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Plant Physiology -- 01/24/08
Lecture 4
Today
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remember to check the weblog, do the reading, first quiz Friday
after you do them, check the answers on web for easy problem set (#2)
first problem set (mostly about solutions) due today (in lab is OK)
download Favorite Water Relations stories from web (required, not on this
quiz, but on first test)
water potential (water activity on the scale centered on zero): terms,
examples
o adding pressure (+ and -), solutes
o what happens when we do both
o moving water between cells and in pipes
see how hydrophilic solids affect water activity (potential)
What water potential tells us
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overall chemical activity of water potential in the system
compared to that of pure water (reference = 0)
takes effects of solutes into account (always lower WP)
takes pressure into account (+ or -)
all potential given in same units: bars or MPa
We have looked at 2 processes
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bulk flow, water potential driven movement across membranes
bulk flow
o moves everything together by pressure differences
o goes through tubes with no membranes in the way
o important in all macro-organism biology
o good way to move lots of material fast and far
across membranes (biological or dialysis)
o requires difference in water potential for movement
o involves both pressure and solute effects
o process of solute-driven water movement called osmosis
o why did you learn about this before without thinking of water potential?
 didn't pay attention to process, just to the inputs, structures,
conditions, outputs
also ignored negative pressure to make life simple
 but pressure counts, some in animal bodies, and especially in
plant bodies (much more pressure + and -)
why are we learning about water potential now?
 physiology is about the processes, which require understanding
the energy to do them
 typical plant water movements employ both pressure and solutes
 osmosis is driven by (energy) water potential differences, not
concentration differences
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o
Osmosis PPT here, starting with #7
Reconciling units – osmotic pressure -- think osmotic (=solute) potential and
reverse sign
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solute potential of water is from the solvent’s point of view (more solute =
more reduced water activity)
osmotic pressure is named from the solute’s point of view (more solute =
more increased osmotic activity)
 there is no actual pressure there (unless it develops)
magnitude of both goes up when there are lots of solutes
o when there are lots of solutes, SP goes down (negative – water tied up)
o when lots of solutes, OP goes up (positive – much solute activity)
same phenomena, just different conventions and perspectives
plant scientists focus on water movement & water energy, thus solute
potential of (effect on) water
remember the difference between magnitude and sign
better yet, stick with solute potential (=osmotic potential)
How can we convince ourselves that water can be under tension?
 clay cup on transpiring tree story
 Wei's pressure probe studies
 boat propellers
 the sounds
Take home messages on why and where water moves
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pressure and solute differences can move water
water potential is influenced by both pressure and solutes
solute and pressure effects on water add up
none of these ideas are specific to plants, just well developed for them
Time to get real
 typical live cells with negative SP, positive PP, negative WP
 xylem (pipes, dead cell walls) with SP near zero, PP usually negative
(sometimes positive)
 Summary sheet in course manual
Put some plant tissue in water (lab)
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if tissue water potential is < zero (neg), water will move into tissue
if tissue water potential is zero, no net movement (molecules move both
ways), tissue is at equilibrium for water potential
tissue water potential is never naturally positive in plants in nature--outside
pressures not positive, and solutes subtract
Thai radish flowers – example of applied water relations
Historically many alternative terms of this equation
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Greek letter version (modern) Ψw = ψp + ψs
(pronounced "sigh"--should
look like pitchfork)
biophysicists gave us this (modern) system: consistent terminology
WP
= PP + SP
also found: PSI = P - B (write under standard)
various combinations
have to be careful reading literature
Special case--the maple syrup story (download from website)
Another way we can lower water potential
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clay particles (or cotton) and charges
water stuck to clay particles
water tied up--reduces the water potential at clay surface (only)
loose water is unaffected—no change in water potential
different from solutes, which have an effect distributed throughout the solution
Stir some clay into water
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no effect on bulk solution--only at clay particle surface
not dissolved, most water unaffected
we can remove "free water" by evaporation
get to a little water and lots of clay
can't remove remaining water easily--tied to clay
NOW surface forces are important -- reduce water activity
we call this factor matric potential (from "matrix")
When is matric potential important?
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not when there is liquid water present (=0)
only a factor in nearly-dry systems (all water attached to matrix or to attached
water attached to matrix)
seeds--very dry, with little free water
external surfaces of cell walls, where they contact air
soils--the major factor controlling water potential (PP=0, SP usually about 0)
In English
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surfaces always tie up water
this lowers water potential
matric potential ranges between 0 (wet) and a very low value (-1000 bars, for
instance)
pressure potential and solute potential are distributed through solutions
matric potential only acts near solid surfaces -- nearby loose water not
affected
matric potential is a combination of direct and indirect surface effects
Building up the water potential equation one more level
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WP = PP + SP + MP
each factor acts similarly (-2 = -2)
water doesn’t know which thing is affecting it
Greek letter equivalent PSIw = psip + psis + psim