Compartment analysis of plant cells by means of turgor pressure relaxation: II. Experimental results onChara corallina
The hydraulic conductivity of the intracellularly-perfused internodal cell ofNitella flexilis was measured by establishing and maintaining osmotic and hydrostatic pressure gradients between the inside and the outside of the cell. The osmotic filtration coefficient (L PD ) determined at zero hydrostatic pressure difference varied between 1.55 and 2.32×10−5 cm/sec/atm. Under internal perfusion conditions no polarity between endosmotic and exosmotic flow was observed. The overall hydrostatic filtration coefficient (L P ) was determined with a step change in hydrostatic pressure up to 0.2 atm, while the osmotic pressure difference was maintained at zero.L P was considerably greater than theL PD , i.e., 14.1 to 19.2×10−5 cm/sec/atm. Theoverall L P of such internodes, which showed protoplasmic streaming and action potentials was the same as that of the isolated cell walls, the latter being 13.2 to 19.9×10−5 cm/sec/atm. Some of these results are consistent with previous results onNitella using different techniques. The situation inNitella where at abnormallylow internal pressure the barrier to hydrostatic pressure-driven water flow does not reside in the plasmalemma but in an in-series structure is comparable to that in the squid axon where the normal internal pressure is close to zero. An interpretation is offered for the finding in the alga that athigh internal pressures the plasmalemma becomes the rate-limiting structure for hydrostatic pressure-driven water flow. It is suggested that the internal pressure pushes a large fraction of the plasmalemma against skeletal nonporous regions of the cell wall. This suggestion entailing a pressure-dependent cell wall-plasmalemma juxtaposition was also deployed in interpreting previous observations in plant cells on water flow polarity (i.e., observations showing that exosmotic rates are less than endosmotic).