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Landscape Architecture (E.J.S.), University ofMinnesota, St. Paul, Minnesota 55108 MATERIALS AND METHODSThis study was undertaken to quantify the effect of aluminum and calcium on membrane permeability. The influence of Ca2" (0.2-3.7 millimolar) and Al' (0-3.7 millimolar) on the permeability of root cortical cells of Quercus rubra was measured using three nonelectrolytes (urea, methyl urea, and ethyl urea) Aluminum, as Al3", can interfere with physiological processes and be toxic to plant cells (1,5,(8)(9)(10)15). Plant cell membranes may be a site of primary lesions from Al3" (1,10). Hofler (10) and Bohm-Tuchy (1) showed that Al3" changed the attachment of the plasmalemma to the cell wall causing the membrane to assume a convex shape on plasmolysis. Bohm-Tuchy (1) also suggested that Al3" solidifies the outer region of the protoplasmic layer. Vierstra and Haug (19) using EPR spectroscopy showed that Al3" decreased membrane fluidity in isolated membranes and intact cells of a thermophilic bacterium.Ca2" is another ion well known to influence the integrity and functions of membranes (13,14). Recent work (9) shows negative interactions between Al3+ and Ca2" with regard to calmodulin activity and other physiologic processes (5, 15).The capacity of Al3" and other cations to alter the biochemical and biophysical properties of both the lipid and protein portions of membranes is well reviewed by Haug (8). Changes in the lipid portion of the membrane could result in an alteration of permeability to nonelectrolytes and to water. A major objective of this study was to measure how Al3+ altered the combined permeability of the plasma and vacuolar membranes in the root cortex cells of red oak. A related objective was to determine whether the effects of Al3" vary with the concentration of Ca2". Permeability Measurements. The solute permeability of the plasma membrane and tonoplast in series was measured by the techniques described by Stadelmann (17). Immediately after sectioning, the tissue was sequentially exposed to increasing concentrations of sucrose. The sections were in 0.1 M sucrose for 1 h, in 0.2, 0.3, and 0.4 M for 30 min each, and 0.5 M for 1 h. At this osmolarity (0.59 osm) all intact cells were plasmolyzed. The tissue sections in 0.5 M sucrose were transferred in a droplet of the same solution into a perfusion chamber (21). In the chamber, the sucrose solution was replaced with isotonic solutions or urea family permeators (urea, methyl urea, or ethyl urea

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