Permeation of Uncharged Organic Molecules and Water Through Tomato Roots

Permeation of ethylene glycol, D-mannitol, L glucose, and raffinose through excised tomato roots was investigated. Solute transport from external solution to the xylem sap was determined at various rates of exudate flow achieved in turn by application of different pressures to the solution surrounding the roots. At an applied pressure of two bar, steady-state solute concentrations expressed as a percentage of external concentrations were, 52 per cent for ethylene glycol, 3–4 per cent for mannitol and glucose, and I per cent for raffinose. Such relationships between the chemical structure of the solute and its concentration in the xylem sap are similar to those demonstrated by other workers for solute permeation into single cells. Thus in the present experiments most of these solutes presumably flowed through at least one membrane before reaching the xylem. The data also indicate that the flow of water via wholly extracellular pathways was slight, at most, I per cent of the total flow reaching the xylem via this route.