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