Effects of Osmotic Pressure on Exudation from Corn Roots

Abstract

For single, unbranched roots of corn (Zea mays L., Pioneer Hybrid 309-B), the rate of exudation per square centimetre of root surface (Jw) decreased logarithmically with increases in the external osmotic pressure (IIo) . The formula Jw = Lp(IIx - IIo), where Lp is the conductivity of the root to osmotic water movement and IIx is the osmotic pressure of the xylem sap, is useful but has limitations. Lp may vary with IIo, and IIx is difficult to measure because the exudate is not always a reliable sample of the xylem sap. The rate of exudation was predicted by the relation Jw = ce -kIIo. The constants c and k were determined (c = 18 [mu]l/cm2/18hr; k = 0.94/bar) for roots grown in Hoagland''s solution to which various amounts of sodium chloride had been added to increase IIo. Roots preconditioned to a high salinity were compared to roots which were grown in Hoagland''s solution and then tested at higher osmotic pressures. The similarities of Jw and the differences in the osmotic pressures of the xylem exudates for these 2 sets of roots indicated that salt was being removed from the xylem sap in the upper part of the root.