Mechanism of Embolism Reversal in Vascular Plants: Studies at the Level of Individual Vessels

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Plants must continually supply their leaves with water that is absorbed by the roots and transported to the leaves through the stem. The driving force for this comes from the evaporation of water from leaf surfaces, which exerts a "pull" on the water column. This allows the plant to draw water from the soil through hollow xylem cells which form conduits for water transport through the stem -- essentially using the stem as a straw. Because the water is under tension, air is sucked into the xylem occasionally, causing an embolism that blocks water flow. It is well known that air embolism occurs in plants, sometimes to the extent that water delivery to leaves is significantly impaired. The goal of this study is to understand the mechanisms by which plants may be able to repair air-filled conduits such that they are able to reuse them in water transport. It is only in the past decade that researchers have realized that plants can reverse embolism, and there is some evidence that this repair may occur even during the day when the water in the xylem is under substantial tension. The mechanisms by which embolisms are repaired, however, remain poorly understood. This proposal outlines studies to understand this key physiological process as well as to explore factors limiting the capacity for repair in different species. A major objective of the proposed work is to use magnetic resonance imaging to monitor the dynamics of embolism and embolism reversal in intact plants. The major limitation to date in understanding embolism repair has been the lack of any method to examine this process in vivo. In addition, the PIs will use a suite of new approaches that allow the determination of hydraulic properties of individual water transport conduits. Water availability is a major factor limiting agricultural production and plays an important role in determining ecological interactions in both agricultural and natural systems. This study will increase our understanding of how plants survive periods of low water availability and improve our ability to predict plant responses to environmental factors such as drought and temperature extremes.