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Plant Physiology 65:74-77 (1980)
© 1980 American Society of Plant Biologists

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Effect of Valinomycin and Gramicidin D on the Reflection Coefficient of Soybean Root Systems 1

Albert H. Markhart, III2 and Aubrey W. Naylor

Department of Botany, Duke University, Durham, North Carolina 27706

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.

2 Current address: Botanisches Institut der Tierärztlichen Hochschule, Hannover, D-3000 Hannover 71, Federal Republic of Germany.

1 This research was supported by National Science Foundation Grants PCM76-1142 A01-2 to P. J. Kramer and DEB77-15845 to H. Hellmers for support of the Duke University Phytotron.






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Copyright © 1980 by the American Society of Plant Biologists