1. Skotomorphogenesis Seed germination Genes and enzymes Embryo and Seed development Plant life cycle Photomorphogenesis Photoreceptors Phytochrome Cells and cell growth Phytochrome: regulation of light responses Photosynthesis: light reaction Photosynthesis: carbon fixation Photorespiration Primary & secondary metabolism Respiration Mineral Nutrition Water Physiological Properties of Water HORT 301 – Plant Physiology October 14, 2009 Taiz and Zeiger – Chapter 3, Chapter 15 (p. 364-372) paul.m.hasegawa.1@purdue.edu

2. Water (H 2 O) - most limiting plant resource

3. Molecular structure of water

4. Interaction (hydrogen bonding) between water molecules

5. Properties of water Biochemical solvent Temperature buffering and cooling High specific heat - thermal energy required to raise temperature of a substance High latent heat of vaporization – thermal energy required for molecular state change from the liquid to the gas phase Cohesion and tensile strength

6. Adhesion – attraction of water molecules to a solid phase Surface tension – negative pressure (pull) created at the water-air interface

7. Capillarity – water movement up xylem vessels

8. Osmosis – diffusion of water across a semi-permeable membrane Water transport into and out of cells

9. Water diffuses across a plant membrane from high to low water concentration Osmosis is facilitated by water channels (aquaporins)

10. Osmosis is driven by the water potential (Ψ w ) gradient Osmosis: higher → lower water concentration (lower → higher solute concentration) higher → lower Ψ w (more negative)

11. Water potential (Ψ w ) components - Ψ w = Ψ s + Ψ p Ψ w (water potential) Ψ s (solute (osmotic) potential) – solute concentration effect on Ψ w van’t Hoff equation, Ψ s = -RTc s Ψ p (hydrostatic pressure/pressure potential/turgor pressure)

12. Solute effects on water potential (Ψ w ) of a solution Solution of 0.1 M sucrose - Ψ w = -0.244 MPa Ψ s = RTc s = -0.244 MPa, Ψ p = 0 MPa -0.244 MPa (Ψ w ) = -0.244 (Ψ s ) + 0 (Ψ p ) Pure water - water potential (Ψ w ) = 0 Ψ w = Ψ s + Ψ p = 0 Ψ s = 0 and Ψ p = 0 Ψ w = Ψ s when Ψ p = 0

13. Water transport is driven by the water potential (Ψ w ) gradient Water moves from lower to higher to lower solute concentration Ψ p drives cell expansion and volume increase is due to uptake of water, Ψ w gradient between the apoplast and symplast (0.1 M sucrose solution)

14. Water movement from the cell – apoplastic water potential becomes more negative

15. Water movement into and out of cells summary: Water moves from higher (less negative) to lower (more negative) Ψ w Ψ w(apolast) = Ψ w(symplast) No water movement Ψ w(apoplast) higher (less negative) than Ψ w(symplast) Water moves into the cell and turgor pressure increases Ψ w(apoplast) lower (more negative) than Ψ w (sympast) Turgor pressure decreases and water moves out of the cell

16. Turgor pressure drives cell expansion PLANT BIOLOGY, Smith et al. Figure 3-66.

17.  w gradient facilitates water uptake for cell volume increase/expansion (fresh weight gain) Turgor and growth rate Turgor Pressure (MPa) Growth rate is defined by the formula: GR = m(  p –Y)

18. Plant water status affects critical physiological functions – water potential (Ψ w ) is a “signal” for numerous physiological processes