Limiting diameters of pores and the surface structure of plant cell walls.

Tepfer and Taylor (1) claimed that our earlier estimates (2) of the limiting pore size of plant cell walls were substantially in error. I will demonstrate that the experimental results that Tepfer and Taylor used to make their claim were not feasible and will resolve the controversy that they believe exists. We used a solute exclusion technique to determine the size of the pores in the walls of plant cells that allow molecules to freely penetrate the wall; we estimated that these pores were about 40 A and suggested that molecules larger than this would have difficulty passing through the walls of living plant cells (2). Tepfer and Taylor (1) constructed a gel permeation column from cell walls of homogenized bean hypocotyls and found that proteins larger than those we would predict to pass through a cell wall did permeate a sizable portion of the wall matrix. Although they stated that such a gel permeation column could not determine the diameters of pores which allow molecules to pass completely through a cell wall, they did suggest that wall shrinkage in hypertonic solutions reduces the pore size artifactually, and that in turgid cells ovalbumin, a molecule with a diameter much larger than 40 A, would penetrate the cell wall of a living cell. They reported that epidermal cells of oat roots shrank but did not plasmolyze in solutions of 0.2M sucrose, whereas 0.3M sucrose caused plasmolysis. They reasoned that for cells placed in 0.2M sucrose plus 5 percent ovalbumin, the sucrose would supply most of the osmotic potential required for plasmolysis, and the ovalbumin would cause little osmotic shrinkage. Tepfer and Taylor (1) stated that 0.2M sucrose plus 5 percent ovalbumin caused plasmolysis and concluded that "under these conditions it appears that the ovalbumin was able to penetrate the cell wall and hence cause plasmolysis." I determined the osmotic potentials of such solutions by freezing-point-depression osmometry; the 0.2M sucrose is about -5.4 bar, and 0.3M sucrose is about -8.2 bar (Fig. 1). Observation of plasmolysis by microscopy with a 0.1M increase in external solute concentration (about 3.0 bars) is reasonable. Since the osmotic potentials among the individual root cells are heterogeneous, observation of incipient plasmolysis of a population of cells would be difficult with increases in solute concentration much less than 0.1M.