Hypoxia Induces Membrane Depolarization and Potassium Loss from Wheat Roots but does not Increase their Permeability to Sorbitol

Buwalda, F., Thomson, C. J., Steigner, W., Barrett-Lennard, E. G., Gibbs, J. and Greenway, H. 1988. Hypoxia induces membrane depolarization and potassium loss from wheat roots but does not increase their permeability to sorbitol.—J. exp. Bot. 39: 1169-1183. This paper deals with the responses of roots of wheat (Triticum aestivum L.) to hypoxia with special emphasis on the effects of severe 02 deficiency on membrane integrity, loss of K+ from the root and root membrane potentials. Seminal and crown roots of 26-d-old plants exposed to severe hypoxia (0 003 mol 02 m 3)for 3 h or 10 d prior to excision and subsequently exposed to hypoxic solutions, had slightly lower rates of sorbitol influx and a slightly smaller apparent free space than roots in aerated solutions. These results indicate that neither a few hours nor a 10-d exposure to hypoxia had adverse effects on the membrane integrity of the bulk of the cells in the roots. However, both 6-d-old seedlings and 26-d-old plants lost K+ from the roots following their transfer from aerated to hypoxic nutrient solutions. In the 26-d-old plants, which were of high nutritional status, there was a net K+ efflux from the roots to the external solution. In contrast, with the 6-d-old seedlings, which were of low nutritional status, the decrease in the K+ content of the roots was smaller than the net K+ uptake to the shoots. Exposure of excised roots to 0008 mol 02 m~3 caused a rapid and reversible membrane depolarization from — 120 to —80 mV. These data and the magnitude of the net effluxes strongly suggest that K+ losses during the early stages of hypoxia are due to membrane depolarization rather than to increases in the permeability of membranes to K + . Key words—Hypoxia, membrane integrity, membrane potentials, seminal and crown roots. Correspondence to: Agronomy Group, School of Agriculture, The University of Western Australia, Nedlands, W.A. 6009, Australia.