Analysis of the effect of permeant solutes on the hydraulic resistance of the plasma membrane in cells of Chara corallina

In the cells of Chara corallina, permeant monohydric alcohols including methanol, ethanol and 1-propanol increased the hydraulic resistance of the membrane (Lp_m^-1). We found that the relative value of the hydraulic resistance (^rLp_m^-1) was linearly dependent on the concentration (C_s) of the alcohol. The relationship is expressed in the equation: ^rLp_m^-1 = ρ_mC_s + 1, where ρ_m is the hydraulic resistance modifier coefficient of the membrane. Ye et al. (2004) showed that membrane-permeant glycol ethers also increased Lp^-1. We used their data to estimate Lp_m^-1 and ^rLp_m^-1. The values of ^rLp_m^-1 fit the above relation we found for alcohols. When we plotted the ρ_m values of all the permeant alcohols and glycol ethers against their molecular weights (MW), we obtained a linear curve with a slope of 0.014 M^-1/MW and with a correlation coefficient of 0.99. We analyzed the influence of the permeant solutes on the relative hydraulic resistance of the membrane (^rLp_m^-1) as a function of the external (π₀) and internal (π_i) osmotic pressures. The analysis showed that the hydraulic resistance modifier coefficients (ρ_m) were linearly related to the MW of the permeant solutes with a slope of 0.012 M^-1/MW and with a correlation coefficient of 0.84. The linear relationship between the effects of permeating solutes on the hydraulic resistance modifier coefficient (ρ_m) and the MW can be explained in terms of the effect of the effective osmotic pressure on the hydraulic conductivity of water channels. The result of the analysis suggests that the osmotic pressure and not the size of the permeant solute as proposed by (Ye et al., J Exp Bot 55:449-461, 2004) is the decisive factor in a solute's influence on hydraulic conductivity. Thus, characean water channels (aquaporins) respond to permeant solutes with essentially the same mechanism as to impermeant solutes.