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Water Metabolism in Plants: Roles, Movements, and Absorption

Discover the critical roles of water in plant metabolism, including movement mechanisms such as diffusion, bulk flow, and osmosis, as well as the absorption pathways in roots through the apoplast, symplast, and transmembrane. Learn about the driving forces of root water absorption, including root pressure and transpirational pull, and the significance of water potential gradients in passive and active absorption processes. Explore the essential concepts of water potential and osmotic potential within plant cells and understand the importance of turgor pressure for plant growth and development.

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Water Metabolism in Plants: Roles, Movements, and Absorption

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  1. Chapter 2 Water Metabolism

  2. Plant Water Relations • For every g of organic matter made by the plant, approximately 500 g of water is transpired by the plant. • Leaves loose up to ~ 100% of their water/hr.

  3. Amounts of water in plants • Living plants =~ 80 to 95% water. • Vegetables =~ 85 to 95% water • Sapwood =~ 35 to 75% water • Seeds =~ 5 to 15% water

  4. Roles of Water in Plant cells • Medium for biochemical reactions. • Physical support. • • Turgor pressure. • Thermal regulation. • Transport of water, nutrients and other molecules.

  5. Status of water in plants • bound water • free water • Cytoplasm • Protein = 60% • Hydrophilic • Hydrophobic

  6. How Does Water Move? • – Passive Processes • Diffusion - down a concentration gradient. • Bulk Flow - down a pressure-driven gradient. • Osmosis - down both a concentration & pressure driven gradient (= water potential). • • Across a selectively semi-permeable membrane.

  7. Diffusion • Diffusion - directed movement - high to low concentration (higher to lower free energy). • Random thermal motion. • Significant role in: • • short-distance movement

  8. Bulk Flow = Mass Flow • The concerted movement of groups of molecules by mass, most often in response to a pressure gradient. • Examples: • • Water moving through the xylem. • • A river flowing, rain falling. • Controled by : • • Aquaporin (水孔蛋白）

  9. A B A. Diffusion B．Bulk flow

  10. Osmosis • Movement of water across a selectively semi-permeable membrane in response to chemical and pressure gradients (together = water potential). • • Selectively semi-permeable membrane • prevents exchange of solutes, but • allows passage of water.

  11. Concept • Water Potential • Water potential = the free energy associated with water = potential for doing work. • Water moves down a water potential gradient (from higher to lower potentials) = gives up energy as it moves.

  12. Water Potential • Water potential is not an absolute value. It is relative to a reference state. • • Reference state = pure water at ambient • temperature and pressure. • The water potential within a cell is usually less than the reference state.

  13. Plant cell—osmotic system • The Plant Cell • • Cell wall –permeable • • Membrane and protoplasm – semi-permeable.

  14. If a cell is placed in a: • Hypotonic Solution (lower solute concentration) – water rushes in and generates turgor pressure. • Hypertonic Solution (higher solute concentration) - water flows out of the cell. • Isotonic Solution (equal solute concentration) - equilibrium.

  15. Concepts • Plasmolysis (质壁分离)- protoplast shrinks away from the cell wall. • • Turgor pressure = 0

  16. Water Potential of the plant cell

  17. Osmotic Potential • Ψs = - RTCs • Ψs = osmotic potential. • R = the gas constant (0.00832). • T = temperature. • Cs = solute concentration (mol L-1). dissolved solutes Ψs

  18. Pressure Potential • Cell expands with water and pushes the cell membrane against the rigid cell wall => pressure. • • Turgor pressure = positive hydrostatic • pressure inside cells pressing against the • cell wall. • Water continues to enter the cell until pressure counteracts the negative osmotic pressure.

  19. Water Movements in Cells • Water moves in and out of cells because of differences in water potential between the cell and the surrounding solution. • Water moves from cell to cell in the same manner.

  20. Water movement from cell to cell

  21. Root Water Absorption Root Hairs • Thin-walled outgrowths of epidermal cells. • Provide a large root surface area and close contact between root & soil. • Greatly increase contact of root with soil H2O. • Small diameter permits them to penetrate capillary spaces.

  22. Vertical section of root tip

  23. Transversal section

  24. Root Water Absorption • Pathways • Apoplast • Transmembrane • Symplast • Driving force • Root pressure • Transpirational pull • Soil factors affecting root water absorption • Available water • Aeration conditions • Temperature • Solute concentration

  25. 3 Pathways: Water Movement Inside the Root • Via the Apoplast – e.g. through cell walls and intercellular spaces. • Via the Symplast – e.g. passing from cell to cell via the plasmodesma. • Via Transmembrane Pathway – e.g. through membranes (cell membrane and vacuole membrane).

  26. Apoplast • Symplast • Apoplast and Symplast pathway

  27. Transmembrane Pathway

  28. Driving force for root water absorption Active absorption • Root Pressure • Roots actively absorb and transport ions into the xylem. • As solutes build up, water potential decreases and water flows into the cells => increases pressure = Root pressure.

  29. Bleeding because of root pressure

  30. Driving force for root water absorption Water potential gradient Transpirational pull :Passive absorption • Transpiration:Evaporated Water moves from the leaf to the outside. • Primarily through the stomata. • Water potential gradient from leaf to root

  31. Effects of Soil on Root Water Absorption: Available water • Sand: low • • Large Diameter • = 20-2000 µm. • • Low Surface Area • = <1-10 m2. • • Large spaces • between particles • =water drains. • • Lower Field • Capacity. • Clay: high • • Small Diameter • =<2 µm. • • Large Surface Area • =100-1000 m2. • • Small spaces • between particles • =holds water. • • Greater Field • Capacity.

  32. Effects of Soil on Root Water Absorption: Aeration conditions • CO2 • • anaerobic respiration • • alcoholism • • water absorption inhibited

  33. Low T • • Viscidity • • Respiration • • Growth • • Enzyme • High T • • Aging • • Enzyme Effects of Soil on Root Water Absorption: Temperature

  34. Low solute • • Water potential • High solute • • Water potential water absorption inhibited Effects of Soil on Root Water Absorption: Solute concentration

  35. Transpiration

  36. Absorbed water by plant Metabolism Loss 95%—99% 1％—5％ Through: 1) Liquid state (e.g. bleeding, guttation） 2) Evaporated state (transpiration) Major way

  37. Transpiration (蒸腾作用) • The loss of evaporated water from the plant to the outside air via diffusion. • • Most is lost through the stomata (>90%). • • But also through the cuticle (表皮), lenticels (皮孔), etc (5-10%).

  38. Significance of transpiration • Responsible for the movement of water and nutrients through the plant. • Cooling of the leaves. • Improving gas exchange.

  39. Dicot Monocot

  40. Guard Cells • Take up water (cells swell) = opens pore. • Lose water – closes pore.

  41. Mechanisms for stomatal movement • Starch–sugar interconversion • Potassium ion uptake • Malate production

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