1. Modeling current-voltage (I/V) characteristics of salt-tolerant charophyte Lamprothamnium in steady state and at the time of turgor regulation Mary J Beilby and Virginia A. Shepherd School of Physics, Biophysics, The University of NSW, Sydney Australia

2. Materials and Methods amplifier Membrane PD Voltage commands Membrane current Current – voltage analysis 1sec 0 PD 200 mV 0 current 4  A

3. Modeling 6 cells in 1/3 seawater: pump state i background = g background (V – V background ) 7 cells in background state

4. 0.2 SW 77.0 2.0 86.0 140.5 Medium Na + K + Cl - osmolarity mM mM mM mOsmol.kg -1 0.4 SW 154.0 4.1 172.0 281.0 0.5 SW 175.0 8.0 201.0 536.0 full SW 350.0 16.0 400.0 1072.0 Steady state I/V characteristics in range of salinities For details see: Beilby and Shepherd, 2001, J. Membrane Biol. 181: 77

5. Hypertonic regulation: from 0.2 SW to 0.4 SW time: 5 min 21 min 41 min 2 hr 34 min 3 hr 30 min

6. Modeling 0.5 SW, 9 mM K + : cell in K + state i background = g background (V – V background )

7. Hypotonic regulation: 1/3 SW to 1/6 SW cell in K + state, Cl - current inhibited by La 3+ time: 3 min 10 min 12 min 15 min 22 min 85 min (For details see: Beilby and Shepherd, 2001, Aust. J. Plant Physiol. 28: 541- 550 )

8. Conclusions Steady state conductance rises with salinity mainly due to i background, V background is not affected by salinity Hypertonic regulation turns on i irc before i pump i irc is carried by K +. Hypotonic regulation turns on i K independently of i Cl The increase in [K + ] cyt precedes rise in N K P K V background depolarizes and g background increases at the time of hypotonic regulation The proton pump works harder to keep the membrane PD negative in more saline media Future work Transporter for i background ? H + channels? Stretch activated channels? Is i irc the detector of hypertonic stress? Electroneutrality ? Signal for tonoplast K + channels to open ? H + channels involved in hypotonic regulation? What is the limiting pump conductance? This presentation can be found on http://www.phys.unsw.edu.au/~mjb/