Turgor-Volume Regulation and Cellular Water Relations of Nicotiana tabacum Roots Grown in High Salinities

Abstract

Nicotiana tabacum plants were grown in solution culture with salinity treatments of 1, 100 and 200 mol m^-3 [NaCl], in Hoagland solution. After several weeks, solute concentrations and osmotic pressure of cell sap from the roots were measured. Increases in cellular [Na⁺] and [Cl^-] and a smaller reduction in [K⁺] accounted for the difference in sap osmotic pressure between the 200 mol m^-3 and 1 mol m^-3 treatments. Turgor pressure (P) of fully expanded cortex cells measured with the pressure probe were 0.48 MPa in 1 mol m^-3, 0.24 MPa in 100 mol m^-3, 0.20 MPa in 200 mol m^-3, and these values agreed with those calculated by difference between internal and external osmotic pressure. Low values of volumetric elastic modulus (ε), ranging from 1.2 MPa to 3.0 MPa at P = 0.42 MPa were obtained, which accounted for long equilibration times to changes in water potential. There was no effect of high salinities on ε after accounting for the fact that ε was a function of P and neither was there an effect on hydraulic conductivity (L_p), which ranged between 1.9 × 10^-8 and 24.1 × 10^-8 m s^-1 Mpa^-1. At 200 mol m^-3 [NaCl]_o, and to a lesser degree at 100 mol m^-3 [NaCl]_o, root hairs became deformed to resemble spherical bladders (mean diameter = 88 µm) which displayed similar P and water relations to other epidermal cells and cortex cells.

In other experiments the response to a sudden reduction in [NaCl], from 200 to 1 mol m^-3 was studied. P of cortex cells first rapidly increased from about 0.15 MPa to 0.53 MPa and then slowly declined with a half time of about 35 min to a new steady state of 0.3 MPa. This level was maintained in intact roots for at least 48 h. The rate of the initial increase in P is limited by water flow into the cells while the slow decline is limited by solute efflux from the cells with water following osmotically. The efflux was mainly in response to reduced external osmotic pressure rather than [NaCl]_o. Efflux of Na⁺, K⁺ and Cl^- accounted for the decrease in internal osmotic pressure but it is possible that the membrane also became more permeable to sugars. With the exception of bladder hairs, the overall integrity of the cell membrane was maintained since L_p did not increase and P declined smoothly to the new level with no evidence of rupture and resealing of the membrane. It is argued that the loss of solutes after the step down in external osmotic pressure consists of turgor or volume regulation in which solute permeability increases steeply as turgor or volume goes above a threshold.