Water in the plant and in the environment Bio 164/264 January 23, 2007 C. Field Why do land plants lose water? How do plants manage the compromise between water loss and CO 2 uptake? How does water move in the environment? How does water get to the top of tall trees? How do plants refill cavitated xylem? Readings for this week 1/23: Campbell & Norman, ch 3,4,9 1/25: Campbell & Norman, p

Gradients of water and CO 2 CO 2 –Outside~380 ppm –Inside~230 ppm –Gradient~150 ppm Water –Inside~3 kPa = 30,000 ppm –Outside~1 kPa = 10,000 ppm –Gradient~2 kPa = 20,000 ppm Ratio20,000/150 = 133 –Diffusion penalty~1.5 (Graham’s law) –Max water use efficiency = 1 CO 2 /200 H 2 O

Fick’s Law – what drives diffusion? Flux = gradient* conductance = gradient/resistance conductance = 1/resistance Conductance: defined as the proportionality constant between flux and concentration gradient E = g w  w = g w (w i -w o ) =  w/r w = (w i -w o )/r w A = g c  c = g c (c a -c i ) =  c/r c = (c a -c i )/r c E/A = g w  w/g c  c = 1.5  w/  c A/E =  c/(1.5  w)

Where does water move? 1.downhill 2.from regions of higher pressure to regions of lower pressure 3.from regions of lower solute concentration to regions of higher solute concentration 4.From regions where it is stuck less tightly to regions where it is stuck more tightly

The chemical potential of water –Defined relative to pure water at a reference height --  t = 0 –  p = pressure potential –  o = osmotic potential –  m = matric potential –  g = gravitational potential

Gravitational potential (  g ) How much pressure (force/area) is exerted by a water column of height h? How hard would you need to push at the bottom to hold a water column of that height? How hard would you need to pull at the top? –  g =  gh where  = density = 1 g cm -3 or 1000 kg m -3, g = 9.8 m s -2

A brief diversion – units n is a unit of force = massacceleration (kgms -2 ) Pa is a unit of pressure = force/area (nm -2 ) J is a unit of energy or work = force distance (nm) Atmospheric pressure –  14.7 psi  1 bar = 10 5 Pa = 0.1 MPa Therefore, atmospheric pressure is sufficient to lift a water column 10 m. Water to the top of a 100 m tree has a gravitational potential of 1 MPa! Energy vs pressure units? –C&N express P in J kg -1 to emphasize the connection with free energy –Note that J m -3 has the same dimensions as pressure –For water, 1000 kg = 1 m 3

Pressure potential Positive hydrostatic pressure drives water outward

Osmotic potential (  o ) -- the van't Hoff relationship C = concentration (mol m -3 )  = osmotic coefficient = ions per molecule R = Universal gas constant = J mol -1 K -1 T = temperature (k) Osmotic potential of sea water: about 480 mM Na + and 560 mM Cl -

Matric potential How tightly is the water stuck to the soil? –Water in the smallest crevices is held the tightest –  m = aw -b

Water potential in the gas phase? Work required to create a volume dV of water vapor –dU = dQ – pdV From the ideal gas law, we know – Substituting for V – Integrate from e = e s to e – Since  = energy per volume,  = U/nV w –V w = molal volume = 18 ml mol -1 = m 3 mol -1

How does water get to the to the top of tall trees? The atmosphere is typically very dry, with respect to the soil/plant system –There is essentially always a potential moving water from the liquid phase inside plants to the gas phase outside. RH  w (MPa)

Water has considerable tensile strength if 25% of H bonds intact, strength ~ 30 MPa ~ 10% the strength of copper or aluminum Water has plenty of tensile strength to be “pulled” to the top of a tree under tension. But if the column breaks (cavitates) the strand is broken, and the water won’t move.

Cavitation Structural details Isolation Refilling Hydraulic architecture

<- Raven, 1998 ^

All images from James D. Mauseth 1998 Botany

Raven et al., Fig 31-7,8 primary wall secondary wall

Healthy plant cells are under positive hydrostatic pressure

Strategies for dealing with water stress Avoidance Tolerance Forage for additional resources

Hydraulic lift Roots can move water from wet to dry layers of the soil

Rooting depth

Measuring water potential Pressure bomb – Scholander et al Thermocouple psychrometer Pressure plate

For your reading pleasure Scholander, P. F., H. T. Hammel, E. D. Bradstreet, and E. A. Hemmingsen, 1965: Sap pressure in vascular plants. Science, 148, Zwieniecki, M., P. Melcher, and N. Holbrook, 2001: Hydrogel control of xylem hydraulic resistance in plants. Science, 291, Angeles, G., B. Bond, J. S. Boyer, T. Brodribb, J. R. Brooks, M. J. Burns, J. Cavender-Bares, M. Clearwater, H. Cochard, J. Comstock, S. D. Davis, J. C. Domec, L. Donovan, F. Ewers, B. Gartner, U. Hacke, T. Hinckley, N. M. Holbrook, H. G. Jones, K. Kavanagh, B. Law, J. Lopez-Portillo, C. Lovisolo, T. Martin, J. Martinez- Vilalta, S. Mayr, F. C. Meinzer, P. Melcher, M. Mencuccini, S. Mulkey, A. Nardini, H. S. Neufeld, J. Passioura, W. T. Pockman, R. B. Pratt, S. Rambal, H. Richter, L. Sack, S. Salleo, A. Schubert, P. Schulte, J. P. Sparks, J. Sperry, R. Teskey, and M. Tyree, 2004: The Cohesion-Tension theory. New Phytologist, 163, Sack, L., and N. M. Holbrook, 2006: Leaf hydraulics. ANNUAL REVIEW OF PLANT BIOLOGY, 57, Hacke, U. G., J. S. Sperry, J. K. Wheeler, and L. Castro, 2006: Scaling of angiosperm xylem structure with safety and efficiency. Tree Physiology, 26, Pittermann, J., J. S. Sperry, U. G. Hacke, J. K. Wheeler, and E. H. Sikkema, 2005: Torus-margo pits help conifers compete with angiosperms. Science (Washington D C), 310, Sperry, J. S., 2004: Coordinating stomatal and xylem functioning - an evolutionary perspective. New Phytologist, 162, Sperry, J. S., and U. G. Hacke, 2004: Analysis of circular bordered pit function - I. Angiosperm vessels with homogenous pit membranes. American Journal of Botany, 91, Holbrook, N. M., M. J. Burns, and C. B. Field, 1995: Negative xylem pressures in plants: A test of the balancing pressure technique. Science, 270, Holbrook, N. M., and M. A. Zwieniecki, 1999: Embolism repair and xylem tension: Do we need a miracle? Plant Physiology, 120, 7-10.