Transcript Lecture 5
Mar. 1stst, 2011 B4730/5730 Plant Physiological Ecology Organ Water Relations 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 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 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 Katsuhara et al Funct Pl Bio 2008 Sunflower Mesophyll Cells Hydrated and Dehydrated (Kramer and Boyer 1995) Chara corallina Ye et al. PCE 2005 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 Trotel et al Plant Science 1996; Brassica rapa leaf discs (RLD) 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 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 Agave deserti; North et al. 2004 PCE Root Water and Aquaporins Transport Agave deserti; North et al. 2004 PCE 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 Niinemets Ecology 2001 Glyricidia sepium Brodribb & Holbrook Plant Phys. 2003 Brodribb & Holbrook Plant Phys. 2003; dotted lines 80% and 20% maximum gs Salt Effects Phillyrea latifolia; Tattini et al. 2002