XYLEM PRESSURE RESPONSE IN MAIZE ROOTS SUBJECTED TO OSMOTIC-STRESS - DETERMINATION OF RADIAL REFLECTION COEFFICIENTS BY USE OF THE XYLEM PRESSURE PROBE

The absolute pressure in conducting xylem vessels of roots of 2-week-o ld, slowly transpiring intact maize plants (bathed in nutrition medium ) was determined to be +0.024 +/- 0.044 MPa using the xylem pressure p robe, When the roots were subjected to osmotic stress (NaCl, KCl or su crose), the xylem pressure decreased immediately and became more negat ive, However, the response of xylem pressure to osmotic stress was con siderably attenuated, indicating that the radial reflection coefficien ts, sigma(r), of the maize root for these solutes were rather low (bet ween 0.2 and 0.4 depending on the concentration of the osmoticum). The low values of sigma(r) may be caused (partly) by unstirred layer effe cts, In repeated osmoticum/nutrition regimes a complex pattern of chan ges in xylem pressure was observed which was apparently linked to the interplay between transpiration and (passive and/or active) solute loa ding of the xylem. These processes were not observed when the roots we re subjected to osmotic stress after excision, In this case, a biphasi c response was observed comparable to that found for excised roots usi ng the root pressure probe.