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Water Movement in Plants: Understanding Transpiration Process

Explore the intricate process of transpiration in plants as water moves from soil to air through plant tissues. Learn about the route, forces, and special properties involved. Discover the role of energy and water potential in driving this essential process. Understand how cells, xylem, and membranes contribute to water movement, and the phenomenon of capillary action. Delve into the consequences of capillary action failure and its effects on air-filled xylem cells. Follow the fascinating research journey of Dr. Mel Tyree, a prominent scientist in plant physiology.

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Water Movement in Plants: Understanding Transpiration Process

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  1. Transpiration UNI Plant Physiology 2008 www.uni.edu/berg

  2. Water moves through plants Into the air Through the plant From soil to plant www.uni.edu/berg

  3. Key questions • What is the route? (cell types) • Why does it move? (Energy source) • Whole system (soil/plant/air) • Locally (cell to cell) • What are the forces involved? • What special properties are involved? • Special properties of water • Special properties of the plant • How can the plant control it? www.uni.edu/berg

  4. The route From leaf cells to air From leaf xylem to leaf cells From stem xylem to leaf xylem From root xylem to stem xylem From root cells to root xylem From soil to root cells www.uni.edu/berg

  5. What drives this? (Energy) • Differences make things move • Also need a route • For water in plants • Water potential (pressure and solute effects) differences move water across membranes • Pressure differences cause bulk flow in xylem • Evaporating water to air • Water potential differences again www.uni.edu/berg

  6. Overall water movement • Water potential • Air < plant <soil • Water flows “downhill” energetically • From higher to lower water potential • From soil to plant to air • Difference between soil and air drives transpiration • Plant provides the conduit www.uni.edu/berg

  7. Cell-to-cell movement • Live cells (with central vacuole) • Membranes present • Water potential difference drives movement • Live cell and xylem “cell” • Membrane present • Water potential difference drives movement • Xylem “cells” • NO Membrane present • Pressure potential difference drives movement www.uni.edu/berg

  8. Air, wall & tracheid Xylem cell wall Air in space between this cell and next cell Interface between xylem sap and air Xylem sap in pore in wall Xylem sap inside xylem “cell” www.uni.edu/berg

  9. Capillary action holds water in Well watered plant—not much tension on water in tracheid Plant starts to dry—more tension on water in tracheid Drier still—more tension Quite dry—more tension www.uni.edu/berg

  10. Failure of capillary action • Tension in sap (water) pulls air/sap interface closer toward interior of “cell” • If tension is too great, the air/sap interface is pulled out the bottom of the capillary pore • The air quickly expands to fill the tracheid www.uni.edu/berg

  11. Failure of capillary action Very fast, and makes a popping sound (too high frequency to hear) www.uni.edu/berg Air-filled xylem “cell”

  12. Sounds of air entry • Dr. Mel Tyree • University of Toronto, then University of Vermont, then US Forest Service • Also discovered the maple sap story www.uni.edu/berg

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