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Balancing different functional demands in plant vascular tissue: Do conduit connections affect embolism propagation and transport efficiency?
Transporting water efficiently is one of the chief functions of plant vascular systems. Highly efficient transport is generally beneficial; however, tradeoffs arise because vascular systems have multiple functional roles that interact with one another. One example of a tradeoff may be related to how plants respond to water deficits. Some plants growing in arid and semi-arid environments form xylem networks that are able to tolerate water deficiencies and cope with embolism spread. Distinguishing patterns in embolism spread is important to understand how plants are vulnerable to water stress and what tradeoffs are involved in water stress resistance. Flowering plants found in mediterranean-type ecosystems differ in conduit (vessels and tracheids) presence and abundance. Little is known about how embolism spreads in plant vascular systems and the role tracheids play in the spread of emboli in vessel-bearing plants. I hypothesized that patterns in vessel embolism spread are linked to the type of conduit connections. I assumed that the propagation of embolism most commonly occurs through vessel-to-vessel connections. Therefore, I predicted that the presence of tracheids minimizes embolism spread compared to species with only vessels that have greater numbers of vessel-to-vessel connections. By contrast, greater vessel-to-vessel connections promote greater hydraulic transport efficiency. To test this hypothesis, I quantified embolism spread using computer assisted tomography (microCT) and hydraulic efficiency (hydraulic conductivity per unit xylem area; Ks) in six different species of woody plants that differ in tracheid presence, vessel structure, and hydraulic function. In Chapter 1, I briefly review how plants respond to water stress and what effects have been noticed to occur within their xylem conduits as a result. In Chapter 2, I present microCT, hydraulic efficiency, and percentage loss in conductivity (PLC) results from dehydration treatments for six species native to mediterranean-type climate regions in California, Cercoparpus betuloides, Malosma laurina, and Heteromeles arbutifolia; and the Mediterranean Basin, Laurus nobilis, Olea europaea, and Cistus ladanifer. In Chapter 3, I summarize my findings in the broader context of plant vascular function. My chief conclusion is that the connections between vessels are a key determinant of the tradeoff between hydraulic safety from cavitation and transport efficiency.
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