Analysis of the Exudation Process in Tomato Plants

We started from the supposition that the exudation is brought about by two processes: (1) an active transport of salt to the xylem, the ‘salt secretion’, a process that depends on metabolism and is sensitive to inhibitors, and (2) a water transport effected by the osmotic suction of the sap in the xylem. It was demonstrated that by enhancing the concentration of the medium the exudation rate rapidly decreases. This is followed by a slower rise to a new level which is the consequence of the salt secretion that continues and enhances the osmotic value of the exudation sap. After some time secondary changes of the exudation rate appear. In lowering the concentration of the medium identical changes occur, but now in the reverse direction. By tracing the influence of addition of substances different in mol. weight and lipoid solubility it could be established that the gradual change was the result of salt secretion and not of permeation of the added substance. From this simple supposition about the causes of the exudation process we derived formulae for calculating the salt secretion, the osmotic value of the exudation sap, and the conductivity for water of the root system. Though most observations are in harmony with this concept, there is a difficulty that too low a value is mostly found for the osmotic value of the exudation sap. The possible causes of this deviation are discussed. There is no necessity to accept an active water secretion as well as active salt secretion since salt and water transport cannot be separated. Important factors in exudation are water conductivity of the protoplasm and salt secretion into the xylem. Both factors are influenced by the osmotic value of the outer solution (D.P.D. of the medium) and by the presence of ions in the outer solution. The two factors must have an identical influence on the transport of water into the transpiration stream.