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

Ca exerts a strongly (lep)ressive effect upoil the iollmetal)olic diffusive enitry of \Na iilto thle cells of the zonie of cell division (0-1.8 iiimi fromii the root tip) of the primlary root of Zcal iwiays (9). Uinder the experimental concditions imiposed in this previous Nvork the maximum effect of Ca was achieved at a relatively, very low concentratioln, about 0.10 meq ler liter. It was postulated that Ca is active at the cell stirface where it stabilizes anid alters the permiieabilitv of a barrier to nonmetabolic ion entry. presumably the outer cell membrane. This suppositioln 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 involvement of Ca with the cell surface rather than competitioln of Ca with Na for adsorption sites deeper in the cell. Our previous paper (9) was concerne(d onlyv with the effect of Ca (anld Sr) uiponi nonoMnetal)olic 'Na uptake since ollly the zonle of cell divisioi1 was inivestigated anld this tisstue displays no capacity for mlletabolic accumulationi of Na, Ca, Sr or Cl ioins (5, 6, 8). \Ve are aware that nonle of the exl)erimenital criteria currelntly emiiployed to distingtuish betw\eenl imletabolic alid noimetabolic ionl uptake is entirely free of objectioni. Ftor instanice. lowerinig the temperature drastically or admiiniisterinig imletabolic poisonls may well affect cell permeabilitas well as preventing operation of metabolic accumulation mechanisms. However, the simplest explanation of the following observations would seenm 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 260 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 althouglh we have not yet attempted to confirm this) in the nmedium comies to an apparent equilibrium wvith