Ion uptake and electrical potentials in plants

In the field of ion transport, animal physiologists have made substantial progress during the last decade and a great deal of information is now available concerning nerve, muscle, and various epithelial tissues. Before the techniques evolved by the animal physiologists were applied to plants a considerable delay occurred which may have been in part due to the morphological complexities of plant cells. Recently, however, these techniques have been applied with considerable success to plant cells, especially to the large internodal cells of the Characeae. It is now clear that the plant cell may be regarded as a three compartment system consisting of the cell wall, the protoplasm, and the vacuole. The cell wall acts as a physical extension of the environment but the protoplasm is bounded by the plasmalemma and tonoplast membranes which act as the diffusion barriers containing the sites at which ?active transports takes place. Electrolytes move from one compartment to another by diffusion, mass flow, solvent drag, or active transport but, in systems in which osmotic pressure is constant, diffusion and active transport are the main processes involved. It is now almost universally recognised that the electrochemical potential gradient, not simply the concentration gradient, is the driving force on an ion moving passively across a membrane, and several measurements of the driving forces on particular ions have been made. However, the forces measured are usually between the vacuole and the external medium and not the forces across the individual membranes. When the ionic fluxes have also been measured it has been possible to suggest whether active transport takes place. In a few cases, connections with metabolism have been established.Several works on the ionic relations of plants have been published recently including books by Briggs, Hope, and Robertson (1961), Sutcliffe (1962), and Jennings (1963). The general approach adopted in this thesis is that outlined in a review by Dainty (1962). It is not yet possible to treat the problem of ion transport in plants at the molecular level but, by using a suitable member of the Characeae, an attempt has been made to determine more accurately the forces acting on the ions, the membranes at which active transport takes place, and the parameters which control the electrical properties of the membranes. Electrical measurements have also been made on a higher plant organ, the exuding root system, in order to obtain evidence on the controversial problem of transport into the xylem