An Examination of the Refractometric Method for Determining the Water Potential of Plant Tissues

Some sources of error in refractometric determinations of waterpotential are examined and methods of correcting for them oravoiding them are discussed, as well as their relevance to Shardakov’s(Schlieren) method. The sources of the exudation from cut tissues,which causes a rise in the refractive index of water and affectsthat of solutions, are investigated. The exudation consistsmainly of sap released from cut and damaged cells. The amountof sap exuded is independent of the concentration of the externalsolution. A method of estimating the extent of the error thusincurred in water potential determinations is described. Errorarising also from the admixture of cell-wall water to the referencesolutions, lowering their effective concentration, and othersources of error are briefly described. A method of eliminating the curvature in graphs of change ofrefractive index plotted against molarity is suggested. Themethod permits the data from all concentrations to be used indetermining the null point for water exchange, instead of thosenear to the null point only.