THE FUNCTION OF WATER CHANNELS IN CHARA - THE TEMPERATURE-DEPENDENCE OF WATER AND SOLUTE FLOWS PROVIDES EVIDENCE FOR COMPOSITE MEMBRANE-TRANSPORT AND FOR A SLIPPAGE OF SMALL ORGANIC SOLUTES ACROSS WATER CHANNELS

Using the cell pressure probe, the effects of temperature on hydraulic conductivity (Lp; osmotic water permeability), solute permeability (p ermeability coefficient, P-s), and reflection coefficients (sigma(s)) were measured on internodes of Chara corallina, Klein ex Willd., em R. D.W.. For the first time, complete sets of transport coefficients were obtained in the range between 10 and 35 degrees C which provided evid ence about pathways of water and solutes as they move across the plasm a membrane (water channel and bilayer arrays). Test solutes used to ch eck for the selectivity of water channels were monohydric alcohols of different molecular size and shape (ethanol, n-propanol, iso-propanol, and tert-butanol) and heavy water (HDO). Within the limits of accurac y, Q(10) values for Lp and for the diffusive water permeability (P-d) were identical (Q(10) for Lp=1.29 +/- 0.17 (+/- SD; n = 15 cells) and Q(10) for P-d = 1.25 +/- 0.16 (n = 5 cells)). The Q(10) values were eq uivalent to activation energies of E-a = 16.8 +/- 6.4 and 16.6 +/- 10. 0 kJ . mol(-1), respectively, which is similar to that of self-diffusi on or of viscous flow of water. The Q(10) values and activation energi es for P-s of the alcohols were significantly larger (ethanol: Q(10) = 1.68 +/- 0.16, E-a = 37.1 +/- 5.9 kJ . mol(-1); n-propanol: Q(10) = 1 .75 +/- 0.40, E-a = 43.1 +/- 15.3 kJ . mol(-1) iso-propanol: Q(10) = 2 .12 +/- 0.42, E-a = 52.2 +/- 14.6 kJ . mol(-1); tert-butanol: Q(10) = 2.13 +/- 0.56, E-a = 51.6 +/- 17.1 kJ . mol(-1); +/- SD, n = 5 to 6 ce lls). Effects of temperature on reflection coefficients were most pron ounced. With increasing temperature, sigma(s) values of the alcohols d ecreased and those of HDO increased. The data indicate that water and solutes use different pathways when crossing the membrane. Ordinary an d isotopic water use water channels and the other test solutes use the bilayer array (composite transport model of membrane). Changes in sig ma(s) values with temperature were found to be a sensitive measure for the open/closed state of water channels. The decrease of sigma(s) wit h temperature was theoretically predicted from the temperature depende nce of P-s and Lp. Differences between predicted and measured values o f sigma(s) allowed estimation of the bypass flow (slippage) of solutes through water channels which did not completely exclude test solutes. The permeability of channels depended on the structure and size of te st solutes. It is concluded that water channels are much less selectiv e than is usually thought. Since water channels represent single-file or no-pass pores, solutes drag along considerable amounts of water as they diffuse across channels. This results in low overall values of si gma(s). The sigma(s) of HDO was extremely low. Its response to tempera ture was opposite to that for the sigma(s) of the alcohols. This sugge sted a stronger effect of temperature on the hydraulic (osmotic) than on the diffusive water flow across individual water channels, i.e. a d ifferential sensitivity of different mechanisms to temperature.