Suitability of the Osmotic Shock Procedure for the Analysis of Membrane Transport in Root Tips ofZea maysL.

Abstract

Root tips of Zea mays L. previously subjected to osmotic shock were studied as to uptake and efflux of glucose and uptake of phosphate, as well as release of protein and other Lowry-positive substances. The osmotic shock procedure applied was similar to that frequently used in order to release periplasmic proteins from bacteria. After treatment with hypertonic solutions of sorbitol (>0·3 M) the uptake of phosphate and glucose is reduced and protein is released into the medium. All osmotic treatments leading to a significant reduction of either glucose or phosphate uptake are accompanied by a strongly increased release of glucose and Lowry-positive substances. Damage of plasma membrane barrier function can be directly observed by the uptake of Evans Blue (Gaff and Okong'o-Ogola, 1971) into the cells involving mainly older vacuolated rhizodermal cells. The occurrence and extent of the alterations of membrane semipermeability are strongly dependent on the duration of exposure to and the concentration of the sorbitol solution used. The velocity of the osmotic transitions (from hypotonic to hypertonic and vice versa) is of minor importance. Sensibility of cells to osmotic stress increases with differentiation and vacuolation. Conditions for a selective reduction of phosphate uptake seem to occur when only meristem and near tip regions subjected to a mild osmotic shock are used.