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Water Relations In Plants

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Water Relations In Plants

  1. 1. Plant Ecophysiology 3. Water Relations in Plants; Dr. Ahad Madani Ph.D in plant Physiology Madani_ahad@yahoo.com +989127108119 Telegram: @DrAmadani
  2. 2. BISC 367 The Importance of Water • Physiological aspects
  3. 3. Turgor of living cells changes depending on solute concentration and total water potential.(.1M, or -0.244 MPa) cell cell solution
  4. 4. Reducing cell volume concentrates solutes and reduces YS.
  5. 5. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 36.3
  6. 6. BISC 367 Yp is sensitive to small changes in cell volume • Relates to rigid cell wall, illustrated by Hofler diagram – Plot of Yw & its components against relative cell vol. • Initial drop in cell vol (5%) is accompanied by a sharp drop in Yp and Yw • As cell vol falls <90%, decreased Yw is accounted for by a lowered Ys as [solute] increases
  7. 7. BISC 367 Yp is sensitive to small changes in cell volume • Slope of Yp curve yields the volumetric elastic modulus (e) – e is a function of the rigidity of the cell wall – High value indicates a rigid wall for which a small vol. change translates into a large drop in Yp – e decreases as Yp falls b/c walls are rigid only when Yp is high
  8. 8. • Lateral transport of minerals and water in roots Figure 36.9 1 2 3 Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls. Minerals and water that cross the plasma membranes of root hairs enter the symplast. As soil solution moves along the apoplast, some water and minerals are transported into the protoplasts of cells of the epidermis and cortex and then move inward via the symplast. Within the transverse and radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks the passage of water and dissolved minerals. Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder. Endodermal cells and also parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The xylem vessels transport the water and minerals upward into the shoot system. Casparian strip Pathway along apoplast Pathway through symplast Plasma membrane Apoplastic route Symplastic route Root hair Epidermis Cortex Endodermis Vascular cylinder Vessels (xylem) Casparian strip Endodermal cell 4 5 2 1
  9. 9. 16 H2O and minerals Endodermis Xylem Phloem Casparian strip Cell membrane Endodermal cell apoplastic route symplastic route H2O and minerals H2O and minerals H2O and minerals Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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  11. 11. Water molecule Root hair Soil particle Water Water uptake from soil The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution Cohesion and Adhesion in the Ascent of Xylem Sap
  12. 12. Adhesion by hydrogen bonding Cell wall Xylem cells Cohesion by hydrogen bonding Cohesion and adhesion in the xylem Transpirational pull is facilitated by cohesion of water molecules to each other and adhesion of water molecules to cell walls
  13. 13. Fig. 36-15c Xylem sap Mesophyll cells Stoma Water molecule Atmosphere Transpiration Drought stress or freezing can cause cavitation, the formation of a water vapor pocket by a break in the chain of water molecules
  14. 14. Fig. 36-15 Outside air ψ = −100.0 Mpa Leaf ψ (air spaces) = −7.0 Mpa Leaf ψ (cell walls) = −1.0 Mpa Trunk xylem ψ = −0.8 Mpa Trunk xylem ψ = −0.6 Mpa Soil ψ = −0.3 Mpa Xylem sap Mesophyll cells StomaStoma Water molecule Transpiration Atmosphere Adhesion by hydrogen bonding Cell wall Xylem cells Cohesion and adhesion in the xylem Cohesion by hydrogen bonding Water molecule Root hair Soil particle Water Water uptake from soil Waterpotentialgradient
  15. 15. • Ascent of xylem sap Xylem sapOutside air Y = –100.0 MPa Leaf Y (air spaces) = –7.0 MPa Leaf Y (cell walls) = –1.0 MPa Trunk xylem Y = – 0.8 MPa Waterpotentialgradient Root xylem Y = – 0.6 MPa Soil Y = – 0.3 MPa Mesophyll cells Stoma Water molecule Atmosphere Transpiration Xylem cells Adhesion Cell wall Cohesion, by hydrogen bonding Water molecule Root hair Soil particle Water Cohesion and adhesion in the xylem Water uptake from soilFigure 36.13
  16. 16. BISC 367 Measuring Yw A leaf or shoot is excised and placed in the chamber • Cutting the leaf breaks the tension in the xylem causing water to retreat into the surrounding cells Pressurizing the leaf chamber returns water to the cut surface of the petiole • The amount of pressure to return water to the cut surface equals the tension (Yp) present in the xylem (but is opposite in sign) before excision Values obtained approximate the tension in the xylem and are used as a measure of Yw • Strictly speaking to know the actual Yw some xylem sap should be collected Scholander’s pressure bomb From Plant Physiology on-line (http://4e.plantphys.net/)
  17. 17. Fig. 10.10 Phloem solution moves along a gradient of pressure generated by a solute concentration difference between source and sink ends of the pathway
  18. 18. Sugar is loaded at sources and unloaded at sinks... consuming ATP (energy) source: leaf sink: root, flower, emerging CB 36.20
  19. 19. BISC 367 Movement of water into a plant cell occurs by osmosis • 2 mechanisms: – Diffusion across the membrane – Bulk flow across aquaporins (water filled pores)
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