Age dependence of the hydraulic resistances of the plasma membrane and the tonoplast (vacuolar membrane) in cells of Chara corallina

Hydraulic resistances (reciprocals of hydraulic conductivities) of the cell (Lp⁻¹), the cell wall (Lp_w⁻¹), the membrane (Lp_m⁻¹), the plasma membrane (Lp_pm⁻¹), and the tonoplast (Lp_tp⁻¹) were determined in individual internodal cells of Chara corallina and their dependence on the cell age was studied. The thickness of the cell wall (d) was adopted as an index of the cell age, since the cell wall of spring-grown young cells (sg-cells) was found to be significantly thinner than that of winter-spent old cells (ws-cells). Both Lp_w⁻¹ and Lp_m⁻¹ were found to increase with cell age. Since Lp_m⁻¹ is the sum of Lp_pm⁻¹ and Lp_tp⁻¹, their dependence on the wall thickness was studied. It was found that both Lp_pm⁻¹ and Lp_tp⁻¹ increase with cell age using d as a proxy and that the former is distinctly higher than the latter. The ratio Lp_pm⁻¹/Lp_tp⁻¹ amounts to 30 for 5 μm of d, indicating that the tonoplast is a negligible barrier to osmotic water flow. The ratio decreases with the increase in d and amounts to 5.0 for 11 μm of d, showing that the tonoplast ages faster than the plasma membrane. The physiological meaning of the age dependence of hydraulic resistance of the tonoplast was discussed in terms of the role of the vacuole in the osmoregulation of the cytoplasm.