Effect of valinomycin and gramicidin D on the reflection coefficient of soybean root systems

Valinomycin and gramicidin D were used to test the hypothesis that a lipoprotein membrane is the osmotic barrier in mature root systems. Hydroponically grown soybean (var. Ranson) root systems were pressurized in steps between 0.2 and 5.0 bars at 25 C. Steady-state flow rates and exudate osmotic potentials were measured at each pressure. Valinomycin or gramicidin D to a final concentration of 2.6 or 5.0 micromolar, respectively, was injected into the nutrient solution, and steady-state values were again measured between 0.2 and 5.0 bars. Both ionophores reduced the x intercept of the straight line portion of the flux rate versus pressure curve, and increased the exudate osmotic potential at infinite flow rate. Valinomycin lowered the reflection coefficient from 0.91 to 0.76, whereas gramicidin D lowered the reflection coefficient from 0.86 to 0.81. The results support the hypothesis that one or more lipoprotein membranes in the root system function in regulation of ion movement from the ambient solution to the stele during high transpiration rates.