WATER TRANSPORT ACROSS ROOTS

Usually, roots are looked at as rather perfect osmometers with the endodermis being the 'root membrane' which is equivalent to the plasma membrane of cells. However, this 'single-equivalent-membrane model' of the root does not explain the findings of a variable hydraulic resistance of roots as well as of differences between hydraulic and osmotic water flow and of low reflection coefficients of roots. Recent work with the root pressure probe is reviewed and discussed which indicates that the simple osmometer model of the root has to be extended by incorporating its composite structure, i.e. the fact that there are different parallel pathways for water in the root, namely, the cell-to-cell and apoplasmic path. The new 'composite transport model of the root' readily explains the experimental findings mentioned above. Pressure probe work with roots in which the endodermis was punctured to create an additional parallel path as well as anatomical studies support the model.