allocations que sur le marché des droits d'eau. La mise en place de droits différenciés peut conduire à une diminution de la coût-efficacité du mécanisme en raison de l'augmentation des échanges sur le marché des droits d'eau et donc des coûts de transaction payés par les agriculteurs quand ceux-ci sont élevés sur le marché des droits. / With climate change, agriculture will have to adapt to face increasingly frequent and more severe droughts. Agriculture is the largest consumer of water and will face most of the water restrictions in the coming years. Existing water allocation mechanisms (pricing, water markets, rationing scheme) often fail to take into account the high variability of the resource, whereas those mechanisms have strong impacts on the risk born by agricultural water users. Water allocation schemes can contribute to reduce the cost of drought risk for farmers and society by integrating risk management mechanisms. We address this question in two different contexts: France, where water is managed by public authorities that are responsible for the rationing scheme and define the allocation of water between farmers in case of scarcity; Australia, where water markets have been implemented in order to allow water transfers between farmers. This Ph.D. dissertation is composed of 3 articles (chapter 2, 3 and 5 in English) and background information (chapter 1 and 4 in French). Chapter 1 describes the organization of irrigation water allocation in France. Chapters 2 and 3 analyze the interactions between the restrictions rules used in France to share the resource in crisis times and the self-insurance strategies of farmers. We investigate the particular example of on-farm storage, an increasingly widespread risk management tool used by French farmers. Chapter 2 models the decisions of agents heterogeneous in their risk tolerance that can substitute the use of a a secure private alternative to the use of a common pool resource of uncertain size. We find the optimal sharing rule under two assumptions concerning the regulator: i) the social planner defines simultaneously the sharing rule and the diversification choices in order to maximize social welfare; ii) the policy maker uses the sharing rule as an instrument to induce each agent to make the optimal diversification decision. Chapter 3 uses laboratory experiment to analyze how the sharing rule impacts the individual trade-off between relying on a free but uncertain resource and investing in a secure alternative resource. Rules are compared on the basis of their ability to facilitate coordination on the optimal investment in the secure resource. Chapter 4 presents how irrigation water is allocated between users in Australia in case of drought. Chapter 5 studies with a laboratory experiment, designed such as capturing the main characteristics of Australian water markets, whether offering different levels of security for water rights could potentially allow water users to manage the risks of supply uncertainty better. We find that a two security levels system improves both allocative efficiency and risk management, but only when transactions costs are higher in the market for water allocation than in the market for water rights. A system with different levels of security for water rights can be detrimental to cost-effectiveness by increasing the number of costly trades on the water rights market.

M.Sc. (Botany) / Please refer to full text to view abstract

Investigations of three low-water requiring landscape species produced the following results: (1) Although Calliandra californica flowered under photoperiods from 12 to 16 hours, plants produced more elongation under 16 hour days. Plants grew taller and developed greener foliage under irradiances of 600 μmol·m⁻²·s⁻¹. Branching was not stimulated by foliar sprays of BA, PBA, or BA + GA₄₊₇. (2) Caesalpinia pulcherrima increased biomass under 16 hour days, but were stunted and chlorotic under irradiances of 1950 μmol·m⁻²·s⁻¹. Internodal lengths were restricted with drenches of 3.75 mg · pot-1 uniconazole, and plants sprayed with 500 mg·liter⁻¹ PBA developed more axillary branching without negative elongating effects. (3) Justicia specigera gained more height under 12 hour days, and produced greener foliage, more elongation, and faster flowering under irradiances of 600 μmol·m⁻²·s⁻¹. Plants showed restricted internodal elongation and fewer flowers when drenched with 5.0 mg·pot⁻¹ uniconazole, and developed more axillary branching with no detrimental elongation effects when sprayed with 100 mg·liter⁻¹ BA + GA₄₊₇.