Apparent Free Space of Plant Roots

Abstract

Although it is now generally recognized that solutes may permeate some appreciable fraction of the volume of certain plant organs by free diffusion (4, 6, 10, 19), the magnitude and physiological significance of free space remain highly debatable. Free space measurements have been questioned more with regard to the adequacy of removal of superficial solution than for any other reason. The retention of a very thin film, especially on fine roots after blotting or centrifuging may falsely magnify the free space value (20). To obviate this difficulty, the authors have developed a technique for tagging the external medium with a colloid which does not penetrate free space. India ink particles (Higgins American India Ink No. 4415, Waterproof Black, Higgins Ink Company , Brooklyn, N. Y.) are not adsorbed to or repelled from root surfaces under any of the experimental conditions described. This was confirmed by microscopic examination of roots mounted in India ink following a variety of pretreatments including leaching in distilled water and/or equilibration with NaCl solutions. By measuring the amount of India ink carried over on the surface of the root, the volume of root medium superficially retained by the root is determined. The quantity of test solute present in this volume of medium is subtracted from the total quantity of test solute carried over with the root to give the amount of solute in the free space of the root. Using this technique, the apparent free space values were measured for roots of a number of plant species for which data are available in the literature. In the course of these preliminary studies, variable free space values were found for pea and bean roots depending on experimental conditions. These conditions (duration of washing of roots in distilled water, nature of test solute, and duration of equilibration with solute) were therefore examined critically so that a reasonable explanation of the observed effects could be given. The term "apparent free space" (AFS) includes a built-in apology: The user concedes that AFS describes a hypothetical partial volume which is a true measure of the free space only if the test solute con-'