Water makes up ~95% of a plant’s volume, and in non-woody tissues, 99%. Water is important for: dissolving and transport of all nutrients into and around.

Water makes up ~95% of a plant’s volume, and in non-woody tissues, 99%.

Water is important for:• dissolving and transport of all nutrients into and around the plant body (circulation)• cooling the plant by evaporation from surfaces• hydrostatic structure

plants ‘wilt’ when theylose turgor pressure:turgor develops when wateris attracted osmotically intocells but cell walls push backon entering water, creatingturgor

In plants: water moves passively down potential energy gradients.

Over short distances (cell-cell), water moves mainly by diffusion along water potential gradients established by osmosis and resultant turgor.

Over longer distances (through xylem, for instance) water moves by bulk flow, driven by pressure gradients.

For solutions separated by semi-permeable membranes:

Osmosis is the diffusion of water through a semi-permeable membrane. More specifically, it is the movement of water across a semi-permeable membrane from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration). It is a physical process in which a solvent moves, without input of energy, across a semi-permeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations. Osmosis releases energy, and can be made to do work.

For plant cells, cells are bound by cell walls that are rigid, and resist changes in volume. As a result, water potential of a cell (Yw) is determined not only by solute

potential (Ys), but also by pressure (Yp)!

Yw = Ys + Yp

(a) Solute potential is the tendency of water to moveby osmosis.

(b) Pressure potential is the tendency of water to move inresponse to pressure.

Solute potential insidecell and in surroundingsolution is the same.No net movement of water.

Cell is placed in pure water.The cell’s solute potential islow relative to its surroundings.Water moves into cell via osmosis.

Solute Pure water

Cell

Watermovement

Isotonic solution Hypotonic solution

Inside of cell

Turgor pressure

Plasma membrane

Expanding volume of cellpushes membrane out.

Turgor pressure isan important sourceof pressure onwater in cells

Cell wall

Wall pressure

Stiff cell wall pushes back withequal and opposite force.

Outside of cell

(a) Solute potentials differ.

(b) Solute and pressure potentials differ.

Water moves left to right—from area with high waterpotential to area with lowwater potential

Water moves into cell—from areawith high water potential to areawith low water potential

Water potentials are equal—no net movement

Water potentials are equal—no net movement

Pure water Solution Pure water Flaccid cell

0 MPa 0 MPaP S

0 MPa1.0 MPa

1.0 MPa

P S

0 MPa1.0 MPa

1.0 MPa

Pure water Solution Pure water Turgid cell

0 MPa 0 MPaP S

1.0 MPa1.0 MPa

0.0 MPa

P S

1.0 MPa1.0 MPa

0.0 MPa

This is a diagram of anosmometer. The two sidesare separated by a semi-permeable membrane.Plant cells operate like osmometers with walls pushing back on water tryingto enter

Stomata open when turgor pressure increasesin the guard cells. • guard cells attract water into them by lowering their Yw

• the ‘glue’ between guard cells has been digested away, and the remaining wall is very stiff, causing turgid guard cells to bow out, opening the pore.

Please see “cell water potentialworksheet” posted on our web site

Factors causing stomata to open or close:

Open Close

Light Lack of light (dark)

Low CO2 level High CO2 level

Circadian rhythm Circadian rhythm

Toxins (e.g. fusicoccin) Hormones (e.g. auxin)

Abscisic Acid

Humidity

How?for opening:

1 – proton pump (H+ ATPase) hyperpolarizes cell (Em – 180 mV) acidifies the cell wall space2 – potassium (K+) is taken up passively3 – anions are taken up by H+/A- symport4 – cell Ys and Yw becomes more negative5 – cell takes up water, turgor goes up

for closing:

inhibit pumpopen anion channelslose soluteslose turgor

Water moves from soil through the plant to the atmosphere --- passively --- movingdown a water potential gradient.Over short distances, water moves cell-cell by osmosis (across semipermeable membranes). Much of the long-distance flow is by bulk flow (through volumes without membranes) driven by pressure gradients.

The pathway of transpirational water flow is throughthe xylem, a complex tissue making up part of the vascularbundles (veins). Phloem is the other tissue in the veins.

Xylem is a complex tissue; it contains several other tissues, each comprised of specializedcell types.

conducting tissue – tracheids or vessel elementsparenchyma – parenchyma cellssupport tissue -- fibers

When plants are transpiring, the water in the xylem is under tension.

When transpiration is not happening (at night,or in very humid conditions) the vascular parenchyma in roots and stems can develop‘root pressure’ which appears as ‘guttation’that is, droplets of water exuding from the ends of veins in leaves.

A ‘pressure bomb’ will measurehow much pressure it takes to makethe sap return to the petiole surface;this is called the balancing pressure, andis opposite in sign to the tension thatexisted before the leaf was cut.