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Plant Water Relations: Components and Regulation

Explore the components of water potential and cell water relations in plant physiology, including osmotic and hydrostatic potentials. Learn about vacuoles, symplast, and apoplast water movement mechanisms. Discover adaptation mechanisms like osmotic adjustment and leaf and root anatomy's role in water relations. Understand pressure-volume curves and osmotic effects on plant tissues.

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Plant Water Relations: Components and Regulation

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  1. Mar. 1stst, 2011B4730/5730Plant Physiological Ecology Organ Water Relations

  2. Components of Water Potential • By definition, 0 (units) is water potential of pure water under standard conditions • Osmotic (solute) potential (Ψπ) is the water potential in solution due to dissolved materials • Hydrostatic or pressure potential (ΨP) is the physical pressure potential • Matric potential (ΨM) force with which water is adsorbed onto surfaces, especially soil • Gravity potential (Ψg) force of water due to gravity • Water Potential in energy or pressure units • Ψ = Ψπ + ΨP + ΨM +Ψg

  3. Water Relations of Cells • Ψ in cells can only be regulated by Ψπ and ΨP, but ΨP must be positive • Osmotic regulation through compatible solutes • Changes in cell-wall elasticity • greater elasticity allows more loss of water before ΨP is zero • ε = dV/dΨ • Reflection coefficient δ • Selectivity of a solute • Water moves across membranes faster than predicted by diffusion gradients • aquaporins

  4. Vacuoles, Apoplast, Symplast • Vacuoles moderate cell water relations • Provide water and solute storage • Reservoir • Apoplast is a continuous aqueous connection that does not cross cell membranes • Cell wall included in apoplast • Symplast is a sometimes discontinuous aqueous connection of water that must cross at least one membrane • plasmodesmata

  5. Katsuhara et al Funct Pl Bio 2008

  6. Sunflower Mesophyll Cells Hydrated and Dehydrated (Kramer and Boyer 1995) Chara corallina Ye et al. PCE 2005

  7. Osmotic Adjustment • Cell water balance may be maintained by lowering osmotic (solute) potential • Compatible solutes • Mineral adjustments • Ion charge and size • Organic synthesis, especially proline • Energy and N expensive • Hypotheses for adaptation include N storage and osmotic adjustment

  8. Trotel et al Plant Science 1996; Brassica rapa leaf discs (RLD)

  9. Leaf Anatomy and Water Relations • Water transported to leaves through xylem in veins • Phloem connection • Leaf water potential drives water transport to leaves • Low water potential changes cell biochemistry • Stomates restrict water loss • Do not respond directly to vpd • Unknown water potential signal

  10. Root Anatomy and Water Relations • Tradeoffs between safety and efficiency • Exodermis • Roots hairs, mycorrhizae • Casparian strip • Apoplast and symplast • Transcellular • Endodermis • Root branching • Stele • Secondary growth, cambium • Xylem and phloem • Root cap

  11. Agave deserti; North et al. 2004 PCE

  12. Root Water and Aquaporins Transport Agave deserti; North et al. 2004 PCE

  13. Pressure Volume Curves • Relationship between tissue pressure and volume describe effect of dehydration • Uses Boyle-Mariotte Law that PV=constant • Ψ is substitute for pressure • Extrapolation to Ψπ • Relative Water Content substitute for volume • Graphs drawn with 1/RWC • ε derived from nonlinearity between full turgor and turgor loss point • Osmotic adjustment shifts turgor loss point to lower RWC

  14. Niinemets Ecology 2001

  15. Glyricidia sepium Brodribb & Holbrook Plant Phys. 2003

  16. Brodribb & Holbrook Plant Phys. 2003; dotted lines 80% and 20% maximum gs

  17. Salt Effects Phillyrea latifolia; Tattini et al. 2002

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