Determination of cell-wall porosity by microscopy: walls of cultured cells and pollen tubes

In previous studies the porosity of the plant cell wall has been estimated to lie between 3-5 and 6 nm, depending on the cell type investigated and the method used. Morinda citrifolia cells from cultures at various stages of growth were incubated for 60 min with a colloidal gold sol (CGS) of size range 1 -9-7-9 nm. Penetration of the cell wall by these gold particles was limited to the log phase of the culture growth cycle, with maximum uptake occurring in early log phase. The size of particles observed within the early log phase cells ranged from 2-5 to 5-7 nm. Since the cell wall was not, as in other methods, disrupted or altered, we conclude that the wall pores are up to 5-7 nm mean diameter at this early stage, but decrease rapidly as the culture ages. These results were confirmed with observations on the uptake of FITC-labelled dextrans. FD 20S dextrans (c. 6-6 nm diameter) failed to penetrate the wall, while FD 10S (c. 4-5 nm diameter) was taken up across the wall and into the protoplast. Further work using this approach demonstrated that pollen tubes of Nicotiana sp. were less permeable than those of Tradescantia sp. With the former, FD 20S only penetrated to the cytoplasm very slowly (18 h to become detectable), while with the latter, uptake was more rapid (1-2 h to become detectable).