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Vessel contents during transpiration - embolisms and refilling
American Journal of Botany
|June 29, 2011
Summary
Plant xylem vessels can refill themselves with water during active transpiration, challenging previous theories. This study shows plants actively manage water refilling based on water stress levels, suggesting a new understanding of plant hydraulics.
Area of Science:
- Plant Physiology
- Plant Hydraulics
- Xylem Function
Background:
- Previous observations suggested embolized plant vessels refill during active transpiration.
- The Cohesion Theory posits that water columns in xylem are under tension and break easily.
- This study investigates the dynamics of vessel embolism and refilling in sunflower plants.
Purpose of the Study:
- To test the unexpected observation of vessel refilling during active transpiration.
- To examine the relationship between water potential, transpiration rate, and vessel embolism.
- To evaluate the validity of the Cohesion Theory and support the Compensating Pressure Theory.
Main Methods:
- Sunflower petioles were snap-frozen at 2-hour intervals during transpiration.
- Cryo-scanning electron microscopy was used to assess vessel contents (liquid or gas/embolism).
- Concurrent measurements included irradiance, leaf temperature, transpiration rate, and leaf water potential.
Main Results:
- Vessel embolism varied significantly throughout the day, from 40% at 0900 to 4% at 1500.
- Water was observed being extruded through vessel pits, indicating active refilling.
- Embolisms were repaired on a short timescale (minutes) and adjusted by the plant on a longer timescale (hours) based on water stress.
Conclusions:
- Plant water columns break under relatively small tensions, contradicting the Cohesion Theory.
- Embolized vessels are actively refilled throughout the day, with refilling vigor adjusted by water stress.
- The pressure chamber measurement reflects the plant's response to water stress (refilling vigor), not tension itself, supporting the Compensating Pressure Theory.
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