Comparison Between Osmotic and Hydrostatic Water Flows in a Higher Plant Cell: Determination of Hydraulic Conductivities and Reflection Coefficients in Isolated Epidermis of Tradescantia virginiana

Abstract

Hydraulic conductivity (L_p), volumetric elastic modulus (ε) and reflection coefficients (δ) have been determined for cells from isolated strips of the lower epidermis of leaves of Tradescantia virginiana using the pressure probe. L_p was (6.4 ± 4.5) × 10^-8 ms^-1 Mpa^-1 [(6.4 ± 4.5) × 10^-7 cm s^-1 bar^-1; mean ± s.d., n = 15 cells] and was independent of the cell turgor pressure (P) and of osmotic pressure of the bathing medium. P in Johnson's solution (π° = 0.09 MPa) was 0.42-0.67 MPa (4.2-6.7 bar), which was somewhat larger than in the intact tissue. ε increased linearly with increasing P in the pressure range from zero to full turgor. Reflection coefficients of some non-electrolytes were determined by measuring the ΔP in response to a change in external osmotic pressure (Δπ°) after the addition of the solutes. The data were corrected for solute flow. For sucrose, mannitol, urea, acetamide, formamide, glycerol and ethylene glycol, δ was close to unity and the cells behaved like ideal osmometers. For the monohydroxyalcohols n-propanol ( δ = -0.58), isopropanol (δ = 0.26), ethanol (δ = 0.25) and methanol (δ = 0.15), rather low reflection coefficients were found which were even negative for some solutes and cells. Values of δ obtained from measuring the inital water (volume) flow were in agreement with those determined from the ΔP/Δπ° ratios. For the rapidly permeating substances, the changes in turgor after the addition of solute were transient and the equilibration of solutes between cell and medium could be measured using the probe. Although unstirred layers may affect the equilibration of solute it should, in principle, be possible to use the technique for the determination of permeability coefficients of membranes of higher plant cells.