New and improved techniques for assessing drought tolerance in plants

Evaluating how plants respond to dry conditions is critical to managing and predicting the effects of climate change on vegetation. Water in the soil and plant are connected via the xylem, a network of dead and elongated hollow cells that provide conduits through which water flows during transpiration. Water movement is a tug of war between the competing water status of the plant and the soil; when conditions are favourable, water is pulled from the soil under a 'metastable' state of tension established via transpiration and capillary action in the leaf. As soil moisture declines during drought, capillary action in the soil microstructure resists the movement of water into the plant causing tension in the xylem to rise. Under tension the xylem is highly vulnerable to bubble nucleation, which results in rapid embolism of xylem conduits, reducing xylem hydraulic conductance. This limits the supply of water for transpiration and photosynthesis. Excessive drought conditions cause catastrophic damage as embolism propagates through the xylem leading to plant dieback and death. The tension at which this occurs defines the absolute threshold of drought tolerance, and across species there are a range of thresholds reflecting the diversity in adaptation to different levels of water availability. Traits incorporating these thresholds can be used as powerful predictors of drought survival under extreme climate conditions, however the limited number of accurate and reliable measurements currently available significantly underrepresents the diversity of vegetation. Although there are a number of techniques for assessing xylem vulnerability, most are only suitable for in-depth study of xylem anatomy and physiology, and the few that have the capacity for broad-scale measurements are limited in access, restricted to short-vesseled species and can be susceptible to artefacts under some conditions. The aim of this thesis was to find a technique that could be used for accurately measuring the vulnerability of xylem to embolism in large numbers of species or individuals. In the first chapter I provide a brief introduction to the evolutionary history of the xylem, and the development of our understanding of xylem physiology. In detail I describe the mechanisms of xylem water transport, and discuss the limitations in the context of climate change. Finally, current methodologies for assessing xylem vulnerability are reviewed. In the second chapter I explore a novel approach for reducing the number of measurements required to assess intraspecific variation, and use the approach to demonstrate significant variation in drought tolerance between two populations of Eucalyptus globulus. In the third chapter I develop procedures to expediate the process of assessing xylem vulnerability using a desktop scanner, and use the procedure to find significant differences in vulnerability between juvenile and adult Eucalyptus globulus. In the fourth chapter I develop a novel system based on the latest optical method of vulnerability assessment. Combining modern electronics, 3D printing technology and collaborative platforms, I develop a cheap, accurate, reliable, widely accessible and Open Source device for assessing drought tolerance, with a broad appeal to plant physiologists, ecologists, forestry managers and educators. This system provides the only current mechanism by which a large and diverse number of species can be measured without significant investment in facilities and training. In the final chapter I evaluate the approaches presented in the thesis and discuss future applications.