Water is important for: 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 they lose turgor pressure: turgor develops when water is attracted osmotically into cells but cell walls push back on entering water, creating turgor Figure 37.4 Wilting Occurs when Water Loss Leads to Loss of Turgor Pressure.

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 move by osmosis. Solute potential inside cell and in surrounding solution is the same. No net movement of water. Cell is placed in pure water. The cell’s solute potential is low relative to its surroundings. Water moves into cell via osmosis. Solute Pure water Water movement Cell Isotonic solution Hypotonic solution (b) Pressure potential is the tendency of water to move in response to pressure. Turgor pressure is an important source of pressure on water in cells Inside of cell Expanding volume of cell pushes membrane out. Figure 37.1 Water Potential Has Two Major Components: Solute Potential and Pressure Potential. Turgor pressure Plasma membrane Cell wall Wall pressure Stiff cell wall pushes back with equal and opposite force. Outside of cell

osmometer. The two sides are separated by a semi- permeable membrane. (a) Solute potentials differ. This is a diagram of an osmometer. The two sides are separated by a semi- permeable membrane. Plant cells operate like osmometers with walls pushing back on water trying to enter Pure water Solution Pure water Flaccid cell   0 MPa P  0 MPa   0 MPa P  0 MPa S  1.0 MPa S  1.0 MPa   1.0 MPa   1.0 MPa Water moves left to right— from area with high water potential to area with low water potential Water moves into cell—from area with high water potential to area with low water potential (b) Solute and pressure potentials differ. Figure 37.2 Solute Potential and Pressure Potential Interact. Pure water Solution Pure water Turgid cell   0 MPa P  1.0 MPa   0 MPa P  1.0 MPa S  1.0 MPa S  1.0 MPa   0.0 MPa   0.0 MPa Water potentials are equal— no net movement Water potentials are equal— no net movement

Stomata open when turgor pressure increases in 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 potential worksheet” posted on our web site

Factors causing stomata to open or close: Light Lack of light (dark) Low CO2 level High CO2 level 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 space 2 – potassium (K+) is taken up passively 3 – anions are taken up by H+/A- symport 4 – cell Ys and Yw becomes more negative 5 – cell takes up water, turgor goes up for closing: inhibit pump open anion channels lose solutes lose turgor

down a water potential gradient. Water moves from soil through the plant to the atmosphere --- passively --- moving down 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. Figure 37.5 A Water-Potential Gradient Exists between Soil, Plants, and Atmosphere.

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

Xylem is a complex tissue; it contains several other tissues, each comprised of specialized cell types. conducting tissue – tracheids or vessel elements parenchyma – parenchyma cells support 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 measure how much pressure it takes to make the sap return to the petiole surface; this is called the balancing pressure, and is opposite in sign to the tension that existed before the leaf was cut. Figure 37.8 Root Pressure Causes Guttation.