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Ca exerts a strongly depressive effect upon the noi-metabolic diffusive entry of Na into the cells of the zone of cell division (0-1.8 mm from the root tip) of the primary root of Zea mays (9). Under the experimental conditions imposed in this previous work the maximum effect of Ca was achieved at a relatively very low concentration, about 0.10 meq per liter. It was postulated that Ca is active at the cell surface where it stabilizes and alters the permeability of a barrier to nonmetabolic ion entry, presumably the outer cell membrane. This supposition is based upon early work dealing with the penetration of dyes into animal cells (1) and recent work with plants (3, 4,10, 12). The low amounts of Ca which are effective also suggest an involveimenit of Ca with the cell sturface rather than coml)etition of Ca with Na for adsorption sites deeper in the cell. Our previous paper (9) was concerned only with the effect of Ca (and Sr) upon nonimetabolic Na uptake since only the zone of cell division was investigated and this tissue displays no capacity for metabolic accumulation of Na, Ca, Sr or Cl ionls (5, 6, 8). We are aware that none of the experimental criteria currently employed to distinguish between metabolic and nonmetabolic ion uptake is entirely free of objection. For instance, lowering the temperature drastically or administering metabolic poisons may well affect cell permeability as well as preventing operation of metabolic accumulation mechanisms. However, the simplest explanation of the following observations would seem to be that ion uptake by the tip section is entirely free of any direct connection with a metabolic ion pump: 1) Between 0? and 26? uptake is independent of temperature. 2) Labeled ions are eluted from the tissue into unlabeled solutions at about the same rate as that at which they were originally taken up. 3) Uptake of Cl is minute compared with uptake of cations. 4) Anerobiosis actually increases greatly the rate at which Sr (and probably other ions although we have not yet attempted to confirm this) in the medium comes to an apparent equilibrium with that in the tissue. 5) Although Ca stimulates the respiration of this tissue (7) it depresses the uptake of Rb, an ion whose uptake into mature tissue is stimulated by Ca (12). The fact that Ca decreases the permeability of cells to other solutes has beeln known for a long time. Equally well known is the seemingly incongruous fact first pointed out by Viets in 1944 (17) that Ca (and other polyvalent cations) often stimulate the uptake of K and some other alkali metal ionls. It is clear that any reasonable hypothesis concerning the effect of Ca upon ion uptake must accommodate both of these facts. Our purpose here is to suggest or perhaps merely to reemphasize that Ca exerts 2 essentially distinct and antagonistic effects upon ion uptake. The first of these is a nonspecific depressant effect not directly linked with ion accumulation mechanisms and resulting from stabilization of the cell membrane with a consequent decline in permeability. The second effect we believe to be a specific one resulting from an involvement of Ca with the metabolic accumulation mechanismus of certain alkali cations and resulting in an increased rate of uptake of these ions. To this end we have investigated the effect of Ca upon Na and Rb uptake in both the zone of cell division investigated previously and the zone of cell elongation and vacuolation (1.8-3.8 mm from the root tip) where metabolic ion accumtilation takes place vigorously (5,6,8). Rb and Na were chosen as ions which respectively do and do not display the "Viets effect" at physiological pH's (10). We have coupled this with a study of the loss of endogenous K during ion uptake. Since the appearance of Viets' paper (17) the nature of the effect of Ca upon ion uptake has been the subject of many investigations (3, 4, 9, 10, 12, 14, 18). Like other aspects of ion uptake by plants it remains mysterious. Complete understanding must await identification of the carriers or other agencies responsible for metabolic ion uptake. A few papers which are especially pertinent should be mentioned. In 1957 Kahn and Hanson (12) reported that whereas Ca stimulated K uptake by excised maize roots it depressed K uptake by soybean roots. They analysed their data by the kinetic treatment of Epstein and Hagen (2). This analysis indicated that in both species Ca exerted a stimulatory and a depressant effect upon K uptake. The stimulatory effect predominated in maize. It was ascribed to an increased 1 Received November 2, 1964. 2 This report is based on work performed under Contract No. AT-(11-1) with the United States Atomic Energy Commission.

Effects of Ca Upon Metabolic and Nonmetabolic Uptake of Na and Rb by Root Segments of Zea mays