Effect of osmotic stress on turgor pressure in mung bean root cells

Turgor pressure in cells of the elongating region of intact mung bean roots was directly measured by using the pressure-probe technique. After the external osmotic pressure had been increased from 0 MPa to 0.5 MPa, turgor pressure rapidly decreased by about 0.5 MPa from 0.65 MPa to 0.14 MPa and root elongation stopped. Subsequent turgor regulation was clearly confirmed, which followed the osmotic adjustment to maintain a constant difference in the osmotic pressure between root-cell sap and the external medium (Δ II). It took at least 6 h for turgor pressure to recover to an adjusted constant level of about 0.5 MPa due to turgor regulation, but rootelongation resumed within only an hour after the osmotic treatment. Therefore, the resumption of root elongation under osmotic stress could not have been directly connected with turgor regulation. Furthermore, since the amounts of decrease in turgor pressure just after applications of various degrees of osmotic stress could be interpreted in relation to those in Δ II, hydraulic conductivity between the inside and the outside of root cells must be large enough to attain water potential equilibrium rapidly in response to osmotic stress. We conclude that turgor pressure in the cells of the elongating region of mung bean roots is determined mainly by Δ II because of water potential equilibrium.